Mechanical Properties of Metals

1. Introduction

Often materials are subject to forces (loads) when they are used. Mechanical engineers calculate those forces and material scientists how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions.

Materials scientists learn about these mechanical properties by testing materials. Results from the tests depend on the size and shape of material to be tested (specimen), how it is held, and the way of performing the test. That is why we use common procedures, or standards, which are published by the ASTM.

2. Concepts of Stress and Strain

To compare specimens of different sizes, the load is calculated per unit area, also called normalization to the area. Force divided by area is called stress. In tension and compression tests, the relevant area is that perpendicular to the force. In shear or torsion tests, the area is perpendicular to the axis of rotation.

= F/A0 tensile or compressive stress

= F/A0 shear stress

The unit is the Megapascal = 106 Newtons/m2.

There is a change in dimensions, or deformation elongation, L as a result of a tensile or compressive stress. To enable comparison with specimens of different length, the elongation is also normalized, this time to the length L. This is called strain, .

 = L/L

The change in dimensions is the reason we use A0 to indicate the initial area since it changes during deformation. One could divide force by the actual area, this is called true stress (see Sec. 6.7).

For torsional or shear stresses, the deformation is the angle of twist, Fig. 6.1) and the shear strain is given by:

 = tg 

3. Stress—Strain Behavior

Elastic deformation. When the stress is removed, the material returns to the dimension it had before the load was applied. Valid for small strains (except the case of rubbers).

Deformation is reversible, non permanent

Plastic deformation. When the stress is removed, the material does not return to its previous dimension but there is a permanent, irreversible deformation.

In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke's law:

= E 

That is, E is the slope of the stress-strain curve. E is Young's modulus or modulus of elasticity. In some cases, the relationship is not linear so that E can be defined alternatively as the local slope:

E = d/d

Shear stresses produce strains according to:

= G 

where G is the shear modulus.

Elastic moduli measure the stiffness of the material. They are related to the second derivative of the interatomic potential, or the first derivative of the force vs. internuclear distance (Fig. 6.6). By examining these curves we can tell which material has a higher modulus. Due to thermal vibrations the elastic modulus decreases with temperature. E is large for ceramics (stronger ionic bond) and small for polymers (weak covalent bond). Since the interatomic distances depend on direction in the crystal, E depends on direction (i.e., it is anisotropic) for single crystals. For randomlyoriented policrystals, E is isotropic.

4. Anelasticity

Here the behavior is elastic but not the stress-strain curve is not immediately reversible. It takes a while for the strain to return to zero. The effect is normally small for metals but can be significant for polymers.

5. Elastic Properties of Materials

Materials subject to tension shrink laterally. Those subject to compression, bulge. The ratio of lateral and axial strains is called the Poisson's ratio .

= lateral/axial

The elastic modulus, shear modulus and Poisson's ratio are related by E = 2G(1+)

6. Tensile Properties

Yield point. If the stress is too large, the strain deviates from being proportional to the stress. The point at which this happens is the yield point because there the material yields, deforming permanently (plastically).

Yield stress. Hooke's law is not valid beyond the yield point. The stress at the yield point is called yield stress, and is an important measure of the mechanical properties of materials. In practice, the yield stress is chosen as that causing a permanent strain of 0.002 (strain offset, Fig. 6.9.)

The yield stress measures the resistance to plastic deformation.

The reason for plastic deformation, in normal materials, is not that the atomic bond is stretched beyond repair, but the motion of dislocations, which involves breaking and reforming bonds.

Plastic deformation is caused by the motion of dislocations.

Tensile strength. When stress continues in the plastic regime, the stress-strain passes through a maximum, called the tensile strength (TS) , and then falls as the material starts to develop aneck and it finally breaks at the fracture point (Fig. 6.10).

Note that it is called strength, not stress, but the units are the same, MPa.

For structural applications, the yield stress is usually a more important property than the tensile strength, since once the it is passed, the structure has deformed beyond acceptable limits.

Ductility. The ability to deform before braking. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation max or maximum area reduction.

%EL = max x 100 %    

%AR = (A0 - Af)/A0

These are measured after fracture (repositioning the two pieces back together).

Resilience. Capacity to absorb energy elastically. The energy per unit volume is the

area under the strain-stress curve in the elastic region.

Toughness. Ability to absorb energy up to fracture. The energy per unit volume is the total area under the strain-stress curve. It is measured by an impact test (Ch. 8).

7. True Stress and Strain

When one applies a constant tensile force the material will break after reaching the tensile strength. The material starts necking (the transverse area decreases) but the stress cannot increase beyond TS. The ratio of the force to the initial area, what we normally do, is called the engineering stress. If the ratio is to the actual area (that changes with stress) one obtains the true stress.

8. Elastic Recovery During Plastic Deformation

If a material is taken beyond the yield point (it is deformed plastically) and the stress is then released, the material ends up with a permanent strain. If the stress is reapplied, the material again responds elastically at the beginning up to a new yield point that is higher than the original yield point (strain hardening, Ch. 7.10). The amount of elastic strain that

it will take before reaching the yield point is called elastic strain recovery (Fig. 6. 16).

9. Compressive, Shear, and Torsional Deformation

Compressive and shear stresses give similar behavior to tensile stresses, but in the case of compressive stresses there is no maximum in the  curve, since no necking occurs. 

10.Hardness

Hardness is the resistance to plastic deformation (e.g., a local dent or scratch). Thus, it is a measure of plastic deformation, as is the tensile strength, so they are well correlated. Historically, it was measured on an empirically scale, determined by the ability of a material to scratch another, diamond being the hardest and talc the softer. Now we use standard tests, where a ball, or point is pressed into a material and the size of the dent is measured. There are a few different hardness tests: Rockwell, Brinell, Vickers, etc. They are popular because they are easy and non-destructive (except for the small dent).

11.Variability of Material Properties

Tests do not produce exactly the same result because of variations in the test equipment, procedures, operator bias, specimen fabrication, etc. But, even if all those parameters are controlled within strict limits, a variation remains in the materials, due to uncontrolled variations during fabrication, non homogenous composition and structure, etc. The measured mechanical properties will show scatter, which is often distributed in a Gaussian curve (bell-shaped), that is characterized by the mean value and the standard deviation (width).

12.Design/Safety Factors

To take into account variability of properties, designers use, instead of an average value of, say, the tensile strength, the probability that the yield strength is above the minimum value tolerable. This leads to the use of a safety factor N > 1 (typ. 1.2 - 4). Thus, a working value for the tensile strength would be W =TS / N.

Important Terms:

AnelasticityDuctilityElastic deformationElastic recoveryEngineering strainEngineering stressHardnessModulus of elasticityPlastic deformationPoisson’s ratioProportional limitShearTensile strengthToughnessYieldingYield strength

Not tested: true stress-true stain relationships, details of the different types of hardness tests, but should know that hardness for a given material correlates with tensile strength. Variability of material properties

 

Chapter 1. Introduction

1 .1 Historical Perspective

Materials are so important in the development of civilization that we associate Ages with them. In the origin of human life on Earth, the Stone Age, people used only natural materials, like stone, clay, skins, and wood. When people found copper and how to make it harder by alloying, the Bronze Age started about 3000 BC. The use of iron and steel, a stronger material that gave advantage in wars started at about 1200 BC. The next big step was the discovery of a cheap process to make steel around 1850, which enabled the railroads and the building of the modern infrastructure of the industrial world.

1.2 Materials Science and Engineering

Understanding of how materials behave like they do, and why they differ in properties was only possible with the atomistic understanding allowed by quantum mechanics, that first explained atoms and then solids starting in the 1930s. The combination of physics, chemistry, and the focus on the relationship between the properties of a material and its microstructure is the domain of Materials Science. The development of this science allowed designing materials and provided a knowledge base for the engineering applications (Materials Engineering).

Structure:

At the atomic level: arrangement of atoms in different ways. (Gives different properties for graphite than diamond both forms of carbon.)

At the microscopic level: arrangement of small grains of material that can be identified by microscopy. (Gives different optical properties to transparent vs. frosted glass.)

Properties are the way the material responds to the environment. For instance, the mechanical, electrical and magnetic properties are the responses to mechanical, electrical and magnetic forces, respectively. Other important properties are thermal (transmission of heat, heat capacity), optical (absorption, transmission and scattering of light), and the chemical stability in contact with the environment (like corrosion resistance).

Processing of materials is the application of heat (heat treatment), mechanical forces, etc. to affect their microstructure and, therefore, their properties.

1.3 Why Study Materials Science and Engineering?

To be able to select a material for a given use based on considerations of cost and performance.

To understand the limits of materials and the change of their properties with use. To be able to create a new material that will have some desirable properties.

All engineering disciplines need to know about materials. Even the most "immaterial", like software or system engineering depend on the development of new materials, which in turn alter the economics, like software-hardware trade-offs. Increasing applications of system engineering are in materials manufacturing (industrial engineering) and complex environmental systems.

1.4 Classification of Materials

Like many other things, materials are classified in groups, so that our brain can handle the complexity. One could classify them according to structure, or properties, or use. The one that we will use is according to the way the atoms are bound together:

Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.

Semiconductors: the bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

Ceramics: atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Examples: glass, porcelain, many minerals.

Polymers: are bound by covalent forces and also by weak van der Waals forces, and usually based on H, C and other non-metallic elements. They decompose at moderate temperatures (100 – 400 C), and are lightweight. Other properties vary greatly. Examples: plastics (nylon, Teflon, polyester) and rubber.

Other categories are not based on bonding. A particular microstructure identifies composites, made of different materials in intimate contact (example: fiberglass, concrete, wood) to achieve specific properties. Biomaterials can be any type of material that is biocompatible and used, for instance, to replace human body parts.

1.5 Advanced Materials

Materials used in "High-Tec" applications, usually designed for maximum performance, and normally expensive. Examples are titanium alloys for supersonic airplanes, magnetic alloys for computer disks, special ceramics for the heat shield of the space shuttle, etc.

1.6 Modern Material's Needs

Engine efficiency increases at high temperatures: requires high temperature structural materials

Use of nuclear energy requires solving problem with residues, or advances in nuclear waste processing.

Hypersonic flight requires materials that are light, strong and resist high temperatures. Optical communications require optical fibers that absorb light negligibly. Civil construction – materials for unbreakable windows. Structures: materials that are strong like metals and resist corrosion like plastics.

 2.2 Fundamental Concepts

Atoms are composed of electrons, protons, and neutrons. Electron and protons are negative and positive charges of the same magnitude, 1.6 × 10-19 Coulombs.

The mass of the electron is negligible with respect to those of the proton and the neutron, which form the nucleus of the atom. The unit of mass is an atomic mass unit (amu) = 1.66 × 10-27 kg, and equals 1/12 the mass of a carbon atom. The Carbon nucleus has Z=6, and A=6, where Z is the number of protons, and A the number of neutrons. Neutrons and protons have very similar masses, roughly equal to 1 amu. A neutral atom has the same number of electrons and protons, Z.

A mole is the amount of matter that has a mass in grams equal to the atomic mass in amu of the atoms. Thus, a mole of carbon has a mass of 12 grams. The number of atoms in a mole is called the Avogadro number, Nav = 6.023 × 1023. Note that Nav = 1 gram/1 amu.

Calculating n, the number of atoms per cm3 in a piece of material of density  (g/cm3).

n = Nav ×  / M

where M is the atomic mass in amu (grams per mol). Thus, for graphite (carbon) with a density  = 1.8 g/cm3, M =12, we get 6 × 1023 atoms/mol × 1.8 g/cm3 / 12 g/mol) = 9 × 1022 C/cm3.

For a molecular solid like ice, one uses the molecular mass, M(H2O) = 18. With a density of 1 g/cm3, one obtains n = 3.3 × 1022 H2O/cm3. Note that since the water molecule contains 3 atoms, this is equivalent to 9.9 × 1022 atoms/cm3.

Most solids have atomic densities around 6 × 1022 atoms/cm3. The cube root of that number gives the number of atoms per centimeter, about 39 million. The mean distance between atoms is the inverse of that, or 0.25 nm. This is an important number that gives the scale of atomic structures in solids.

2.3 Electrons in Atoms

The forces in the atom are repulsions between electrons and attraction between electrons and protons. The neutrons play no significant role. Thus, Z is what characterizes the atom.

The electrons form a cloud around the neutron, of radius of 0.05 – 2 nanometers. Electrons do not move in circular orbits, as in popular drawings, but in 'fuzzy' orbits. We cannot tell how it moves, but only say what is the probability of finding it at some distance from the nucleus. According to quantum mechanics, only certain orbits are allowed (thus, the idea of a mini planetary system is not correct). The orbits are identified by a principal quantum number n, which can be related to the size, n = 0 is the smallest; n = 1, 2 .. are larger. (They are "quantized" or discrete, being specified by integers). The angular momentum l is quantized, and so is the projection in a specific direction m. The structure of the atom is determined by the Pauli exclusion principle, only two electrons can be placed in an orbit with a givenn, l, m – one for each spin. Table 2.1 in the textbook gives the number of electrons in each shell (given by n) and subshells (given by l).

 2.4 The Periodic Table

Elements are categorized by placing them in the periodic table. Elements in a column share similar properties. The noble gases have closed shells, and so they do not gain or lose electrons near another atom. Alkalis can easily lose an electron and become a closed shell; halogens can easily gain one to form a negative ion, again with a closed shell. The propensity to form closed shells occurs in molecules, when they share electrons to close a molecular shell. Examples are H2, N2, and NaCl.

The ability to gain or lose electrons is termed electronegativity or electropositivity, an important factor in ionic bonds.

2.5 Bonding Forces and Energies

The Coulomb forces are simple: attractive between electrons and nuclei, repulsive between electrons and between nuclei. The force between atoms is given by a sum of all the individual forces, and the fact that the electrons are located outside the atom and the nucleus in the center.

When two atoms come very close, the force between them is always repulsive, because the electrons stay outside and the nuclei repel each other. Unless both atoms are ions of the same charge (e.g., both negative) the forces between atoms is always attractive at large internuclear distances r. Since the force is repulsive at small r, and attractive at small r, there is a distance at which the force is zero. This is the equilibrium distance at which the atoms prefer to stay.

The interaction energy is the potential energy between the atoms. It is negative if the atoms are bound and positive if they can move away from each other. The interaction energy is the integral of the force over the separation distance, so these two quantities are directly related. The interaction energy is a minimum at the equilibrium position. This value of the energy is called the bond energy, and is the energy needed to separate completely to infinity (the work that needs to be done to overcome the attractive force.) The strongest the bond energy, the hardest is to move the atoms, for instance the hardest it is to melt the solid, or to evaporate its atoms.

2.6 Primary Interatomic Bonds

Ionic Bonding

This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl - and less around Na, forming Na+. Ionic bonds are the strongest bonds. In real solids, ionic bonding is usually combined with covalent bonding. In this case, the fractional ionic bonding is defined as %ionic = 100 × [1 – exp(-0.25 (XA – XB)2], where XA and XB are the electronegativities of the two atoms, A and B, forming the molecule.

Covalent Bonding

In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding.

Metallic Bonding

In metals, the atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place, in a similar way that the electrons in between the H atoms in the H2 molecule bind the protons.

2.7 Secondary Bonding (Van der Waals)

Fluctuating Induced Dipole Bonds

Since the electrons may be on one side of the atom or the other, a dipole is formed: the + nucleus at the center, and the electron outside. Since the electron moves, the dipole fluctuates. This fluctuation in atom A produces a fluctuating electric field that is felt by the electrons of an adjacent atom, B. Atom B then polarizes so that its outer electrons are on the side of the atom closest to the + side (or opposite to the – side) of the dipole in A. This bond is called van der Waals bonding.

Polar Molecule-Induced Dipole Bonds

A polar molecule like H2O (Hs are partially +, O is partially – ), will induce a dipole in a nearby atom, leading to bonding.

Permanent Dipole Bonds

This is the case of the hydrogen bond in ice. The H end of the molecule is positively charged and can bond to the negative side of another dipolar molecule, like the O side of the H2O dipole.

2.8 Molecules

If molecules formed a closed shell due to covalent bonding (like H2, N2) then the interaction between molecules is weak, of the van der Waals type. Thus, molecular solids usually have very low melting points.

Review - Classification of materials

See table-chapter2.gif

Terms:

Atomic mass unit (amu) Atomic number Atomic weight Bonding energy Coulombic force Covalent bond Dipole (electric) Electron state Electronegative Electropositive Hydrogen bond Ionic bond Metallic bond Mole Molecule Periodic table Polar molecule Primary bonding Secondary bonding Valence electron

 Crystal Structures

3.2 Fundamental Concepts

Atoms self-organize in crystals, most of the time. The crystalline lattice, is a periodic array of the atoms. When the solid is not crystalline, it is called amorphous. Examples of crystalline solids are metals, diamond and other precious stones, ice, graphite. Examples of amorphous solids are glass, amorphous carbon (a-C), amorphous Si, most plastics

To discuss crystalline structures it is useful to consider atoms as being hard spheres, with well-defined radii. In this scheme, the shortest distance between two like atoms is one diameter.

3.3 Unit Cells

The unit cell is the smallest structure that repeats itself by translation through the crystal. We construct these symmetrical units with the hard spheres. The most

common types of unit cells are the faced-centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP). Other types exist, particularly among minerals. The simple cube (SC) is often used for didactical purpose, no material has this structure.

3.4 Metallic Crystal Structures

Important properties of the unit cells are

The type of atoms and their radii R. cell dimensions (side a in cubic cells, side of base a and height c in HCP) in

terms of R. n, number of atoms per unit cell. For an atom that is shared with m adjacent

unit cells, we only count a fraction of the atom, 1/m. CN, the coordination number, which is the number of closest neighbors to

which an atom is bonded. APF, the atomic packing factor, which is the fraction of the volume of the cell

actually occupied by the hard spheres. APF = Sum of atomic volumes/Volume of cell.

Unit Cell n CN a/R APF

SC 1 6 2 0.52

BCC 2 8 4 3 0.68

FCC 4 12 2 2 0.74

HCP 6 12 0.74

The closest packed direction in a BCC cell is along the diagonal of the cube; in a FCC cell is along the diagonal of a face of the cube.

3.5 Density Computations

The density of a solid is that of the unit cell, obtained by dividing the mass of the atoms (n atoms x Matom) and dividing by Vc the volume of the cell (a3 in the case of a

cube). If the mass of the atom is given in amu (A), then we have to divide it by the Avogadro number to get Matom. Thus, the formula for the density is:

3.6 Polymorphism and Allotropy

Some materials may exist in more than one crystal structure, this is called polymorphism. If the material is an elemental solid, it is called allotropy. An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.

3.7 – 3.10 Crystallography – Not Covered

3.11 Close-Packed Crystal Structures

The FCC and HCP are related, and have the same APF. They are built by packing spheres on top of each other, in the hollow sites (Fig. 3.12 of book). The packing is alternate between two types of sites,ABABAB.. in the HCP structure, and alternates between three types of positions, ABCABC… in the FCC crystals.

Crystalline and Non-Crystalline Materials

3.12 Single Crystals

Crystals can be single crystals where the whole solid is one crystal. Then it has a regular geometric structure with flat faces.

3.13 Polycrystalline Materials

A solid can be composed of many crystalline grains, not aligned with each other. It is called polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called agrain boundary.

3.14 Anisotropy

Different directions in the crystal have a different packing. For instance, atoms along the edge FCC crystals are more separated than along the face diagonal. This causes anisotropy in the properties of crystals; for instance, the deformation depends on the direction in which a stress is applied.

3.15 X-Ray Diffraction Determination of Crystalline Structure – not covered

3.16 Non-Crystalline Solids

In amorphous solids, there is no long-range order. But amorphous does not mean random, since the distance between atoms cannot be smaller than the size of the hard spheres. Also, in many cases there is some form of short-range order. For instance, the tetragonal order of crystalline SiO2 (quartz) is still apparent in amorphous SiO2 (silica glass.)

Terms:

Allotropy Amorphous Anisotropy Atomic packing factor (APF) Body-centered cubic (BCC) Coordination number Crystal structure Crystalline Face-centered cubic (FCC) Grain Grain boundary Hexagonal close-packed (HCP) Isotropic Lattice parameter Non-crystalline Polycrystalline Polymorphism Single crystal Unit cell

 

Imperfections in Solids4.1 Introduction

Materials are often stronger when they have defects.  The study of defects is divided according to their dimension:

0D (zero dimension) – point defects: vacancies and interstitials. Impurities.

1D – linear defects: dislocations (edge, screw, mixed)

2D – grain boundaries, surfaces.

3D – extended defects: pores, cracks.

Point Defects4.2 Vacancies and Self-Interstitials

A vacancy is a lattice position that is vacant because the atom is missing. It is created when the solid is formed. There are other ways of making a vacancy, but they also occur naturally as a result of thermal vibrations.

An interstitial is an atom that occupies a place outside the normal lattice position. It may be the same type of atom as the others (self interstitial) or an impurity atom.

In the case of vacancies and interstitials, there is a change in the coordination of atoms around the defect. This means that the forces are not balanced in the same way as for other atoms in the solid, which results in lattice distortion around the defect.

The number of vacancies formed by thermal agitation follows the law:

NV = NA × exp(-QV/kT)

where NA is the total number of atoms in the solid, QV is the energy required to form a vacancy, k is Boltzmann constant, and T the temperature in Kelvin (note, not in oC or oF).

When QV is given in joules, k = 1.38 × 10-23 J/atom-K. When using eV as the unit of energy, k = 8.62 × 10-5 eV/atom-K.

Note that kT(300 K) = 0.025 eV (room temperature) is much smaller than typical vacancy formation energies. For instance, QV(Cu) = 0.9 eV/atom. This means that NV/NA at room temperature is exp(-36) = 2.3 × 10-16, an insignificant number. Thus, a high temperature is needed to have a high thermal concentration of vacancies. Even so, NV/NA is typically only about 0.0001 at the melting point.

4.3 Impurities in Solids

All real solids are impure. A very high purity material, say 99.9999% pure (called 6N – six nines) contains ~ 6 × 1016 impurities per cm3.

Impurities are often added to materials to improve the properties. For instance, carbon added in small amounts to iron makes steel, which is stronger than iron. Boron impurities added to silicon drastically change its electrical properties.

Solid solutions are made of a host, the solvent or matrix) which dissolves the solute (minor component). The ability to dissolve is called solubility. Solid solutions are:

homogeneous maintain crystal structure contain randomly dispersed impurities (substitutional or interstitial)

Factors for high solubility

Similar atomic size (to within 15%) Similar crystal structure Similar electronegativity (otherwise a compound is formed) Similar valence

Composition can be expressed in weight percent, useful when making the solution, and in atomic percent, useful when trying to understand the material at the atomic level.

Miscellaneous Imperfections4.4 Dislocations—Linear Defects

Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. They occur in high density and are very important in mechanical properties of material. They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction.

Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line.

Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

4.5 Interfacial Defects

The environment of an atom at a surface differs from that of an atom in the bulk, in that the number of neighbors (coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes).

The density of atoms in the region including the grain boundary is smaller than the bulk value, since void space occurs in the interface.

Surfaces and interfaces are very reactive and it is usual that impurities segregate there. Since energy is required to form a surface, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

Twin boundaries: not covered

4.6 Bulk or Volume Defects

A typical volume defect is porosity, often introduced in the solid during processing. A common example is snow, which is highly porous ice.

4.7 Atomic Vibrations

Atomic vibrations occur, even at zero temperature (a quantum mechanical effect) and increase in amplitude with temperature. Vibrations displace transiently atoms from their regular lattice site, which destroys the perfect periodicity we discussed in Chapter 3.

Macroscopic Examination

Sections 4.8 to 4-10 were not covered.

 

Terms:

Alloy

Atom percent

Atomic vibration

Boltzmann’s constant

Burgers vector

Composition

Dislocation line

Edge dislocation

Grain size

Imperfection

Interstitial solid solution

Microstructure

Point defect

Screw dislocation

Self-Interstitial

Solid solution

Solute

Solvent

Substitutional solid solution

Vacancy

Weight percent

 

 

5.1 Introduction

Many important reactions and processes in materials occur by the motion of atoms in the solid (transport), which happens by diffusion.

Inhomogeneous materials can become homogeneous by diffusion, if the temperature is high enough (temperature is needed to overcome energy barriers to atomic motion.

5.2 Diffusion Mechanisms

Atom diffusion can occur by the motion of vacancies (vacancy diffusion) or impurities (impurity diffusion). The energy barrier is that due to nearby atoms which need to move to let the atoms go by. This is more easily achieved when the atoms vibrate strongly, that is, at high temperatures.

There is a difference between diffusion and net diffusion. In a homogeneous material, atoms also diffuse but this motion is hard to detect. This is because atoms move randomly and there will be an equal number of atoms moving in one direction than in another. In inhomogeneous materials, the effect of diffusion is readily seen by a change in concentration with time. In this case there is a net diffusion. Net diffusion occurs because, although all atoms are moving randomly, there are more atoms moving in regions where their concentration is higher.

5.3 Steady-State Diffusion

The flux of diffusing atoms, J, is expressed either in number of atoms per unit area and per unit time (e.g., atoms/m2-second) or in terms of mass flux (e.g., kg/m2-second).

Steady state diffusion means that J does not depend on time. In this case, Fick’s first law holds that the flux along direction x is:

J = – D dC/dx

Where dC/dx is the gradient of the concentration C, and D is the diffusion constant. The concentration gradient is often called the driving force in diffusion (but it is not a force in the mechanistic sense). The minus sign in the equation means that diffusion is down the concentration gradient.

5.4 Nonsteady-State Diffusion

This is the case when the diffusion flux depends on time, which means that a type of atoms accumulates in a region or that it is depleted from a region (which may cause them to accumulate in another region). 

5.5 Factors That Influence Diffusion

As stated above, there is a barrier to diffusion created by neighboring atoms that need to move to let the diffusing atom pass. Thus, atomic vibrations created by temperature assist diffusion. Also,

smaller atoms diffuse more readily than big ones, and diffusion is faster in open lattices or in open directions. Similar to the case of vacancy formation, the effect of temperature in diffusion is given by a Boltzmann factor: D = D0 ×exp(–Qd/kT).

5.6 Other Diffusion Paths

Diffusion occurs more easily along surfaces, and voids in the material (short circuits like dislocations and grain boundaries) because less atoms need to move to let the diffusing atom pass. Short circuits are often unimportant because they constitute a negligible part of the total area of the material normal to the diffusion flux. .

 

Terms: Activation energy

Concentration gradient

Diffusion

Diffusion coefficient

Diffusion flux

Driving force

Fick’s first and second laws

Interdiffusion

Interstitial diffusion

Self-diffusion

Steady-state diffusion

Vacancy diffusion

 

 

 Chapter 7. Dislocations and Strengthening Mechanisms

1. Introduction

The key idea of the chapter is that plastic deformation is due to the motion of a large number of dislocations. The motion is called slip. Thus, the strength (resistance to deformation) can be improved by putting obstacles to slip.

2. Basic Concepts

Dislocations can be edge dislocations, screw dislocations and exist in combination of the two (Ch. 4.4). Their motion (slip) occurs by sequential bond breaking and bond reforming (Fig. 7.1). The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area.

3. Characteristics of Dislocations

There is strain around a dislocation which influences how they interact with other dislocations, impurities, etc. There is compression near the extra plane (higher atomic density) and tensionfollowing the dislocation line (Fig. 7.4)

Dislocations interact among themselves (Fig. 7.5). When they are in the same plane, they repel if they have the same sign and annihilate if they have opposite signs (leaving behind a perfect crystal). In general, when dislocations are close and their strain fields add to a larger value, they repel, because being close increases the potential energy (it takes energy to strain a region of the material).

The number of dislocations increases dramatically during plastic deformation. Dislocations spawn from existing dislocations, and from defects, grain boundaries and surface irregularities.

4. Slip Systems

In single crystals there are preferred planes where dislocations move (slip planes). There they do not move in any direction, but in preferred crystallographic directions (slip direction). The set of slip planes and directions constitute slip systems.

The slip planes are those of highest packing density. How do we explain this? Since the distance between atoms is shorter than the average, the distance perpendicular to the plane has to be longer than average. Being relatively far apart, the atoms can move more easily with respect to the

atoms of the adjacent plane. (We did not discuss direction and plane nomenclature for slip systems.)

BCC and FCC crystals have more slip systems, that is more ways for dislocation to propagate. Thus, those crystals are more ductile than HCP crystals (HCP crystals are more brittle).

5. Slip in Single Crystals

A tensile stress will have components in any plane that is not perpendicular to the stress. These components are resolved shear stresses. Their magnitude depends on orientation (see Fig. 7.7).  

R = cos cos 

If the shear stress reaches the critical resolved shear stress CRSS, slip (plastic deformation) can start. The stress needed is:

y=CRSS / (cos cos )max

at the angles at which CRSS is a maximum. The minimum stress needed for yielding is when  =  = 45 degrees: y=CRSS. Thus, dislocations will occur first at slip planes oriented close to this angle with respect to the applied stress (Figs. 7.8 and 7.9).

6. Plastic Deformation of Polycrystalline Materials

Slip directions vary from crystal to crystal. When plastic deformation occurs in a grain, it will be constrained by its neighbors which may be less favorably oriented. As a result, polycrystalline metals are stronger than single crystals (the exception is the perfect single crystal, as in whiskers.)

7. Deformation by Twinning

This topic is not included.

Mechanisms of Strengthening in Metals

General principles. Ability to deform plastically depends on ability of dislocations to move. Strengthening consists in hindering dislocation motion. We discuss the methods of grain-size reduction, solid-solution alloying and strain hardening. These are for single-phase metals. We

discuss others when treating alloys. Ordinarily, strengthening reduces ductility.

8. Strengthening by Grain Size Reduction

This is based on the fact that it is difficult for a dislocation to pass into another grain, especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity in slip planes. For high-angle grain boundaries, stress at end of slip plane may trigger new dislocations in adjacent grains. Small angle grain boundaries are not effective in blocking dislocations.

The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Usually, the yield strength varies with grain size d according to:

y = 0 + ky / d1/2

Grain size can be controlled by the rate of solidification and by plastic deformation.

9. Solid-Solution Strengthening

Adding another element that goes into interstitial or substitutional positions in a solution increases strength. The impurity atoms cause lattice strain (Figs. 7.17 and 7.18) which can "anchor" dislocations. This occurs when the strain caused by the alloying element compensates that of the dislocation, thus achieving a state of low potential energy. It costs strain energy for the dislocation to move away from this state (which is like a potential well). The scarcity of energy at low temperatures is why slip is hindered.

Pure metals are almost always softer than their alloys.

10.Strain Hardening

Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working). (This is different from hot working is the shaping of materials at high temperatures where large deformation is possible.) Strain hardening

(work hardening) is the reason for the elastic recovery discussed in Ch. 6.8.

The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one.

The measure of strain hardening is the percent cold work (%CW), given by the relative reduction of the original area, A0 to the final value Ad :

%CW = 100 (A0–Ad)/A0

Recovery, recrystallization and Grain Growth

Plastic deformation causes 1) change in grain size, 2) strain hardening, 3) increase in the dislocation density. Restoration to the state before cold-work is done by heating through two processes: recovery and recrystallization. These may be followed by grain growth.

11.Recovery

Heating  increased diffusion  enhanced dislocation motion  relieves internal strain energy and reduces the number of dislocation. The electrical and thermal conductivity are restored to the values existing before cold working.

12.Recrystallization

Strained grains of cold-worked metal are replaced, upon heating, by more regularly-spaced grains. This occurs through short-range diffusion enabled by the high temperature. Since recrystallization occurs by diffusion, the important parameters are both temperature and time.

The material becomes softer, weaker, but more ductile (Fig. 7.22).

Recrystallization temperature: is that at which the process is complete in one hour. It is typically 1/3 to 1/2 of the melting temperature. It falls as the %CW is increased. Below a "critical deformation", recrystallization does not occur.

13.Grain Growth

The growth of grain size with temperature can occur in all polycrystalline materials. It occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster at higher temperatures. The "driving force" is the reduction of energy, which is proportional to the total area. Big grains grow at the expense of the small ones.

Important Terms:

Cold workingCritical resolved shear stressDislocation densityGrain growthLattice strainRecoveryRecrystallizationRecrystallization temperatureResolved shear stressSlipSlip systemStrain hardeningSolid-solution strengthening 

 

Chapter 8. Failure

1. Introduction

Failure of materials may have huge costs. Causes included improper materials selection or processing, the improper design of components, and improper use.

2. Fundamentals of Fracture

Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small.

Steps in fracture (response to stress):

track formation track propagation

Ductile vs. brittle fracture

  Ductile Brittle

deformation extensive little

track propagation slow, needs stress fast

type of materials most metals (not too cold) ceramics, ice, cold metals

warning permanent elongation none

strain energy higher lower

fractured surface rough smoother

necking yes no

Ductile Fracture

Stages of ductile fracture

Initial necking small cavity formation (microvoids) void growth (elipsoid) by coalescence into a crack fast crack propagation around neck. Shear strain at 45o

final shear fracture (cup and cone)

The interior surface is fibrous, irregular, which signify plastic deformation.

Brittle Fracture

There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (or faceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular.

5. Principles of Fracture Mechanics

 

Fracture occurs due to stress concentration at flaws, like surface scratches, voids, etc. If a is the length of the void and  the radius of curvature, the enhanced stress near the flaw is:

m  2 0 (a/)1/2

where 0 is the applied macroscopic stress. Note that a is 1/2 the length of the flaw, not the full length for an internal flaw, but the full length for a surface flaw. The stress concentration factor is:

Kt = m/0  2 (a/)1/2

Because of this enhancement, flaws with small radius of curvature are called stress raisers.

6. Impact Fracture Testing

Normalized tests, like the Charpy and Izod tests measure the impact energy required to fracture a notched specimen with a hammer mounted on a pendulum. The energy is measured by the change in potential energy (height) of the pendulum. This energy is called notch toughness.

Ductile to brittle transition occurs in materials when the temperature is dropped below a transition temperature. Alloying usually increases the ductile-brittle transition temperature (Fig. 8.19.) For ceramics, this type of transition occurs at much higher temperatures than for metals.

Fatigue

Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in bridges, airplanes, machine components, etc. The characteristics are:

long period of cyclic strain the most usual (90%) of metallic failures (happens also in ceramics and

polymers) is brittle-like even in ductile metals, with little plastic deformation it occurs in stages involving the initiation and propagation of cracks.

Cyclic Stresses

These are characterized by maximum, minimum and mean stress, the stress amplitude, and the stress ratio (Fig. 8.20).

The S—N Curve

S—N curves (stress-number of cycles to failure) are obtained using apparatus like the one shown in Fig. 8.21. Different types of S—N curves are shown in Fig. 8.22.

Fatigue limit (endurance limit) occurs for some materials (like some ferrous and Ti allows). In this case, the S—N curve becomes horizontal at large N . This

means that there is a maximum stress amplitude (the fatigue limit) below which the material never fails, no matter how large the number of cycles is.

For other materials (e.g., non-ferrous) the S—N curve continues to fall with N.

Failure by fatigue shows substantial variability (Fig. 8.23).

Failure at low loads is in the elastic strain regime, requires a large number of cycles (typ. 104 to 105). At high loads (plastic regime), one has low-cycle fatigue (N < 104 - 105 cycles).

Crack Initiation and Propagation

Stages is fatigue failure:

I. crack initiation at high stress points (stress raisers)

II. propagation (incremental in each cycle)

III. final failure by fracture

Nfinal = Ninitiation + Npropagation

Stage I - propagation

slow along crystallographic planes of high shear stress flat and featureless fatigue surface

 

Stage II - propagation

crack propagates by repetive plastic blunting and sharpening of the crack tip. (Fig. 8.25.)

. Crack Propagation Rate (not covered)

. Factors That Affect Fatigue Life

Mean stress (lower fatigue life with increasing mean). Surface defects (scratches, sharp transitions and edges). Solution:

polish to remove machining flaws add residual compressive stress (e.g., by shot peening.) case harden, by carburizing, nitriding (exposing to appropriate gas at high

temperature)

. Environmental Effects

Thermal cycling causes expansion and contraction, hence thermal stress, if component is restrained. Solution:

o eliminate restraint by designo use materials with low thermal expansion coefficients.

Corrosion fatigue. Chemical reactions induced pits which act as stress raisers. Corrosion also enhances crack propagation. Solutions:

o decrease corrosiveness of medium, if possible.o add protective surface coating.o add residual compressive stresses.

Creep

Creep is the time-varying plastic deformation of a material stressed at high temperatures. Examples: turbine blades, steam generators. Keys are the time dependence of the strain and the high temperature.

. Generalized Creep Behavior

At a constant stress, the strain increases initially fast with time (primary or transient deformation), then increases more slowly in the secondary region at a steady rate (creep rate). Finally the strain increases fast and leads to failure in the tertiary region. Characteristics:

Creep rate: d/dt Time to failure.

. Stress and Temperature Effects

Creep becomes more pronounced at higher temperatures (Fig. 8.37). There is essentially no creep at temperatures below 40% of the melting point.

Creep increases at higher applied stresses.

The behavior can be characterized by the following expression, where K, n and Qc are constants for a given material:

d/dt = K n exp(-Qc/RT)

. Data Extrapolation Methods (not covered.)

. Alloys for High-Temperature Use

These are needed for turbines in jet engines, hypersonic airplanes, nuclear reactors, etc. The important factors are a high melting temperature, a high elastic modulus and large grain size (the latter is opposite to what is desirable in low-temperature materials).

Some creep resistant materials are stainless steels, refractory metal alloys (containing elements of high melting point, like Nb, Mo, W, Ta), and superalloys (based on Co, Ni, Fe.)

Terms:

Brittle fractureCharpy testCorrosion fatigueCreepDuctile fractureDuctile-to-brittle transitionFatigueFatigue lifeFatigue limitFatigue strengthFracture mechanicsFracture toughnessImpact energyIntergranular fractureIzod testStress intensity factorStress raiserThermal fatigueTransgranular fracture

 

9.1 Introduction

Definitions

Component: pure metal or compound (e.g., Cu, Zn in Cu-Zn alloy, sugar, water, in a syrup.)

Solvent: host or major component in solution.

Solute: dissolved, minor component in solution.

System: set of possible alloys from same component (e.g., iron-carbon system.)

Solubility Limit: Maximum solute concentration that can be dissolved at a given temperature.

Phase: part with homogeneous physical and chemical characteristics

9.2 Solubility Limit

Effect of temperature on solubility limit. Maximum content: saturation. Exceeding maximum content (like when cooling) leads to precipitation.

9.3 Phases

One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers.

A two-component alloy is called binary. One with three components, ternary.

9.4 Microstructure

The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.

A binary alloy may be

a. a single solid solutionb. two separated, essentially pure components.c. two separated solid solutions.d. a chemical compound, together with a solid solution.

The way to tell is to cut the material, polish it to a mirror finish, etch it a weak acid (components etch at a different rate) and observe the surface under a microscope.

9.5 Phase Equilibria

Equilibrium is the state of minimum energy. It is achieved given sufficient time. But the time to achieve equilibrium may be so long (the kinetics is so slow) that a state that is not at an energy minimum may have a long life and appear to be stable. This is called a metastable state.

A less strict, operational, definition of equilibrium is that of a system that does not change with time during observation.

Equilibrium Phase Diagrams

Give the relationship of composition of a solution as a function of temperatures and the quantities of phases in equilibrium. These diagrams do not indicate the dynamics when one phase transforms into another. Sometimes diagrams are given with pressure as one of the variables. In the phase diagrams we will discuss, pressure is assumed to be constant at one atmosphere.

9.6 Binary Isomorphous Systems

This very simple case is one complete liquid and solid solubility, an isomorphous system. The example is the Cu-Ni alloy of Fig. 9.2a. The complete solubility occurs because both Cu and Ni have the same crystal structure (FCC), near the same radii, electronegativity and valence.

The liquidus line separates the liquid phase from solid or solid + liquid phases. That is, the solution is liquid above the liquidus line.

The solidus line is that below which the solution is completely solid (does not contain a liquid phase.)

Interpretation of phase diagrams

Concentrations: Tie-line method

a. locate composition and temperature in diagramb. In two phase region draw tie line or isothermc. note intersection with phase boundaries. Read compositions.

Fractions: lever rule

a. construct tie line (isotherm)b. obtain ratios of line segments lengths.

Note: the fractions are inversely proportional to the length to the boundary for the particular phase. If the point in the diagram is close to the phase line, the fraction of that phase is large.

Development of microstructure in isomorphous alloys

a) Equilibrium cooling

Solidification in the solid + liquid phase occurs gradually upon cooling from the liquidus line. The composition of the solid and the liquid change gradually during cooling (as can be determined by the tie-line method.) Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line.

b) Non-equilibrium cooling

Solidification in the solid + liquid phase also occurs gradually. The composition of the liquid phase evolves by diffusion, following the equilibrium values that can be derived from the tie-line method. However, diffusion in the solid state is very slow. Hence, the new layers that solidify on top of the grains have the equilibrium composition at that temperature but once they are solid their composition does not change. This lead to the formation of layered (cored) grains (Fig. 9.14) and to the invalidity of the tie-line method to determine the composition of the solid phase (it still works for the liquid phase, where diffusion is fast.)

9.7 Binary Eutectic Systems

Interpretation: Obtain phases present, concentration of phases and their fraction (%).

Solvus line: limit of solubility

Eutectic or invariant point. Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature.

Note:

the melting point of the eutectic alloy is lower than that of the components (eutectic = easy to melt in Greek).

At most two phases can be in equilibrium within a phase field.

Single-phase regions are separated by 2-phase regions.

Development of microstructure in eutectic alloys

Case of lead-tin alloys, figures 9.9–9.14. A layered, eutectic structure develops when cooling below the eutectic temperature. Alloys which are to the left of the eutectic concentration (hipoeutectic) or to the right (hypereutectic) form a proeutectic phase before reaching the eutectic temperature, while in the solid + liquid region. The eutectic structure then adds when the remaining liquid is solidified when cooling

further. The eutectic microstructure is lamellar (layered) due to the reduced diffusion distances in the solid state.

To obtain the concentration of the eutectic microstructure in the final solid solution, one draws a vertical line at the eutectic concentration and applies the lever rule treating the eutectic as a separate phase (Fig. 9.16).

9.8 Equilibrium Diagrams Having Intermediate Phases or Compounds

A terminal phase or terminal solution is one that exists in the extremes of concentration (0 and 100%) of the phase diagram. One that exists in the middle, separated from the extremes, is called anintermediate phase or solid solution.

An important phase is the intermetallic compound, that has a precise chemical compositions. When using the lever rules, intermetallic compounds are treated like any other phase, except they appear not as a wide region but as a vertical line.

9.9 Eutectoid and Peritectic Reactions

The eutectoid (eutectic-like) reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. There are associated eutectoid temperature (or temperature), eutectoid phase, eutectoid and proeutectoid microstructures.

Solid Phase 1  Solid Phase 2 + Solid Phase 3

The peritectic reaction also involves three solid in equilibrium, the transition is from a solid + liquid phase to a different solid phase when cooling. The inverse reaction occurs when heating.

Solid Phase 1 + liquid  Solid Phase 2

9.10 Congruent Phase Transformations

Another classification scheme. Congruent transformation is one where there is no change in composition, like allotropic transformations (e.g., Fe to -Fe) or melting transitions in pure solids.

9.11 Ceramic and Ternary Phase Diagrams

Ternary phase diagrams are three-dimensional. Ceramic phase diagrams will be discussed in Ch. 13.

9.12 The Gibbs Phase Rule (not discussed)

The Iron–Carbon Diagram9.13 The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

This is one of the most important alloys for structural applications. The diagram Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C diagram. Concentrations are usually given in weight percent. The possible phases are:

-ferrite (BCC) Fe-C solution -austenite (FCC) Fe-C solution -ferrite (BCC) Fe-C solution liquid Fe-C solution Fe3C (iron carbide) or cementite. An intermetallic compound.

The maximum solubility of C in - ferrite is 0.022 wt%. ferrite is only stable at high temperatures. It is not important in practice. Austenite has a maximum C concentration of 2.14 wt %. It is not stable below the eutectic temperature (727 C) unless cooled rapidly (Chapter 10). Cementite is in reality metastable, decomposing into -Fe and C when heated for several years between 650 and 770 C.

For their role in mechanical properties of the alloy, it is important to note that:

Ferrite is soft and ductile

Cementite is hard and brittle

Thus, combining these two phases in solution an alloy can be obtained with intermediate properties. (Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.)

9.14 Development of Microstructures in Iron—Carbon Alloys

The eutectoid composition of austenite is 0.76 wt %. When it cools slowly it forms perlite, a lamellar or layered structure of two phases: -ferrite and cementite (Fe3C).

Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid perlite. Hypereutectoid alloys contain proeutectoid cementite plus perlite.

Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite. The new microstructures that form are discussed in Ch. 10.

9.15 The Influence of Other Alloying Elements

As mentioned in section 7.9, alloying strengthens metals by hindering the motion of dislocations. Thus, the strength of Fe–C alloys increase with C content and also with the addition of other elements.

 

Terms:

Austenite

Cementite

Component

Congruent transformation

Equilibrium

Eutectic phase

Eutectic reaction

Eutectic structure

Eutectoid reaction

Ferrite

Free energy

Hypereutectoid alloy

Hypoeutectoid alloy

Intermediate solid solution

Intermetallic compound

Invariant point

Isomorphous

Lever rule

Liquidus line

Metastable

Microconstituent

Pearlite

Peritectic reaction

Phase

Phase diagram

Phase equilibrium

Primary phase

Proeutectoid cementite

Proeutectoid ferrite

Solidus line

Solubility limit

Solvus line

System

Terminal solid solution

Tie line

 

10.1 Introduction

The goal is to obtain specific microstructures that will improve the mechanical properties of a metal, in addition to grain-size refinement, solid-solution strengthening, and strain-hardening.

10.2 Basic Concepts

Phase transformations that involve a change in the microstructure can occur through:

Diffusion

Maintaining the type and number of phases (e.g., solidification of a pure metal, allotropic transformation, recrystallization, grain growth.

Alteration of phase composition (e.g., eutectoid reactions, see 10.5)

Diffusionless

Production of metastable phases (e.g., martensitic transformation, see 10.5)

10.3 The Kinetics of Solid-State Reactions

Change in composition implies atomic rearrangement, which requires diffusion. Atoms are displaced by random walk. The displacement of a given atom, d, is not linear in time t (as would be for a straight trajectory) but is proportional to the square root of time, due to the tortuous path: d = c(Dt) 1/2 where c is a constant and D the diffusion constant. This time-dependence of the rate at which the reaction (phase transformation) occurs is what is meant by the term reaction kinetics.

D is called a constant because it does not depend on time, but it depends on temperature as we have seen in Ch. 5. Diffusion occurs faster at high temperatures.

Phase transformation requires two processes: nucleation and growth. Nucleation involves the formation of very small particles, or nuclei (e.g., grain boundaries, defects). This is similar to rain happening when water molecules condensed around dust particles. During growth, the nuclei grow in size at the expense of the surrounding material.

The kinetic behavior often has the S-shape form of Fig. 10.1, when plotting percent of material transformed vs. the logarithm of time. The nucleation phase is seen as an incubation period, where nothing seems to happen. Usually the transformation rate has the form r = A e-Q/RT (similar to the temperature dependence of the diffusion constant), in which case it is said to be thermally activated.

10.4 Multiphase Transformations

To describe phase transformations that occur during cooling, equilibrium phase diagrams are inadequate if the transformation rate is slow compared to the cooling rate. This is usually the

case in practice, so that equilibrium microstructures are seldom obtained. This means that the transformations are delayed (e.g., case of supercooling), and metastable states are formed. We then need to know the effect of timeon phase transformations.

Microstructural and Property Changes in Fe-C Alloys

10.5 Isothermal Transformation Diagrams

We use as an example the cooling of an eutectoid alloy (0.76 wt% C) from the austenite (- phase) to pearlite, that contains ferrite () plus cementite (Fe3C or iron carbide). When cooling proceeds below the eutectoid temperature (727 oC) nucleation of pearlite starts. The S-shaped curves (fraction of pearlite vs. log. time, fig. 10.3) are displaced to longer times at higher temperatures showing that the transformation is dominated by nucleation (the nucleation period is longer at higher temperatures) and not by diffusion (which occurs faster at higher temperatures).

The family of S-shaped curves at different temperatures can be used to construct the TTT (Time-Temperature-Transformation) diagrams (e.g., fig. 10.4.) For these diagrams to apply, one needs to cool the material quickly to a given temperature To before the transformation occurs, and keep it at that temperature over time. The horizontal line that indicates constant temperature To intercepts the TTT curves on the left (beginning of the transformation) and the right (end of the transformation); thus one can read from the diagrams when the transformation occurs. The formation of pearlite shown in fig. 10.4 also indicates that the transformation occurs sooner at low temperatures, which is an indication that it is controlled by the rate of nucleation. At low temperatures, nucleation occurs fast and grain growth is reduced (since it occurs by diffusion, which is hindered at low temperatures). This reduced grain growth leads to fine-grained microstructure (fine pearlite). At higher temperatures, diffusion allows for larger grain growth, thus leading to coarse pearlite.

At lower temperatures nucleation starts to become slower, and a new phase is formed, bainite. Since diffusion is low at low temperatures, this phase has a very fine (microscopic) microstructure.

Spheroidite is a coarse phase that forms at temperatures close to the eutectoid temperature. The relatively high temperatures caused a slow nucleation but enhances the growth of the nuclei leading to large grains.

A very important structure is martensite, which forms when cooling austenite very fast (quenching) to below a maximum temperature that is required for the transformation. It forms nearly instantaneously when the required low temperature is reached; since no thermal activation is needed, this is called an athermal transformation. Martensite is a different phase, a body-centered tetragonal (BCT) structure with interstitial C atoms. Martensite is metastable and decomposes into ferrite and pearlite but this is extremely slow (and not noticeable) at room temperature.

In the examples, we used an eutectoid composition. For hypo- and hypereutectoid alloys, the analysis is the same, but the proeutectoid phase that forms before cooling through the eutectoid temperature is also part of the final microstructure.

10.6 Continuous Cooling Transformation Diagrams - not covered

10.7 Mechanical Behavior of Fe-C Alloys

The strength and hardness of the different microstructures is inversely related to the size of the microstructures. Thus, spheroidite is softest, fine pearlite is stronger than coarse pearlite, bainite is stronger than pearlite and martensite is the strongest of all. The stronger and harder the phase the more brittle it becomes.

10.8 Tempered Martensite

Martensite is so brittle that it needs to be modified in many practical cases. This is done by heating it to 250-650 oC for some time (tempering) which produces tempered martensite, an extremely fine-grained and well dispersed cementite grains in a ferrite matrix.

Terms:

Alloy steel

Athermal transformation

Bainite

Coarse pearlite

Fine pearlite

Isothermal transformation diagram

Kinetics

Martensite

Nucleation

Phase transformation

Plain carbon steel

Spheroidite

Supercooling

Superheating

Tempered martensite

Thermally activated transformation

Transformation rate

Annealing Processes

11.1 Introduction 

Annealing is a heat treatment where the material is taken to a high temperature, kept there for some time and then cooled.  High temperatures allow diffusion processes to occur fast.  The time at the high temperature (soaking time) is long enough to allow the desired transformation to occur.  Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneities.  Benefits of annealing are:

relieve stresses increase softness, ductility and toughness produce a specific microstructure

11.2 Process Annealing 

Deforming a piece that has been strengthened by cold working requires a lot of energy.  Reverting the effect of cold work by process annealing eases further deformation.  Heating allows recovery and recrystallization but is usually limited to avoid excessive grain growth and oxidation. 

11.3 Stress Relief

Stresses resulting from machining operations of non-uniform cooling can be eliminated by stress relief annealing at moderately low temperatures, such that the effect of cold working and other heat treatments is maintained.

11.4 Annealing of Ferrous Alloys

Normalizing (or austenitizing) consists in taking the Fe-C alloy to the austenitic phase which makes the grain size more uniform, followed by cooling in air. 

Full anneal involves taking hypoeutectoid alloys to the austenite phase and hypereutectoid alloys over the eutectoid temperature (Fig. 11.1) to soften pieces which have been hardened by plastic deformation, and which need to be machined.

Spheroidizing consists in prolongued heating just below the eutectoid temperature, which results

in the soft spheroidite structure discussed in Sect. 10.5. This achieves maximum softness that minimizes the energy needed in subsequent forming operations. 

Heat Treatment of Steels

1.5 Hardenability 

To achieve a full conversion of austenite into hard martensite, cooling needs to be fast enough to avoid partial conversion into perlite or bainite.  If the piece is thick, the interior may cool too slowly so that full martensitic conversion is not achieved.  Thus, the martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the piece.  Hardenability is the ability of the material to be hardened by forming martensite. 

Hardenability is measured by the Jominy end-quench test (Fig. 11.2).  Hardenability is then given as the dependence of hardness on distance from the quenched end.  High hardenability means that the hardness curve is relatively flat. 

11.6 Influence of Quenching Medium, Specimen Size, and Geometry

The cooling rate depends on the cooling medium.  Cooling is fastest using water, then oil, and then air.  Fast cooling brings the danger of warping and formation of cracks, since it is usually accompanied by large thermal gradients. 

The shape and size of the piece, together with the heat capacity and heat conductivity are important in determining the cooling rate for different parts of the metal piece.  Heat capacity is the energy content of a heated mass, which needs to be removed for cooling.  Heat conductivity measures how fast this energy is transported to the colder regions of the piece. 

Precipitation Hardening

Hardening can be enhanced by extremely small precipitates that hinder dislocation motion.  The precipitates form when the solubility limit is exceeded.  Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time. 

11.7 Heat Treatments

Precipitation hardening is achieved by:   

a) solution heat treatment where all the solute atoms are dissolved to form a single-phase solution.

b) rapid cooling across the solvus line to exceed the solubility limit. This leads to a supersaturated solid   solution that remains stable (metastable) due to the low temperatures,

which prevent diffusion.

c) precipitation heat treatment where the supersaturated solution is heated to an intermediate temperature to induce precipitation and kept there for some time (aging).

If the process is continued for a very long time, eventually the hardness decreases.  This is called overaging. 

The requirements for precipitation hardening are: 

appreciable maximum solubility solubility curve that falls fast with temperature composition of the alloy that is less than the maximum solubility

11.8 Mechanism of Hardening 

Strengthening involves the formation of a large number of microscopic nuclei, called zones.  It is accelerated at high temperatures.  Hardening occurs because the deformation of the lattice around the precipitates hinder slip.  Aging that occurs at room temperature is called natural aging, to distinguish from the artificial aging caused by premeditated heating. 

11.9 Miscellaneous Considerations 

Since forming, machining, etc. uses more energy when the material is hard, the steps in the processing of alloys are usually:

solution heat treat and quench do needed cold working before hardening do precipitation hardening

Exposure of precipitation-hardened alloys to high temperatures may lead to loss of strength by overaging.

Terms:

Annealing  Artificial aging  Austenitizing  Full annealing  Hardenability  Jominy end-quench test  Overaging Natural aging  Precipitation hardening 

Precipitation heat treatment Process annealing Solution heat treatment Spheroidizing Stress relief 

  

13.1 Introduction 

Ceramics are inorganic and non-metallic materials that are commonly electrical and thermal insulators, brittle and composed of more than one element (e.g., two in Al2O3)

Ceramic Structures

13.2 Crystal Structures 

Ceramic bonds are mixed, ionic and covalent, with a proportion that depends on the particular ceramics. The ionic character is given by the difference of electronegativity between the cations (+) and anions (-).  Covalent bonds involve sharing of valence electrons.  Very ionic crystals usually involve cations which are alkalis or alkaline-earths (first two columns of the periodic table) and oxygen or halogens as anions. 

The building criteria for the crystal structure are two:   

maintain neutrality charge balance dictates chemical formula achieve closest packing

the condition for minimum energy implies maximum attraction and minimum repulsion. This leads to contact, configurations where anions have the highest number of cation neighbors and viceversa. 

The parameter that is important in determining contact is the ratio of cation to anion radii, rC/rA. Table 13.2 gives the coordination number and geometry as a function of rC/rA. For example, in the NaCl structure (Fig. 13.2), rC = rNa = 0.102 nm,  rA =rCl.= 0.181 nm, so rC/rA.= 0.56.  From table 13.2 this implies coordination number = 6, as observed for this rock-salt structure. 

Other structures were shown in class, but will not be included in the test. 

13.3 Silicate Ceramics

Oxygen and Silicon are the most abundant elements in Earth’s crust.  Their combination (silicates) occur in rocks, soils, clays and sand. The bond is weekly ionic, with Si4+ as the cation

and O2- as the anion.  rSi = 0.04 nm,  rO.= 0.14 nm, so rC/rA = 0.286.  From table 13.2 this implies coordination number = 4, that is tetrahedral coordination (Fig. 13.9). 

The tetrahedron is charged: Si4+ + 4 O2-  (Si O4)4-.  Silicates differ on how the tetrahedra are arranged.  In silica, (SiO2), every oxygen atom is shared by adjacent tetrahedra.  Silica can be crystalline (e.g., quartz) or amorphous, as in glass. 

Soda glasses melt at lower temperature than amorphous SiO2 because the addition of Na2O (soda) breaks the tetrahedral network.  A lower melting point makes it easy to form glass to make, for instance, bottles. 

13.4 Carbon 

Carbon is not really a ceramic, but an allotropic form, diamond, may be thought as a type of ceramic.  Diamond has very interesting and even unusual properties:   

diamond-cubic structure (like Si, Ge) covalent C-C bonds highest hardness of any material known very high thermal conductivity (unlike ceramics) transparent in the visible and infrared, with high index of refraction semiconductor (can be doped to make electronic devices) metastable (transforms to carbon when heated)

Synthetic diamonds are made by application of high temperatures and pressures or by chemical vapor deposition.  Future applications of this latter, cheaper production method include hard coatings for metal tools, ultra-low friction coatings for space applications, and microelectronics.

Graphite has a layered structure with very strong hexagonal bonding within the planar layers (using 3 of the 3 bonding electrons) and weak, van der Waals bonding between layers using the fourth electron.  This leads to easy interplanar cleavage and applications as a lubricant and for writing (pencils).  Graphite is a good electrical conductor and chemically stable even at high temperatures.  Applications include furnaces, rocket nozzles, electrodes in batteries. 

A recently (1985) discovered formed of carbon is the C60 molecule, also known as fullerene or bucky-ball (after the architect Buckminster Fuller who designed the geodesic structure that C60 resembles.)  Fullerenes and related structures like bucky-onions amd nanotubes are exceptionally strong. Future applications are as a structural material and possibly in microelectronics, due to the unusual properties that result when fullerenes are doped with other atoms. 

13.5 Imperfections in Ceramics

Imperfections include point defects and impurities.  Their formation is strongly affected by the condition of charge neutrality (creation of unbalanced charges requires the expenditure of a large amount of energy. 

Non-stoichiometry refers to a change in composition so that the elements in the ceramic are not in the proportion appropriate for the compound (condition known as stoichiometry).  To minimize energy, the effect of non-stoichiometry is a redistribution of the atomic charges (Fig. 13.1).

Charge neutral defects include the Frenkel and Schottky defects.  A Frenkel-defect is a vacancy- interstitial pair of cations (placing large anions in an interstitial position requires a lot of energy in lattice distortion). A Schottky-defect is the a pair of nearby cation and anion vacancies. 

Introduction of impurity atoms in the lattice is likely in conditions where the charge is maintained.  This is the case of electronegative impurities that substitute a lattice anions or electropositive substitutional impurities. This is more likely for similar ionic radii since this minimizes the energy required for lattice distortion.  Defects will appear if the charge of the impurities is not balanced. 

13.6 Ceramic Phase Diagrams (not covered)

13.7 Brittle Fracture of Ceramics

The brittle fracture of ceramics limits applications.  It occurs due to the unavoidable presence of microscopic flaws (micro-cracks, internal pores, and atmospheric contaminants) that result during cooling from the melt.  The flaws need to crack formation, and crack propagation (perpendicular to the applied stress) is usually transgranular, along cleavage planes. The flaws cannot be closely controlled in manufacturing; this leads to a large variability (scatter) in the fracture strength of ceramic materials. 

The compressive strength is typically ten times the tensile strength.  This makes ceramics good structural materials under compression (e.g., bricks in houses, stone blocks in the pyramids), but not in conditions of tensile stress, such as under flexure. 

Plastic deformation in crystalline ceramics is by slip, which is difficult due to the structure and the strong local (electrostatic) potentials.  There is very little plastic deformation before fracture. 

Non-crystalline ceramics, like common glass deform by viscous flow (like very high-density liquids).  Viscosity decreases strongly with increases temperature. 

13.8 Stress-Strain Behavior (not covered)

13.9 Mechanisms of Plastic Deformation (not covered)

13.10 Miscellaneous Mechanical Considerations (not covered) 

Terms:

Anion Cation  Defect structure  Frenkel defect Electroneutrality Octahedral position Schottky defect Stoichiometry Tetrahedral position Viscosity  14.1 Introduction

Ceramics properties that are different from those of metals lead to different uses.  In structures, designs must be done for compressive loads.  The transparency to light of many ceramics leads to optical uses, like in windows, photographic cameras, telescopes and microscopes. Good thermal insulation leads to use in ovens, the exterior tiles of the Shuttle orbiter, etc.  Good electrical isolation are used to support conductors in electrical and electronic applications.  The good chemical inertness shows in the stability of the structures thousands of years old. 

14.2 Glass Properties

A special characteristic of glasses is that solidification is gradual, through a viscous stage, without a clear melting temperature.  The specific volume does not have an abrupt transition at a temperature but rather shows a change in slope at the glass-transition temperature (Fig. 14.3). 

The melting point,  working point, softening point and annealing point are defined in terms of viscosity, rather than temperature (Fig. 14.4), and depend on glass composition.. 

14.3 Glass Forming (not covered)

14.4 Heat Treating Glasses 

Similar to the case of metals, annealing is used at elevated temperatures is used to remove stresses, like those caused by inhomogeneous temperatures during cooling. 

Strengthening by glass tempering is done by heating the glass above the glass transition temperature but below the softening point and then quenched in an air jet or oil bath. The interior, which cools later than the outside, tries to contract while in a plastic state after the exterior has become rigid.  This causes residual compressive stresses on the surface and tensile stresses inside.  To fracture, a crack has first to overcome the residual compressive stress, making tempered glass less susceptible to fracture.  This improvement leads to use in automobile windshields, glass doors, eyeglass lenses, etc. 

14.5 -  14.12 – Not included. 

Terms:

Glass tempering Glass transition temperature  Melting point Thermal tempering

 15.1 Introduction

Polymers are common in nature, in the form of wood, rubber, cotton, leather, wood, silk, proteins, enzymes, starches, cellulose.  Artificial polymers are made mostly from oil. Their use has grown exponentially, especially after WW2.  The key factor is the very low production cost and useful properties (e.g., combination of transparency and flexibility, long elongation). 

15.2 Hydrocarbon Molecules

Most polymers are organic, and formed from hydrocarbon molecules.  These molecules can have single, double, or triple carbon bonds.  A saturated hydrocarbon is one where all bonds are single, that is, the number of atoms is maximum (or saturated).  Among this type are the paraffin compounds, CnH2n+2 (Table 15.1).  In contrast, non-saturated hydrocarbons contain some double and triple bonds. 

Isomers are molecules that contain the same molecules but in a different arrangement.  An example is butane and isobutane. 

15.3 Polymer Molecules 

Polymer molecules are huge, macromolecules that have internal covalent bonds. For most polymers, these molecules form very long chains. The backbone is a string of carbon atoms, often single bonded. 

Polymers are composed of basic structures called mer units.  A molecule with just one mer is a monomer. 

15.4 The Chemistry of Polymer Molecules 

Examples of polymers are polyvinyl chloride (PVC), poly-tetra-chloro-ethylene (PTFE or Teflon), polypropylene, nylon and polystyrene.  Chains are represented straight but in practice they have a three-dimensional, zig-zag structure (Fig. 15.1b).

When all the mers are the same, the molecule is called a homopolymer.  When there is more than one type of mer present, the molecule is a copolymer. 

15.5 Molecular Weight 

The mass of a polymer is not fixed, but is distributed around a mean value, since polymer molecules have different lengths. The average molecular weight can be obtained by averaging

the masses with the fraction of times they appear (number-average) or with the mass fraction of the molecules (called, improperly, a weight fraction). 

The degree of polymerization is the average number of mer units, and is obtained by dividing the average mass of the polymer by the mass of a mer unit. 

Polymers of low mass are liquid or gases, those of very high mass (called high-polymers, are solid).  Waxes, paraffins and resins have intermediate masses. 

15.6 Molecular Shape

Polymers are usually not linear; bending and rotations can occur around single C-C bonds (double and triple bonds are very rigid) (Fig. 15.5).  Random kings and coils lead to entanglement, like in the spaghetti structure shown in Fig. 15.6. 

15.7 Molecular Structure

Typical structures are (Fig. 15.7):

linear (end-to-end, flexible, like PVC, nylon) branched cross-linked (due to radiation, vulcanization, etc.) network (similar to highly cross-linked structures).

15.8 Molecular Configurations

The regularity and symmetry of the side-groups can affect strongly the properties of polymers. Side groups are atoms or molecules with free bonds, called free-radicals, like H, O, methyl, etc. 

If the radicals are linked in the same order, the configuration is called isostatic 

In a stereoisomer in a syndiotactic configuration, the radical groups alternative sides in the chain. 

In the atactic configuration, the radical groups are positioned at random. 

15.9 Copolymers

Copolymers, polymers with at least two different types of mers can differ in the way the mers are arranged. Fig. 15.9 shows different arrangements: random, alternating, block, and graft.

15.10 Polymer Crystallinity 

Crystallinity in polymers is more complex than in metals (fig. 15.10). Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material. . 

Chain disorder or misalignment, which is common, leads to amorphous material since twisting, kinking and coiling prevent strict ordering required in the crystalline state.  Thus, linear polymers with small side groups, which are not too long form crystalline regions easier than branched, network, atactic polymers, random copolymers, or polymers with bulky side groups. 

Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density. 

15.11 Polymer Crystals

Different models have been proposed to describe the arrangement of molecules in semicrytalline polymers. In the fringed-micelle model, the crystallites (micelles) are embedded in an amorphous matrix (Fig.15.11). Polymer single crystals grown are shaped in regular platelets (lamellae) (Fig. 15.12). Spherulites (Fig. 15.4) are chain-folded crystallites in an amorphous matrix that grow radially in spherical shape “grains”.

Terms:

Alternating copolymer Atactic configuration Bifunctional mer Block copolymer Branched polymer Chain-folded model Cis (structure) Copolymer Crosslinked polymer Crystallite Degree of polymerization Graft copolymer Homopolymer Isomerism Isotactic configuration Linear polymer Macromolecule Mer Molecular chemistry Molecular structure Molecular weight Monomer Network polymer Polymer Polymer crystallinity

Random copolymer Saturated Spherulite Stereoisomerism Syndiotactic configuration Trans (structure) Trifunctional mer Unsaturated

16.1 Introduction

16.2 Stress-Strain Behavior

The description of stress-strain behavior is similar to that of metals, but a very important consideration for polymers is that the mechanical properties depend on the strain rate, temperature, and environmental conditions.

The stress-strain behavior can be brittle, plastic and highly elastic (elastomeric or rubber-like), see Fig. 16. 1. Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than those of metals, but elongation can be up to 1000 % in some cases. The tensile strength is defined at the fracture point (Fig. 16.2) and can be lower than the yield strength.

Mechanical properties change dramatically with temperature, going from glass-like brittle behavior at low temperatures (like in the liquid-nitrogen demonstration) to a rubber-like behavior at high temperatures (Fig. 16.3).

In general, decreasing the strain rate has the same influence on the strain-strength characteristics as increasing the temperature: the material becomes softer and more ductile.

16.3 Deformation of Semicrystalline Polymers

Many semicrystalline polymers have the spherulitic structure and deform in the following steps (Fig. 16.4):

elongation of amorphous tie chains tilting of lamellar chain folds towards the tensile direction separation of crystalline block segments orientation of segments and tie chains in the tensile direction

The macroscopic deformation involves an upper and lower yield point and necking. Unlike the case of metals, the neck gets stronger since the deformation aligns the chains so increasing the tensile stress leads to the growth of the neck. (Fig. 16.5).

16.4 Factors that Influence the Mechanical Properties of Polymers

The tensile modulus decreases with increasing temperature or diminishing strain rate. 

Obstacles to the steps mentioned in 16.4 strengthen the polymer. Examples are cross-linking (aligned chains have more van der Waals inter-chain bonds) and a large mass (longer molecules have more inter-chain bonds). Crystallinity increases strength as the secondary bonding is enhanced when the molecular chains are closely packed and parallel. Pre-deformation by drawing, analogous to strain hardening in metals, increases strength by orienting the molecular chains.  For undrawn polymers, heating increases the tensile modulus and yield strength, and reduces the ductility - opposite of what happens in metals. 

16.5 Crystallization, Melting, and Glass Transition Phenomena

Crystallization rates are governed by the same type of S-curves we saw in the case of metals (Fig. 16.7). Nucleation becomes slower at higher temperatures.

The melting behavior of semicrystalline polymers is intermediate between that of crystalline materials (sharp density change at a melting temperature) and that of a pure amorphous material (slight change in slope of density at the glass-transition temperature). The glass transition temperature is between 0.5 and 0.8 of the melting temperature.

The melting temperature increases with the rate of heating, thickness of the lamellae, and depends on the temperature at which the polymer was crystallized.

Melting involves breaking of the inter-chain bonds, so the glass and melting temperatures depend on:

chain stiffness (e.g., single vs. double bonds) size, shape of side groups size of molecule side branches, defects cross-linking

Rigid chains have higher melting temperatures.

16.6 Thermoplastic and Thermosetting Polymers

Thermoplastic polymers (thermoplasts) soften reversibly when heated (harden when cooled back)

Thermosetting polymers (thermosets) harden permanently when heated, as cross-linking hinder bending and rotations. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts.

16.7 Viscoelasticity

At low temperatures, amorphous polymers deform elastically, like glass, at small elongation. At high temperatures the behavior is viscous, like liquids. At intermediate temperatures, the behavior, like a rubbery solid, is termed viscoelastic.

Viscoelasticity is characterized by the viscoelastic relaxation modulus

    Er = (t)/0.

If the material is strained to a value 0.it is found that the stress needs to be reduced with time to maintain this constant value of strain (see figs. 16.11 and 16.12).

In viscoelastic creep, the stress is kept constant at 0 and the change of deformation with time t) is measured. The time-dependent creep modulus is given by

    Ec = 0/(t).

16.8 Deformation and Elastomers

Elastomers can be deformed to very large strains and the spring back elastically to the original length, a behavior first observed in natural rubber. Elastic elongation is due to uncoiling, untwisting and straightening of chains in the stress direction.

To be elastomeric, the polymer needs to meet several criteria:

must not crystallize easily have relatively free chain rotations delayed plastic deformation by cross-linking (achieved by vulcanization). be above the glass transition temperature

16.9 Fracture of Polymers

As other mechanical properties, the fracture strength of polymers is much lower than that of metals. Fracture also starts with cracks at flaws, scratches, etc. Fracture involves breaking of covalent bonds in the chains. Thermoplasts can have both brittle and ductile fracture behaviors. Glassy thermosets have brittle fracture at low temperatures and ductile fracture at high temperatures.

Glassy thremoplasts often suffer grazing before brittle fracture. Crazes are associated with regions of highly localized yielding which leads to the formation of interconnected microvoids (Fig. 16.15). Crazing absorbs energy thus increasing the fracture strength of the polymer.

16.10 Miscellaneous Characteristics

Polymers are brittle at low temperatures and have low impact strengths (Izod or Charpy tests), and a brittle to ductile transition over a narrow temperature range.

Fatigue is similar to the case of metals but at reduced loads and is more sensitive to frequency due to heating which leads to softening.

16.11 Polymerization

Polymerization is the synthesis of high polymers from raw materials like oil or coal. It may occur by:

addition (chain-reaction) polymerization, where monomer units are attached one at a time condensation polymerization, by stepwise intermolecular chemical reactions that produce

the mer units.

16.12 – 16.14 – not covered

16.15 Elastomers

In vulcanization, crosslinking of the elastomeric polymer is achieved by an irreversible chemical reaction usually at high temperatures (hence ‘vulcan’), and usually involving the addition of sulfur compounds. The S atoms are the ones that form the bridge cross-links. Elastomers are thermosetting due to the cross-linking.

Rubbers become harder and extend less with increasing sulfur content. For automobile applications, synthetic rubbers are strengthened by adding carbon black.

In silicone rubbers, the backbone C atoms are replaced by a chain of alternating silicon and oxygen atoms. These elastomers are also cross-linked and are stable to higher temperatures than C-based elastomers.

16.16 –16.18 – not covered

Terms:

Addition polymerizationCondensation polymerizationElastomerGlass transition temperaturePlasticRelaxation modulusThermoplastic polymerThermoplastic elastomerThermosetting polymerUltrahigh molecular weight polyethyleneViscoelasticityVulcanization

  

17.1 Introduction

The idea is that by combining two or more distinct materials one can engineer a new material with the desired combination of properties (e.g., light, strong, corrosion resistant). The idea that a better combination of properties can be achieved is called the principle of combined action.

New - High-tech materials, engineered to specific applications

Old - brick-straw composites, paper, known for > 5000 years.

A type of composite that has been discussed is perlitic steel, which combines hard, brittle cementite with soft, ductile ferrite to get a superior material.

Natural composites: wood (polymer-polymer), bones (polymer-ceramics).

Usual composites have just two phases:

matrix (continuous) dispersed phase (particulates, fibers)

Properties of composites depend on

properties of phases geometry of dispersed phase (particle size, distribution, orientation) amount of phase

Classification of composites: three main categories:

particle-reinforced (large-particle and dispersion-strengthened) fiber-reinforced (continuous (aligned) and short fibers (aligned or random) structural (laminates and sandwich panels)

Particle-reinforced composites

These are the cheapest and most widely used. They fall in two categories depending on the size of the particles:

large-particle composites, which act by restraining the movement of the matrix, if well bonded.

dispersion-strengthened composites, containing 10-100 nm particles, similar to what was discussed under precipitation hardening. The matrix bears the major portion of the applied load and the small particles hinder dislocation motion, limiting plastic deformation.

17.2 Large-Particle Composites

Properties are a combination of those of the components. The rule of mixtures predicts that an upper limit of the elastic modulus of the composite is given in terms of the elastic moduli of the matrix (Em) and the particulate (Ep) phases by:

    Ec = EmVm + EpVp

where Vm and Vp are the volume fraction of the two phases. A lower bound is given by:

    Ec = EmEp / (EpVm + EmVp)

Fig. 17.3 - modulus of composite of WC particles in Cu matrix vs. WC concentration.

Concrete

The most common large-particle composite is concrete, made of a cement matrix that bonds particles of different size (gravel and sand.) Cement was already known to the Egyptians and the Greek. Romans made cement by mixing lime (CaO) with volcanic ice.

In its general from, cement is a fine mixture of lime, alumina, silica, and water. Portland cement is a fine powder of chalk, clay and lime-bearing minerals fired to 1500o C (calcinated). It forms a paste when dissolved in water. It sets into a solid in minutes and hardens slowly (takes 4 months for full strength). Properties depend on how well it is mixed, and the amount of water: too little - incomplete bonding, too much - excessive porosity.

The advantage of cement is that it can be poured in place, it hardens at room temperature and even under water, and it is very cheap. The disadvantages are that it is weak and brittle, and that water in the pores can produce crack when it freezes in cold weather.

Concrete is cement strengthened by adding particulates. The use of different size (stone and sand) allows better packing factor than when using particles of similar size.

Concrete is improved by making the pores smaller (using finer powder, adding polymeric lubricants, and applying pressure during hardening.

Reinforced concrete is obtained by adding steel rods, wires, mesh. Steel has the advantage of a similar thermal expansion coefficient, so there is reduced danger of cracking due to thermal stresses. Pre-stressed concrete is obtained by applying tensile stress to the steel rods while the cement is setting and hardening. When the tensile stress is removed, the concrete is left under compressive stress, enabling it to sustain tensile loads without fracturing. Pre-stressed concrete shapes are usually prefabricated. A common use is in railroad or highway bridges.

Cermets

are composites of ceramic particles (strong, brittle) in a metal matrix (soft, ductile) that enhances toughness. For instance, tungsten carbide or titanium carbide ceramics in Co or Ni. They are used for cutting tools for hardened steels. 

Reinforced rubber

is obtained by strengthening with 20-50 nm carbon-black particles. Used in auto tires.

17.3 Dispersion-Strengthened Composites

Use of very hard, small particles to strengthen metals and metal alloys. The effect is like precipitation hardening but not so strong. Particles like oxides do not react so the strengthening action is retained at high temperatures.

Fiber-reinforced composites

In many applications, like in aircraft parts, there is a need for high strength per unit weight (specific strength). This can be achieved by composites consisting of a low-density (and soft) matrix reinforced with stiff fibers.

The strength depends on the fiber length and its orientation with respect to the stress direction.

The efficiency of load transfer between matrix and fiber depends on the interfacial bond.

17.4 Influence of Fiber Length

Normally the matrix has a much lower modulus than the fiber so it strains more. This occurs at a distance from the fiber. Right next to the fiber, the strain is limited by the fiber. Thus, for a composite under tension, a shear stress appears in the matrix that pulls from the fiber. The pull is uniform over the area of the fiber. This makes the force on the fiber be minimum at the ends and maximum in the middle, like in the tug-of-war game.

To achieve effective strengthening and stiffening, the fibers must be larger than a critical length lc, defined as the minimum length at which the center of the fiber reaches the ultimate (tensile) strength f, when the matrix achieves the maximum shear strength m:

    lc = sf d /2 tm

Since it is proportional to the diameter of the fiber d, a more unified condition for effective strengthening is that the aspect ratio of the fiber is l/d > f /2 m.

17.5 Influence of Fiber Orientation

The composite is stronger along the direction of orientation of the fibers and weakest in a direction perpendicular to the fiber. For discontinuous, random fibers, the properties are isotropic.

17.6 The Fiber Phase (not covered)

17.7 The Matrix Phase (not covered)

17.8 Polymer Matrix Composites

Largest and most diverse use of composites due to ease of fabrication, low cost and good properties.

Glass-fiber reinforced composites (GFRC) are strong, corrosion resistant and lightweight, but not very stiff and cannot be used at high temperatures. Applications include auto and boat bodies, aircraft components.

Carbon-fiber reinforced composites (CFRC) use carbon fibers, which have the highest specific module (module divided by weight). CFRC are strong, inert, allow high temperature use. Applications include fishing rods, golf clubs, aircraft components.

Kevlar, and aremid-fiber composite (Fig. 17.9) can be used as textile fibers. Applications include bullet-proof vests, tires, brake and clutch linings.

Wood

This is one of the oldest and the most widely used structural material. It is a composite of strong and flexible cellulose fibers (linear polymer) surrounded and held together by a matrix of lignin and other polymers. The properties are anisotropic and vary widely among types of wood. Wood is ten times stronger in the axial direction than in the radial or tangential directions.

17.9 Metal-Matrix Composites (not covered)

17.10 Ceramic-Matrix Composites (not covered)

17.11 Carbon-Carbon Composites (not covered)

17.12 Hybrid Composites (not covered)

17.13 Processing of Fiber-Reinforced Composites (not covered)

17.14 Laminar Composites

Sheets (panels) with different orientation of high strength directions are stacked and glued together, producing a material with more isotropic strength in the plane. Examples are plywood and modern skis.

17.15 Sandwich Panels

Strong, stiff end sheets are bonded to lightweight core structure, for instance honeycomb (Fig. 17.16) which provides strength to shear. It is used in roofs, walls, and aircraft structures.

Terms:

Ceramic-matrix compositeCermetConcreteDispersed phaseDispersion-strengthened compositeFiberFiber-reinforced compositeHybrid compositeLaminar compositeLarge-particle compositeLongitudinal directionMatrix phaseMetal-matrix compositePolymer-matrix compositePre-stressed concretePrinciple of combined actionReinforced concreteRule of mixturesSandwich panelSpecific modulusSpecific strengthStructural compositeTransverse directionWhisker 

 Electrical Conduction

19.2 Ohm’s Law

When an electric potential V is applied across a material, a current of magnitude I flows. In most metals, at low values of V, the current is proportional to V, according to Ohm's law:

    I = V/R

where R is the electrical resistance. R depends on the intrinsic resistivity of the material and on the geometry (length l and area A through which the current passes).

    R = l/A

19.3 Electrical Conductivity

The electrical conductivity is the inverse of the resistivity: 

The electric field in the material is EV/l, Ohm's law can then be expressed in terms of the current density j = I/A as:

    j =  E

The conductivity is one of the properties of materials that varies most widely, from 107 (-m) typical of metals to 10-20 (-m) for good electrical insulators. Semiconductors have conductivities in the range 10-6 to 104 (-m).

19.4 Electronic and Ionic Conduction

In metals, the current is carried by electrons, and hence the name electronic conduction. In ionic crystals, the charge carriers are ions, thus the name ionic conduction (see Sect. 19.15).

19.5 Energy Band Structures in Solids

When atoms come together to form a solid, their valence electrons interact due to Coulomb forces, and they also feel the electric field produced by their own nucleus and that of the other atoms. In addition, two specific quantum mechanical effects happen. First, by Heisenberg's uncertainty principle, constraining the electrons to a small volume raises their energy, this is called promotion. The second effect, due to the Pauli exclusion principle, limits the number of electrons that can have the same property (which include the energy). As a result of all these effects, the valence electrons of atoms form wide valence bands when they form a solid. The bands are separated by gaps, where electrons cannot exist. The precise location of the bands and band gaps depends on the type of atom (e.g., Si vs. Al), the distance between atoms in the solid, and the atomic arrangement (e.g., carbon vs. diamond).

In semiconductors and insulators, the valence band is filled, and no more electrons can be added, following Pauli's principle. Electrical conduction requires that electrons be able to gain energy in an electric field; this is not possible in these materials because that would imply that the electrons are promoted into the forbidden band gap.

In metals, the electrons occupy states up to the Fermi level. Conduction occurs by promoting electrons into the conduction band, that starts at the Fermi level, separated by the valence band by an infinitesimal amount.

19.6 Conduction in Terms of Band and Atomic Bonding Models

Conduction in metals is by electrons in the conduction band. Conduction in insulators is by electrons in the conduction band and by holes in the valence band. Holes are vacant states in the valence band that are created when an electron is removed.

In metals there are empty states just above the Fermi levels, where electrons can be promoted. The promotion energy is negligibly small so that at any temperature electrons can be found in the

conduction band. The number of electrons participating in electrical conduction is extremely small.

In insulators, there is an energy gap between the valence and conduction bands, so energy is needed to promote an electron to the conduction band. This energy may come from heat, or from energetic radiation, like light of sufficiently small wavelength.

A working definition for the difference between semiconductors and insulators is that in semiconductors, electrons can reach the conduction band at ordinary temperatures, where in insulators they cannot. The probability that an electron reaches the conduction band is about exp(-Eg/2kT) where Eg is the band gap and kT has the usual meaning. If this probability is, say, < 10-24 one would not find a single electron in the conduction band in a solid of 1 cubic centimeter. This requires Eg/2kT > 55. At room temperature, 2kT = 0.05 eV; thus Eg > 2.8 eV can be used as the condition for an insulator.

Besides having relatively small Eg, semiconductors have covalent bond, whereas insulators usually are partially ionic bonded.

19.7 Electron Mobility

Electrons are accelerated in an electric field E, in the opposite direction to the field because of their negative charge. The force acting on the electron is -eE, where e is the electric charge. This force produces a constant acceleration so that, in the absence of obstacles (in vacuum, like inside a TV tube) the electron speeds up continuously in an electric field. In a solid, the situation is different. The electrons scatter by collisions with atoms and vacancies that change drastically their direction of motion. Thus electrons move randomly but with a net drift in the direction opposite to the electric field. The drift velocity is constant, equal to the electric field times a constant called the mobility ,

vd= – e E

which means that there is a friction force proportional to velocity. This friction translates into energy that goes into the lattice as heat. This is the way that electric heaters work.

The electrical conductivity is:

 = n |e| e

where n is the concentration of electrons (n is used to indicate that the carriers of electricity are negative particles).

19.8 Electrical Resistivity of Metals

The resistivity then depends on collisions. Quantum mechanics tells us that electrons behave like waves. One of the effects of this is that electrons do not scatter from a perfect lattice. They scatter by defects, which can be:

o atoms displaced by lattice vibrationso vacancies and interstitialso dislocations, grain boundarieso impurities

One can express the total resistivity tot by the Matthiessen rule, as a sum of resistivities due to thermal vibrations, impurities and dislocations. Fig. 19.8 illustrates how the resistivity increases with temperature, with deformation, and with alloying.

19.9 Electrical Characteristics of Commercial Alloys

The best material for electrical conduction (lower resistivity) is silver. Since it is very expensive, copper is preferred, at an only modest increase in . To achieve low  it is necessary to remove gases occluded in the metal during fabrication. Copper is soft so, for applications where mechanical strength is important, the alloy CuBe is used, which has a nearly as good . When weight is important one uses Al, which is half as good as Cu. Al is also more resistant to corrosion.

When high resistivity materials are needed, like in electrical heaters, especially those that operate at high temperature, nichrome (NiCr) or graphite are used.

19.10 Intrinsic Semiconduction

Semiconductors can be intrinsic or extrinsic. Intrinsic means that electrical conductivity does not depend on impurities, thus intrinsic means pure. In extrinsic semiconductors the conductivity depends on the concentration of impurities.

Conduction is by electrons and holes. In an electric field, electrons and holes move in opposite direction because they have opposite charges. The conductivity of an intrinsic semiconductor is:

n |e| e + p |e| h

where p is the hole concentration and h the hole mobility. One finds that electrons move much faster than holes:

e > h

In an intrinsic semiconductor, a hole is produced by the promotion of each electron to the conduction band. Thus:

    n = p

Thus, 2 n |e| (e + h) (only for intrinsic semiconductors).

19.11 Extrinsic Semiconduction

Unlike intrinsic semiconductors, an extrinsic semiconductor may have different concentrations of holes and electrons. It is called p-type if p>n and n-type if n>p. They are made by doping, the addition of a very small concentration of impurity atoms. Two common methods of doping are diffusion and ion implantation.

Excess electron carriers are produced by substitutional impurities that have more valence electron per atom than the semiconductor matrix. For instance phosphorous, with 5 valence electrons, is an electrondonor in Si since only 4 electrons are used to bond to the Si lattice when it substitutes for a Si atom. Thus, elements in columns V and VI of the periodic table are donors for semiconductors in the IV column, Si and Ge. The energy level of the donor state is close to the conduction band, so that the electron is promoted (ionized) easily at room temperature, leaving a hole (the ionized donor) behind. Since this hole is unlike a hole in the matrix, it does not move easily by capturing electrons from adjacent atoms. This means that the conduction occurs mainly by the donated electrons (thus n-type).

Excess holes are produced by substitutional impurities that have fewer valence electrons per atom than the matrix. This is the case of elements of group II and III in column IV semiconductors, like B in Si. The bond with the neighbors is incomplete and so they can capture or accept electrons from adjacent silicon atoms. They are called acceptors. The energy level of the acceptor is close to the valence band, so that an electron may easily hop from the valence band to complete the bond leaving a hole behind. This means that conduction occurs mainly by the holes (thus p-type).

19.12 The Temperature Variation of Conductivity and Carrier Concentration

Temperature causes electrons to be promoted to the conduction band and from donor levels, or holes to acceptor levels. The dependence of conductivity on temperature is like other thermally activated processes:

 = A exp(–Eg/2kT)

where A is a constant (the mobility varies much more slowly with temperature). Plotting ln  vs. 1/T produces a straight line of slope Eg/2k from which the band gap energy can be determined. Extrinsic semiconductors have, in addition to this dependence, one due to the thermal promotion of electrons from donor levels or holes from acceptor levels. The dependence on temperature is also exponential but it eventually saturates at high temperatures where all the donors are emptied or all the acceptors are filled.

This means that at low temperatures, extrinsic semiconductors have larger conductivity than intrinsic semiconductors. At high temperatures, both the impurity levels and valence electrons are ionized, but since the impurities are very low in number and they are exhausted, eventually the behavior is dominated by the intrinsic type of conductivity.

19.13 The Hall Effect (not covered)

19.14 Semiconductor Devices

A semiconductor diode is made by the intimate junction of a p-type and an n-type semiconductor (an n-p junction). Unlike a metal, the intensity of the electrical current that passes through the material depends on the polarity of the applied voltage. If the positive side of a battery is connected to the p-side, a situation called forward bias, a large amount of current can flow since holes and electrons are pushed into the junction region, where they recombine (annihilate). If the polarity of the voltage is flipped, the diode operates under reverse bias. Holes and electrons are removed from the region of the junction, which therefore becomes depleted of carriers and behaves like an insulator. For this reason, the current is very small under reverse bias. The asymmetric current-voltage characteristics of diodes (Fig. 19.20) is used to convert alternating current into direct current. This is called rectification.

A p-n-p junction transistor contains two diodes back-to-back. The central region is very thin and is called the base. A small voltage applied to the base has a large effect on the current passing through the transistor, and this can be used to amplify electrical signals (Fig. 19.22). Another common device is the MOSFET transistor where a gate serves the function of the base in a junction transistor. Control of the current through the transistor is by means of the electric field induced by the gate, which is isolated electrically by an oxide layer.

19.15 Conduction in Ionic Materials

In ionic materials, the band gap is too large for thermal electron promotion. Cation vacancies allow ionic motion in the direction of an applied electric field, this is referred to as ionic conduction. High temperatures produce more vacancies and higher ionic conductivity.

At low temperatures, electrical conduction in insulators is usually along the surface, due to the deposition of moisture that contains impurity ions.

19.16 Electrical Properties of Polymers

Polymers are usually good insulators but can be made to conduct by doping. Teflon is an exceptionally good insulator.

Dielectric Behavior

A dielectric is an electrical insulator that can be made to exhibit an electric dipole structure (displace the negative and positive charge so that their center of gravity is different).

19.17 Capacitance

When two parallel plates of area A, separated by a small distance l, are charged by +Q, –Q, an electric field develops between the plates

    E = D/

where D = Q/A.  is called the vacuum permittivity and the relative permittivity, or dielectric constant ( = 1 for vacuum). In terms of the voltage between the plates, V = E l,

    V = Dl/= Q l/A= Q / C

The constant C= A/l is called the capacitance of the plates.

19.18 Field Vectors and Polarization

The dipole moment of a pair of positive and negative charges (+q and –q) separated at a distance d is p = qd. If an electric field is applied, the dipole tends to align so that the positive charge points in the field direction. Dipoles between the plates of a capacitor will produce an electric field that opposes the applied field. For a given applied voltage V, there will be an increase in the charge in the plates by an amountQ' so that the total charge becomes Q = Q' + Q0, where Q0 is the charge of a vacuum capacitor with the same V. With Q' = PA, the charge density becomes D = D0 E + P, where the polarization P = (–1) E.

19.19 Types of Polarization

Three types of polarization can be caused by an electric field:

Electronic polarization: the electrons in atoms are displaced relative to the nucleus. Ionic polarization: cations and anions in an ionic crystal are displaced with respect to

each other. Orientation polarization: permanent dipoles (like H2O) are aligned.

19.20 Frequency Dependence of the Dielectric Constant

Electrons have much smaller mass than ions, so they respond more rapidly to a changing electric field. For electric field that oscillates at very high frequencies (such as light) only electronic polarization can occur. At smaller frequencies, the relative displacement of positive and negative ions can occur. Orientation of permanent dipoles, which require the rotation of a molecule can occur only if the oscillation is relatively slow (MHz range or slower). The time needed by the specific polarization to occur is called the relaxation time.

19.21 Dielectric Strength

Very high electric fields (>108 V/m) can free electrons from atoms, and accelerate them to such high energies that they can, in turn, free other electrons, in an avalanche process (or electrical discharge). This is called dielectric breakdown, and the field necessary to start the is called the dielectric strength or breakdown strength.

19.22 Dielectric Materials

Capacitors require dielectrics of high  that can function at high frequencies (small relaxation times). Many of the ceramics have these properties, like mica, glass, and porcelain). Polymers usually have lower 

19.23 Ferroelectricity

Ferroelectric materials are ceramics that exhibit permanent polarization in the absence of an electric field. This is due to the asymmetric location of positive and negative charges within the unit cell. Two possible arrangements of this asymmetry results in two distinct polarizations, which can be used to code "0" and "1" in ferroelectric memories. A typical ferroelectric is barium titanate, BaTiO3, where the Ti4+is in the center of the unit cell and four O2- in the central plane can be displaced to one side or the other of this central ion (Fig. 19.33).

19.24 Piezoelectricity

In a piezolectric material, like quartz, an applied mechanical stress causes electric polarization by the relative displacement of anions and cations.

Terms:

Acceptor stateCapacitanceConduction bandConductivity, electricalDielectricDielectric constantDielectric displacementDielectric strengthDiodeDipole, electricDonor stateDopingElectrical resistanceElectron energy bandEnergy band gapExtrinsic semiconductorFermi energyFerroelectricForward biasFree electronHoleInsulatorIntegrated circuitIntrinsic semiconductorIonic conductionJunction transistorMatthiessen’s ruleMetalMobilityMOSFETOhm’s lawPermittivity

PiezoelectricPolarizationPolarization, electronicPolarization, ionicPolarization, orientationRectifying junctionRelaxation frequencyResistivity, electricalReverse biasSemiconductorValence band

 

PROPERTIES OF MATERIALS - 1V. Ryan © 2005 - 2009

 

When studying materials and especially when selecting materials for a project / design, it is important to understand key properties. The most important properties are outlined below. 

PDF FILE - CLICK HERE FOR PRINTABLE EXERCISE 

STRENGTH  The ability of a material to stand up to forces being applied without it bending, breaking, shattering or deforming in any way.

Our technology technician (Ed) demonstrates the ‘strength’ of a material by performing a hand stand on a strong piece of timber (wood). It does not bend even under his weight. He has eaten pies and drunk a large amount of beer for twenty years and yet the strong material does not bend, flex or deform (change shape) in any way.

  

   ELASTICITY  The ability of a material to absorb force and flex in different directions, returning to its original position.

Our technology technician demonstrates the ‘elasticity’ of a material by springing up and down on a piece of steel rod. Do not try this at home as an accident may result. Ed our technician is an expert at demonstrating this property as it is his hobby.

  

PLASTICITY  The ability of a material to be change in shape permanently.    

Our technology technician and his twin brother demonstrate the ‘plasticity’ of a molten aluminium by pouring it into a mould. Once the aluminium has cooled down, it can be removed from the casting sand. It has a new shape.

Our technician is often seen scavenging in dust bins after aluminum drinks cans. He then melts them down to form blocks (ingots) of aluminium to sell to scrap metal dealers.   

     DUCTILITY  The ability of a material to change shape (deform) usually by stretching along its length.

Our technician stretches the lead above his head. As it stretches if deforms (changes shape).

Ed thinks he is a strong man, little does he realise that lead is a very soft metal and stretches very easily. He performs these tricks in local pubs in an attempt to pass himself off as a ‘hard man’.

  

   TENSILE STRENGTH  The ability of a material to stretch without breaking or snapping.

Our technology technician demonstrates ‘tensile strength’ by stretching a piece of steel until it snaps. Ed thinks he is incredibly strong. However, his friends at work have substituted a sausage in place of the steel.

  

MALLEABILITY  

The ability of a material to be reshaped in all directions without cracking

Our technology technician demonstrates the ‘malleability’ of a material by heating a piece of mild steel until it is red hot. He then beats it with a large forging hammer to reshape it. Because of the high temperature it reaches while heating the steel becomes malleable, it can be reshaped permanently.

Ed often heats up steel, because he likes the colour and it matches his complexion after he has run up the stairs.

  

   

TOUGHNESS  

A characteristic of a material that does not break or shatter when receiving a blow or under a sudden shock.

  

Our technology technician demonstrates the ‘toughness’ of a material by hitting a piece or material to see if it will break or shatter. 

Ed has been known to test authentic Chinese Ming Dynasty pottery with the same technique. This is why he is often arrested in Museums and has been banned from the local Antique dealers.

   

HARDNESS  

The ability of a material to resist scratching, wear and tear and indentation.

Our technology technician, dressed in a kilt, slides along the floor to see if it will scratch. It will be considered to hard wearing if it resists scratching.

Ed has been known to fall over. Not a sight for sore eyes.

  

   

  

   

CONDUCTIVITY  

The ability of a material to conduct electricity.

Our technology technician demonstrates ‘conductivity’ by pressing live wires against either side of his head (PLEASE NOTE - THIS WILL KILL NORMAL PEOPLE). 

Ed survives because his skull is empty.

  

   

   

How The World Is Made

Fundamentals Of Metal Castingo Metal Casting Basics: Molds, Patterns, Cores, and Gating o The Metal Casting Operation o The Effect of Gases and Material Selection on Metal Casting o Gating Systems and Mold Design /Troubleshooting

Expendable Mold Castingo Sand Casting o Plaster Mold Casting o Ceramic Mold Casting o Shell Molding o Vacuum Casting or V-process o Expanded Polystyrene Casting o Investment Casting

Permanent Mold Castingo Basic Permanent Mold Casting o Slush Casting o Pressure Casting o Vacuum Permanent Mold Casting o Die Casting o Hot Chamber Die Casting o Cold Chamber Die Casting o True Centrifugal Casting

o Semicentrifugal Casting o Centrifuge Casting o Ingot Manufacture o Continuous Casting

Fundamentals Of Metal Formingo Metal Forming Basics

Bulk Deformationo Metal Rolling

Shape Rolling Ring Rolling Thread Rolling Rotary Tube Piercing

o Metal Forging Forging Hammers Hydraulic And Mechanical Presses Heading Or Upset Forging Piercing Sizing Roll Forging Swaging Hobbing Metal Ball Manufacture Orbital Forging Ring Forging Riveting Coining  Isothermal Forging Trimming High Energy Forming

o Extrusion Extrusion Design

Impact Extrusion Hydrostatic Extrusion

o Metal Drawing Sheet Metal Forming

o Sheet Metal Basics o Sheet Metal Cutting o Sheet Metal Bending o Deep Drawing Sheet Metal o Ironing Sheet Metal o Metal Spinning o Rubber Forming Of Sheet Metal o High Energy Rate Forming

Powder Metallurgy Pressing And Sintering Alternative Powder Processing Methods Design And Applications Of Powder Metallurgy

Polymer Casting Injection Molding

o Multi-Injection Moldingo Reaction-Injection Molding

Compression Molding Transfer Molding Blow Molding Rotational Molding Extrusion Production of Sheet and Film Thermoforming

Casting of Ceramicso Slip Castingo Doctor-Blade Process

Forming of Ceramics and Glasses

Machining Basics Turning Milling Drilling Reaming Broaching Shaping Sawing Planning Grinding

Manufacturing processes are applicable in all areas of our lives, so much that we often don't realize or think about it. From the cars we drive, the containers our food comes in, the TV's, computers and other devices we use, power tools, heaters, air conditioners, the pipes that deliver our water and the list goes on and on to include just about everything defining our modern society. These things are all manufactured or built from manufactured components. Manufacturing equipment itself must also be manufactured. The manufacturing process used is determined by a variety of factors.

Manufacturing Process: The Fundamental IdeaThe fundamental idea of manufacturing or production is to create, (or produce), something that has a useful form. This form is most likely predetermined and calculated, with a certain physical geometry. Usually this geometry has certain tolerances that it must meet in order to be considered acceptable. A tolerance outlines the geometric accuracy that must be achieved in the manufacturing process. The "tightness" of the tolerances, or in other words the allowed variance between the manufactured product and the ideal product, is a function of the particular application of the product.

Goals and Core Principles For All Processes1. Meeting performance requirements (ie. tolerances, strength, weight, ect.)2. Meeting cost of production requirements3. Ability to reproduce constant quality during mass production4. Large manufactured components should have uniform material properties throughout

the component

The Primary Consideration For A Manufacturing Process

When different manufacturing processes and methods are considered,develop an understanding of the relationship between the process used and the properties of the finished product. For this it is important to know what conditions a particular process will subject a material to and how different manufacturing materials respond to different conditions, (ie. stress, heat).

Manufacturing MaterialsAll manufactured products are made from some sort of material. Similar to the geometric tolerance, the properties of the material of the final manufactured product are of utmost importance. Hence, those who are interested in manufacturing should be very concerned with material selection. An extremely wide variety of materials are available to the manufacturer today. The manufacturer must consider the properties of these materials with respect to the desired properties of the manufactured goods. Simultaneously, one must also consider manufacturing process. Although the properties of a material may be great, it may not be able to effectively, or economically, be processed into a useful form. Also, since the microscopic structure of materials is often changed through different manufacturing processes -dependent upon the process- variations in manufacturing technique may yield different results in the end product. Therefore, a constant feedback must exist between manufacturing process and materials optimization.

Basic Concepts:

Types of MaterialsMaterials can be classified into 3 basic types.

1. Metals2. Ceramics3. Polymers

What are the properties of metals, the properties of ceramics, the properties of polymers?

Metals: Metals are hard, malleable, (meaning capable of being shaped), and somewhat flexible materials. Metals are also very strong. Their combination of strength and flexibility makes them useful in structural applications. When the surface of a metal is polished it has a lustrous appearance; although this surface luster is usually obscured by the presence of dirt, grease and salt. Metals are not transparent to visible light. Also, metals are extremely good conductors of electricity and heat.

Ceramics: Ceramics are very hard and strong, but lack flexibility making them brittle. Ceramics are extremely resistant to high temperatures and chemicals. Ceramics can typically withstand more brutal environments than metals or polymers. Ceramics are usually not good conductors of electricity or heat.

Polymers: Polymers are mostly soft and not as strong as metals or ceramics. Polymers can be extremely flexible. Low density and viscous behavior under elevated temperatures are typical polymer traits. Polymers can be insulative to electricity.

What are metals made of, what are ceramics made of, what are polymers made of?Or in other words, what is the basic microstructure of metals, what is the basic microstructure of ceramics, what is the basic microstructure of polymers?

Metals: A metal is most likely a pure metallic element, (like iron), or an alloy, which is a combination of two or more metallic elements, (like copper-nickel), the atoms of a metal, similar to the atoms of a ceramic or polymer, are held together by electrical forces. The electrical bonding in metals is termed metallic bonding. The simplest explanation for these

types of bonding forces would be positively charged ion cores of the element, (nucleus's of the atoms and all electrons not in the valence level), held together by a surrounding "sea" of electrons, (valence electrons from the atoms). With the electrons in the "sea" moving about, not bound to any particular atom. This is what gives metals their properties such malleability and high conductivity. Metal manufacturing processes usually begin in a

Figure 1 

Ceramics: Ceramics are compounds between metallic and non-metallic elements. The atomic bonds are usually ionic, where one atom, (non-metal), holds the electrons from another, (metal). The non-metal is then negatively charged and the metal positively charged. The opposite charge causes them to bond together electrically. Sometimes the forces are partially covalent. Covalent bonding means the electrons are shared by both atoms, in this case electrical forces between the two atoms still result from the difference in charge, holding them together. To simplify think of a building framework structure. This is what gives ceramics their properties such as strength and low flexibility.

Figure 2 

Polymers: Polymers are often composed of organic compounds and consist of long hydro-carbon chains. Chains of carbon, hydrogen and often other elements or compounds covalently bonded together.

Figure 3 

In figure 3, (a) represents a simple hydrocarbon chain, each group of hydrogen and carbon is called a mer, there are 13 mers shown in the diagram, the dotted lines indicate that the pattern is continuing indefinitely. Polymers chains often contain thousands upon thousands of mers each. The [R] in (b) indicates a variable element or group of elements that could occupy a

certain position in the chain. The [X] in (c) also represents another variable element or group that could occupy another position, this one being at the end or beginning of a polymer chain. The chains themselves bond to each other through secondary bonding forces. To simplify polymer structure, think of a bowl of spaghetti.

Figure 4 

This is a polymer with a random or amorphous microstructure.

Figure 5 

This is a polymer with a very high degree of order or a crystalline microstructure.

When heat is applied, the weaker secondary bonds, (between the strands), begin to break and the chains start to slide easier over one another. However, the stronger covalent bonds, (the strands themselves), stay intact until a much higher temperature. This is what causes polymers to become increasingly viscous as temperature goes up.

Manufacturing Processes

This is a summary of the basic and most commonly used manufacturing processes in industry today. Any of these processes can be employed to produce a manufactured part. Also, remember when deciding how to produce manufactured items, a part may require a combination of these processes to facilitate its completion. For example, a cast part may require some machining before it becomes the final product. Or, a part may be produced through a powder metallurgy process, then undergo some kind of metal forming operation. The following describes the methods and techniques involved in each of these manufacturing processes. Always keep in mind how material properties relate to manufacturing process. Most manufacturing processes described below are for metals. Manufacturing processes for polymers and ceramics will be discussed separately, each given its respective section. These processes are often similar in nature to those for metals, (ie. polymers are essentially both cast and formed in different techniques), however they are different enough to be classified independently.

Casting: Metal casting is definitely one of the oldest manufacturing processes. Castings have been found dating back 6000 years. Fundamentally, casting involves filling a mold with molten material. This material, upon solidification, takes the shape of the mold. There are two basic types of metal casting processes, expendable mold and permanent moldcan be made into the same shape as the final product, being the only process required. Or sometimes, casting is the first manufacturing process in the production of a multi-process manufactured part. Metal casting can be used to make parts with complicated geometry, both internal and external. With casting, intricate parts can be made in a single piece. Metal casting can produce very small parts like jewelry, or enormous parts weighing several hundred tons, like components for very large machinery. Although careful influence of casting parameters and technique can help control material properties; a general disadvantage to metal casting is that the final product tends to contain more flaws and has a lower strength and ductility compared to that of other manufacturing processes, such as metal forming.

Forming: The category of manufacturing by metal forming includes a large group of processes that use force to induce a shape change in a metal, by mechanical working and plastic deformation. The most desirable quality of a manufacturing material as a candidate for a metal forming process is high ductility and malleability and a lower yield strength of the material. When working with metals, an increase in temperature will result in a higher ductility and a lower yield strength. In manufacturing industry, metals are often formed at elevated temperatures. In addition to shape change, the metal forming process will usually change the mechanical properties of the part's material. Metal forming can close up vacancies within the metal, break up and distribute impurities and establish new, stronger grain boundaries. For these reasons, the metal forming process is known to produce parts with superior mechanical properties. With relation to temperature there are 3 types of forming. Cold working, (room temperature), warm working and hot working. Also, with relation to the surface area-to-volume of a material there are 2 main categories,deformation and sheet forming. 

Powder Processing: Powder processing is a manufacturing technique that produces parts from the powder of certain materials. The powders are pressed into the desired shape, (called pressing), and heated sufficiently to cause the particles to bond together into a solid component, (called sintering). Powder processing is common for metal materials, however ceramics may also be subject to powder processing techniques. There are many advantages to powder processing. With powder processing you can obtain consistent dimensional control of the product, keeping relatively tight tolerances, (+/- .005"). It also can produce parts with good surface finish. Parts can therefore be made into their final shape, requiring no further manufacturing processes. With powder processing there is very little waste of material. Since powder processing can be automated, it minimizes the need for labor, requiring small amounts of skilled labor. Metals that are difficult to work with other processes can be shaped easily, (ie. tungsten). Also, certain alloy combinations and cermets that can not be formed any other way, can be produced with this technique. Lastly, parts can be produced with a controlled level of porosity, due to the nature of the process. Powder processes also have a number of disadvantages. The first is high cost. Powders are expensive compared to solid material, they are also difficult to store. Sintering furnaces and special presses are more complicated to construct than conventional machinery. Tooling is also very expensive. Since powders do not easily flow laterally in a die when pressed, there are geometric limitations to the parts that can be manufactured. Powder parts may have inferior mechanical properties, (unless they undergo a forging process). Finally, variations in material density throughout the part may be a problem, especially with more intricate geometries. Powder processing manufacturing is ideal for producing large quantities of moderately complex, small to medium size parts that do not require strong mechanical properties in the part's material. This is not true of some alternative powder processes, such as hot isostatic pressing, that can manufacture parts with superior mechanical properties. A process such as hot isostatic pressing, however, would not be efficient in the manufacture of large quantities of parts.

Machining: In machining, a manufactured part is created to its desired geometric dimensions by the removal of excess material from a work piece, via a force exerted through a certain material removal tool. Qualities of a desirable manufacturing material for this purpose would be:

1) Lower Shear Strength (to make cutting easier) 2) Shock Resistant (to withstand impact loading) 3) Material must not have a tendency to stick to the cutting tool 4) Material removed should separate from the work easily and completely

A material's relative ability to be machined is called machinability. Ceramics have high shear strengths, making them difficult to cut. Also, they are not shock resistant, which causes them to fracture from the impact loading between the tool and work piece. Polymers, although having low yield strengths, melt from the heat generated in the process, causing them to stick to the tool. In addition, high ductility in polymers can make material removal difficult and machining is based on material removal. For these reasons, ceramics and polymers have poor machinability. Machining is generally applicable to metals. Machinability varies among metals, hardened metals present a particular problem, due to a very high shear strength. Often, metals are machined as close to their final shape as possible before being hardened. That way, the hardened material only has to undergo minimal finishing operations. This type of manufacturing process has many advantages. Machining can produce extreme dimensional accuracy, often more so than any other process alone, (tolerances of less than .001"). Also, it can produce sharp corners and flatness on a part that may not be able to be created through other processes. Machining accuracy allows it to produce surface finish and smoothness that can not be achieved any other way. By combining different machining operations, very complex parts can be manufactured. This type of manufacturing process does have disadvantages. This is a material removal process, thus wastes material. Although economical if the number of parts to be produced is small; labor, energy, equipment and scrap cost are relatively high for large runs. Machining is very applicable for finishing operations on manufactured goods. 

Top

Mechanical PropertiesThe mechanical properties of a material describe how it will react to physical forces. Mechanical properties occur as a result of the physical properties inherent to each material, and are determined through a series of standardized mechanical tests.

 

Strength

Strength has several definitions depending on the material type and application. Before choosing a material based on its published or measured strength it is important to understand the manner in which strength is defined and how it is measured. When designing for strength, material class and mode of loading are important considerations.

For metals the most common measure of strength is the yield strength. For most polymers it is more convenient to measure the failure strength, the stress at the point where the stress strain curve becomes obviously non-linear. Strength, for ceramics however, is more difficult to define. Failure in ceramics is highly dependent on the mode of loading. The typical failure strength in compression is fifteen times the failure strength in tension. The more common reported value is the compressive failure strength.

 

Elastic limit

The elastic limit is the highest stress at which all deformation strains are fully recoverable. For most materials and applications this can be considered the practical limit to the maximum stress a component can withstand and still function as designed. Beyond the elastic limit permanent strains are likely to deform the material to the point where its function is impaired.

 

Proportional limit

The proportional limit is the highest stress at which stress is linearly proportional to strain. This is the same as the elastic limit for most materials. Some materials may show a slight deviation from proportionality while still under recoverable strain. In these cases the proportional limit is preferred as a maximum stress level because deformation becomes less predictable above it.

 

Yield Strength

The yield strength is the minimum stress which produces permanent plastic deformation. This is perhaps the most common material property reported for structural materials because of the ease and relative accuracy of its measurement. The yield strength is usually defined at a specific amount of plastic strain, or offset, which may vary by material and or specification. The offset is the amount that the stress-strain curve deviates from the linear elastic line. The most common offset for structural metals is 0.2%.

 

Ultimate Tensile Strength

The ultimate tensile strength is an engineering value calculated by dividing the maximum load on a material experienced during a tensile test by the initial cross section of the test sample. When viewed in light of the other tensile test data the ultimate tensile strength helps to provide a good indication of a material's toughness but is not by

itself a useful design limit. Conversely this can be construed as the minimum stress that is necessary to ensure the failure of a material.

 

True Fracture Strength

The true fracture strength is the load at fracture divided by the cross sectional area of the sample. Like the ultimate tensile strength the true fracture strength can help an engineer to predict the behavior of the material but is not itself a practical strength limit. Because the tensile test seeks to standardize variables such as specimen geometry, strain rate and uniformity of stress it can be considered a kind of best case scenario of failure.

 

Ductility

Ductility is a measure of how much deformation or strain a material can withstand before breaking. The most common measure of ductility is the percentage of change in length of a tensile sample after breaking. This is generally reported as % El or percent elongation. The R.A. or reduction of area of the sample also gives some indication of ductility.

 

Toughness

Toughness describes a material's resistance to fracture. It is often expressed in terms of the amount of energy a material can absorb before fracture. Tough materials can absorb a considerable amount of energy before fracture while brittle materials absorb very little. Neither strong materials such as glass or very ductile materials such as taffy can absorb large amounts of energy before failure. Toughness is not a single property but rather a combination of strength and ductility.

The toughness of a material can be related to the total area under its stress-strain curve. A comparison of the relative magnitudes of the yield strength, ultimate tensile strength and percent elongation of different material will give a good indication of their relative toughness. Materials with high yield strength and high ductility have high toughness. Integrated stress-strain data is not readily available for most materials so other test methods have been devised to help quantify toughness. The most common test for toughness is the Charpy impact test.

In crystalline materials the toughness is strongly dependent on crystal structure. Face centered cubic materials are typically ductile while hexagonal close packed materials tend to be brittle. Body centered cubic materials often display dramatic variation in the mode of failure with temperature. In many materials the toughness is temperature dependent. Generally materials are more brittle at lower temperatures and more ductile at higher temperatures. The temperature at which the transition takes place is known as the DBTT, or ductile to brittle transition temperature. The DBTT is measured by performing a series of Charpy impact tests at various temperatures to determine the ranges of brittle and ductile behavior. Use of alloys below their transition temperature is avoided due to the risk of catastrophic failure.

 

Fatigue ratio

The dimensionless fatigue ratio f is the ratio of the stress required to cause failure after a specific number of cycles to the yield stress of a material. Fatigue tests are generally run through 107 or 108 cycles. A high fatigue ratio indicates materials which are more susceptible to crack growth during cyclic loading.

 

Loss coefficient

The loss coefficient is an other important material parameter in cyclic loading. It is the fraction of mechanical energy lost in a stress strain cycle. The loss coefficient for each material is a function of the frequency of the cycle. A high loss coefficient can be desirable for damping vibrations while a low loss coefficient transmits energy more efficiently. The loss coefficient is also an important factor in resisting fatigue failure. If the loss coefficient is too high, cyclic loading will dissipate energy into the material leading to fatigue failure.

Job shopFrom Wikipedia, the free encyclopedia

In the United Kingdom, "job shop" can also be a colloquialism for a Job Centre.

Job shops are typically small manufacturing systems that handle job production, that is, custom/bespoke or semi-custom/bespoke manufacturing processes such as small to medium-size customer orders or batch jobs. Job shops typically move on to different jobs (possibly with different customers) when each job is completed. In job shops machines are aggregated in shops by the nature of skills and technological processes involved, each shop therefore may contain different machines, which gives this production system processing flexibility, since jobs are not necessarily constrained to a single machine. In computer science the problem of job shop scheduling is considered strongly NP-hard.

In a job shop product flow is twisted, also notice that in this drawing each shop contains a single machine.

A typical example would be a machine shop, which may make parts for local industrial machinery, farm machinery and implements, boats and ships, or even batches of specialized components for the aircraft industry. Other types of common job shops are grinding, honing, jig-boring, gear manufacturing, and fabrication shops.

The opposite would be continuous flow manufactures such as textile, steel,food manufacturing and manual labor.

Contents  [hide] 

1   Advantages 2   Disadvantages 3   See also 4   Further reading 5   External links

Advantages[edit]

High flexibility in product engineering High expansion flexibility (machines are easily added or substituted) High production volume elasticity (due to small increments to productive capacity) Low obsolescence (machines are typically multipurpose) High robustness to machine failures

Compare to transfer line

Disadvantages[edit]

Very hard scheduling due to high product variability and twisted production flow Low capacity utilization

Compare to transfer line

See also[edit]

Job shop scheduling Production line Transfer line Workflow

Further reading[edit]

A. Portioli, A. Pozzetti, Progettazione dei sistemi produttivi, Hoepli 2003 N.A. Buzacott, G.E. Shanthikumar, Stochastic models of manufacturing systems, Prentice Hall,

1993

Physical Properties of Material

A material undergoes transition under the influence of temperature and pressure, and these changes are physical in nature, because their molecules remain intact. During our school days, we were asked to distinguish physical and chemical changes. At that stage, we began to think in more details than what our senses have detected. Having the ability to distinguish physical

The Challenger Accident

Challenger, launched at 11:38 am ESTafter a freezing Florida night,exploded 73 seconds after liftoffdue to the failure of an O-ring sealon the right solid rocket boosterkilling 7 passengers.Redesign of the seal and modificationof the space shuttle took almost 3 years.The replacement Endeavor resumed flight Sept. 29, 1988.

properties from chemical properties is indeed a good beginning in the study of materials.

The Challenger disaster is due to a human failure to recognize the limitation of the property of o-ring material.

Effects of Temperature on Substances

When temperature rises, a typical substance changes from solid to liquid and then to vapor, at a constant pressure. Some substance has several crystal forms in the solid state. The glassy state is also considered a solid. Transitions from one solid to another solid form, from solid to liquid, from liquid to vapor, from vapor to solid etc. are called phase transitions.

Phase transitions from solid to liquid, and from liquid to vapor absorb heat. The temperature of a system usually does not change as long as two phases are present. The (phase) transition temperature from solid to liquid is called the melting point whereas the temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling point. Thus, the measured boiling point depends on the atmosphere pressure. For compounds that decompose at high temperature, boiling point can be either specified at lower pressure or be replaced by the decomposition temperature. Thus, conditions as well as the value of boiling point listed in literature must be taken into account for application considerations. Boiling points of mixtures change with composition. The boiling points of some common mixtures are listed in handbooks, and boiling points can be used to assess the composition of a mixture or the purity of a compound.

However, a glassy material becomes soft in a wide range of temperatures. The temperature at which the material becomes soft (behave molten like) is called glassy temperature, but it may be a range of temperatures. Behavior of a substance as the temperature changes must be carefully considered in its applications. Behavior of a mixture as temperature rises is different from its components. There is no theoretical way to predict the behavior of a mixture from its components, even if its exact composition is known. Addition of one or more materials usually changes the melting or glassy temperature of a substance. Thus, we often employ a blend (mixture) of

Phase Transition at Constant Pressure

Temperature

Vapor

Boiling pointHeat of vaporization

Liquid

Melting pointHeat of fussion

Solid

materials whose behavior is acceptable within the desirable range of temperatures. Antifreeze for automobile radiator and deicing liquid for airplanes are examples of this application.

The behavior of mixtures as temperature and pressure change often requires a phase diagram to represent, and there are several models of two-component systems. A phase diagram of a many-component system requires a lengthy study.

Combined Effect of Temperature and Pressure on the Behavior of Material

When the temperature remains constant, the pressure also affects the behavior of a material. The volume of a gas changes as the pressure changes even if temperature remains the same. When temperature is close to the melting point of a substance, a liquid may solidify or a solid may melt as the pressure changes. A diagram showing the temperature and pressure combined effect on a system is called a phase diagram.

One-component phase diagrams for water and carbon dioxide are given here.

At pressure below 5.1 atm, solid and gas carbon dioxide coexist, but the vapor pressure depends on the temperature. The variation of vapor pressure is represented by a line, which separates the Dry ice phase from the CO2 Gas phase. The vapor pressure of dry ice at 194.6 K (-78.5°C) the pressure is 1 atm, and at 216.7 K (-56.4°C) the pressure is 5.11 atm. The line separates the conditions for the formation of solid and vapor. A similar curve borders between liquid and gas CO2, whereas a line separates dry ice from the liquid phase. At 216.7 K, vapor pressures of solid and liquid CO2 are the same, 5.11 atm. At, 5.11 atm and 216.7 K, all three phases coexist, and the condition is called the triple point.

Phase rules in soil science Phase equilibria in one-component systems pdf file Phase equilibrium: Pure substance Chapter 5 of a book Phase and phase diagrams Phase equilibria Chemical energy Use FIND to search for "tin disease" after

Thermal expansion coefficient

A substance expands on heating. For a rod, the lengthening of a unit length per degree Kelvin is the linear thermal expansion coefficient. This factor affects the substance performance in machines or structural assemblies. Thermal expansion causes tight fitted parts to break and moving part to jam, in any machine. The problem is serious if different material is used. When a large body of glass is subject to local heating or cooling, it breaks up due to expansion or shrinkage. Thermal expansion also causes distortion, and some thermometers are made of two strips of different metals. Thermal properties must be considered in any engineering constructions such as railroad, bridges, pipelines, and buildings, especially in areas where temperatures go to extreme values.

Heat and Electric Conductance

Transmissions of energy and electric charge across a body of material give rise to heat and electric conductance respectively. The rate of flow across a unit-area section when the temperature or electric potential difference applied to the wire of unit length is called the thermal or electric conductance coefficient. Metals are usually good conductors of both, and their conductance coefficients are high. Insulation material for heat and electricity should have low conductance, whearas metals have high conductance.

The reciprocal of electric conductance is called electric resistance; thus, the higher the conductance, the lower the resistance. Electric resistance for some familiar materials are given in the table here. Note the large range of 1015 among these substances. Aluminium and copper are very good conductors, and their resistances are very low, in the order of 10-8, almost 100 times smaller than that of tungsten, W. Germanium, Ge, and silicon, Si, are typical semiconductors, whereas sulfur and phosphorous are insulation material.

Linear thermal expansioncoefficient per K at room

temperature of some substances

Aluminum 24

Copper 17

Diamond 1

Glass 11

Pyrex glass 3

Rubber, hard 80

Electric resistance of someelements

MetalResistance/ohm m

Al 2.4x10-8

Au 2.05x10-8

W 5.65x10-6

Ge 0.46

Si 0.03

S (yellow) 2x1015

P (white) 109 Note the range of 1015

Magnetic Properties

A magnetic field strength is measured in Tesla (T) and gauss (G, 1 T = 10,000 G). The Earth magnetic field is 0.5 G. When a material is placed into a magnetic field H, a magnetic field of different intensity B is produced inside the material. The ratio B/H is called the magnetic susceptibility. The higher the magnetic susceptibility, the easier the material is magnetized. Most substances are diamagnetic. The magnetic fields (B) within the bodies of these substances when they are placed in a magnetic field (H) are less than that of an empty space (vacuum).; thus their magnetic susceptibilities (B/H ratio) are less than 1. When a body of paramagnetic substance is placed in a magnetic field, the intensity of the field within the body is slightly larger than that of the applied field. The magnetic susceptibilities of paramagnetic substances are slightly greater than 1.

Iron, cobalt and nickel are some ferromagnetic substances, there are some other alloys and oxides that behave this way. They possess a spontaneous magnetic moment. A magnetic field is present in these materials even in the absence of an external magnetic field. However, ferromagnetism is temperature dependent, and above the so called Curie temperatures of the substances, magnetism vanishes. The Curie temperature or Curie point of a substance is unique. The Curie points for Fe, Co, and Ni are 1043, 1400, and 630 K respectively.

Ferromagnetism are due to the presence of magnetic domains in the substance, and when these domains line up parallel to each other, they give a net magnetic field. If the domains line up antiparallel to each other at the Curie point, the substance is said to be antiferromagnetic. The magnetic susceptibility reaches a maximum at Curie temperature for antferromagnetic material. For example, FeO, MnO, CoO, NiO, FeF2, FeCl2, a-Mn, Cr2O3 etc. are some of the antiferromagnetic substances.

Ferromagnetic substances play important roles in recording tapes and disks for audio, video, and computer signals. Furthermore, ferromagnetic materials are used in permanent magnets, which are used in motors, antenna, and speakers. Recent development in strong magnets enables communication equipment and computers to be miniaturized.

Density

The mass per unit volume (cm3 = mL, m3 etc.) of is called density, an intensive property. Often, specific gravity is given. Specific gravity is the ratio of density of a substance compared to that of water. As a ratio, it has no units. Since density of water is 1.00 g/mL, specific gravity is the density in g/mL. Other units to use are kg/L

or 103 kg m-3. Specific gravity for a few common substances are given here: Au, 19.3; mercury, 13.6; alcohol, 0.7893; benzene, 0.8786. Do you know which element has the highest density?

Dielectric Constant

The dielectric constant e of a medium is its ability to reduce the force F of attraction of charged (q1 and q2) particles separated at distance r, compared to vacuum. It is usually defined by the equation, F = q1q2 / (e r). A substance with large dielectric constant placed between two plates to which an electric voltage has been applied will result in a weak electric field within it. Water, due to its polar nature, has a rather large dielectric constant, 80.4. At the atomic scale, water molecules weaken the attraction between Na+ and Cl- ions, resulting in dissolving it. Dielectric constants for some familiar substances are: H2O, 80.4; methanol, 33.6; benzene, 2.3; H2 at 20 K, 1.23.

Heat capacity

The amount of energy required to raise the temperature of a substance by 1 K is the heat capacity. If the substance has a unit mass, the amount is referred to as specific heat capacity, or specific heat. For example, it takes 1 cal (4.184 J) to raise the temperature of 1 g water by 1 K. Thus, the specific heat for water is 1 cal g -1 K-1 (75 J mol-1 K-1). Specific heat of water is large compared to most other substances, for example: Cu, 24.4 J mol-1 K-1. This large heat capacity of water affect the weather, making temperatures in areas close to large bodies of water more steadier than large dry land.

Refractive Index

The ratio of light speed in vacuum to its speed in the medium is refractive index. Light travel slower in any medium than in vacuum. Thus, refractive index is always greater than unity (1), and light beam usually bents when entering from air to another medium. This value depends on the wavelength of the light used, and the property is important for material used in optics instrument such as eye glasses. The higher the refractive index, the thinner the glasses to achieve the same power. The difference in refractive indexes between two liquid gives rise to the visible boundary between layers.

Refractive index of some common substances

water 1.3

benzene 1.5011

ethanol 1.359

quartz 1.5*

NaCl solid 1.5

*wavelength dependent

Difference in refractive indexes of lights of different wavelengths can be separated using a prism. Refractive indexes for some familiar substances are given in a box. It should also be kept in mind that index of refraction changes with dissolved substance and concentration.

Solubility

The amount of substance dissolved in 100 mL of solvent is called solubility. However, units for solubility might be specified in some other fashion. Solubility depends on temperature, and the variation can be used to separate components in a solid mixture. Sodium acetate trihydrate, CH3COONa-3H2O, when heated will melt in the sense that it dissolves in its water of crystallization. This liquid remains liquid till about -15 C (258 K), and when crystallization does take place after triggered by cold hand, heat is released providing a source of heat. This property provides a winter hand warmer pack for skiers or winter travelers.

Washing and cleaning also involve solubility, and the formulation of an effective cleaning agent requires the knowledge of many substances. Substances can be classified according to polarity. Water, ammonia (NH3), and methanol (CH3OH) are polar, because their molecules have negative and positive sites, whereas methane (CH4), gasoline, and motor oil are non-polar. Regarding solubility, a rule of thumb readslike dissolves like, which means that polar solvents dissolve polar substances and non-polar solvents dissolve non-polar substances. An organic compound with a polar group attached to non-polar chain will bring water molecules to a non-polar surface, and hence it can be used as a detergent or wetting agent. This rule of thumb has potential for both domestic and industrial applications.

Optical activity

The ability of certain substances to rotate the plane of polarized light as it passes through a crystal, liquid, or solution is generally referred to as the optical activity. Substances possessing this activity usually lack a center of symmetry (see crystal symmetry), and they have two isomers as mirror images of each other. The two isomers, called dextrorotatory (d, right hand) or laevorotory (l, left hand) isomers, rotate the polarized light in opposite directions. Thus, equi-molar or racemic mixture of the two appears optically inactive. For example, sugar, tartaric acid, and aminoacids are optically active compounds.

Viscosity and Surface Tension

Viscosity and surface tension are properties of liquid state. The former is a measure of its resistance to flow. Molasses, glycerin, oil, softened glass have high viscosity, and water, gasoline, ethanol have low viscosity. The SI units for viscosity is N.s /m2, but the unit poise (P, a cgs unit) have been used for a long time, and is more common, and 1 P = 0.1 N¸s m-2. Viscosity usually decreases with increase in temperature, and softened glass has a viscosity more than 1014 N¸s m-2.

Surface tension results from intermolecular attraction, the higher of which, the higher surface tension. Energy required to stretch the surface by some defined unit is called surface tension, and whose unit is N.m/m2 (= N/m). Like viscosity, surface tension decrease with increase temperature. Surface tension causes the dew and raindrops to be round, and the idea to manufacture perfect spheres in zero gravity zone is making use of surface tension. Soap reduces surface tension of water and soapy water forms bubbles when air is blown into it.

Activities:

Ask the class to give examples of physical properties.

Describe an application of a material based on any one of the physical properties.

Point out two physical properties that has not been mentioned here.

What substance has the highest dielectric constant?

What is a beam of polarized light?

Give a sketch of the molecular structure of an aminoacid.

Learning guide

Describe phase transitions Describe the phase diagram of water or carbon dioxide. What is dielectric constant? What is the dielectric constant for water and

methanol? Why do ionic substances such as NaCl and CaCO3 have higher solubility in water than in methanol?

©cchieh@uwaterloo.ca

List of materials propertiesFrom Wikipedia, the free encyclopedia

A materials property is an intensive, often quantitative, property of a solid or quasi-solid. Quantitative properties may be used as a metric by which the benefits of one material versus another can be assessed, thereby aiding in materials selection.

A property may be a constant or may be a function of one or more independent variables, such as temperature. Materials properties often vary to some degree according to the direction in the material in which they are measured, a condition referred to as anisotropy. Materials properties that relate two different physical phenomena often behave linearly (or approximately so) in a given operating range, and may then be modeled as a constant for that range. This linearization can significantly simplify the differential constitutive equations that the property describes.

Some materials properties are used in relevant equations to predict the attributes of a system a priori. For example, if a material of a known specific heat gains or loses a known amount of heat, the temperature change of that material can be determined. Materials properties are most reliably measured by standardized test methods. Many such test methods have been documented by their respective user communities and published through ASTM International.

Contents  [hide] 

1   Acoustical properties 2   Atomic properties 3   Chemical properties 4   Electrical properties 5   Environmental properties 6   Magnetic properties 7   Manufacturing properties 8   Mechanical properties 9   Optical properties 10   Radiological properties 11   Thermal properties 12   See also

Acoustical properties[edit]

Acoustical absorption Speed of sound

Atomic properties[edit]

Atomic mass Atomic number  - applies to pure elements only Atomic weight  - applies to individual isotopes or specific mixtures of isotopes of a given element.

Chemical properties[edit]Main article: Chemical property

Corrosion  resistance Hygroscopy pH Reactivity Specific internal surface area Surface energy Surface tension

Electrical properties[edit]

Dielectric constant Dielectric strength Electrical conductivity Permeability Permittivity Piezoelectric  constants Seebeck coefficient

Environmental properties[edit]

Embodied energy Embodied water

Magnetic properties[edit]

Curie Point Diamagnetism Hysteresis Permeability

Manufacturing properties[edit]

Castability Extruding  temperature and pressure Hardness Machinability rating Machining speeds and feeds

Mechanical properties[edit]

Compressive strength  : stress a material can withstand before compressive failure (MPa) Ductility  : Ability of a material to deform under tensile load (% elongation) Fatigue limit  : Maximum stress a material can withstand under repeated loading (MPa) Flexural modulus Flexural strength Fracture toughness  : Energy absorbed by unit area before the fracture of material (J/m^2) Hardness  : Ability to withstand surface indentation (e.g. Brinell hardness number)

Plasticity (physics)  : Ability of a material to undergo irreversible deformations (-) Poisson's ratio  : Ratio of lateral strain to axial strain (no units) Resilience  : Ability of a material to absorb energy when it is deformed elastically (MPa) Shear modulus  : Ratio of shear stress to shear strain (MPa) Shear strain  :in the angle between two perpendicular lines in a plane Shear strength  : Maximum shear stress a material can withstand Specific modulus  : Modulus per unit volume (MPa/ m^3) Specific strength   : Strength per unit density (Nm/kg) Specific weight   : Weight per unit volume (N/m^3) Tensile strength  : Maximum tensile stress a material can withstand before failure (MPa) Yield strength  : The stress at which a material starts to yield (MPa) Young's modulus  : Ratio of linear stress to linear strain (MPa) Coefficient of friction  (also depends on surface finish) Coefficient of restitution Roughness strength

Optical properties[edit]

Absorptivity Color Luminosity Photosensitivity Reflectivity Refractive index Scattering Transmittance

Radiological properties[edit]

Neutron cross-section Specific activity

Thermal properties[edit]

Autoignition temperature Binary phase diagram Boiling point Coefficient of thermal expansion Critical temperature Curie point Emissivity Eutectic point Flammability Flash point Glass transition temperature Heat of fusion

Heat of vaporization Inversion temperature Melting point Phase diagram Pyrophoricity Solidus Specific heat Thermal conductivity Thermal diffusivity Thermal expansion Seebeck coefficient Triple point Vapor pressure Vicat softening point

Physical propertyFrom Wikipedia, the free encyclopedia

For the legal concept, see Tangible property.

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2013)

A physical property is any property that is measurable whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its transformations or evolutions between its momentary states. Physical properties are often referred to as observables. They are not modal properties.

Physical properties are often characterized as intensive and extensive properties. An intensive property does not depend on the size or extent of the system, nor on the amount of matter in the object, while an extensive property shows an additive relationship. These classifications are in general only valid in cases when smaller subdivisions of the sample do not interact in some physical or chemical process when combined.

Properties may also be classified with respect to the directionality of their nature. For example, isotropic properties do not change with the direction of observation, and anisotropic properties do have spatial variance.

It may be difficult to determine whether a given property is a material property or not. Color, for example, can be seen and measured; however, what one perceives as color is really an interpretation of the reflective properties of a surface and the light used to illuminate it. In this sense, many ostensibly physical properties are called supervenient. A supervenient property is one which is actual, but is secondary to some underlying reality. This is similar to the way in which objects are supervenient on atomic structure. A cup might have the physical properties of mass, shape, color, temperature, etc., but these properties are supervenient on the underlying atomic structure, which may in turn be supervenient on an underlying quantum structure.

Physical properties are contrasted with chemical properties which determine the way a material behaves in a chemical reaction.

Contents

  [hide] 

1   List of properties 2   See also 3   Bibliography 4   External links

List of properties[edit]

The physical properties of an object that are traditionally defined by classical mechanics are often called mechanical properties. The physical properties of an object may include, but are not limited to:

absorption (physical) Absorption (electromagnetic) albedo angular momentum area brittleness boiling point capacitance color concentration density dielectric ductility distribution efficacy elasticity

electric charge electrical conductivity electrical impedance electric field electric potential emission flow rate fluidity frequency hardness inductance Intrinsic impedance intensity irradiance length

location luminance Luminescence luster malleability magnetic field magnetic flux mass melting point moment momentum opacity permeability permittivity plasticity pressure

See alsoChemical propertyFrom Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

A chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity.[1] Simply speaking, chemical properties cannot be determined just by viewing or touching the substance; the substance's internal structure must be affected for its chemical properties to be investigated. However a catalytic property would also be a chemical property.

Chemical properties can be contrasted with physical properties, which can be discerned without changing the substance's structure. However, for many properties within the scope of physical chemistry, and other disciplines at the boundary between chemistry and physics, the distinction may be a matter of researcher'sperspective. Material properties, both physical and chemical, can be

viewed as supervenient; i.e., secondary to the underlying reality. Several layers of superveniency[clarification needed] are possible.

Chemical properties can be used for building chemical classifications. They can also be useful to identify an unknown substance or to separate or purify it from other substances. Materials science will normally consider the chemical properties of a substance to guide its applications.

Examples of chemical properties[edit]

Heat of combustion Enthalpy of formation Toxicity Chemical stability  in a given environment Flammability  (The ability to burn) Preferred oxidation state(s) Coordination number Half life gas

IngotFrom Wikipedia, the free encyclopedia

Aluminium ingot after ejection from mold

An ingot is a material, usually metal, that is cast into a shape suitable for further processing.[1] Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.[2]

Contents  [hide] 

1   Uses 2   Types

o 2.1   Single crystal o 2.2   Copper alloys

3   Manufacture 4   Historical ingots 5   Cultural references 6   See also 7   Notes

8   References

Uses[edit]

Ingots require a second procedure of shaping, such as cold/hot working, cutting or milling to produce a useful final product. Additionally ingots (of less common materials) can be used as currency, or as a currency reserve as with gold bars.

Types[edit]

Metal, either pure or alloy, heated past its melting point and cast into a bar or block using a mold chill method. Polycrystalline and single crystal ingots are made from semiconductor materials by pulling from a molten melt. Uses include the formation of photovoltaic cells from silicon ingots by cutting the ingot into flats, known as wafers.

Single crystal[edit]See also: Boule (crystal)

Single crystal ingots (called boules) of materials are grown (crystal growth) using methods such as the Czochralski process or Bridgeman technique.

The boules may be either semiconductors—for the electronic industry, or non-conducting inorganic compounds for industrial and jewelry use, e.g., synthetic ruby, sapphire etc.

Single crystal ingots of metal are produced in similar fashion to that used to produce high purity semiconductor ingots,[3] i.e. by vacuum induction refining. Single crystal ingots of engineering metals are of interest due to their very high strength due to lack of grain boundaries. The method of production is via single crystaldendrite and not via simple casting. Possible uses include turbine blades.

Copper alloys[edit]

In the United States, the brass and bronze ingot making industry started in the early 19th century. The US brass industry grew to be the number one producer by the 1850s.[4] During colonial times the brass and bronze industries were almost non-existent because the British demanded all copper ore be sent to Britain for processing.[5] Copper based alloy ingots weighed approximately 20 pounds (9.1 kg).[6][7]

Manufacture[edit]

Crystalline structure of mold cast ingot

See also: Deoxidized steel

Ingots are manufactured by the freezing of a molten liquid (known as the melt) in a mold. The manufacture of ingots has several aims. Firstly, the mold is designed to completely solidify and form an appropriate grain structure required for later processing, as the structure formed by the freezing melt controls the physical properties of the material. Secondly, the shape and size of the mold is designed to allow for ease of ingot handling and downstream processing. Finally the mold is designed to minimize melt wastage and aid ejection of the ingot, as losing either melt or ingot increases manufacturing costs of finished products.

A variety of designs exist for the mold, which may be selected to suit the physical properties of the liquid melt and the solidification process. Molds may exist in top, horizontal or bottom-up pouring and may be fluted or flat walled. The fluted design increases heat transfer owing to a larger contact area. Molds may be either solid "massive" design, sand cast (e.g. for pig iron) or water-cooled shells, depending upon heat transfer requirements. Ingot molds are tapered to prevent the formation of cracks due to uneven cooling. Crack or void formation occurs as the liquid to solid transition has an associated volume change for a constant mass of material. Formation of these ingot defects may render the cast ingot useless, and may need to be re-melted, recycled or discarded.

Teeming ingots at a steel mill

The physical structure of a crystalline material is largely determined by the method of cooling and precipitation of the molten metal. During the pouring process, metal in contact with the ingot walls rapidly cools and forms either a columnar structure, or possibly a "chill zone" of equiaxed dendrites, depending upon the liquid being cooled and the cooling rate of the mold.[8]

For a top-poured ingot, as the liquid cools within the mold, differential volume effects cause the top of the liquid to recede leaving a curved surface at the mold top which may eventually be required to be machined from the ingot. The mold cooling effect creates an advancing solidification front, which has several associated zones, closer to the wall there is a solid zone which draws heat from the solidifying melt, for alloys there may exist a "mushy" zone, which is the result of solid-liquid equilibrium regions in the alloy's phase diagram, and a liquid region. The rate of front advancement controls the time that dendrites or nuclei have to form in the solidification region. The width of the mushy zone in an alloy may be controlled by tuning the heat transfer properties of the mold, or adjusting the liquid melt alloy compositions.

Continuous casting methods for ingot processing also exist, whereby a stationary front of solidification is formed by the continual take-off of cooled solid material, and the addition of molten liquid to the casting process.[9]

Approximately 70 percent of aluminium ingots in the U.S. are cast using the direct chill casting process, which reduces cracking. A total of 5 percent of ingots must be scrapped because of stress induced cracks and butt deformation.[10]

Historical ingots[edit]

Ancient copper ingot from Zakros, Crete. The ingot is shaped in the form of an animal skin, a typical shape of copper ingots from these times.

 

The mold of the Ancient Chinese gold and silversycee, measured in tael. One of the Chinese names is 金元寶.

 

Lead ingots from Roman Britain on display at theWells and Mendip Museum.

 

Pig iron ingot from Norrhyttan, Dalarna,Sweden.

Plano-convex ingots are widely distributed archaeological artifacts which are studied to provide information on the history of metallurgy.

Cultural references[edit]

The Chinese New Year food Jiaozi was made to symbolize the ingot.

The eighth letter in the Ogham alphabet is Tinne meaning "ingot".

See also[edit]

Billet (manufacturing) —typically refers to a large ingot of less precious metal – both are forms of bar stock

Gold bar Oxhide ingot Tin ingot Wafer etching

NotesSlagFrom Wikipedia, the free encyclopedia

For other uses, see Slag (disambiguation).

A path through a slag heap inClarkdale, Arizona, showing the striations from the rusting corrugated sheets retaining

it.

The Manufacture of Iron -- Carting Away the Scoriæ (slag), an 1873 wood engraving

Slag run-off from one of the open hearth furnaces of a steel mill,Republic Steel, Youngstown, Ohio, November 1941.

Slag is drawn off the furnace just before the molten steel is poured into ladles for ingotting.

Slag is the glass-like by-product left over after a desired metal has been separated (i.e., smelted) from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals. While slags are generally used to remove waste in metal smelting, they can also serve other purposes, such as assisting in thetemperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.

Contents  [hide] 

1   Ore smelting 2   Ancient uses 3   Modern uses

4   Basic slag 5   See also 6   References 7   Further reading

Ore smelting[edit]

In nature, the ores of metals such as iron, copper, lead, nickel and other metals are found in impure states, often oxidizedand mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. Slag is the collection of compounds that are removed. In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In this case, the slag is termed synthetic. A good example is steelmaking slag:quicklime and magnesite are introduced for refractory protection, neutralising the alumina and silica separated from the metal, and assist in the removal of sulfur and phosphorus from the steel.

Ferrous and non-ferrous smelting processes produce different slags. The smelting of copper and lead in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores, and separates them as iron-silicate-based slags. Slag from steel mills in ferrous smelting, on the other hand, is designed to minimize iron loss and so mainly contains oxides of calcium, silicon, magnesium, and aluminium. Any sandy component or quartz component of the original ore automatically carries through the smelting process as silicon dioxide.

As the slag is channeled out of the furnace, water is poured over it. This rapid cooling, often from a temperature of around 2,600 °F (1,430 °C), is the start of the granulating process. This process causes several chemical reactions to take place within the material, and gives the slag its cementitious properties.

The water carries the slag in its slurry format to a large agitation tank, from where it is pumped along a piping system into a number of gravel based filter beds. The filter beds then retain the slag granules, while the water filters away and is returned to the system.

When the filtering process is complete, the remaining slag granules, which now give the appearance of coarse beach sand, can be scooped out of the filter bed and transferred to the grinding facility where they are ground into particles that are finer than Portland cement.

Ancient uses[edit]

Early slag from Denmark, ca. 200-500 AD

During the Bronze Age of the Mediterranean there were a vast number of differential metallurgical processes in use. A slag by-product of such workings was a colorful, glassy, vitreous material found on the surfaces of slag from ancient copper foundries. It was primarily blue or green and was formerly chipped away and melted down to make glassware products and jewelry. It was also ground into powder to add to glazes for use in ceramics. Some of the earliest such uses for the by-products of slag have been found in ancient Egypt.[1]

Historically, the re-smelting of iron ore slag was common practice, as improved smelting techniques permitted greater iron yields--in some case exceeding that which was originally achieved. During the early 20th century, iron ore slag was also ground to a powder and used to make 'Agate Glass', also known as 'Slag Glass'.[2]

Modern uses[edit]

Ground granulated slag is often used in concrete in combination with Portland cement as part of a blended cement. Ground granulated slag reacts with water to produce cementitious properties. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability. Since the unit volume of Portland cement is reduced, this concrete is less vulnerable to alkali-silica and sulfate attack.[citation needed]

This previously unwanted recycled product is used in the manufacture of high performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure.[3]The slag can also be used to create fibers used as an insulation material named slag wool.

Basic slag[edit]

Basic slag is a byproduct of steelmaking using the basic version of the Bessemer process or the Linz-Donawitz process. It is largely limestone or dolomite which has absorbed phosphate from the iron ore being smelted. Because of the slowly released phosphate content, and because of its liming effect, it is valued as fertilizer in gardens and farms in steelmaking areas. However, the most important application is construction.[4]

See also[edit]

Wikimedia Commons has

media related to Slag.

Dross Fly ash Ground granulated blast furnace slag Pozzolan Spoil heap Tailings Slag (welding)

BrazingFrom Wikipedia, the free encyclopedia

This article is about the metal-joining process. For the cooking technique, see braising.

Brazing practice

This article's lead section may not adequately summarize key points of its contents.Please consider expanding the lead to provide an accessible overview of all important aspects of the article. (August 2010)

Brazing is a metal-joining process whereby a filler metal is heated above melting point and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the workpieces together.[1] It is similar to soldering, except the temperatures used to melt the filler metal are higher for brazing.

Contents  [hide] 

1   Fundamentals o 1.1   Flux o 1.2   Filler materials o 1.3   Atmosphere

2   Common techniques o 2.1   Torch brazing o 2.2   Furnace brazing o 2.3   Silver brazing o 2.4   Braze welding o 2.5   Cast iron "welding" o 2.6   Vacuum brazing

o 2.7   Dip brazing 3   Heating methods 4   Advantages and disadvantages 5   Filler metals

o 5.1   Braze families o 5.2   Role of elements o 5.3   Melting behavior o 5.4   Interaction with base metals o 5.5   Preform

6   See also 7   References

o 7.1   Bibliography 8   Further reading 9   External links

Fundamentals[edit]

In order to obtain high-quality brazed joints, parts must be closely fitted, and the base metals must be exceptionally clean and free of oxides. In most cases, joint clearances of 0.03 to 0.08 mm (0.0012 to 0.0031 in) are recommended for the best capillary action and joint strength.[2] However, in some brazing operations it is not uncommon to have joint clearances around 0.6 mm (0.024 in). Cleanliness of the brazing surfaces is also important, as any contamination can cause poor wetting (flow). The two main methods for cleaning parts, prior to brazing, are chemical cleaning and abrasive or mechanical cleaning. In the case of mechanical cleaning, it is important to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry.[2]

Another consideration that cannot be overlooked is the effect of temperature and time on the quality of brazed joints. As the temperature of the braze alloy is increased, the alloying and wetting action of the filler metal increases as well. In general, the brazing temperature selected must be above the melting point of the filler metal. However, there are several factors that influence the joint designer's temperature selection. The best temperature is usually selected so as to: (1) be the lowest possible braze temperature, (2) minimize any heat effects on the assembly, (3) keep filler metal/base metal interactions to a minimum, and (4) maximize the life of any fixtures or jigs used.[2] In some cases, a higher temperature may be selected to allow for other factors in the design (e.g. to allow use of a different filler metal, or to control metallurgical effects, or to sufficiently remove surface contamination). The effect of time on the brazed joint primarily affects the extent to which the aforementioned effects are present; however, in general most production processes are selected to minimize brazing time and the associated costs. This is not always the case, however, since in some non-production settings, time and cost are secondary to other joint attributes (e.g. strength, appearance).

Flux[edit]

In the case of brazing operations not contained within an inert or reducing atmosphere environment (i.e. a furnace), flux is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, liquid, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core. In either case, the flux flows into the joint when applied to the heated joint and is displaced by the molten filler metal entering the joint. Excess flux should be removed when the cycle is completed because flux left in the joint can lead to corrosion, impede joint inspection, and prevent further surface finishing operations. Phosphorus-containing brazing alloys can be self-fluxing when

joining copper to copper.[3] Fluxes are generally selected based on their performance on particular base metals. To be effective, the flux must be chemically compatible with both the base metal and the filler metal being used. Self-fluxing phosphorus filler alloys produce brittle phosphides if used on iron or nickel.[3] As a general rule, longer brazing cycles should use less active fluxes than short brazing operations.[4]

Filler materials[edit]

A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.

Braze alloy is generally available as rod, ribbon, powder, paste, cream, wire and preforms (such as stamped washers).[5] Depending on the application, the filler material can be pre-placed at the desired location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used as they are the easiest to apply while heating. In the case of furnace brazing, alloy is usually placed beforehand since the process is usually highly automated.[5] Some of the more common types of filler metals used are

Aluminum-silicon Copper Copper-silver Copper-zinc (brass) Gold -silver Nickel  alloy Silver [1] [6] Amorphous brazing foil  using nickel, iron, copper, silicon, boron, phosphorus, etc.

Atmosphere[edit]

As brazing work requires high temperatures, oxidation of the metal surface occurs in an oxygen-containing atmosphere. This may necessitate the use of an atmospheric environment other than air. The commonly used atmospheres are[7][8]

Air : Simple and economical. Many materials susceptible to oxidation and buildup of scale. Acid cleaning bath or mechanical cleaning can be used to remove the oxidation after work. Flux tends to be employed to counteract the oxidation, but it may weaken the joint.

Combusted fuel gas (low hydrogen, AWS type 1, "exothermic generated atmospheres"): 87% N2, 11–12% CO2, 5-1% CO, 5-1% H2. For silver, copper-phosphorus and copper-zinc filler metals. For brazing copper and brass.

Combusted fuel gas (decarburizing, AWS type 2, "endothermic generated atmospheres"): 70–71% N2, 5–6% CO2, 9–10% CO, 14–15% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium carbon steels.

Combusted fuel gas (dried, AWS type 3, "endothermic generated atmospheres"): 73–75% N2, 10–11% CO, 15–16% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.

Combusted fuel gas (dried, decarburizing, AWS type 4): 41–45% N2, 17–19% CO, 38–40% H2. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.

Ammonia  (AWS type 5, also called forming gas): Dissociated ammonia (75% hydrogen, 25% nitrogen) can be used for many types of brazing and annealing. Inexpensive. For copper, silver,

nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys.

Nitrogen+hydrogen, cryogenic or purified (AWS type 6A): 70–99% N2, 1–30% H2. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals.

Nitrogen+hydrogen+carbon monoxide, cryogenic or purified (AWS type 6B): 70–99% N2, 2–20% H2, 1–10% CO. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.

Nitrogen , cryogenic or purified (AWS type 6C): Non-oxidizing, economical. At high temperatures can react with some metals, e.g. certain steels, forming nitrides. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.

Hydrogen  (AWS type 7): Strong deoxidizer, highly thermally conductive. Can be used for copper brazing and annealing steel. May cause hydrogen embrittlement to some alloys. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys, cobalt alloys, tungsten alloys, and carbides.

Inorganic vapors (various volatile fluorides, AWS type 8): Special purpose. Can be mixed with atmospheres AWS 1–5 to replace flux. Used for silver-brazing of brasses.

Noble gas  (usually argon, AWS type 9): Non-oxidizing, more expensive than nitrogen. Inert. Parts must be very clean, gas must be pure. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels chromium alloys, titanium, zirconium, hafnium.

Noble gas+hydrogen (AWS type 9A) Vacuum : Requires evacuating the work chamber. Expensive. Unsuitable (or requires special

care) for metals with high vapor pressure, e.g. silver, zinc, phosphorus, cadmium, and manganese. Used for highest-quality joints, for e.g. aerospace applications.

Common techniques[edit]

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Brazing and soldering processes classification chart[9]

Torch brazing[edit]

Torch brazing is by far the most common method of mechanized brazing in use. It is best used in small production volumes or in specialized operations, and in some countries, it accounts for a majority of the brazing taking place. There are three main categories of torch brazing in use:[10] manual, machine, and automatic torch brazing.

Manual torch brazing is a procedure where the heat is applied using a gas flame placed on or near the joint being brazed. The torch can either be hand held or held in a fixed position depending on whether the operation is completely manual or has some level of automation. Manual brazing is most commonly used on small production volumes or in applications where the part size or configuration makes other brazing methods impossible.[10] The main drawback is the high labor cost associated with the method as well as the operator skill required to obtain quality brazed joints. The use of flux or self-fluxing material is required to prevent oxidation. Torch brazing of copper can be done without the use of flux if it is brazed with a torch using oxygen and hydrogen gas, rather than oxygen and other flammable gases.

Machine torch brazing is commonly used where a repetitive braze operation is being carried out. This method is a mix of both automated and manual operations with an operator often placing brazes material, flux and jigging parts while the machine mechanism carries out the actual braze.[10] The advantage of this method is that it reduces the high labor and skill requirement of manual brazing. The use of flux is also required for this method as there is no protective atmosphere, and it is best suited to small to medium production volumes.

Automatic torch brazing is a method that almost eliminates the need for manual labor in the brazing operation, except for loading and unloading of the machine. The main advantages of this method are: a high production rate, uniform braze quality, and reduced operating cost. The equipment used

is essentially the same as that used for Machine torch brazing, with the main difference being that the machinery replaces the operator in the part preparation.[10]

Furnace brazing[edit]

Furnace brazing schematic

Furnace brazing is a semi-automatic process used widely in industrial brazing operations due to its adaptability to mass production and use of unskilled labor. There are many advantages of furnace brazing over other heating methods that make it ideal for mass production. One main advantage is the ease with which it can produce large numbers of small parts that are easily jigged or self-locating.[11] The process also offers the benefits of a controlled heat cycle (allowing use of parts that might distort under localized heating) and no need for post braze cleaning. Common atmospheres used include: inert, reducing or vacuum atmospheres all of which protect the part from oxidation. Some other advantages include: low unit cost when used in mass production, close temperature control, and the ability to braze multiple joints at once. Furnaces are typically heated using either electric, gas or oil depending on the type of furnace and application. However, some of the disadvantages of this method include: high capital equipment cost, more difficult design considerations and high power consumption.[11]

There are four main types of furnaces used in brazing operations: batch type; continuous; retort with controlled atmosphere; and vacuum.

Batch type furnaces have relatively low initial equipment costs and heat each part load separately. It is capable of being turned on and off at will which reduces operating expenses when not in use. These furnaces are well suited to medium to large volume production and offer a large degree of flexibility in type of parts that can be brazed.[11] Either controlled atmospheres or flux can be used to control oxidation and cleanliness of parts.

Continuous type furnaces are best suited to a steady flow of similar-sized parts through the furnace.[11] These furnaces are often conveyor fed, allowing parts to be moved through the hot zone at a controlled speed. It is common to use either controlled atmosphere or pre-applied flux in continuous furnaces. In particular, these furnaces offer the benefit of very low manual labor requirements and so are best suited to large scale production operations.

Retort-type furnaces differ from other batch-type furnaces in that they make use of a sealed lining called a "retort". The retort is generally sealed with either a gasket or is welded shut and filled completely with the desired atmosphere and then heated externally by conventional heating elements.[11] Due to the high temperatures involved, the retort is usually made of heat resistant alloys that resist oxidation. Retort furnaces are often either used in a batch or semi-continuous versions.

Vacuum furnaces is a relatively economical method of oxide prevention and is most often used to braze materials with very stable oxides (aluminum, titanium andzirconium) that cannot be brazed in atmosphere furnaces. Vacuum brazing is also used heavily with refractory materials and other exotic alloy combinations unsuited to atmosphere furnaces. Due to the absence of flux or a reducing atmosphere, the part cleanliness is critical when brazing in a vacuum. The three main types of vacuum furnace are: single-wall hot retort, double-walled hot retort, and cold-wall retort. Typical

vacuum levels for brazing range from pressures of 1.3 to 0.13pascals (10−2 to 10−3 Torr) to 0.00013 Pa (10−6 Torr) or lower.[11] Vacuum furnaces are most commonly batch-type, and they are suited to medium and high production volumes.

Silver brazing[edit]

Silver brazing, sometimes known as a silver soldering or hard soldering, is brazing using a silver alloy based filler. These silver alloys consist of many different percentages of silver and other metals, such as copper, zinc and cadmium.

Brazing is widely used in the tool industry to fasten 'hard metal' (carbide, ceramics, cermet, and similar) tips to tools such as saw blades. "Pretinning" is often done: the braze alloy is melted onto the hard metal tip, which is placed next to the steel and remelted. Pretinning gets around the problem that hard metals are hard to wet.

Brazed hard metal joints are typically two to seven mils thick. The braze alloy joins the materials and compensates for the difference in their expansion rates. In addition it provides a cushion between the hard carbide tip and the hard steel which softens impact and prevents tip loss and damage, much as the suspension on a vehicle helps prevent damage to both the tires and the vehicle. Finally the braze alloy joins the other two materials to create a composite structure, much as layers of wood and glue create plywood.

The standard for braze joint strength in many industries is a joint that is stronger than either base material, so that when under stress, one or other of the base materials fails before the joint.

One special silver brazing method is called pinbrazing or pin brazing. It has been developed especially for connecting cables to railway track or for cathodic protection installations. The method uses a silver- and flux-containing brazing pin which is melted down in the eye of a cable lug. The equipment is normally powered from batteries.

Braze welding[edit]

Braze welding is the use of a bronze or brass filler rod coated with flux to join steel workpieces. The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, acetylene or methylacetylene-propadiene (MAP) gas fuel is commonly used. The name comes from the fact that no capillary action is used.

Braze welding has many advantages over fusion welding. It allows the joining of dissimilar metals, minimization of heat distortion, and can reduce the need for extensive pre-heating. Additionally, since the metals joined are not melted in the process, the components retain their original shape; edges and contours are not eroded or changed by the formation of a fillet. Another effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.

Carbide, cermet and ceramic tips are plated and then joined to steel to make tipped band saws. The plating acts as a braze alloy.

Cast iron "welding"[edit]

The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods is also available. Ductile cast iron pipe may be also "cadwelded," a process which connects joints by means of a small copper wire fused into the iron when previously ground down to the bare metal, parallel to the iron joints being formed as per hub pipe with neoprene gasket seals. The purpose behind this operation is to use electricity along the copper for keeping underground pipes warm in cold climates.

Vacuum brazing[edit]

Vacuum brazing is a material joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. The process can be expensive because it must be performed inside a vacuum chamber vessel. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.

Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.

Dip brazing[edit]

Dip brazing is especially suited for brazing aluminum because air is excluded, thus preventing the formation of oxides. The parts to be joined are fixtured and the brazing compound applied to the mating surfaces, typically in slurry form. Then the assemblies are dipped into a bath of molten salt (typically NaCl, KCl and other compounds) which functions both as heat transfer medium and flux.

Heating methods[edit]

This section requires expansion.

(September 2009)

There are many heating methods available to accomplish brazing operations. The most important factor in choosing a heating method is achieving efficient transfer of heat throughout the joint and doing so within the heat capacity of the individual base metals used. The geometry of the braze joint is also a crucial factor to consider, as is the rate and volume of production required. The easiest way to categorize brazing methods is to group them by heating method. Here are some of the most common:[1][12]

Torch brazing Furnace brazing Induction brazing Dip brazing Resistance brazing Infrared brazing Blanket brazing Electron beam and laser brazing Braze welding

Advantages and disadvantages[edit]

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Brazing has many advantages over other metal-joining techniques, such as welding. Since brazing does not melt the base metal of the joint, it allows much tighter control over tolerances and produces a clean joint without the need for secondary finishing. Additionally, dissimilar metals and non-metals (i.e. metalized ceramics) can be brazed.[13] In general, brazing also produces less thermal distortion than welding due to the uniform heating of a brazed piece. Complex and multi-part assemblies can be brazed cost-effectively. Welded joints must sometimes be ground flush, a costly secondary operation that brazing does not require because it produces a clean joint. Another advantage is that the brazing can be coated or clad for protective purposes. Finally, brazing is easily adapted to mass production and it is easy to automate because the individual process parameters are less sensitive to variation.[14][15]

One of the main disadvantages is: the lack of joint strength as compared to a welded joint due to the softer filler metals used.[1][dubious – discuss] The strength of the brazed joint is likely to be less than that of the base metal(s) but greater than the filler metal.[citation needed][16] Another disadvantage is that brazed joints can be damaged under high service temperatures.[1] Brazed joints require a high degree of base-metal cleanliness when done in an industrial setting. Some brazing applications require the use of adequate fluxing agents to control cleanliness. The joint color is often different from that of the base metal, creating an aesthetic disadvantage.

Filler metals[edit]See also: List of brazing alloys

Some brazes come in the form of trifoils, laminated foils of a carrier metal clad with a layer of braze at each side. The center metal is often copper; its role is to act as a carrier for the alloy, to absorb mechanical stresses due to e.g. differential thermal expansion of dissimilar materials (e.g. a carbide tip and a steel holder), and to act as a diffusion barrier (e.g. to stop diffusion of aluminium from aluminium bronze to steel when brazing these two).

Braze families[edit]

Brazing alloys form several distinct groups; the alloys in the same group have similar properties and uses.[17]

Pure metalsUnalloyed. Often noble metals – silver, gold, palladium.

Ag-CuGood melting properties. Silver enhances flow. Eutectic alloy used for furnace brazing. Copper-rich alloys prone to stress cracking by ammonia.

Ag-ZnSimilar to Cu-Zn, used in jewelry due to high silver content to be compliant with hallmarking. Color matches silver. Resistant to ammonia-containing silver-cleaning fluids.

Cu-Zn (brass)General purpose, used for joining steel and cast iron. Corrosion resistance usually inadequate for copper, silicon bronze, copper-nickel, and stainless steel. Reasonably ductile. High vapor pressure due to volatile zinc, unsuitable for furnace brazing. Copper-rich alloys prone to stress cracking by ammonia.

Ag-Cu-Zn

Lower melting point than Ag-Cu for same Ag content. Combines advantages of Ag-Cu and Cu-Zn. At above 40% Zn the ductility and strength drop, so only lower-zinc alloys of this type are used. At above 25% zinc less ductile copper-zinc and silver-zinc phases appear. Copper content above 60% yields reduced strength and liquidus above 900 °C. Silver content above 85% yields reduced strength, high liquidus and high cost. Copper-rich alloys prone to stress cracking by ammonia. Silver-rich brazes (above 67.5% Ag) are hallmarkable and used in jewellery; alloys with lower silver content are used for engineering purposes. Alloys with copper-zinc ratio of about 60:40 contain the same phases as brass and match its color; they are used for joining brass. Small amount of nickel improves strength and corrosion resistance and promotes wetting of carbides. Addition of manganese together with nickel increases fracture toughness. Addition of cadmium yields Ag-Cu-Zn-Cd alloys with improved fluidity and wetting and lower melting point; however cadmium is toxic. Addition of tin can play mostly the same role.

Cu-PWidely used for copper and copper alloys. Does not require flux for copper. Can be also used with silver, tungsten, and molybdenum. Copper-rich alloys prone to stress cracking by ammonia.

Ag-Cu-PLike Cu-P, with improved flow. Better for larger gaps. More ductile, better electrical conductivity. Copper-rich alloys prone to stress cracking by ammonia.

Au-AgNoble metals. Used in jewelry.

Au-CuContinuous series of solid solutions. Readily wet many metals, including refractory ones. Narrow melting ranges, good fluidity.[18] Frequently used in jewellery. Alloys with 40–90% of gold harden on cooling but stay ductile. Nickel improves ductility. Silver lowers melting point but worsens corrosion resistance; to maintain corrosion resistance gold has to be kept above 60%. High-temperature strength and corrosion resistance can be improved by further alloying, e.g. with chromium, palladium, manganese and molybdenum. Addition of vanadium allows wetting ceramics. Low vapor pressure.

Au-NiContinuous series of solid solutions. Wider melting range than Au-Cu alloys but better corrosion resistance and improved wetting. Frequently alloyed with other metals to reduce proportion of gold while maintaining properties. Copper may be added to lower gold proportion, chromium to compensate for loss of corrosion resistance, and boron for improving wetting impaired by the chromium. Generally no more than 35% Ni is used, as higher Ni/Au ratios have too wide melting range. Low vapor pressure.

Au-Pd

Improved corrosion resistance over Au-Cu and Au-Ni alloys. Used for joining superalloys and refractory metals for high-temperature applications, e.g. jet engines. Expensive. May be substituted for by cobalt-based brazes. Low vapor pressure.

PdGood high-temperature performance, high corrosion resistance (less than gold), high strength (more than gold). usually alloyed with nickel, copper, or silver. Forms solid solutions with most metals, does not form brittle intermetallics. Low vapor pressure.

NiNickel alloys, even more numerous than silver alloys. High strength. Lower cost than silver alloys. Good high-temperature performance, good corrosion resistance in moderately aggressive environments. Often used for stainless steels and heat-resistant alloys. Embrittled with sulfur and some lower-melting point metals, e.g. zinc. Boron, phosphorus, silicon and carbon lower melting point and rapidly diffuse to base metals; this allows diffusion brazing and allows the joint to be used above the brazing temperature. Borides and phosphides form brittle phases; amorphous preforms can be made by rapid solidification.

CoCobalt alloys. Good high-temperature corrosion resistance, possible alternative to Au-Pd brazes. Low workability at low temperatures, preforms prepared by rapid solidification.

Al-Sifor brazing aluminium.

Active alloysContaining active metals, e.g. titanium or vanadium. Used for brazing non-metallic materials, e.g. graphite or ceramics.

Role of elements[edit]

element

role

volatility

corrosion resistance

cost

incompatibility

description

Silver

structural, wetting

volatile

expensive

Enhances capillary flow, improves corrosion resistance of less-noble alloys, worsens corrosion resist

element

role

volatility

corrosion resistance

cost

incompatibility

description

ance of gold and palladium. Relatively expensive. High vapor pressure, problematic in vacuum brazing.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Wets copper. Does not wet nickel and iron. Reduces melting point of many alloys, including gold-coppe

element

role

volatility

corrosion resistance

cost

incompatibility

description

r.

Copper

structural

ammonia

Good mechanical properties. Often used with silver. Dissolves and wets nickel. Somewhat dissolves

element

role

volatility

corrosion resistance

cost

incompatibility

description

and wets iron. Copper-rich alloys sensitive to stress cracking in presence of ammonia.

Zinc structural, mel

volatile

low cheap

Ni Lowers melting

element

role

volatility

corrosion resistance

cost

incompatibility

description

ting, wetting

point. Often used with copper. Susceptible to corrosion. Improves wetting on ferrous metals and on nickel

element

role

volatility

corrosion resistance

cost

incompatibility

description

alloys. Compatible with aluminium. High vapor tension, produces somewhat toxic fumes, requires ventilation;

element

role

volatility

corrosion resistance

cost

incompatibility

description

highly volatile above 500 °C. At high temperatures may boil and create voids. Prone to selective leaching in some

element

role

volatility

corrosion resistance

cost

incompatibility

description

environments, which may cause joint failure. Traces of bismuth and beryllium together with tin or zinc in alumi

element

role

volatility

corrosion resistance

cost

incompatibility

description

nium-based braze destabilize oxide film on aluminium, facilitating its wetting. High affinity to oxygen, prom

element

role

volatility

corrosion resistance

cost

incompatibility

description

otes wetting of copper in air by reduction of the cuprous oxide surface film. Less such benefit in furnace brazin

element

role

volatility

corrosion resistance

cost

incompatibility

description

g with controlled atmosphere. Embrittles nickel. High levels of zinc may result in a brittle alloy.[19]

element

role

volatility

corrosion resistance

cost

incompatibility

description

Aluminium

structural, active

Fe

Usual base for brazing aluminium and its alloys. Embrittles ferrous alloys.

Gold

structural, we

excellent

very expensi

Excellent corrosion

element

role

volatility

corrosion resistance

cost

incompatibility

description

tting

ve

resistance. Very expensive. Wets most metals.

Palladium

structural

excellent

very expensive

Excellent corrosion resistance, though less than gold.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Higher mechanical strength than gold. Good high-temperature strength. Very expensive, though less than

element

role

volatility

corrosion resistance

cost

incompatibility

description

gold. Makes the joint less prone to fail due to intergranular penetration when brazing alloys of nickel, molyb

element

role

volatility

corrosion resistance

cost

incompatibility

description

denum, or tungsten.[20

]Increases high-temperature strength of gold-based alloys.[18] Improves high-temp

element

role

volatility

corrosion resistance

cost

incompatibility

description

erature strength and corrosion resistance of gold-copper alloys. Forms solid solutions with most engineering

element

role

volatility

corrosion resistance

cost

incompatibility

description

metals, does not form brittle intermetallics. High oxidation resistance at high temperatures, especially

element

role

volatility

corrosion resistance

cost

incompatibility

description

Pd-Ni alloys.

Cadmium

structural, wetting, melting

volatile

toxic Lowers melting point, improves fluidity. Toxic. Produces toxic fumes, requires

element

role

volatility

corrosion resistance

cost

incompatibility

description

ventilation. High affinity to oxygen, promotes wetting of copper in air by reduction of the cuprous oxide surfac

element

role

volatility

corrosion resistance

cost

incompatibility

description

e film. Less such benefit in furnace brazing with controlled atmosphere. Allows reducing silver content of

element

role

volatility

corrosion resistance

cost

incompatibility

description

Ag-Cu-Zn alloys. Replaced by tin in more modern alloys.

Lead

structural, melting

Lowers melting point. Toxic. Produces

element

role

volatility

corrosion resistance

cost

incompatibility

description

toxic fumes, requires ventilation.

Tin structural, melting, wetting

Lowers melting point, improves fluidity. Broadens meltin

element

role

volatility

corrosion resistance

cost

incompatibility

description

g range. Can be used with copper, with which it forms bronze. Improves wetting of many difficult-to-wet

element

role

volatility

corrosion resistance

cost

incompatibility

description

metals, e.g. stainless steels and tungsten carbide. Traces of bismuth and beryllium together with tin or

element

role

volatility

corrosion resistance

cost

incompatibility

description

zinc in aluminium-based braze destabilize oxide film on aluminium, facilitating its wetting. Low solubility in zinc, which

element

role

volatility

corrosion resistance

cost

incompatibility

description

limits its content in zinc-bearing alloys.[19]

Bismuth

trace additive

Lowers melting point. May disrupt surface

element

role

volatility

corrosion resistance

cost

incompatibility

description

oxides. Traces of bismuth and beryllium together with tin or zinc in aluminium-based braze destabilize oxide film

element

role

volatility

corrosion resistance

cost

incompatibility

description

on aluminium, facilitating its wetting.[19]

Beryllium

trace additive

Traces of bismuth and beryllium together with tin or zinc in

element

role

volatility

corrosion resistance

cost

incompatibility

description

aluminium-based braze destabilize oxide film on aluminium, facilitating its wetting.[19]

Nickel

structural,

high Zn, S Strong, corros

element

role

volatility

corrosion resistance

cost

incompatibility

description

wetting

ion-resistant. Impedes flow of the melt. Addition to gold-copper alloys improves ductility and resistance to

element

role

volatility

corrosion resistance

cost

incompatibility

description

creep at high temperatures.[18

]Addition to silver allows wetting of silver-tungsten alloys and improves bond

element

role

volatility

corrosion resistance

cost

incompatibility

description

strength. Improves wetting of copper-based brazes. Improves ductility of gold-copper brazes. Impro

element

role

volatility

corrosion resistance

cost

incompatibility

description

ves mechanical properties and corrosion resistance of silver-copper-zinc brazes. Nickel content offsets

element

role

volatility

corrosion resistance

cost

incompatibility

description

brittleness induced by diffusion of aluminium when brazing aluminium-containing alloys, e.g. aluminium bronzes. In

element

role

volatility

corrosion resistance

cost

incompatibility

description

some alloys increases mechanical properties and corrosion resistance, by a combination of solid solution streng

element

role

volatility

corrosion resistance

cost

incompatibility

description

thening, grain refinement, and segregation on fillet surface and in grain boundaries, where it forms a corros

element

role

volatility

corrosion resistance

cost

incompatibility

description

ion-resistant layer. Extensive intersolubility with iron, chromium, manganese, and others; can severely erode

element

role

volatility

corrosion resistance

cost

incompatibility

description

such alloys. Embrittled by zinc, many other low melting point metals, and sulfur.[19]

Chromium

structural

high Corrosion-resist

element

role

volatility

corrosion resistance

cost

incompatibility

description

ant. Increases high-temperature corrosion and strength of gold-based alloys. Added to copper and nickel to

element

role

volatility

corrosion resistance

cost

incompatibility

description

increase corrosion resistance of them and their alloys.[18] Wets oxides, carbides, and graphite; frequ

element

role

volatility

corrosion resistance

cost

incompatibility

description

ently a major alloy component for high-temperature brazing of such materials. Impairs wetting by gold-

element

role

volatility

corrosion resistance

cost

incompatibility

description

nickel alloys, which can be compensated for by addition of boron.[19]

Manganese

structural

volatile

good

cheap

High vapor pressure, unsuitable

element

role

volatility

corrosion resistance

cost

incompatibility

description

for vacuum brazing. In gold-based alloys increases ductility. Increases corrosion resistance of copper and

element

role

volatility

corrosion resistance

cost

incompatibility

description

nickel alloys.[18] Improves high-temperature strength and corrosion resistance of gold-copper alloys. Highe

element

role

volatility

corrosion resistance

cost

incompatibility

description

r manganese content may aggravate tendency to liquation. Manganese in some alloys may tend to cause

element

role

volatility

corrosion resistance

cost

incompatibility

description

porosity in fillets. Tends to react with graphite molds and jigs. Oxidizes easily, requires flux. Lowers meltin

element

role

volatility

corrosion resistance

cost

incompatibility

description

g point of high-copper brazes. Improves mechanical properties and corrosion resistance of silver-

element

role

volatility

corrosion resistance

cost

incompatibility

description

copper-zinc brazes. Cheap, even less expensive than zinc. Part of the Cu-Zn-Mn system is brittle, some

element

role

volatility

corrosion resistance

cost

incompatibility

description

ratios can not be used.[19] In some alloys increases mechanical properties and corrosion resistance, by a combi

element

role

volatility

corrosion resistance

cost

incompatibility

description

nation of solid solution strengthening, grain refinement, and segregation on fillet surface and in grain bound

element

role

volatility

corrosion resistance

cost

incompatibility

description

aries, where it forms a corrosion-resistant layer. Facilitates wetting of cast iron due to its ability to dissol

element

role

volatility

corrosion resistance

cost

incompatibility

description

ve carbon.

Molybdenum

structural

good

Increases high-temperature corrosion and strength of gold-based alloys.[18

]Incre

element

role

volatility

corrosion resistance

cost

incompatibility

description

ases ductility of gold-based alloys, promotes their wetting of refractory materials, namely carbides and graphi

element

role

volatility

corrosion resistance

cost

incompatibility

description

te. When present in alloys being joined, may destabilize the surface oxide layer (by oxidizing and then volatil

element

role

volatility

corrosion resistance

cost

incompatibility

description

izing) and facilitate wetting.

Cobalt

structural

good

Good high-temperature properties and corrosion resistance. In

element

role

volatility

corrosion resistance

cost

incompatibility

description

nuclear applications can absorb neutrons and build up cobalt-60, a potent gamma radiation emitter.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Magnesium

volatile O2 getter

volatile

Addition to aluminium makes the alloy suitable for vacuum brazing. Volatile, though less than zinc. Vaporizatio

element

role

volatility

corrosion resistance

cost

incompatibility

description

n promotes wetting by removing oxides from the surface, vapors act as getter for oxygen in the furnac

element

role

volatility

corrosion resistance

cost

incompatibility

description

e atmosphere.

Indium

melting, wetting

expensive

Lowers melting point. Improves wetting of ferrous alloys by copper-

element

role

volatility

corrosion resistance

cost

incompatibility

description

silver alloys.

Carbon

melting

Lowers melting point. Can form carbides. Can diffuse to the base metal, resulting in

element

role

volatility

corrosion resistance

cost

incompatibility

description

higher remelt temperature, potentially allowing step-brazing with the same alloy. At above 0.1% worsens

element

role

volatility

corrosion resistance

cost

incompatibility

description

corrosion resistance of nickel alloys. Trace amounts present in stainless steel may facilitate reduction of

element

role

volatility

corrosion resistance

cost

incompatibility

description

surface chromium(III) oxide in vacuum and allow fluxless brazing. Diffusion away from the braze increa

element

role

volatility

corrosion resistance

cost

incompatibility

description

ses its remelt temperature; exploited in diffusion brazing.[19]

Silicon

melting, wetting

Ni Lowers melting point. Can form s

element

role

volatility

corrosion resistance

cost

incompatibility

description

ilicides. Improves wetting of copper-based brazes. Promotes flow. Causes intergranular embrittlem

element

role

volatility

corrosion resistance

cost

incompatibility

description

ent of nickel alloys. Rapidly diffuses into the base metals. Diffusion away from the braze increases its remel

element

role

volatility

corrosion resistance

cost

incompatibility

description

t temperature; exploited in diffusion brazing.

Germanium

structural, melting

expensive

Lowers melting point. Expensive. For specia

element

role

volatility

corrosion resistance

cost

incompatibility

description

l applications. May create brittle phases.

Boron

melting, wetting

Ni Lowers melting point. Can form hard and brittle borid

element

role

volatility

corrosion resistance

cost

incompatibility

description

es. Unsuitable for nuclear reactors. Fast diffusion to the base metals. Can diffuse to the base metal, resulti

element

role

volatility

corrosion resistance

cost

incompatibility

description

ng in higher remelt temperature, potentially allowing step-brazing with the same alloy. Can erode some base

element

role

volatility

corrosion resistance

cost

incompatibility

description

materials or penetrate between grain boundaries of many heat-resistant structural alloys, degrading their mech

element

role

volatility

corrosion resistance

cost

incompatibility

description

anical properties. Has to be avoided in nuclear applications due to its interaction with neutrons. Causes interg

element

role

volatility

corrosion resistance

cost

incompatibility

description

ranular embrittlement of nickel alloys. Improves wetting of/by some alloys, can be added to Au-Ni-Cr alloy to

element

role

volatility

corrosion resistance

cost

incompatibility

description

compensate for wetting loss by chromium addition. In low concentrations improves wetting and lowers meltin

element

role

volatility

corrosion resistance

cost

incompatibility

description

g point of nickel brazes. Rapidly diffuses to base materials, may lower their melting point; especially a

element

role

volatility

corrosion resistance

cost

incompatibility

description

concern when brazing thin materials. Diffusion away from the braze increases its remelt temperature; exploi

element

role

volatility

corrosion resistance

cost

incompatibility

description

ted in diffusion brazing.

Mischmetal

trace additive

in amount of about 0.08%, can be used to substitute boron where boron

element

role

volatility

corrosion resistance

cost

incompatibility

description

would have detrimental effects.[19]

Cerium

trace additive

in trace quantities, improves fluidity of brazes. Particularly

element

role

volatility

corrosion resistance

cost

incompatibility

description

useful for alloys of four or more components, where the other additives compromise flow and spreading.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Strontium

trace additive

in trace quantities, refines the grain structure of aluminium-based alloys.

Phosphorus

deoxidizer

H2S, SO2, Ni, Fe, Co

Lowers melting point. Deoxi

element

role

volatility

corrosion resistance

cost

incompatibility

description

dizer, decomposes copper oxide; phosphorus-bearing alloys can be used on copper without flux.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Does not decompose zinc oxide, so flux is needed for brass. Forms brittle phosphides with some metals, e.g. nickel (Ni3P)

element

role

volatility

corrosion resistance

cost

incompatibility

description

and iron, phosphorus alloys unsuitable for brazing alloys bearing iron, nickel or cobalt in amount above 3%.

element

role

volatility

corrosion resistance

cost

incompatibility

description

The phosphides segregate at grain boundaries and cause intergranular embrittlement. (Sometimes the brittle

element

role

volatility

corrosion resistance

cost

incompatibility

description

joint is actually desired, though. Fragmentation grenades can be brazed with phosphorus bearing alloy to produ

element

role

volatility

corrosion resistance

cost

incompatibility

description

ce joints that shatter easily at detonation.) Avoid in environments with presence of sulfur dioxide (e.g.

element

role

volatility

corrosion resistance

cost

incompatibility

description

paper mills) and hydrogen sulfide (e.g. sewers, or close to volcanoes); the phosphorus-rich phase rapidly corro

element

role

volatility

corrosion resistance

cost

incompatibility

description

des in presence of sulfur and the joint fails. Phosphorus can be also present as an impurity introduced from

element

role

volatility

corrosion resistance

cost

incompatibility

description

e.g. electroplating baths.[20] In low concentrations improves wetting and lowers melting point of nickel

element

role

volatility

corrosion resistance

cost

incompatibility

description

brazes. Diffusion away from the braze increases its remelt temperature; exploited in diffusion brazing.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Lithium

deoxidizer

Deoxidizer. Eliminates the need for flux with some materials. Lithium oxide formed by reaction with the

element

role

volatility

corrosion resistance

cost

incompatibility

description

surface oxides is easily displaced by molten braze alloy.[19]

Titanium

structural, active

Most commonly used active metal.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Few percents added to Ag-Cu alloys facilitate wetting of ceramics, e.g. silicon nitride.[21] Most metals, excep

element

role

volatility

corrosion resistance

cost

incompatibility

description

t few (namely silver, copper and gold), form brittle phases with titanium. When brazing ceramics, like other active

element

role

volatility

corrosion resistance

cost

incompatibility

description

metals, titanium reacts with them and forms a complex layer on their surface, which in turn is wetta

element

role

volatility

corrosion resistance

cost

incompatibility

description

ble by the silver-copper braze. Wets oxides, carbides, and graphite; frequently a major alloy component

element

role

volatility

corrosion resistance

cost

incompatibility

description

for high-temperature brazing of such materials.[19]

Zirconium

structural, active

Wets oxides, carbides, and graphite; frequ

element

role

volatility

corrosion resistance

cost

incompatibility

description

ently a major alloy component for high-temperature brazing of such materials.[19]

Hafnium

active

element

role

volatility

corrosion resistance

cost

incompatibility

description

Vanadium

structural, active

Promotes wetting of alumina ceramics by gold-based alloys.[18]

Sulfur

impurity

Compromises integrity of nickel alloys.

element

role

volatility

corrosion resistance

cost

incompatibility

description

Can enter the joints from residues of lubricants, grease or paint. Forms brittle nickel sulfide (Ni3S2) that segregates

element

role

volatility

corrosion resistance

cost

incompatibility

description

at grain boundaries and cause intergranular failure.

Some additives and impurities act at very low levels. Both positive and negative effects can be observed. Strontium at levels of 0.01% refines grain structure of aluminium. Beryllium and bismuth at similar levels help disrupt the passivation layer of aluminium oxide and promote wetting. Carbon at 0.1% impairs corrosion resistance of nickel alloys.

Aluminium can embrittle mild steel at 0.001%, phosphorus at 0.01%.[19]

In some cases, especially for vacuum brazing, high-purity metals and alloys are used. 99.99% and 99.999% purity levels are available commercially.

Care has to be taken to not introduce deletrious impurities from joint contaminations or by dissolution of the base metals during brazing.

Melting behavior[edit]

Alloys with larger span of solidus/liquidus temperatures tend to melt through a "mushy" state, where the alloy is a mixture of solid and liquid material. Some alloys show tendency to liquation, separation of the liquid from the solid portion; for these the heating through the melting range has to be sufficiently fast to avoid this effect. Some alloys show extended plastic range, when only a small portion of the alloy is liquid and most of the material melts at the upper temperature range; these are suitable for bridging large gaps and for forming fillets. Highly fluid alloys are suitable for penetrating deep into narrow gaps and for brazing tight joints with narrow tolerances but are not suitable for filling larger gaps. Alloys with wider melting range are less sensitive to non-uniform clearances.

When the brazing temperature is suitably high, brazing and heat treatment can be done in a single operation simultaneously.

Eutectic alloys melt at single temperature, without mushy region. Eutectic alloys have superior spreading; non-eutectics in the mushy region have high viscosity and at the same time

attack the base metal, with correspondingly lower spreading force. Fine grain size gives eutectics both increased strength and increased ductility. Highly accurate melting temperature allows joining process to be performed only slightly above the alloy's melting point. On solidifying, there is no mushy state where the alloy appears solid but is not yet; the chance of disturbing the joint by manipulation in such state is reduced (assuming the alloy did not significantly change its properties by dissolving the base metal). Eutectic behavior is especially beneficial for solders.[19]

Metals with fine grain structure before melting provide superior wetting to metals with large grains. Alloying additives (e.g. strontium to aluminium) can be added to refine grain structure, and the preforms or foils can be prepared by rapid quenching. Very rapid quenching may provide amorphous metal structure, which possess further advantages.[19]

Interaction with base metals[edit]

Brazing at the Gary Tubular Steel Plant,

1943

For successful wetting, the base metal has to be at least partially soluble in at least one component

of the brazing alloy. The molten alloy therefore tends to attack the base metal and dissolve it, slightly changing its composition in the process. The composition change is reflected in the change of the alloy's melting point and the corresponding change of fluidity. For example, some alloys dissolve both silver and copper; dissolved silver lowers their melting point and increases fluidity, copper has the opposite effect.

The melting point change can be exploited. As the remelt temperature can be increased by enriching the alloy with dissolved base metal, step brazing using the same braze can be possible.

Alloys that do not significantly attack the base metals are more suitable for brazing thin sections.

Nonhomogenous microstructure of the braze may cause non-uniform melting and localized erosions of the base metal.

Wetting of base metals can be improved by adding a suitable metal to the alloy. Tin facilitates wetting of iron, nickel, and many other alloys. Copper wets ferrous metals that silver does not attack, copper-silver alloys can therefore braze steels silver alone won't wet. Zinc improves wetting of ferrous metals, indium as well. Aluminium improves wetting of aluminium alloys. For wetting of ceramics, reactive metals capable of forming chemical compounds with the ceramic (e.g. titanium, vanadium, zirconium...) can be added to the braze.

Dissolution of base metals can cause detrimental changes in the brazing alloy. For example, aluminium dissolved from aluminium bronzes can embrittle the braze; addition of nickel to the braze can offset this.

The effect works both ways; there can be detrimental interactions between the braze alloy and the base metal. Presence of phosphorus in the braze alloy leads to formation of brittle phosphides of iron and nickel, phosphorus-containing alloys are therefore unsuitable for brazing nickel and ferrous alloys. Boron tends to diffuse into the base metals, especially along the grain boundaries, and may form brittle borides. Carbon can negatively influence some steels.

Care has to be taken to avoid galvanic corrosion between the braze and the base metal, and especially between dissimilar base metals being brazed together.

Formation of brittle intermetallic compounds on the alloy interface can cause joint failure. This is discussed more in-depth with solders.

The potentially detrimental phases may be distributed evenly through the volume of the alloy, or be concentrated on the braze-base interface. A thick layer of interfacial intermetallics is usually considered detrimental due to its commonly low fracture toughness and other sub-par mechanical properties. In some situations, e.g. die attaching, it however does not matter much as silicon chips are not typically subjected to mechanical abuse.[19]

On wetting, brazes may liberate elements from the base metal. For example, aluminium-silicon braze wets silicon nitride, dissociates the surface so it can react with silicon, and liberates nitrogen, which may create voids along the joint interface and lower its strength. Titanium-containing nickel-gold braze wets silicon nitride and reacts with its surface, forming

titanium nitride and liberating silicon; silicon then forms brittle nickel silicides and eutectic gold-silicon phase; the resulting joint is weak and melts at much lower temperature than may be expected.[19]

Metals may diffuse from one base alloy to the other one, causing embrittlement or corrosion. An example is diffusion of aluminium from aluminium bronze to a ferrous alloy when joining these. A diffusion barrier, e.g. a copper layer (e.g. in a trimet strip), can be used.

A sacrificial layer of a noble metal can be used on the base metal as an oxygen barrier, preventing formation of oxides and facilitating fluxless brazing. During brazing, the noble metal layer dissolves in the filler metal. Copper or nickel plating of stainless steels performs the same function.[19]

In brazing copper, a reducing atmosphere (or even a reducing flame) may react with the oxygen residues in the metal, which are present as cuprous oxide inclusions, and cause hydrogen embrittlement. The hydrogen present in the flame or atmosphere at high temperature reacts with the oxide, yielding metallic copper and water vapour, steam. The steam bubbles exert high pressure in the metal structure, leading to cracks and joint porosity. Oxygen-free copper is not sensitive to this effect, however the most readily available grades, e.g. electrolytic copper or high-conductivity copper, are. The embrittled joint may then fail catastrophically without any previous sign of deformation or deterioration.[22]

Preform[edit]

A brazing preform is a high quality, precision metal stamping used for a variety of joining applications in manufacturing electronic devices and systems. Typical brazing preform uses include attaching electronic circuitry, packaging electronic devices, providing good thermal and electrical conductivity, and providing an interface for electronic connections. Square, rectangular and disc shaped brazing preforms are commonly used to attach electronic components containing silicon dies to a substrate such as a printed circuit board.

Rectangular frame shaped preforms are often required for the construction of electronic packages while washer shaped brazing preforms are typically utilized to attach lead wires and hermetic feed-throughs to electronic circuits and packages. Some preforms are also used in diodes, rectifiers, optoelectronic devices and components packaging.[23]

Flux (metallurgy)From Wikipedia, the free encyclopedia

Rosin used as flux for soldering

A flux pen used for electronics rework

Multicore solder containing flux

Wire freshly coated with solder, still immersed in molten rosin flux

In metallurgy, a flux (derived from Latin fluxus meaning “flow”), is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

Some of the earliest known fluxes were carbonate of soda, potash, charcoal, coke, borax,[1] lime,[2] lead sulfide [3]  and certain minerals containing phosphorus. Iron ore was also used as a flux in the smelting of copper. These agents served various functions, the simplest being a reducing agent which prevented oxides from forming on the surface of the molten metal, while others absorbed impurities into the slag which could be scraped off the molten metal. As cleaning agents, fluxes facilitatesoldering, brazing, and welding by removing oxidation from the metals to be joined. Common fluxes are: ammonium chlorideor rosin for soldering tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); andborax for brazing or braze-welding ferrous metals.

In the process of smelting, inorganic chlorides, fluorides (see fluorite), limestone and other materials are designated as "fluxes" when added to the contents of a smelting furnace or a cupola for the purpose of purging the metal of chemical impurities such as phosphorus, and of rendering slag more liquid at the smelting temperature. The slag is a liquid mixture ofash, flux, and other impurities. This reduction of slag viscosity with temperature, increasing the flow of slag in smelting, is the original origin of the word flux in metallurgy. Fluxes are also used in foundries for removing impurities from molten nonferrous metals such as aluminum, or for adding desirable trace elements such as titanium.

In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise.

The role of a flux in joining processes is typically dual: dissolving of the oxides on the metal surface, which facilitates wettingby molten metal, and acting as an oxygen barrier by coating the hot surface, preventing its oxidation. In some applications molten flux also serves as a heat transfer medium, facilitating heating of the joint by the soldering tool or molten solder.

Fluxes for soft soldering are typically of organic nature, though inorganic fluxes, usually based on halogenides and/or acids, are also used in non-electronics applications. Fluxes for brazing operate at significantly higher temperatures and are therefore mostly inorganic; the organic compounds tend to be of supplementary nature.

Contents  [hide] 

1   Composition and properties o 1.1   Behavior of activators o 1.2   Rosin fluxes o 1.3   Specifications o 1.4   Examples of special fluxes

2   Drawbacks o 2.1   Dangers o 2.2   Fluxless techniques

3   Uses o 3.1   Soldering

o 3.2   Brazing and silver soldering o 3.3   Smelting

4   Flux recovery 5   Metal salts as flux in hot corrosion 6   List of fluxes 7   See also 8   References 9   External links

Composition and properties[edit]

Organic fluxes typically consist of four major components:[4]

Activators - chemicals disrupting/dissolving the metal oxides. Their role is to expose unoxidized, easily wettable metal surface and aid soldering by other means, e.g. by exchange reactions with the base metals. Highly active fluxes contain chemicals that are corrosive at room temperature. The

compounds used include metal halides (most often zinc chloride orammonium chloride), hydrochloric acid, phosphoric acid, and hydrobromic acid. Salts of mineral acids with amines are also used as aggressive activators. Aggressive fluxes typically facilitate corrosion, require careful removal, and are unsuitable for finer work. Activators for fluxes for soldering and brazing aluminium often contain fluorides.

Milder activators begin to react with oxides only at elevated temperature. Typical compounds used are carboxylic acids (e.g. fatty acids (most often oleic acidand stearic acid), dicarboxylic acids) and sometimes amino acids. Some milder fluxes also contain halides or organohalides.

Vehicles - high-temperature tolerant chemicals in the form of non-volatile liquids or solids with suitable melting point; they are generally liquid at soldering temperatures. Their role is to act as an oxygen barrier to protect the hot metal surface against oxidation, to dissolve the reaction products of activators and oxides and carry them away from the metal surface, and to facilitate heat transfer. Solid vehicles tend to be based on natural or modified rosin (mostly abietic acid, pimaric acid, and other resin acids) or natural or synthetic resins. Water-soluble organic fluxes tend to contain vehicles based on high-boiling polyols -glycols, diethylene glycol and higher polyglycols, polyglycol-based surfactants and glycerol.

Solvents - added to facilitate processing and deposition to the joint. Solvents are typically dried out during preheating before the soldering operation; incomplete solvent removal may lead to boiling off and spattering of solder paste particles or molten solder.

Additives - numerous other chemicals modifying the flux properties. Additives can be surfactants (especially nonionic), corrosion inhibitors, stabilizers andantioxidants, tackifiers, thickeners and other rheological modifiers (especially for solder pastes), plasticizers (especially for flux-cored solders), and dyes.

Inorganic fluxes contain components playing the same role as in organic fluxes. They are more often used in brazing and other high-temperature applications, where organic fluxes have insufficient thermal stability. The chemicals used often simultaneously act as both vehicles and activators; typical examples are borax, borates,fluoroborates, fluorides and chlorides. Halogenides are active at lower temperatures than borates, and are therefore used for brazing of aluminium and magnesium alloys; they are however highly corrosive.

Fluxes have several important properties:

Activity - the ability to dissolve existing oxides on the metal surface and promote wetting with solder. Highly active fluxes are often of acidic and/or corrosive nature.

Corrosivity - the promotion of corrosion by the flux and its residues. Most active fluxes tend to be corrosive at room temperatures and require careful removal. As activity and corrosivity are linked, the preparation of surfaces to be joined should allow use of milder fluxes. Some water-soluble flux residues are hygroscopic, which causes problems with electrical resistance and contributes to corrosion. Fluxes containing halides and mineral acids are highly corrosive and require thorough removal. Some fluxes, especially borax-based brazing ones, form very hard glass-like coatings that are difficult to remove.

Cleanability - the difficulty of removal of flux and its residues after the soldering operation. Fluxes with higher content of solids tend to leave larger amount of residues; thermal decomposition of some vehicles also leads to formation of difficult-to-clean, polymerized and possibly even charred deposits (a problem especially for hand soldering). Some flux residues are soluble in organic solvents, others in water, some in both. Some fluxes are no-clean, as they are sufficiently volatile or undergoing thermal decomposition to volatile products that they do not require the cleaning step. Other fluxes leave non-corrosive residues that can be left in place. However, flux residues can interfere with subsequent operations; they can impair adhesion of conformal coatings, or act as undesired insulation on connectors and contact pads for test equipment.

Residue tack - the stickiness of the surface of the flux residue. When not removed, the flux residue should have smooth, hard surface. Tacky surfaces tend to accumulate dust and particulates, which causes issues with electrical resistance; the particles themselves can be conductive or they can be hygroscopic or corrosive.

Volatility - this property has to be balanced to facilitate easy removal of solvents during the preheating phase but to not require too frequent replenishing of solvent in the process equipment.

Viscosity - especially important for solder pastes, which have to be easy to apply but also thick enough to stay in place without spreading to undesired locations. Solder pastes may also function as a temporary adhesive for keeping electronic parts in place before and during soldering. Fluxes applied by e.g. foam require low viscosity.

Flammability - relevant especially for glycol-based vehicles and for organic solvents. Flux vapors tend to have low autoignition temperature and present a risk of a flash fire when the flux comes in contact with a hot surface.

Solids - the percentage of solid material in the flux. Fluxes with low solids, sometimes as little as 1-2%, are called low solids flux, low-residue flux, or no clean flux. They are often composed of weak organic acids, with addition of small amount of rosin or other resins.

The surface of the tin-based solder is coated predominantly with tin oxides; even in alloys the surface layer tends to become relatively enriched by tin. Fluxes for indium and zinc based solders have different compositions than fluxes for ordinary tin-lead and tin-based solders, due to different soldering temperatures and different chemistry of the oxides involved.

The composition of fluxes is tailored for the required properties - the base metals and their surface preparation (which determine the composition and thickness of surface oxides), the solder (which determines the wetting properties and the soldering temperature), the corrosion resistance and ease of removal, and others.

Organic fluxes are unsuitable for flame soldering and flame brazing, as they tend to char and impair solder flow.

Some metals are classified as "unsolderable" in air, and have to be either coated with another metal before soldering or special fluxes and/or protective atmospheres have to be used. Such metals are beryllium, chromium, magnesium, titanium, and some aluminium alloys.

Fluxes for high-temperature soldering differ from the fluxes for use at lower temperatures. At higher temperatures even relatively mild chemicals have sufficient oxide-disrupting activity, but the metal oxidation rates become fairly high; the barrier function of the vehicle therefore becomes more

important than the fluxing activity. High molecular weight hydrocarbons are often used for this application; a diluent with a lower molecular weight, boiling off during the preheat phase, is usually used to aid application.[5]

Behavior of activators[edit]

The role of the activators is primarily disruption and removal of the oxide layer on the metal surface (and also the molten solder), to facilitate direct contact between the molten solder and metal. The reaction product is usually soluble or at least dispersible in the molten vehicle. The activators are usually either acids, or compounds that release acids at elevated temperature.

The general reaction of oxide removal is:

Metal oxide + Acid → Salt + Water

Salts are ionic in nature and can cause problems from metallic leaching or dendrite growth, with possible product failure. In some cases, particularly in high-reliabilityapplications, flux residues must be removed.

The activity of the activator generally increases with temperature, up to a certain value where activity ceases, either due to thermal decomposition or excessive volatilization. However the oxidation rate of the metals also increases with temperature.

At high temperatures, copper oxide reacts with hydrogen chloride to water-soluble and mechanically weak copper chloride, and with rosin to salts of copper and abietic acid which is soluble in molten rosin.

Some activators may also contain metal ions, capable of exchange reaction with the underlying metal; such fluxes aid soldering by chemically depositing a thin layer of easier solderable metal on the exposed base metal. An example is the group of fluxes containing zinc, tin or cadmium compounds, usually chlorides, sometimes fluorides or fluoroborates.

Common high-activity activators are mineral acids, often together with halides, amines, water and/or alcohols:

hydrochloric acid , most common phosphoric acid , less common, use limited by its polymerization at higher temperatures

Inorganic acids are highly corrosive to metals even at room temperature, which causes issues during storage, handling and applications. As soldering involves high temperatures, compounds that decompose or react with acids as products are frequently used:

zinc chloride , which at high temperatures reacts with moisture, forming oxychloride and hydrochloric acid

ammonium chloride , thermally decomposing to ammonia and hydrochloric acid amine  hydrochlorides, decomposing to the amine and hydrochloric acid

Rosin fluxes[edit]

Electrical solder with a rosin core, visible as a dark spot in the cut end of the solder wire.

The terms resin flux and rosin flux are ambiguous and somewhat interchangeable, with different vendors using different assignments. Generally, fluxes are labeled as rosin if the vehicle they are based on is primarily natural rosin. Some manufactures reserve "rosin" designation for military fluxes based on rosin (R, RMA and RA compositions) and label others as "resin".

Rosin has good flux properties. A mixture of organic acids (resin acids, predominantly abietic acid, with pimaric acid,isopimaric acid, neoabietic acid, dihydroabietic acid, and dehydroabietic acid), rosin is a glassy solid, virtually nonreactive and noncorrosive at normal temperature, but liquid, ionic and mildly reactive to metal oxides at molten state. Rosin tends to soften between 60-70 °C and is fully fluid at around 120 °C; molten rosin is weakly acidic and is able to dissolve thinner layers of surface oxides from copper without further additives. For heavier surface contamination or improved process speed, additional activators can be added.

There are three types of rosin: gum rosin (from pine tree oleoresin), wood rosin (obtained by extraction of tree stumps), and tall oil rosin (obtained from tall oil, a byproduct of kraft paper process). Gum rosin has a milder odor and lower tendency to crystallize from solutions than wood rosin, and is therefore preferred for flux applications. Tall oil rosin finds increased use due to its higher thermal stability and therefore lower tendency to form insoluble thermal decomposition residues. The composition and quality of rosin differs by the tree type, and also by location and even by year. In Europe, rosin for fluxes is usually obtained from a specific type of Portuguese pine, in America a North Carolina variant is used.[6]

Natural rosin can be used as-is, or can be chemically modified by e.g. esterification, polymerization, or hydrogenation. The properties being altered are increased thermal stability, better cleanability, altered solution viscosity, and harder residue (or conversely, softer and more tacky residue). Rosin can be also converted to a water-soluble rosin flux, by formation of an ethoxylated rosin amine, an adduct with a polyglycol and an amine.

One of the early fluxes was a mixture of equal amounts of rosin and vaseline. A more aggressive early composition was a mixture of saturated solution of zinc chloride, alcohol, and glycerol.[7]

Fluxes can be also prepared from synthetic resins, often based on esters of polyols and fatty acids. Such resins have improved fume odor and lower residue tack, but their fluxing activity and solubility tend to be lower than of natural resins.

Rosin fluxes are categorized by grades of activity: L for low, M for moderate, and H for high. There are also other abbreviations for different rosin flux grades:[6][8]

R (Rosin) - pure rosin, no activators, low activity, mildest WW (Water-White) - purest rosin grade, no activators, low activity, sometimes synonymous

with R RMA (Rosin Mildly Activated) - contains mild activators, typically no halides RA (Rosin Activated) - rosin with strong activators, high activity, contains halides OA (Organic Acid) - rosin activated with organic acids, high activity, highly corrosive,

aqueous cleaning SA (Synthetically Activated) - rosin with strong synthetic activators, high activity; formulated

to be easily soluble in organic solvents (chlorofluorocarbons, alcohols) to facilitate cleaning WS (Water-Soluble) - usually based on inorganic or organic halides; highly corrosive

residues

SRA (Superactivated rosin) - rosin with very strong activators, very high activity IA (Inorganic Acid) - rosin activated with inorganic acids (usually hydrochloric acid or

phosphoric acid), highest activities, highly corrosive

R, WW, and RMA grades are used for joints that can not be easily cleaned or where there is too high corrosion risk. More active grades require thorough cleaning of the residues. Improper cleaning can actually aggravate the corrosion by releasing trapped activators from the flux residues.

There are several possible activator groups for rosins:

halide activators (organic halide salts, e.g. dimethylammonium chloride and diethylammonium chloride)

organic acids (monocarboxylic, e.g. formic acid, acetic acid, propionic acid, and dicarboxylic, e.g. oxalic acid, malonic acid, sebacic acid)

Specifications[edit]

Solder fluxes are specified according to several standards.

The most common one in European setting is the ISO 9454-1 (also known as DIN EN 294545-1).[9]

The flux classes according to ISO 9454-1 are specified by four-character code, by flux type, base, activator, and type. The form code is often omitted.

Flux type Base Activator Form

1 Resin 1 Rosin 2 Without rosin 1 Without activator

2 Halide activator 3 Non-halide activator

A Liquid B Solid C Paste

2 Organic 1 Water-soluble 2 Water-insoluble

3 Inorganic

1 Salts 1 Ammonium chloride 2 Without ammonium chloride

2 Acids 1 Phosphoric acid 2 Other acids

3 Alkaline 1 Amines and/or ammonia

Therefore 1.1.2 means rosin flux with halides.

The older specification, still often in use for specifying fluxes in shops, is the older German DIN 8511; the pairing is however not always one-to-one (note the multiple vs. one relation old standard vs. new standard in table below

Residues Old New Description

Strongly corrosive F-SW-11 3.2.2 Inorganic acid other than phosphoric

Residues Old New Description

Strongly corrosive F-SW-12 3.1.1 Ammonium chloride

Strongly corrosive F-SW-13 3.2.1 Phosphoric acid

Weakly corrosive F-SW-21 3.1.1 Ammonium chloride

Weakly corrosive F-SW-22 3.1.2 Inorganic salts without ammonium chloride

Weakly corrosive F-SW-23 2.1.3 Organic water-soluble without halides

Weakly corrosive F-SW-23 2.2.1 Organic water-insoluble without activators

Weakly corrosive F-SW-23 2.2.3 Organic water-insoluble without halides

Weakly corrosive F-SW-24 2.1.1 Organic water-soluble without activators

Weakly corrosive F-SW-24 2.1.3 Organic water-soluble without halides

Weakly corrosive F-SW-24 2.2.3 Organic water-insoluble without halides

Weakly corrosive F-SW-25 2.1.2 Organic water-soluble with halides

Weakly corrosive F-SW-25 2.2.2 Organic water-insoluble with halides

Weakly corrosive F-SW-26 1.1.2 Rosin with halides

Residues Old New Description

Weakly corrosive F-SW-27 1.1.3 Rosin without halides

Weakly corrosive F-SW-28 1.2.2 Rosin-free resin with halides

Non-corrosive F-SW-31 1.1.1 Rosin without activators

Non-corrosive F-SW-32 1.1.3 Rosin without halides

Non-corrosive F-SW-33 1.2.3 Rosin-free resin without halides

Non-corrosive F-SW-34 2.2.3 Organic water-insoluble without halides

One standard increasing used (United States) is J-STD-004 (very similar to DIN EN 61190-1-1). Four characters (two letters, then one letter, and last a number) represent flux composition, flux activity, and whether activators include halides:[10]

Base RO(sin) - RE(sin) - OR(ganic) - IN(organic)

Activity L(ow) - M(oderate)-H(igh)

Halide content <0.05% 0 (yes) - 1 (No)

Any combination is possible, e.g. ROL0, REM1 or ORH0.

Examples of special fluxes[edit]

Some materials are very difficult to solder. In some cases special fluxes have to be employed.

Aluminium and its alloys are difficult to solder due to the formation of the passivation layer of aluminium oxide. The flux has to be able to disrupt this layer and facilitate wetting by solder. Salts or organic complexes of some metals can be used; the salt has to be able to penetrate the cracks in the oxide layer. The metal ions, more noble than aluminium, then undergo a redox reaction, dissolve the surface layer of aluminium and form a deposit there. This intermediate layer of another metal then can be wetted with a solder. One example of such flux is a composition of triethanolamine, fluoroboric acid, and cadmium fluoroborate. More than 1% magnesium in the alloy however impairs the flux action, as the magnesium oxide layer is more refractory. Another possibility is an inorganic flux composed of zinc chloride or tin(II) chloride,[11] ammonium chloride, and a fluoride (e.g. sodium fluoride). Presence of silicon in the alloy impairs the flux effectivity, as silicon does not undergo the exchange reaction aluminium does.

Magnesium alloys  can be potentially soldered at low temperature by using molten acetamide as flux. Acetamide dissolves surface oxides on both aluminium and magnesium; promising experiments were done with its use as a flux for a tin-indium solder on magnesium.

Stainless steel  is another difficult to solder material, due to its stable, self-healing oxide layer and its low thermal conductivity. A solution of zinc chloride in hydrochloric acid is a common flux for stainless steels; it has however to be thoroughly removed afterwards as it would cause pitting corrosion. Another highly effective flux is phosphoric acid; its tendency to polymerize at higher temperatures however limits its applications.

Drawbacks[edit]

Fluxes have several serious drawbacks:

Corrosivity, which is mostly due to the aggressive compounds of the activators; hygroscopic properties of the flux residues may aggravate the effects

Interference with test equipment, which is due to the insulating residues deposited on the test contacts on electronic circuit boards

Interference with machine vision systems when the layer of flux or its remains is too thick or improperly located

Contamination of sensitive parts, e.g. facets of laser diodes, contacts of connectors and mechanical switches, and MEMS assemblies

Deterioration of electrical properties of printed circuit boards, as soldering temperatures are above the glass transition temperature of the board material and flux components (e.g. glycols, or chloride and bromide ions) can diffuse into its matrix; e.g. water-soluble fluxes containing polyethylene glycol were demonstrated to have such impact[12]

Deterioration of high-frequency circuit performance by flux residues Deterioration of surface insulation resistance, which tends to be as much as three orders of

magnitude lower than the bulk resistance of the material Electromigration  and growth of whiskers between nearby traces, aided by ionic residues,

surface moisture and a bias voltage The fumes liberated during soldering may have adverse health effects, and volatile organic

compounds can be outgassed during processing The solvents required for post-soldering cleaning of the boards are expensive and may have

adverse environmental impact

In special cases the drawbacks are sufficiently serious to warrant using fluxless techniques.

Dangers[edit]

Acid flux types (not used in electronics) may contain hydrochloric acid, zinc chloride or ammonium chloride, which are harmful to humans. Therefore, flux should be handled with gloves and goggles, and used with adequate ventilation.

Prolonged exposure to rosin fumes released during soldering can cause occupational asthma (formerly called colophony disease[13] in this context) in sensitive individuals, although it is not known which component of the fumes causes the problem.[14]

While molten solder has low tendency to adhere to organic materials, molten fluxes, especially of the resin/rosin type, adhere well to fingers. A mass of hot sticky flux can transfer more heat to skin and cause more serious burns than a comparable particle of non-adhering molten metal, which can be quickly shaken off. In this regard, molten flux is similar to molten hot glue.

Fluxless techniques[edit]

In some cases the presence of flux is undesirable; flux traces interfere with e.g. precision optics or MEMS assemblies. Flux residues also tend to outgas in vacuum and space applications, and traces of water, ions and organic compounds may adversely affect long-term reliability of hermetic packages. Trapped flux residues are also the cause of most voids in the joints. Flux-less techniques are therefore desirable there.[15]

For successful soldering and brazing, the oxide layer has to be removed from both the surfaces of the materials and the surface of the filler metal preform; the exposed surfaces also have to be protected against oxidation during heating. Flux-coated preforms can also be used to eliminate flux residue entirely from the soldering process.[16]

Protection of the surfaces against further oxidation is relatively simple, by using vacuum or inert atmosphere. Removal of the native oxide layer is more troublesome; physical or chemical cleaning methods have to be employed and the surfaces can be protected by e.g. gold plating. The gold layer has to be sufficiently thick and non-porous to provide protection for reasonable storage time. Thick gold metallization also limits choice of soldering alloys, as tin-based solders dissolve gold and form brittle intermetallics, embrittling the joint. Thicker gold coatings are usually limited to use with indium-based solders and solders with high gold content.

Removal of the oxides from the solder preform is also troublesome. Fortunately some alloys are able to dissolve the surface oxides in their bulk when superheated by several degrees above their melting point; the Sn-Cu1 and Sn-Ag4 require superheating by 18-19 °C, the Sn-Sb5 requires as little as 10 °C, but the Sn-Pb37 alloy requires 77 °C above its melting point to dissolve its surface oxide. The self-dissolved oxide however degrades the solder's properties and increases its viscosity in molten state, this approach is therefore not optimal. Solder preforms are preferred to be with high volume-to-surface ratio, as that limits the amount of oxide being formed. Pastes have to contain smooth spherical particles, preforms are ideally made of round wire. The problem with preforms can be also sidestepped by depositing the solder alloy directly on the surfaces of the parts and/or substrates, by e.g. chemical or electrochemical means.

Protective atmosphere with chemically reducing properties can be beneficial in some cases. Molecular hydrogen can be used to reduce surface oxides of tin and indium at temperatures above 430 and 470 °C; for zinc the temperature is above 500 °C, where zinc is already becoming volatilized. (At lower temperatures the reaction speed is too slow for practical applications.) Very low partial pressures of oxygen and water vapor have to be achieved for the reaction to proceed.

Other reactive atmospheres are also in use. Vapors of formic acid and acetic acid are the most commonly used. Carbon monoxide and halogen gases (e.g. carbon tetrafluoride, sulfur hexafluoride, or dichlorodifluoromethane) require fairly high temperatures for several minutes to be effective.

Atomic hydrogen is much more reactive than molecular hydrogen. In contact with surface oxides it forms hydroxides, water, or hydrogenated complexes, which are volatile at soldering temperatures. The most practical dissociation method is probably an electrical discharge. Argon-hydrogen gas compositions with hydrogen concentration below the low flammable limit can be used, eliminating the safety issues. The operation has to be performed at low pressure, as the stability of atomic hydrogen at atmospheric pressure is insufficient. Such hydrogen plasma can be used for fluxless reflow soldering.

Active atmospheres are relatively common in furnace brazing; due to the high process temperatures the reactions are reasonably fast. The active ingredients are usually carbon monoxide (possibly in the form of combusted fuel gas) and hydrogen. Thermal dissociation of ammonia yields an inexpensive mixture of hydrogen and nitrogen.

Bombardment with atomic particle beams can remove surface layers at a rate of tens of nanometers per minute. The addition of hydrogen to the plasma augments the removal efficiency by chemical mechanisms.

Mechanical agitation is another possibility for disrupting the oxide layer. Ultrasound can be used for assisting tinning and soldering; an ultrasonic transducer can be mounted on the soldering iron, in a solder bath, or in the wave for wave soldering. The oxide disruption and removal involves cavitation effects between the molten solder and the base metal surface. A common application of ultrasound fluxing is in tinning of passive parts (active parts do not cope well with the mechanical stresses involved); even aluminium can be tinned this way. The parts can then be soldered or brazed conventionally.

Mechanical rubbing of a heated surface with molten solder can be used for coating the surface. Both surfaces to be joined can be prepared this way, then placed together and reheated. This technique was formerly used to repair small damages on aluminium aircraft skins.

Very thin layer of zinc can be used for joining aluminium parts. The parts have to be perfectly machined, or pressed together, due to the small volume of filler metal. At high temperature applied for long time, the zinc diffuses away from the joint. The resulting joint does not present a mechanical weakness and is corrosion-resistant. The technique is known as diffusion soldering.

Fluxless brazing of copper alloys can be done with self-fluxing filler metals. Such metals contain an element capable of reaction with oxygen, usually phosphorus. A good example is the family of copper-phosphorus alloys.

Uses[edit]

Soldering[edit]

In soldering of metals, flux serves a threefold purpose: it removes rust from the surfaces to be soldered, it seals out air thus preventing further rust, and by facilitatingamalgamation improves wetting characteristics of the liquid solder. Some fluxes are corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage. Several types of flux are used in electronics.

A number of standards exist to define the various flux types. The principal standard is J-STD-004.

J-STD-004 characterizes the flux by type (e.g. Rosin (RO), Resin (RE), Organic (OR), Inorganic (IN)), its activity (strength of fluxing) and reliability of residue from asurface insulation resistance (SIR) and electromigration standpoint, and whether or not it contains halide activators.

This replaces the old MIL QQS standard which defined fluxes as:

R (Rosin)

RMA (Rosin Mildly Activated)

RA (Rosin Activated)

WS (Water soluble)

Any of these categories (except WS) may be no-clean, or not, depending on the chemistry selected and the standard that the manufacturer requires.

J-STD-004 includes tests for electromigration and surface insulation resistance (which must be greater than 100 MΩ after 168 hours at elevated temperature and humidity with a DC bias applied).

Brazing and silver soldering[edit]

Brazing (sometimes known as silver soldering or hard soldering) requires a much higher temperature than soft soldering, sometimes over 850 °C. As well as removing existing oxides, rapid oxidation of the metal at the elevated temperatures has to be avoided. This means that fluxes need to be more aggressive and to provide a physical barrier.[17] Traditionally borax was used as a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides [18]  as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.

Smelting[edit]Main article: Smelting § Fluxes

A related use of flux is to designate the material added to the contents of a smelting furnace or a cupola for the purpose of purging the metal of impurities, and of rendering the slag more liquid. The flux most commonly used in iron and steel furnaces is limestone, which is charged in the proper proportions with the iron and fuel. The slag is a liquid mixture of ash, flux, and other impurities.

Flux recovery[edit]

During the submerged arc welding process, not all flux turns into slag. Depending on the welding process, 50% to 90% of the flux can be reused.[19]

Metal salts as flux in hot corrosion[edit]

Hot corrosion can affect gas turbines operating in high salt environments (e.g., near the ocean). Salts, including chlorides and sulfates, are ingested by the turbines and deposited in the hot sections of the engine; other elements present in fuels also form salts, e.g. vanadates. The heat from the engine melts these salts which then can flux the passivating oxide layers on the metal components of the engine, allowing corrosion to occur at an accelerated rate.

List of fluxes[edit]

Borax  - for brazing Beeswax Tallow  and lead Paraffin wax Palm oil Zinc chloride  ("Killed Spirits") Zinc chloride and sal ammoniac Olive oil  and sal ammoniac - for iron

Rosin , tallow, olive oil, and zinc chloride - for aluminium Cryolite Cryolite and phosphoric acid Phosphoric acid & alcohol Cryolite and barium chloride Oleic acid Lithium chloride Magnesium chloride Sodium chloride Potassium chloride Unslaked lime

Tech 045 Safety & Allowances Tips   What is an "ergonomic injury"?

Input from the recent ergonomics forums demonstrated to OSHA that there are a wide variety of opinions on how the Agency should define an ergonomic injury and that the definition adopted by OSHA depends on the context. Ergonomic injuries are often described by the term "musculoskeletal disorders" or "MSDs." This is the term of art in scientific literature that refers collectively to a group of injuries and illnesses that affect the musculoskeletal system; there is no single diagnosis for MSDs. As OSHA develops guidance material for specific industries, the agency may narrow the definition as appropriate to address the specific workplace hazards covered. OSHA will work closely with stakeholders to develop definitions for MSDs as part of its overall effort to develop guidance materials.

 

Specific Allowances for Tech 045 Projects The following facilities layout safety-related allowances are not all-

inclusive, but are provided to help you in your layout assignments in Tech 045. Refer to your books, other appropriate materials, or consult the instructor for any allowances not included here.

1) Work height = elbow height +/- 2 inches 2) Operator space = 3 feet X 3 feet (or 3 feet X width of workstation) 3) Operator space should be 3 feet off the aisle for safety 4) Having 3 feet from side to side of operator allows parts to be placed

comfortably next to the operator

5) If two people are working back to back then allow 5 feet between stations

6) Workstation square footage = length X width of workstation 7) Allowance for aisles, work in process etc. = workstation space X

150% 8) Width of aisles running into trailers = 8 feet 9) A semitrailer = 8 feet X 40 feet 10) Small offices for clerks = 100 square feet per person 11)  Access foe machine maintenance = 2 feet around the machine 12)  Parking for small cars = 8 feet  X 15 feet per car 13)  Paring for medium car = 9 feet X 17.5 feet per car 14)  Parking for large cars = 10 feet X 20 feet per car 15)  Single lane driveways = 11 feet wide 16)  Double lane drive ways = 22 feet wide 17)  A parking lot = 250 square feet per number of parking spaces

needed 18)  Personnel and security offices = 200 square feet per office 19)  About one personnel person per 100 employees 20)  About 1 security person per 300 employees 21)  Locker rooms = 4 square feet per employee 22)  Allow one toilet for every 20 employees 23)  One toilet = 15 square feet 24)  One washbasin = 15 square feet 25)  Entryway for toilet = 15 square feet 26)  One urinal = 9 square feet 27)  The number of washbasins = number of toilets 28)  Restroom should not be farther than 200 feet from employees 29)  Lunchrooms = 10 square feet per person 30)  Drinking fountains should not be farther than 200 feet from

employees 31)  Each drinking fountain = 3 feet X 5 feet, or 15 square feet 32)  One way aisle for fork trucks = 4 feet wide 33)  Two-way aisle for fork trucks = 10 or (4+4+2) feet 34)  First aid rooms = 6 feet X 6 feet per room 35)  500 employees justifies one nurse 36)  One nurse would require a 400-square foot area for patients’ needs

37)  Lounges are 25 square feet per driver or user 38)  Locker rooms should be = 4 square feet per employee 39)  Warehouse size = size of product X quantity manufactured daily X

number of days’ supply 40)  tool room or maintenance room = sum of all equipment in it X 200% 41)  Office space needs = number of office personnel X 20 square feet 42)  Office space for general managers & senior executives = 200 –300

square feet 43)  Office space for managers = 150 – 250 square feet 44)  Office space for supervisors = 100 – 200 square feet 45)  Office space for accountants = 75 – 150 square feet 46)  Office space for engineers 100 – 150 square feet 47)  Office space for clerks = 75 – 100 square feet 48)  Overall office space determination = 200 square feet X number of

personnel 49)  For desks in one row, there should be 6 feet from the front of one

desk to the front of the one behind it 50)  For desks in two or more where ingress and egress are confined to

one side, 7 feet should be allowed from the front of one desk to the front of the desk behind it

51)  If employees are back to back, allow a minimum of 4 feet between chairs

52)  Inside aisles within desk areas should be from 3 to 5 feet wide 53)  Intermediate aisles = 4 feet wide 54)  Main aisles = 5 feet wide 55)  A workspace consisting of a desk, chair, and shelf space = 50 – 75

square feet with a 2-foot allowance on the length and width 56)  Reception area = 10 square feet per visitor 57)  Private offices = 100 – 300 square feet per office 58)  The scale of layout = ¼” per foot or 1/8” per foot 59)  Desks should face the same direction 60) Smaller aisles for office layout = 3 – 5 feet 61)  Larger aisles for office layout = 6 – 8 feet  

 

   

ntroduction to Material Handling Equipment

There are thousands of pieces of material handling devices. These equipments vary from the most basic manual too to the most sophisticated computer-controlled material handling systems that can incorporate a wide range of other manufacturing and control functions.

Handling and storing materials involve diverse operations such as hoisting tons of steel with a crane; driving a truck loaded with concrete blocks; carrying bags or materials manually; and stacking palletized bricks or other materials such as drums, barrels, kegs, and lumber. The efficient handling and storing of materials are vital to industry.

In addition to raw materials, these operations provide a continuous flow of parts and assemblies through the workplace and ensure that materials are available when needed. Material handling equipment (MHE) is used for the movement and storage of material within a facility or at a site. MHE can be classified into the following five major categories:

Transport Equipment.  Equipment used to move material from one location to another (e.g.,

between workplaces, between a loading dock and a storage area, etc.). The major subcategories

of transport equipment are conveyors, cranes, and industrial trucks. Material can also be

transported manually using no equipment.

Positioning Equipment.  Equipment used to handle material at a single location so that it is in

the correct position for subsequent handling, machining, transport, or storage. Unlike transport

equipment, positioning equipment is usually used for handling at a single workplace. Material can

also be positioned manually using no equipment.

Unit Load Formation Equipment.  Equipment used to restrict materials so that they maintain

their integrity when handled a single load during transport and for storage. If materials are self-

restraining (e.g., a single part or interlocking parts), then they can be formed into a unit load with

no equipment.

Storage Equipment.  Equipment used for holding or buffering materials over a period of time.

Some storage equipment may include the transport of materials (e.g., the S/R machines of an

AS/RS, or storage carousels). If materials are block stacked directly on the floor, then no storage

equipment is required.

Identification and Control Equipment.  Equipment used to collect and communicate the

information that is used to coordinate the flow of materials within a facility and between a facility

and its suppliers and customers. The identification of materials and associated control can be

performed manually with no specialized equipment.

Did you Know? Traditionally, material-handling equipment may be grouped into 4 general categories and

they are fixed-path system, the fixed area material handling system, variable-path variable-area

equipment and the lastly the fourth category consist of all the auxiliary tools. In the first category (fixed-

path system or also referred to as the continuous flow system) it consists of point-to-point equipments.

This group of equipment serves the material handling need along a fixed path (like on a guided track).

The most common and familiar example of a fixed path system are trains and railroad tracks as trains can

travel from any point to point, serving at any point that is along the track system. Other equipments under

this category include the conveyor systems, powered, gravity-fed, automated guided vehicles and any

other equipments that operate otherwise.

The second category is called the fixed area material handling system and this system can serve at any

point within the three dimensional area or a cub. Examples such as the jib crane or the overhead traveling

cranes can be installed on a floor pedestal to move parts and other material from any point to point in the

x, y, z direction: however, this ability is limited within confines of the equipment. Automated storage and

retrieval system (ASRS) also falls into this category.

AS for the third category, it consists of variable-material variable-area handling equipments or equipments

that can move to any area of the facility.

Examples such as motorized vehicles, forklift trucks and all manual carts can be dragged, driven or

pushed throughout the plant. Lastly, the fourth category consists of all auxiliary tools and equipments

such as pallets, skids, automatic data collection systems, and containers Haha :)

That is all for our introduction to MHE. Although there is still much more categories of equipment in the

industries, our group will be only focusing at the transport equipments and one of the positioning

equipment which is the hoist for this assignment.

Hope that it stirs some interest in you!! Keep a lookout for more of our detailed findings to each of the

equipments. Bye:)cya soon....

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The United States Code of Federal Regulations for Toilet Facilities  Content of  this page is for information only.  It is not official and non-relevant text has been removed  

     Please refer to the OHSA Website for the Official Regulations  

29 CFR 1910.141(c)(1)(i): Toilet Facilities 29 CFR 1910.141(c)(1)(i): Official Interpretation 29 CFR 1910.141(c)(1)(i): Unofficial Discussions 29 CFR 1926.51(c)(4): Sanitation 29 CFR 1926.51(c)(4): Interpretations

29 CFR 1910.141(c)(1)(i): Toilet Facilities

Scope. This section applies to permanent places of employment.

Definitions applicable to this section.

"Toilet facility," means a fixture maintained within a toilet room for the purpose of defecation or urination, or both.

"Toilet room," means a room maintained within or on the premises of any place of employment, containing toilet facilities for use by employees.

"Urinal" means a toilet facility maintained within a toilet room for the sole purpose of urination.

"Water closet" means a toilet facility maintained within a toilet room for the purpose of both defecation and urination and which is flushed with water.

(c) Toilet facilities--(1) General. (i) Except as otherwise indicated in this paragraph (c)(1)(i), toilet facilities, in toilet rooms separate for each sex, shall be provided in all places of employment in accordance with table J-1 of this section. The number of facilities to be provided for each sex shall be based on the number of employees of that sex for whom the facilities are furnished. Where toilet rooms will be occupied by no more than one person at a time, can be locked from the inside, and contain at least one water closet, separate toilet rooms for each sex need not be provided. Where such single-occupancy rooms have more than one toilet facility, only one such facility in each toilet room shall be counted for the purpose of table J-1.

Table J-1------------------------------------------------------------------------Minimum number of Number of employees water closets \1\------------------------------------------------------------------------

1 to 15.................................................... 116 to 35................................................... 236 to 55................................................... 356 to 80................................................... 481 to 110.................................................. 5.111 to 150................................................. 6Over 150................................................... (\2\)------------------------------------------------------------------------

Source: U.S. Department of Labor Occupational Safety and Health AdministrationInterpretation   of   29   CFR   1910.141 ( c )( 1 )(i):   Toilet   Facilities

Interpretation of 29 CFR 1910.141(c)(1)(i): Toilet Facilities Standards Interpretation and Compliance Letters

Record Type: Interpretation Standard Number: 1910.141 Subject: Interpretation of 29 CFR 1910.141(c)(1)(i): Toilet Facilities Information Date: 04/06/1998

April 6, 1998

 

MEMORANDUM FOR: REGIONAL ADMININSTRATORSSTATE DESIGNEES

FROM: JOHN B. MILES, JR., DirectorDirectorate of Compliance Programs

SUBJECT: Interpretation of 29 CFR 1910.141(c)(1)(i): Toilet Facilities

OSHA's sanitation standard for general industry, 29 CFR 1910.141(c)(l)(i), requires employers to provide their employees with toilet facilities:

Except as otherwise indicated in this paragraph (c)(l)(i), toliet facilities, in toilet rooms separate for each sex shall be provided in all places of employment in accordance with Table J-1 of this section .... [emphasis added]

This memorandum explains OSHA's interpretation that this standard requires employers to make toilet facilities

available so that employees can use them when they need to do so. The employer may not impose unreasonable restrictions on employee use of the facilities. OSHA believes this requirement is implicit in the language of the standard and has not previously seen a need to address it more explicitly. Recently, however, OSHA has received requests for clarification of this point and has decided to issue this memorandum to explain its position clearly.

Background

The sanitation standard is intended to ensure that employers provide employees with sanitary and available toilet facilities, so that employees will not suffer the adverse health effects that can result if toilets are not available when employees need them. Individuals vary significantly in the frequency with which they need to urinate and defecate, with pregnant women, women with stress incontinence, and men with prostatic hypertrophy needing to urinate more frequently. Increased frequency of voiding may also be caused by various medications, by environmental factors such as cold, and by high fluid intake, which may be necessary for individuals working in a hot environment. Diet, medication use, and medical condition are among the factors that can affect the frequency of defecation.

Medical studies show the importance of regular urination, with women generally needing to void more frequently than men. Adverse health effects that may result from voluntary urinary retention include increased frequency of urinary tract infections (UTIs), which can lead to more serious infections and, in rare situations, renal damage (see, e.g., Nielsen, A. Waite, W., "Epidemiology of Infrequent Voiding and Associated Symptoms," Scand J Urol Nephrol Supplement 157). UTIs during pregnancy have been associated with low birthweight babies, who are at risk for additional health problems compared to normal weight infants (see, Naeye, R.L., "Causes of the Excess Rates of Perinatal Mortality and the Prematurity in Pregnancies Complicated by Maternity Urinary Tract Infections," New England J. Medicine 1979; 300(15); 819-823). Medical evidence also shows that health problems, including constipation, abdominal pain, diverticuli, and hemorrhoids, can result if individuals delay defecation (see National Institutes of Health (NIH) Publication No. 95-2754, July 1995).

OSHA's field sanitation standard for Agriculture, 29 CFR 1928.110, based its requirement that toilets for farmworkers be located no more than a quarter mile from the location where employees are working on similar findings. This is particularly significant because the field sanitation standard arose out of the only OSHA rulemaking to address explicitly the question of worker need for prompt access to toilet facilities.

The Sanitation Standard

The language and structure of the general industry sanitation standard reflect the Agency's intent that employees be able to use toilet facilities promptly. The standard requires that toilet facilities be "provided" in every workplace. The most basic meaning of "provide" is "make available." See Webster's New World Dictionary, Third College Edition, 1988, defining "provide" as "to make available; to supply (someone with something);" Borton Inc. V. OSHRC, 734 F.2d 508, 510 (l0th Cir. 1984) (usual meaning of provide is "to furnish, supply, or make available");Usery v. Kennecott Copper Corp., 577 F.2d 1113, 1119 (10th Cir, 1978) (same); Secretary v. Baker Concrete Constr. Co., 17 OSH Cas. (BNA) 1236, 1239 (concurring opinion; collecting cases); Contractors Welding of Western New York, Inc., 15 OSH Cas. (BNA) 1249, 1250 (same).1 Toilets that employees are not allowed to use for extended periods cannot be said to be "available" to those employees. Similarly, a clear intent of the requirement in Table J-1 that adequate numbers of toilets be provided for the size of the workforce is to assure that employees will not have to wait in long lines to use those facilities. Timely access is the goal of the standard.

The quoted provision of the standard is followed immediately by a paragraph stating that the toilet provision does not apply to mobile work crews or to locations that are normally unattended, "provided the employees working at these locations have transportation immediately available to nearby toilet facilities which meet the other requirements" of the standard (29 CFR 1910.141(c)(1)(ii) (emphasis supplied). Thus employees who are members of mobile crews, or who work at normally unattended locations must be able to leave their work location

"immediately" for a "nearby" toilet facility. This provision was obviously intended to provide these employees with protection equivalent to that the general provision provides to to employees at fixed worksites. Read together, the two provisions make clear that all employees must have prompt access to toilet facilities.

OSHA has also made this point clear in a number of letters it has issued since the standard was promulgated. For example, in March 1976, OSHA explained to Aeroil Products Company that it would not necessarily violate the standard by having a small single-story building with no toilet facilities separated by 90 feet of pavement from a building that had the required facilities, so long as the employees in the smaller building had "unobstructed free access to the toilet facilities." Later that year, it explained again, in response to a question about toilet facilities at a U-Haul site, "reasonableness in evaluating the availability of sanitary facilities will be the rule." Again in 1983, OSHA responded to a request for a clarification of the standard by stating, "([i]f an employer provides the required toilet facilities ... and provides unobstructed free access to them, it appears the intent of the standard would be met."

In light of the standard's purpose of protecting employees from the hazards created when toilets are not available, it is clear that the standard requires employers to allow employees prompt access to sanitary facilities. Restrictions on access must be reasonable, and may not cause extended delays. For example, a number of employers have instituted signal or relief worker systems for employees working on assembly lines or in other jobs where any employee's absence, even for the brief time it takes to go to the bathroom, would be disruptive. Under these systems, an employee who needs to use the bathroom gives some sort of a signal so that another employee may provide relief while the first employee is away from the work station. As long as there are sufficient relief workers to assure that employees need not wait an unreasonably long time to use the bathroom, OSHA believes that these systems comply with the standard.

Citation Policy

Employee complaints of restrictions on toilet facility use should be evaluated on a case-by-case basis to determine whether the restrictions are reasonable. Careful consideration must be given to the nature of the restriction, including the length of time that employees are required to delay bathroom use, and the employer's explanation for the restriction. In addition, the investigation should examine whether restrictions are general policy or arise only in particular circumstances or with particular supervisors, whether the employer policy recognizes individual medical needs, whether employees have reported adverse health effects, and the frequency with which employees are denied permission to use the toilet facilities. Knowledge of these factors is important not only to determine whether a citation will be issued, but also to decide how any violation will be characterized.

It is important that a uniform approach be taken by all OSHA offices with respect to the interpretation of OSHA's general industry sanitation standard, specifically with regard to the issue of employee use of toilet facilities. Proposed citations for violations of this standard must be forwarded to the Directorate of Compliance Programs (DCP) for review and approval. DCP will consult with the Office of Occupational Medicine. DCP will approve citations if the employer's restrictions are clearly unreasonable, or otherwise not in compliance with the standard.(NOTE: See 08/11/00 Memorandum to RAs attached below.)---Added this note

State Plan States are not required to issue their own interpretation in response to this policy, however they must ensure that State standards and their interpretations remain "at least as effective" as the Federal standard. Regional Administrators shall offer assistance to the States on this issue, including consultation with the Directorate of Compliance programs, at the State's request.

If you have any questions, contact Helen Rogers in the Office of General Industry Compliance at (202) 219-8031/41 x106.

_______________________________________________________

Footnote(1) This decision was later vacated pursuant to a settlement, but the Commission has continued to cite it.See Secretary v. Baker Concrete Constr. Co., supra. The issue in Contractors Welding and the other cited cases has been whether the meaning of the term "provide," in various standards requiring employers to provide certain equipment or other materials, is not limited to making something available, but may also mean that the employer must pay for what it provides and must require it to be used. Those broader meanings are not relevant to this issue, however, where the sanitary facilities the employer is required to provide are a physical part of its workplace, and the question is not whether employees must be required to use those facilities, but whether they will be allowed to do so. (Back to Text)

_______________________________________________________

August 11, 2000---Added this memo

 

MEMORANDUM FOR: REGIONAL ADMININSTRATORS

FROM: RICHARD E. FAIRFAX, DirectorDirectorate of Compliance Programs

SUBJECT: Interpretation of 29 CFR 1910.141(c)(1)(i): Toilet Facilities

On April 6, 1998 we issued an interpretation of 1910.141(c)(1)(i), which requires employers to make toilet facilities available so that employees can use them when they need to do so. A copy of that memorandum is attached.

The 1998 memorandum states that proposed citations for violations of this standard are to be forwarded to the Directorate of Compliance Programs (DCP) for review and approval. Shortly after the interpretation was issued, it was decided that the review and approval was to be at the Regional Office level, but that copies of any citations issued based on the April 6, 1998 interpretation should still be sent to DCP.

This topic continues to generate interest from the public. Early this year we had a Freedom of Information Act (FOIA) request for copies of citations issued. Therefore, please continue to send copies of any citations issued pursuant to the 1998 interpretation to the National Office. If you have any questions, please contact Helen Rogers at (202) 693-1850. The copies should be sent to the following address:

Richard E. Fairfax, DirectorDirectorate of Compliance ProgramsU.S. Department of Labor - OSHA200 Constitution Avenue, NW Room N-3603Washington, DC 20210

Source: Interpretation   of   29   CFR   1910.141 ( c )( 1 )(i):   Toilet   Facilities

Memorandum from John B Miles Jnr. OSHA Directorate of Compliance Programs. OSHA Standards Interpretation and Compliance Letters. Interpretation of 29 CFR 1910.141(c)(I)(I): Toilet facilities. 6 April 1998. 

From Hazards and Workers' Health International NewsletterPO Box 199 Sheffield S1 4YL Englandhttp://www.hazards.org/toiletbreaks.htm

Please relieve me, let me go...

The US pee-breaks memo from John B Miles Jr, head of OSHA's Directorate of Compliance Programs, explained that the standard is necessary "so that employees will not suffer the adverse health effects that can result if toilets are not available when employees need them...

"Medical studies show the importance of regular urination with women generally needing to void more frequently than men. Adverse health effects that may result from voluntary urinary retention include increased frequency of urinary tract infections (UTIs), which can lead to more serious infections and, in rare situations, renal damage. UTIs during pregnancy have been associated with low birthweight babies, who are at risk of for additional health problems compared to normal weight infants.

"Medical evidence also shows that health problems, including constipation, abdominal pain, diverticuli, and haemorrhoids, can result if individuals delay defecation."

UK health and safety law does not contain a specific right to go, but this is implied in employers' general duties to protect the health, safety and welfare of employees in the Health and Safety at Work etc Act 1974, regulation 2. The Workplace (Health, Safety and Welfare) Regulations 1992 require suitable and sufficient, clean and adequately ventilated and lit sanitary conveniences at readily accessible places. Pregnant women have additional rights (Hazards 63).

 

From the Washington Insider: OSHA clarifies ruling about restroom breakshttp://www.aphanet.org/stat/restroom.html

In 1974, the U.S. Occupational Safety and Health Administration (OSHA) released a sanitation standard that required employers to provide their employees with toilet facilities and provided specifications for those facilities. At the time, OSHA believed that the standard was self-explanatory; however, on April 6 of this year, the agency issued another memo-to clarify the intent of the standard as it pertains to allowing workers to use the restroom facilities. The OSHA Sanitation Standard was intended to ensure that employers provide employees with available, sanitary toilet facilities. OSHA issued the interpretive memo to ensure that employees do not experience adverse health effects (urinary tract infection, renal damage, constipation, abdominal pain, hemorrhoids, diverticula) that can occur when they are unable to use (or discouraged from using) the restroom when necessary.

 

http://www.uiowa.edu/~ournews/1998/april/0409osha.html

CONTACT: SCOTT HAUSER100 Old Public LibraryIowa City IA 52242(319) 384-0007; fax (319) 384-0024e-mail: scott-hauser@uiowa.edu

Release: Immediate

OSHA agrees with UI professors: Workers have right to use the bathroom

IOWA CITY, Iowa -- Federal regulations that say employers must provide toilet facilities for their workers also mean that workers must be permitted to use the bathroom when they need to at work, according to new guidelines issued this week by regulators to clarify an issue that two University of Iowa professors raised with OSHA and have been closely watching.

John B. Miles Jr., director of compliance programs for the U.S. Occupational Safety and Health Administration (OSHA), issued a four-page memorandum Monday, April 6 to regional administrators of the agency, spelling out the proper way to interpret a federal rule that says "toilet facilities, in toilet rooms separate for each sex, shall be provided in all places of employment."

"This memorandum explains OSHA's interpretation that this standard requires employers to make toilet facilities available so that employees can use them when they need to do so," the memo says. "The employer may not impose unreasonable restrictions on employee use of the facilities." ...

"Recently, however, OSHA has received requests for clarification of this point and has decided to issue this memorandum to explain its position clearly," Miles wrote.

Marc Linder, professor of law, and Ingrid Nygaard, associate professor of obstetrics and gynecology, initiated OSHA's reexamination of its position on access to bathrooms for workers.

They are the authors of a new book, "Void Where Prohibited: Rest Breaks and the Right to Urinate on Company Time" (Cornell University Press), which argues that many workers are not allowed to go to the bathroom when they need to because of a lack of regulatory enforcement, callousness by some employers who pay little attention to worker rights, and miserly planning in some work environments.

The restrictions lead to personal indignities and health problems for many employees.

Linder has made frequent appeals to state and federal regulators to clarify the regulations.

"One of the points we make in the book is that the 25-year-old standard requiring employers to provide toilets makes no sense unless it includes the obligation to let workers use those toilets," Linder says. "But until OSHA was confronted with the research in the book, and with strong appeals from the United Food and Commercial Workers, the agency was unwilling to enforce the law in a way that guaranteed employees the right to use the bathroom at work." 

  Regulations (Standards - 29 CFR) Sanitation. - 1926.51 

OSHA Regulations (Standards - 29 CFR) - Table of Contents

Standard Number: 1926.51 Standard Title: Sanitation. SubPart Number: D SubPart Title: Occupational Health and Environmental Controls

(c) "Toilets at construction jobsites."

(c)(1) Toilets shall be provided for employees according to the following table:

Table D-1

______________________________________________________________ Number of employees 20 or less.......| 1 20 or more.......| 1 toilet seat and 1 urinal per 40 workers. 200 or more......| 1 toilet seat and 1 urinal per 50 workers._________________|____________________________________________

(c)(2) Under temporary field conditions, provisions shall be made to assure not less than one toilet facility is available.

(c)(3) Job sites, not provided with a sanitary sewer, shall be provided with one of the following toilet facilities unless prohibited by local codes:

(c)(3)(i) Privies (where their use will not contaminate ground or surface water);

..1926.51(c)(3)(ii)

(c)(3)(ii) Chemical toilets;

(c)(3)(iii) Recirculating toilets;

(c)(3)(iv) Combustion toilets.

(f)(3) "Lavatories."

(f)(3)(i) Lavatories shall be made available in all places of employment. The requirements of this subdivision do not apply to mobile crews or to normally unattended work locations if employees working at these locations have transportation readily available to nearby washing facilities which meet the other requirements of this paragraph.

(f)(3)(ii) Each lavatory shall be provided with hot and cold running water, or tepid running water.

(f)(3)(iii) Hand soap or similar cleansing agents shall be provided.

(f)(3)(iv) Individual hand towels or sections thereof, of cloth or paper, warm air blowers or clean individual sections of continuous cloth toweling, convenient to the lavatories, shall be provided.

 OSHA Regulations (Standards - 29 CFR) - Table of Contents

OSHA Standard Interpretations 06/07/2002 - Mobile crews must have prompt access to nearby toilet facilities.

June 7, 2002

[ DELETED TEXT ]

Re: §1926.51(c)(4); sanitation, mobile crews

This is in response to your letter of October 1, 2001, to the Occupational Safety and Health Administration (OSHA) in which you ask for an interpretation of the construction sanitation standard, specifically the provision pertaining to mobile crews. We apologize for the long delay in providing this response.

Question: The requirements of §1926.51(c) for sanitation facilities states that they do not apply to mobile crews "having transportation readily available to nearby toilet facilities." What does "nearby" mean?

Answer:Construction sanitation standard; mobile crews

OSHA's construction sanitation standard is codified at 29 CFR 1926.51. Paragraph (c) of §1926.51, "Toilets at construction jobsites," states:

Toilets shall be provided for employees according to the following table:

Table D-1

 

employees Minimum number of facilities

20 or less . .  . 1.

20 or more . .. . 1 toilet seat and 1 urinal per 40 workers.

200 or more . . . 1 toilet seat and 1 urinal per 50 workers.

However, as you are aware, §1926.51(c)(4) makes the provisions of paragraph (c) inapplicable to "mobile crews 

having transportation readily available to nearby toilet facilities."

When determining whether paragraph (c)(4) applies to a work crew, employers must evaluate the nature of the site and job functions of the crew. Workers who continually or frequently move from jobsite to jobsite on a daily or hourly basis would be considered a "mobile crew." Workers who report to a conventional construction project, where they work for more extended periods of time (days, weeks, or longer), would not be considered a "mobile crew" for purposes of the sanitation standard.

Definition of "nearby"

As noted above, the requirements of (c)(4) are inapplicable when a mobile crew has transportation readily available to "nearby" toilet facilities. As explained below, for purposes of this standard, "nearby" means prompt access -- sufficiently close so that employees can use them when they need to do so.

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Mobile crew employees relying on "nearby" facilities must be afforded access equivalent to that provided by the general provision for employees at fixed worksites. Read together, the two provisions make clear that mobile crews must have prompt access to nearby toilet facilities. For example, in general, toilets would be considered "nearby" if it would take less than 10 minutes to get to them. 

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SOURCE:www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24369

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WARNING AND DISCLAIMER: The American Restroom Association (ARA) is formally incorporated in the State of Maryland and is a subsidiary of the International Paruresis Association  ARA is not qualified to provide legal advice. This web site contains non-vetted information that is un-official and for education only. There are no formal or financial agreements with any persons or entities cited. Some material is from copyrighted sources. This material is for education only and it must be source referenced.

Number of toilets required in buildingsHow many toilets are required for employees who work inside buildings?

The requirements for toilet pans and urinals in buildings are set out in the New Zealand Building Code, which is part of the Building Regulations 1992, administered by the Department of Building and Housing. The requirements depend on the building use, including whether the toilets will be used by employees only, or by members of the public as well.The following tables set out the numbers to be provided in commercial and industrial premises used by employees only.  Refer to the Building Code Approved Document Clause G1: Personal Hygiene for further details and requirements in other situations.

Unisex toilet facilities

Design occupancy Number1–5 1

6–30 2

Greater than 30 add 1 per 40

Separate toilet facilities – females

Design occupancy Number1–10 1

11–50 2

51–90 3

Greater than 90 add 1 per 60

Separate toilet facilities – males (toilet pans only)

Design occupancy Number1–10 1

11–50 2

51–90 3

Greater than 90 add 1 per 60

Separate toilet facilities – males (toilet pans and urinals)

Toilet pans Urinals

Design occupancy Number Design occupancy

Number

1–10 1 1–150 1

11–60 2 151–550 2

61–120 3 Greater than 550 add 1 per 450

Greater than 120 add 1 per 80    

Date Modified: Wednesday, 9 December 2009

Disclaimer: The content on this website covers common problems. It will not answer every question and should not be used as a substitute for legislation or legal advice.State sector employers and employees may be affected by some differences in the laws that apply to them (e.g. State Sector Act 1988).The Department of Labour takes no responsibility for the results of any actions taken on the basis of information on this website, nor for any errors or omissions.

      Chapter 18: Managing Inventory 

Review

1. Explain the various inventory control systems and the advantages and disadvantages of each.

o Inventory represents the largest investment for the typical small business. Unless properly managed, the cost of inventory will strain the firm’s budget and cut into its profitability. The goal of inventory control is to balance the cost of holding and maintaining inventory with meeting customer demand.

o Regardless of the inventory control system selected, business owners must recognize the relevance of the 80/20 rule, which states that roughly 80 percent of the value of the firm’s inventory is in about 20 percent of the items in stock. Because only a small percentage of items account for the majority of the value of the firm’s inventory, managers should focus control on those items.

o Three basic types of inventory control systems are available to the small business owner: perpetual, visual, and partial. Perpetual inventory control systems are designed to maintain a running count of the items in inventory. Although they can be expensive and cumbersome to operate by hand, affordable computerized point-of-sale (POS) terminals that deduct items sold from inventory on hand make perpetual systems feasible for small companies. The visual inventory system is the most common method of controlling merchandise in a small business. This system works best when shortages are not likely to cause major

problems. Partial inventory control systems are most effective for small businesses with limited time and money. These systems operate on the basis of the 80/20 rule.

o The ABC system is a partial system that divides a firm’s inventory into three categories depending on each item’s dollar usage volume (cost per unit multiplied by quantity used per time period). The purpose of classifying items according to their value is to establish the proper degree of control over them. A items are most closely controlled by perpetual inventory control systems; B items use basic analytical tools; and C items are controlled by very simple techniques such as the two-bin system, the level control method, or the tag system.

1. Describe how just-in-time (JIT) and JIT II inventory control techniques work.

o The just-in-time system of inventory control sees excess inventory as a blanket that masks production problems and adds unnecessary costs to the production operation. Under a JIT philosophy, the level of inventory maintained is the measure of efficiency. Materials and parts should not build up as costly inventory. They should flow through the production process without stopping, arriving at the appropriate location just in time.

o JIT II techniques focus on creating a close, harmonious relationship with a company’s suppliers so that both parties benefit from increased efficiency. To work successfully, JIT II requires suppliers and their customers to share what was once closely guarded information in an environment of trust and cooperation. Under JIT II, customers and suppliers work hand in hand, acting more like partners than mere buyers and sellers.

3. Describe some methods for reducing loss from slow-moving inventory.o Managing inventory requires monitoring the company’s inventory

turnover ratio; slow-moving items result in losses from spoilage or obsolescence.

o Slow-moving items can be liquidated by markdowns, eye-catching displays, or quantity discounts.

4. Discuss employee theft and shoplifting and how to prevent them.o Employee theft accounts for the majority of business losses due to theft.

Most small business owners are so busy managing their companies’ daily affairs that they fail to develop reliable security systems. Thus, they provide their employees with prime opportunities to steal.

o The organizational atmosphere may encourage employee theft. The owner sets the organizational tone for security. A complete set of security controls, procedures, and penalties should be developed and enforced. Physical breakdowns in security invite employee theft. Open doors and windows, poor key control, and improper cash controls are major contributors to the problem of employee theft. Employers can build security into their businesses by screening and selecting employees carefully. Orientation programs also help the employee to get started in the right direction. Internal controls, such as division of responsibility, spot checks, and audit procedures, are useful in preventing employee theft.

o Shoplifting is the most common business crime. Fortunately, most shoplifters are amateurs. Juveniles often steal to impress their friends, but prosecution can halt their criminal ways early on. Impulse shoplifters steal because the opportunity suddenly arises. Simple prevention is the

best defense against these shoplifters. Alcoholics, vagrants, and drug addicts steal to supply some need and are usually easiest to detect. Kleptomaniacs have a compelling need to steal. Professionals are in the business of theft and can be very difficult to detect and quite dangerous.

o Three strategies are most useful in deterring shoplifters. First, employees should be trained to look for signs of shoplifting. Second, store layout should be designed with theft deterrence in mind. Finally, anti-theft devices should be installed in the store.

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Lean Six Sigma to Reduce Excess and Obsolete Inventory

James W. Martin February 26, 2010 3Excess and obsolete inventory write-offs are chronic supply chain problems costing businesses billions of dollars

each year. Unfortunately, improvement projects that are deployed to eliminate these problems often have a short-

term focus. In other words, the current levels of excess and obsolete inventory are usually addressed, but not the root

causes of the problem. Often such inventory is reduced by selling it below standard cost or donating it to charitable

organizations. Competing business priorities sometimes keeps businesses from developing effective long-term

solutions to eliminate the root causes, sometimes it is the difficulty in unraveling the complexity of the root causes.

Lean Six Sigma methods have been shown to be very effective in finding and eliminating root causes, and thus

preventing arbitrary year-end reductions in inventory investment.

Higher- and Lower-Level Root CausesAn analysis of excess and obsolete inventory often shows that its major root causes are associated with long lead

times, poor forecasting accuracy, quality problems or design obsolescence. However, these higher-level causes can

be successively broken down into lower-level root causes as shown in the figure below.

As the figure suggests, from an inventory investment perspective, a long lead time may be caused, in part, by large

lot sizes. For example, if the actual lead time or order cycle time is 30 days, but the required lot size for purchase is

90 days of supply (DOS), then this lot size drives a higher average inventory level than lead time by itself. In this

case, the average on-hand inventory (neglecting a safety-stock calculation) increases from 15 to 45 DOS assuming a

constant usage rate. Of course, the actual reasons for large lot sizes would have to be investigated by a Lean Six

Sigma improvement team. The root causes of long lead times also could be due to complicated processes having

numerous rework loops and non-value-adding operations as well as scheduling problems and/or late deliveries.

The second major cause of excess and obsolete inventory is poor demand management practices. Some lower-level

root causes may include inaccurate historical demand data, a poor forecasting modeling methodology or other issues

such as overly optimistic sales projections. Lean Six Sigma projects also can be used to attack lower-level root

causes in this area. Lean Six Sigma is frequently used to improve quality levels to reduce waste and rework caused

by a multitude of diverse factors within a process workflow. Finally, Design for Six Sigma can be used to improve the

design processes for new products or services.

Using DMAIC to Find Root CausesLean Six Sigma improvement teams can drive to the root causes of their excess and obsolete inventory problem

using the DMAIC problem-solving methodology (Define, Measure, Analyze, Improve, Control) in conjunction with

Lean tools as well as process workflow models. In fact, building simple Excel-based inventory models or using off-

the-shelf software, are good ways to identify the key process input variables (KPIVs) or drivers of excess and

obsolete inventory problems. Inventory models follow a generalized Six Sigma root-cause philosophy  � Y = f(x). They

also are effective communication vehicles showing sales, marketing, manufacturing and other supply chain functions,

as well as the impact of lead time and demand management practices on excess and obsolete inventory.

In an actual improvement project, the team begins an inventory analysis by defining the project’s goals in the Define

phase. Using these goals as guidelines, relevant questions are developed to enable the team to understand how the

system operates. Data fields corresponding to these questions are identified and extracted from information

technology (IT) systems. The data fields are then organized in the form of an inventory model to provide the

information necessary to answer the team’s questions and understand the root causes of the inventory problem.

After the Define phase, the team begins to evaluate measurement systems and plan data collection activities. This is

the Measure phase of the project. An important activity in this phase is an on-site physical count by location of

inventoried items associated with the problem. This is done to measure valuation accuracy relative to stated book

value. Measurement analyses also are conducted of management reports and their related workflow systems. These

analyses determine the accuracy of key supply chain metrics such as lead time, lot size, expected demand and its

variation, forecasting accuracy (different from demand variation), on-time delivery and other metrics that may be

related to an inventory investment problem. Unfortunately, supply chain metrics often are scattered across the

several software systems within an organization. These systems include the forecasting module, master production

schedule module, materials requirements planning module, inventory record files, warehouse management system

module and similar IT systems.

After verification of a system’s metrics, the improvement team begins data collection to capture information necessary

to answer the team’s questions developed during the Define phase. Relevant information, which may help the team in

its root-cause investigation, usually includes suppliers, lead times, expected demand and its variation, lot sizes,

storage locations, delivery information, customers and other facts.

Analyzing Data and Using Inventory ModelThe Analyze phase begins after the required data has been collected and a simple inventory model has been created

using classic inventory formulas such as those found in operations management textbooks. These models are used

to analyze an inventory population to understand how key process input variables impact excess and obsolete

inventory investment (i.e., key process output variables). A value stream map also should be constructed as part of

the overall analysis. In fact, in many projects, a value stream map, once quantified, becomes the basis for the

inventory model. This is especially true when the analyses focus on internal process workflows, at system

bottlenecks, rather than finished goods inventories.

In addition, a simple inventory balance is calculated for every item and location of the inventory population based on

each item’s service level, lead time and demand variation. An inventory balance shows which items and locations

may have too much inventory and which items and locations may have too little inventory. In the latter case, inventory

investment must be temporarily increased to meet required customer service levels.

After the team determines the root causes of the excess or obsolete inventory problem, it develops countermeasures

to eliminate these root causes – the project’s Improve phase. In addition, other needs for improvement may be found

as a project winds toward the Improve phase. The Analyze phase often identifies other types of process breakdowns

within the supply chain that may serve as a justification for subsequent improvement projects. Lean tools and

methods are particularly important in the analysis and execution of these types of projects. In fact, the application of

the Lean tool, 5S, or what can loosely be called housekeeping, in the Control phase of a project, can help ensure that

the resultant improvements are sustained over time.

Conclusion: The Typical Project BenefitsTypical benefits of defining and implementing improvement projects to reduce and eliminate excess and obsolete

inventory include higher system accuracy, creation of quantified inventory models showing relationships between

inventory investment versus lead time and demand variation, higher inventory valuation and location accuracies,

higher cycle counting accuracies, and  most importantly permanent reductions in excess and obsolete inventory � �investment.