Material Science Topics

8/8/2019 Material Science Topics

1/16

Selection of Materials

?? Formed shape and dimension

?? Maintain shape in service

?? Maintain properties during use

?? Compatible with other parts: How joined

?? Recycling properties

?? Interaction with the environment

?? Economically made into useful product

Type of Materials

Metals :

?? Aluminium, magnesium, Zinc, copper, lead, iron, titanium, nickel etc

(see periodic table)

?? Good electrical and thermal properties

?? High strength

?? High stiffness?? Ductility and formability

?? Shock resistance

?? Structural and load bearing applications

?? Use of alloys

Ceramics:?? Brick, glass, refractories, Abrasives

?? Low electrical and thermal conductivity

?? Insulators

?? Strong and hard


2/16

?? Brittle

?? May be used for load bearing applications

?? Turbine blades (high temperature applications)

Polymers:

?? Large molecular structures of organic molecules (C and H)

?? Polymerisation

?? Rubber, plastics, adhesives

?? Low electrical and thermal conductivity

?? Low strength?? Not suitable for high temperature applications

?? Thermoplastics and thermosets

Semiconductors:

?? Silicon, Ge, GaAs

?? Brittle?? Electronic components and communication applications

?? Controlled electrical conductivity

?? Transistors, diodes, ICs

Composite Materials

?? Concrete, plywood, fibre glass, Metal matrix composites (MMCs)

?? Two or more materials

?? Take advantage of differing properties

?? Light weight, very strong, stiff

?? May be used in high temperature applications


3/16

?? Shock resistant

Properties = f(structure and processing method)

?? Structure (Microstructure, atomic structure and bonding)

?? Processing method (grain structure, forming)


4/16

Mechanical and Physical properties

Structure of Materials

?? Atomic structure (Atoms and bonding)

?? Atomic arrangement (lattice/crystal structure)

?? Micro structure (grains, alloying)

?? Macro structure (surface properties)

Atomic Structure: The atom

Dalton Model (1808)

?? Matter is composed of tiny particles called atoms


5/16

?? Atoms can neither be subdivided nor changed into another

?? Atoms are incapable of being created or destroyed

?? Atoms of any pure substance are identical with one another in weight,

size, and other properties

Thomson Atomic model (1910)

?? Electron, proton, neutron

?? Charge carried by electrons and protons (1.6 E-19 C)

?? No. of electrons and no. of protons are equal

Bohrs Atomic model (particle like behaviour)

?? Electrons revolve around the nucleus in defined orbits

?? The centripetal force required for rotation is provided by electrostatic

attraction between the electrons and the nucleus

?? Electrons in specific orbits have defined amount of energy

(Quantised)?? Electrons can acquire energy and jump to higher energy levels (orbits)

Atomic number : number of electron or proton in each atom (basis of

periodic table)

Atomic mass : concentrated within the nucleus:

Mass of proton = mass of neutron = 1.67 E -24 g

Mass of electron = 9.11 E -28 g


6/16

Isotope: Same number of protons, different number of neutrons

Since mass of atom is very small: the concept of atomic mass unit One AMU = 1/12 of atomic mass of the most common isotope of Carbon 12.

Atomic weight : weighted average of the atomic masses of the atoms

naturally occurring isotopes (on the basis of AMU/atom)

One mole of any element contains 6.023 E 23 atoms or molecules

1 AMU/atom = 1 g/mol

e.g For iron: Atomic weight = 55.85 amu/atom

= 55.85 g/mol

See derivation of equations based on Bohrs atomic model

?? Velocity of electrons

?? Principle orbits

?? Kinetic and potential energy

?? Energy emission

?? Frequency and wavelength of emission (Lyman, Balmer and other

series)

Limitations of Bohrs model

?? Why certain spectral lines are more dense than others


7/16

?? Energy emission within same orbit

?? Cannot explain atoms with large number of electrons

?? Failed to render any quantitative explanation to chemical bonding

Quantum Mechanics (Wave like behaviour) : Concept of orbital and sub

orbitals1. Principal quantum numbers (K, L, M, N, O)

2. Orbital quantum numbers (s, p, d, e, f)

3. Magnetic quantum numbers (+ 1)

4. Spin quantum number (+)


8/16

Pauli Exclusion principle

No two electrons in an atom can have identical values for all the four

quantum numbers.

Electronic (Electron) Configuration in an atom (Distribution of electrons

in various sub shells)

1. Total number of electrons with the same principal quantum number nis 2n 2

2. In the nth shell, there are n sub shells having different values of orbital

quantum number such as 0, 1, 2, 3, (n-1)

3. Each sub shell can have a maximum of 2(2 l+1) electrons


9/16

Bonding

?? States of matter: Solid, Liquid and Gas

?? Primary Bonds: Ionic, Covalent and Metallic

?? Secondary bonds: Hydrogen bonding, dipoles, Van der Vaals


10/16

Ionic Bonding

Mechanism

Characteristics

?? Strong and hard?? High melting and boiling points

?? Generally brittle in nature and cannot be drawn into sheets or wires

?? Generally non conductors of electricity (melts may conduct)

?? Generally transparent

?? Polar liquids like water can dissolve ionic crystals

Covalent Bonding

Mechanism

Characteristics

?? Found in solid, liquid and gas

?? Directional in nature

??They are soft

?? Have low melting and boiling points

?? Do not conduct electric current

?? Soluble in non polar solvents e.g. benzene

Metallic Bonding

Mechanism

Characteristics

?? Good conductors

?? High thermal and electrical conductivity

?? Moderate to high melting temperature


11/16

?? Opaque

?? Crystalline in nature

Secondary bonds (weaker than primary bonds)?? Dipole

?? Hydrogen bonding


12/16

Lecture 2 Crystalline Structure of Solids

?? Regularity with which atoms or ions are arranged wrt one another

?? Crystalline material: repeating or periodic array over large interatomic

distances?? Applies to all metals, many ceramic materials, certain polymers.

Properties of Materials = f(crystalline structure)

Assumption

Atoms are solid spheres with well defined diameter which are arranged in a3D array of points: also referred to a lattice

Unit cell: Small repeating entity; it is the basic structural unit or building

block of a crystal.

Metallic crystalline structures

?? SC

?? BCC (Fe, Cr)

?? FCC (Fe, Pb, Cu, Al)

?? HCP (Zn)

Lattice Parameters

Bravais Lattice


13/16


14/16

Coordination Number : Number of nearest atoms which are directly

surrounding a given atom

SC 6FCC 12

BCC 8

Inter atomic spacing : half the distance between nearest neighbours in a pure

substance

Number of atoms per unit cell

??SC 1

?? FCC 4

?? BCC 2

Atomic Packing Factor ratio of volume of atoms per unit cell to volume of

unit cell

?? SC 0.52

?? BCC 0.68

?? FCC 0.74

Calculate density, inter atomic spacing etc

Points, Directions and Planes

?? Importance

?? Deformation along directions

Procedure

?? Coordinate System: Miller Indices


15/16


16/16

To denote planes in a crystal?? Find intercepts of the plane on the three axes?? Express them into axial units?? Take numerical parameters of the palne and find ratio of their reciprocals?? Convert these into whole numbers by multiplying with the LCM?? Obtain the miller indices of the plane expressed in (a, b, c)

Example

Notes on Miller indices?? Equally spaced parallel planes have same indices?? Plane parallel of one of the coordinate axes has an intercept of infinity and index

zero?? Miller indices do not define one plane but a set of parallel planes?? Ratio of indices is important (4 2 2) is same as (2 1 1)?? Plane passing through origin has non zero intercepts?? The normal to a plane having index (h k l) is direction [h k l]?? Distance between adjacent planes of a set of parallel planes of indices (h k l) is

given by?? Linear density : fraction of line length passing through atom centres

BCCFCC

?? Planar density :?? Angle between two direction [u1 v1 w1] and [u2 v2 w2]

Determining Crystal structure using X ray diffractio

Material Science Topics

Documents

Transcript of Material Science Topics