Module IIIDeformation of metalsSyllabus Elastic, an elastic and visco elastic behaviour,Plastic deformation, mechanism of slip, slip planes and slip directions, mechanism of twinning, strengthening mechanisms, work hardening, grain boundary hardening, precipitation hardening, cold working, hot working, recovery, recrystallisation and grain growth.University QuestionsExplain Elastic, an elastic and visco elastic behavior (5) Nov 2007What is meant by work hardening? What are the stages of work hardening? (8) May 2007Compare the mechanisms of slip and twinning with neat sketches. (10) May 2007Explain the mechanism of slip and twinning with neat sketches (10) April 2008Explain the recovery , recrystallisation and grain growth with neat sketches (10) April 2008Explain the mechanism of twinning (10) April 2009 IntroductionChange produced in a metal without fracture even after removal of external forces is called deformation.The property which enables metals to deform without any fracture is called plasticity.When external force is applied to a plastic metal, it undergoes first elastic deformation, thereafter permanent deformation and finally fracture. Against this a brittle material like cast iron will fracture suddenly without showing any plastic deformation.The property of material which enables it to regain its original shape and size after removal of the forces, within the elastic limits is the elasticity. The metal deformation mainly is of the following two types Elastic deformationPlastic deformationElastic deformationFully recoverable deformation which is time independent is called elastic deformation. Fully recoverable but time dependent deformation is called anelastic deformation. When both recoverable and permanent deformation occur together and are time dependent is called viscoelastic deformation. This behavior is very common in non-crystalline organic polymers. The time dependent permanent deformation is called viscous flow Plastic deformationPlastic deformation is observed when the stress is more than the elastic limit.Plastic deformation takes place by two processes namely slip and twinning. Deformation by slipSlip is that mechanism wherein one part of the crystal moves, glides or slips over another part along certain planes known as slip planes.A slip plane is a crystallographic plane in which either the slip is likely to take place or in which slip has already taken place

Deformation by twinningSliding of one plane of atoms over the next plane is known as twinning.The total movement at any point is proportional to the distance from the twinning plane.Twinning differs from slip in the sense that in twinning each plane of atoms moves a definite distance instead of a complete block of atoms as in the case of slip. Each of two parts produced by twinning are symmetrical about the twinning plane and are mirror image of each other.Twinning is generally caused due to impact, thermal treatment etc.Twinning occurs in Body Centered Cubic (B.C.C) and hexagonal close packed (H.C.P) structure. There are two types of twinningTwinning produced due to applied stress under shock loading known as strain twins or mechanical twins.Twinning produced as a result of annealing process known as annealing twins. Difference between twinning and slip SlNo Twinning Slip1 Each plane of atoms moves a definite Distance All atoms in one block moves the same distance2 Under microscope twinning appears as broad lines or bands Slip appears as thin lines3 The crystal orientation difference takes place across the twin plane The orientation of crystal above and below the slip plane is the same after deformation4 Stress required to produce twinning is more Stress required to produce slip is less Strengthening mechanismsWhen a material is subjected to plastic deformation, a certain amount of work done on it is stored internally as strain energy. This additional energy in a crystal results in strengthening of solids.Strain hardening or work hardening is a phenomenon which results in an increase in hardness and strength of a metal subjected to plastic deformation (cold working) at temperatures lower than the recrystallisation range.Strain hardening is very commonly employed both on pure metals and on alloys as means of improving the useful mechanical properties such as strength and hardness.Strain hardening reduces ductility (formability) and plasticity.Work hardeningGrain boundary hardeningAge hardening and Precipitation hardeningIn case of some alloys , there is increase in hardness with time at room temperature or after heating to slightly higher temperature.Cold working When a metal is subjected to mechanical processes below the recrystallisation temperature it undergoes plastic deformation. The process is termed as cold working.

Most of the cold working processes are performed at room temperature.Since the process is below the recrystallisation temperature, there is no recrystallisation of grains.Strength and hardness increases with corresponding decreases in ductility.While the metal is deformed, it gives rise to severe stresses inside the metal which are known as residual stresses. This residual stress increases hardness and corrosion resistance of the metal.The various cold working processes are Drawing, Shearing, Hobbing, Shot peening, cold extrusion, Bending etc.Hot working When plastic deformation of metal is carried out at temperature above the recrystallisation temperature, the processes performed on metals are termed as hot working.Hot working process can be considered as simultaneous combination of cold working and annealing.Because of higher temperature scaling, oxidation of the metal surface take place. So fine dimensional tolerance cannot be achieved compared to cold working.The various hot working operations are Rolling, Forging, Pipe welding, Hot spinning, Hot extrusion etc. Comparison of cold working and hot working SlNo Cold working Hot working1 Carried out below the recrystallizationTemperature of metal. So grains are permanently distorted Carried out above recrystallisation temperature. So it can be regarded as a combination of deformation and recovery process2 Hardening is not eliminated. So cold working is always accompanied by strain hardening Hardening due to plastic deformation is completely eliminated by recovery and recrystallisation3 Crystallization does not take place. So no refinement of crystals Refinement of crystals occurs4 Uniformity of material is lost. So properties are affected Uniformity of materials occurs.5 Chances of crack propagation is more Cracks, unoxidised blowholes etc are welded up.6 Cold working increases ultimate tensile strength, yield point, hardness, fatigue strength etc. Corrosion resistance is decreased Properties are not affected7 Internal and residual stresses are produced Internal and residual stresses are not produced8 Energy required for plastic deformation is more Energy required is less because at high temperature , metals become soft and ductile9 More stress is required for deformation Less stress required for deformation10 No oxidation occurs .So pickling is not required Heavy oxidation occurs. So pickling is required to remove oxide layer

11 Surface decarburization in steels does not occur Surface decarburization in steels is likely to occur due to higher temperature12 Surface finish is good Surface finish is not so good13 Dimensional tolerance is maintained It is difficult to control dimensions because of contraction occurring during cooling14 Handling of materials is easy Handling of materials is difficultRecovery, recrystallisation and grain growth In general, as a result of cold working the hardness, yield strength, ultimate strength and electrical resistance increases while ductility and plasticity are reduced It is imperative that the metal be somehow returned to a condition approximately that prior to deformation. This can be achieved by subjecting the material to annealing process which involves heating the material above the critical temperature followed by slow cooling, so that the system more closely approaches equilibrium and the properties of metal, prior to deformation are gradually recovered. During annealing the metal tries to reach equilibrium through 3 stagesRecoveryRecrystallisationGrain growth (1) RecoveryThis involves heating the metal to about 0.1Tm where Tm is the melting temperature of the metal. Recovery is brought about by the movement of dislocations due to vacancy diffusion, so that dislocations can climb out of their slip planes. As a result, the dislocations of the same sign align themselves to form small angle sub grain boundaries and the process is called polygonisation.Recovery produces little change in mechanical properties of the metal. The principal effect is the reduction in internal or residual stresses. The micro structure is relatively unaffected. There is also a slight increase in electrical conductivity.Recovery is an important method for releasing the internal stresses in castings, forgings, welded and fabricated equipment, boiler tubes etc. without lowering the strength acquired during cold working. Recovery is often referred to as a Stress –relief annealing(2) RecrystallisationFor cold worked metals and alloys recrystallization usually occurs at temperatures of about 0.3Tm for pure metals and 0.5Tm for alloys. Recrystallization results in the arrangement of the atoms and molecules of the solid into an entirely new set of crystals. Thus a new set of grains form and grow in size. The distorted elongated grains formed during cold working disappear and new equiaxial grains are formed. These new crystals generally appear at the most drastically deformed portions of the grains, that is usually the grain boundaries and slip planes.As shown in figure recrystallisation has a marked effect on properties and also the microstructure. Mechanical properties like ultimate strength and hardness decreases while ductility increases. Usually grain refinement takes place. (3) Grain growthGrain growth takes place during the final stages of annealing. This occurs at higher temperatures nearer to critical temperature and longer times. Grain growth takes place by

interaction between the adjacent grains. That is the new equiaxial grains formed during recrystallization when heated further, will grow in size at the expense of the neighboring grains. During grain growth some of the small grains become still smaller and are ultimately absorbed by the larger grains which grow further in size.Grain growth results in increase in grain size, gain in ductility with a decrease in strength and hardnessRecovery, recrystallisation and grain growth In general, as a result of cold working the hardness, yield strength, ultimate strength and electrical resistance increases while ductility and plasticity are reduced It is imperative that the metal be somehow returned to a condition approximately that prior to deformation. This can be achieved by subjecting the material to annealing process which involves heating the material above the critical temperature followed by slow cooling, so that the system more closely approaches equilibrium and the properties of metal, prior to deformation are gradually recovered. During annealing the metal tries to reach equilibrium through 3 stagesRecoveryRecrystallisationGrain growth (1) RecoveryThis involves heating the metal to about 0.1Tm where Tm is the melting temperature of the metal. Recovery is brought about by the movement of dislocations due to vacancy diffusion, so that dislocations can climb out of their slip planes. As a result, the dislocations of the same sign align themselves to form small angle sub grain boundaries and the process is called polygonisation.Recovery produces little change in mechanical properties of the metal. The principal effect is the reduction in internal or residual stresses. The micro structure is relatively unaffected. There is also a slight increase in electrical conductivity.Recovery is an important method for releasing the internal stresses in castings, forgings, welded and fabricated equipment, boiler tubes etc. without lowering the strength acquired during cold working. Recovery is often referred to as a Stress –relief annealing(2) RecrystallisationFor cold worked metals and alloys recrystallization usually occurs at temperatures of about 0.3Tm for pure metals and 0.5Tm for alloys. Recrystallization results in the arrangement of the atoms and molecules of the solid into an entirely new set of crystals. Thus a new set of grains form and grow in size. The distorted elongated grains formed during cold working disappear and new equiaxial grains are formed. These new crystals generally appear at the most drastically deformed portions of the grains, that is usually the grain boundaries and slip planes.As shown in figure recrystallisation has a marked effect on properties and also the microstructure. Mechanical properties like ultimate strength and hardness decreases while ductility increases. Usually grain refinement takes place. (3) Grain growth

Grain growth takes place during the final stages of annealing. This occurs at higher temperatures nearer to critical temperature and longer times. Grain growth takes place by interaction between the adjacent grains. That is the new equiaxial grains formed during recrystallization when heated further, will grow in size at the expense of the neighboring grains. During grain growth some of the small grains become still smaller and are ultimately absorbed by the larger grains which grow further in size.Grain growth results in increase in grain size, gain in ductility with a decrease in strength and hardnessNB: FIGURES WILL BE UPLOADED SOON