Basic Concepts (Structure of Solids)

Basic Concepts (Structure of Solids) Crystal Structure of materialsFCC: Ni, Cu, Ag, Pt, Au, Pb, Al (soft) BCC: V, Mo, Ta, W (hard material)HCP: Mg, ZnCobalt HCP < 4200C, FCC > 4200CChromium HCP < 20oC , BCC >20oCGlass- AmorphousBCC-Ferrite or - iron -ferrite or -ironFCC- Austenite or -iron

GATE 2011The crystal structure of austenite is(a) body centered cubic(b) face centered cubic(c) hexagonal closed packed(d) body centered tetragonalAns. (b) Austenite has FCC Crystal structure.

3IES 2011Match List I with List II and select the correct answer using the code given below the lists :

CodesA B C D A B C D(a) 1 4 3 2(b) 2 4 3 1(c) 1 3 4 2(d) 2 3 4 1

List I List IIA. Alpha iron1. FCCB. Zinc2. BCCC. Glass3. HCPD. Copper4. AmorphousAns. (d)4IES-2003Match List-I (Crystal Structure) with List-II (Example) and select the correct answer using the codes given below the Lists:List-IList-II(Crystal Structure)(Example)A.Simple Cubic1. ZincB.Body-centered Cubic2. CopperC.Face-centered Cubic3. Alpha iron at room temperatureD.Hexagonal Close Packed4. ManganeseCodes:ABCDABCD(a) 4312(b) 4321(c) 3421(d) 3412

Ans. (b)5IES-1998Match List-I with List-II and select the correct answer using the codes given below the lists:List-I List-II(Material) (Structure) A.Charcoal 1.F.C.CB.Graphite 2.H.C.PC.Chromium 3.AmorphousD.Copper 4.B.C.CCode:ABCDABCD(a)3214(b)3241(c)2341(d)2314

Ans. (b)6IES-2001Match List-I (Name of the Element) with List-II (Crystal Structure) and select the correct answer using the codes given below the lists:List IList IIA.Fluorspar1. Body-centered cubicB.Alpha-Iron2. Hexagonal closed packedC.Silver3. Simple cubicD.Zinc4. Face-centered cubicCodes:ABCDABCD(a) 3 2 4 1 (b) 4 132(c) 4 2 3 1 (d) 3 1 4 2

Ans. (d)7IES-2006Match List-I (Element) with List-II (Crystal Structure) and select the correct answer using the code given below the Lists:List - I List - IIA.Alpha Iron 1.Hexagonal closed packedB.Copper 2.Body-centred cubicC.Zinc 3.AmorphousD.Glass 4.Face-centred cubicCodes:ABC DAB C D(a) 2 3 1 4 (b) 1 4 2 3(c) 2 4 1 3 (d) 1 3 2 4

Ans. (c)8Plastic deformationFollowing the elastic deformation, material undergoes plastic deformation.Also characterized by relation between stress and strain at constant strain rate and temperature.Microscopically, it involves breaking atomic bonds, moving atoms, then restoration of bonds.Stress-Strain relation here is complex because of atomic plane movement, dislocation movement, and the obstacles they encounter.Crystalline solids deform by processes slip and twinning in particular directions.ContdAmorphous solids deform by viscous flow mechanism without any directionality.Because of the complexity involved, theory of plasticity neglects the following effects:Anelastic strain, which is time dependent recoverable strain.Hysteresis behavior resulting from loading and unloading of material.Bauschinger effect dependence of yield stress on loading path and direction.Equations relating stress and strain are called constitutive equations.ContdA true stress-strain curve is called flow curve as it gives the stress required to cause the material to flow plastically to certain strain.Because of the complexity involved, there have been many stress-strain relations proposed.

SlipSlip is the prominent mechanism of plastic deformation in metals. It involves sliding of blocks of crystal over one other along definite crystallographic planes, called slip planes.In physical words it is analogous to a deck of cards when it is pushed from one end.Slip occurs when shear stress applied exceeds a critical value. During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step, but the orientation of the crystal remains the same.Steps observable under microscope as straight lines are called slip lines.TwinningThe second important mechanism of plastic deformation is twinning.It results when a portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way. The twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry is called twinning plane. Each atom in the twinned region moves by a homogeneous shear a distance proportional to its distance from the twin plane. The lattice strains involved in twinning are small, usually in order of fraction of inter-atomic distance, thus resulting in very small gross plastic deformation. ContdThe important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.Twinning generally occurs when slip is restricted, because the stress necessary for twinning is usually higher than that for slip.Thus, some HCP metals with limited number of slip systems may preferably twin. Also, BCC metals twin at low temperatures because slip is difficult. Of course, twinning and slip may occur sequentially or even concurrently in some cases.

IES 2007What is the movement of block of atoms along certain crystallographic planes and directions, termed as?(a)Glide (b)Twinning(c)Slip(d)Jog

Ans. (c)15IES-2005The B.C.C. and H.C.P. metals undergo plastic deformation by:(a)Slip(b)Twinning(c)Edge dislocation(d)Twinning in combination with slip

Ans. (d)16IES-1998Assertion (A): Plastic deformation in metals and alloys is a permanent deformation under load. This property is useful in obtaining products by cold rolling.Reason (R): Plastic or permanent deformation in metal or alloy is caused by movement or dislocations.(a)Both A and R are individually true and R is the correct explanation of A(b)Both A and R are individually true but R is not the correct explanation of A (c)A is true but R is false(d)A is false but R is true

Ans. (c) The deformation of metals, which is caused by the displacement of the atom is achieved by one or both of the processes called slip and twinning. 17Atomic StructureAtoms consist of a relatively dense nucleus composed of positively charged protons and neutral particles of nearly identical mass, known as neutrons. Surrounding the nucleus are the negatively charged electrons, which have only 1/1839 times the mass of a neutron and appear in numbers equal to the protons, to maintain a net charge balance. The light electrons that surround the nucleus play a far more significant role in determining material properties. Again, experiments reveal that the electrons are arranged in a characteristic structure consisting of shells and subshells, each possessing a distinctive energy. Upon absorbing a small amount of energy, an electron can jump to a higher-energy shell farther from the nucleus. ContdThe reverse jump can also occur with the concurrent release of a distinct amount, or quantum, of energy.The number of electrons surrounding the nucleus of a neutral atom is called the atomic number.More important, however, are those electrons in the outermost shell or subshell, known as valence electrons. These are influential in determining chemical properties, electrical conductivity, some mechanical properties, the nature of interatomic bonding, atom size, and optical characteristics.Atomic BondsGeneral characteristics of materials joined by ionic bonds include moderate to high strength, high hardness, brittleness, high melting point, and low electrical conductivity. A second type of primary bond is the covalent type. Like the ionic bond, the covalent bond tends to produce materials with high strength and high melting point.Atom movement within the framework material (plastic deformation) requires the breaking of discrete bonds, thereby making the material characteristically brittle.Electrical conductivity depends on bond strength, ranging from conductive tin (weak covalent bonding), through semiconducting silicon and germanium, to insulating diamond (carbon).Engineering materials possessing ionic or covalent bonds tend to be ceramic (refractories or abrasives) or polymeric in nature.

ContdA third type of primary bond can form when a complete outer shell cannot be formed by either electron transfer or electron sharing. This bond is known as the metallic bond.If there are only a few valence electrons (one, two, or three) in each of the atoms in an aggregate, these electrons can easily be removed while the remainder are held firmly to the nucleus. These highly-mobile,"free" electrons account for the high electrical and thermal conductivity values as well as the opaque property (free electrons can absorb the discrete energies of light radiation) observed in metals. Moreover, they provide the "cement" required for the positive-negative-positive attractions that result in bonding. Bond strength, and therefore material strength, varies over a wide range. ContdMore significant, however, is the observation that the positive ions can move within the structure without the breaking of discrete bonds. Materials bonded by metallic bonds can therefore be deformed by atom-movement mechanisms and produce a deformed material that is every bit as strong as the original. This phenomenon is the basis of metal plasticity, ductility, and many of the shaping processes used in the fabrication of metal products.IES-2008Assertion (A): Elements are classified into metals and non-metals on the basis of their atomic weights.Reason (R): The valence electron structures contribute to the primary bonding between the atoms to form aggregates.(a)Both A and R are true and R is the correct explanation of A(b)Both A and R are true but R is NOT the correct explanation of A(c)A is true but R is false(d)A is false but R is true

Ans.(d)23IES-2003Assertion (A): Unlike in the case of ionic bonds, the co-ordination numbers for covalently bonded atoms are not controlled by the radii ratio.Reason (R): A covalent bond has a specific direction of bonding in space.(a)Both A and R are individually true and R is the correct explanation of A(b)Both A and R are individually true but R is not the correct explanation of A (c)A is true but R is false(d)A is false but R is true

Ans. (c)24IES 2011Solid material chemical bonds are :(a) Ionic, molecular and fusion(b) Covalent, fusion and fission(c) Ionic, covalent and molecular(d) Fission, molecular and ionicAns. (c)25Development of a grain structureWhen a metal solidifies, a small particle of solid forms from the liquid with a lattice structure characteristic of the given material. This particle then acts like a seed or nucleus and grows as other atoms in the vicinity attach themselves. The basic crystalline unit is repeated throughout space.In actual solidification, many nuclei form independently at various locations throughout the liquid and have random orientations with respect to one another. Each then grows until it begins to interfere with its neighbours.Since adjacent lattice structures have different alignments or orientations, growth cannot produce a single continuous structure. ContdThe small, continuous volumes of solid are known as cristals or grains, and the surfaces that divide them (i.e., the surfaces of crystalline discontinuity) are known as grain boundaries.The process by which a grain structure is produced Upon solidification is one of nucleation and growth.Grains are the smallest of the structural units in a metal that are observable with ordinary light microscopy. The atoms in the grain boundaries are more loosely bonded and tend to react with the chemical more readily than those that are part of the grain interior.ContdThe number and size of the grains in a metal vary with the rate of nucleation and the rate of growth.The greater the nucleation rate, the smaller the resulting grains. Conversely, the greater the rate of growth, the larger the grain.Because the resulting grain structure will influence certain mechanical and physical properties, it is an important property for an engineer to both control and specify. One means of specification is through the ASTM (American Society for Testing and Materials) grain size number, defined as:

where N is the number of grains per square inch visible in a prepared specimen at lOOX and n is the ASTM grain-size number. Low ASTM numbers mean a few massive grains;high numbers refer to materials with many small grains.

IES-2002Chemicals attack atoms within grain boundaries preferentially because they have(a)Lower energy than those in the grains(b)Higher energy than those in the grains(c)Higher number of atoms than in the grains(d)Lower number of atoms than in the grains

Ans. (b)29Fracture of metalsIf the plastic deformation of a metal is extended too far, the metal may ultimately fracture.These types of fractures are known as ductile fractures, noting that the initial response to the applied load was one of plastic deformation.Another possibility, however, is where fracture precedes plastic deformation, occuring in a sudden, catastrophic manner, and propagating rapidly through the material. These fractures, known as, brittle fractures, are most common with metals having the bcc or hcp crystal structures. Whether the fracture is ductile or brittle, however, often depends on the specific conditions of material, temperature,state of stress, and rate of loading.Fracture strength depends only on the basic crystal structure .GATE-2010The material property which depends only on the basic crystal structure is(a)Fatigue strength(b)Work hardening(c)Fracture strength(d)Elastic constant

Ans. (c) The material property which depends only on the basic crystal structure is fracture strength. Elastic constant depends not only on material parameters but also on the experimental geometry. 31IES-1992Which of the following statement is true about brittle fracture?(a)High temperature and low strain rates favour brittle fracture(b)Many metal with HCP crystal structure commonly show brittle fracture(c)Brittle fracture is always preceded by noise(d)Cup and cone formation is characteristic for brittle materials

Ans. (b)

32Cold working, recrystallization and hot workingDuring deformation, a portion of the deformation energy becomes stored within the material in the form of additional dislocations and increased grain boundary surface area. If a deformed polycrystalline metal is subsequently heated to a high-enough temperature, the material will seek to lower its energy. New, equiaxed (spherical-shaped) crystals will nucleate and grow out of the original structure . This process of reducing the internal energy through the formation of new crystals is known as recrysrallization. The temperature at which recrystallization takes place is different for each metal and also varies with the amount of prior deformation. ContdThe greater the amount of prior deformation, the more stored energy, and the lower the recrystallization temperature. However, there is a lower limit below which recrystallization will not take place in a reasonable amount of time. This temperature can often be estimated by taking 0.4 times the melting point of the metal when the melting point is expressed in an absolute temperature scale.This is also the temperature at which atomic diffusion (atom movement within the solid) becomes significant, indicating that diffusion is an important mechanism in recrystallization.ContdWhen metals are plastically deformed below their recrystallization temperature, the process is called cold working. The metal strain hardens and the structure consists of distorted grains. If the deformation is continued, the metal may fracture. Therefore, we find it common practice to recrystallize material after certain amounts of cold work.Ductility is restored, and the material is ready for further deformation. The heating process is known as a recrystallization anneal and enables deformation to be carried out to great lengths without the danger of fracture.ContdIf metals are deformed at temperatures sufficiently above the recrystallization, the process is known as hot working.Deformation and recrystallization can take place simultaneously,and large deformations are possible.Since a recrystallized grain structure is constantly forming, the final product will not exhibit strain hardening.Contd

Fig. Recrystallization of 70-30 brass: (a) cold-worked 33%; (b)heated at 580C (1075F) for 3 seconds, (c) 4 seconds, and (d) 8 secondsPlastic deformation in polycrystalline metalsGross plastic deformation of a polycrystalline specimen corresponds to the comparable distortion of the individual grains by means of slip. Although some grains may be oriented favourably for slip, yielding cannot occur unless the unfavourably oriented neighbouring grains can also slip.Thus in a polycrystalline aggregate, individual grains provide a mutual geometrical constraint on one other, and this precludes plastic deformation at low applied stresses.That is to initiate plastic deformation, polycrystalline metals require higher stresses than for equivalent single crystals, where stress depends on orientation of the crystal.Much of this increase is attributed to geometrical reasons.ContdSlip in polycrystalline material involves generation, movement and (re-)arrangement of dislocations.The second important mechanism of plastic deformation is twinning. It results when a portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way.The twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry is called twinning plane.