上海交通大学：《材料科学基础》教学资源_英文题库习题_Deformation & Recrystallization.pdf_大学文库

annealing.) 6.Discuss the role of stacking fault energy in the microstructural and mechanical-property changes which result from the cold working of an annealed alloy and from the recovery of a cold-working alloy. 7.The catalyst in an automotive exhaust system consists of many very fine particles (10nm diam.)of metal (Pt)on a ceramic (alumina)substrate.The fineness is necessary to provide the maximum surface area per unit volume of precious metal. A dominant mechanism of deterioration for such a catalyst is the coarsening of these particles.This process parallels Ostwald ripening in solids. Discuss,with approximate equations,how the rate of coarsening would depend on:surface tension,surface diffusion coefficient,concentration of absorbed metal atoms,and temperature. 8.A metal-forming operation involves the compression of a cylinder of a fec metal to give a 50%reduction in height.(At room temperature the flow stress of the fcc metals shows little strain rate sensitivity.)This operation is done at room temperature where dynamic recovery occurs.At both temperatures the compression is done at several different strain rates with the max.and min.rates differing by an order of magnitude. (a)On a plot of load vs.strain rate show how the max.load required for the compression varies with strain rate.Show the plot for room temperature and elevated temperature on the same graph. (b)How would you expect the microstructure to differ for samples done at the two temperatures?The examination is made with a TEM using samples deformed at the slowest strain rate. (c)What would you see in the microstructure that would indicate the sample had undergone dynamic recrystallization instead of dynamic recovery? 9 (a)Define,using words and any figures you find appropriate,the following qualifying words as applied to continuum material behavior.For example,a material model may be called"rigid plastic."For each word of this type,show the meaning clearly. rigid visco-elastic visco-plastic anelastic elastic plastic ideal plastic (b)Discuss which kinds of laws listed in part (a)would be most appropriate for each of the following continuum analysis problems and why.Include in your material description only the most important effect:that is,the ones that are essential to a reasonably accurate solution. 1)Forming of an automobile fender from plain-carbon steel at room temperature. 2)Hot rolling of aluminum slabs. 3)Analysis of buckling initiation in vertical support columns. 4)Interaction energies between separated dislocations in a single crystal

annealing.) 6. Discuss the role of stacking fault energy in the microstructural and mechanical-property changes which result from the cold working of an annealed alloy and from the recovery of a cold-working alloy. 7. The catalyst in an automotive exhaust system consists of many very fine particles (10nm diam.) of metal (Pt) on a ceramic (alumina) substrate. The fineness is necessary to provide the maximum surface area per unit volume of precious metal. A dominant mechanism of deterioration for such a catalyst is the coarsening of these particles. This process parallels Ostwald ripening in solids. Discuss, with approximate equations, how the rate of coarsening would depend on: surface tension, surface diffusion coefficient , concentration of absorbed metal atoms, and temperature. 8. A metal-forming operation involves the compression of a cylinder of a fcc metal to give a 50% reduction in height. (At room temperature the flow stress of the fcc metals shows little strain rate sensitivity.) This operation is done at room temperature where dynamic recovery occurs. At both temperatures the compression is done at several different strain rates with the max. and min. rates differing by an order of magnitude. (a) On a plot of load vs. strain rate show how the max. load required for the compression varies with strain rate. Show the plot for room temperature and elevated temperature on the same graph. (b) How would you expect the microstructure to differ for samples done at the two temperatures? The examination is made with a TEM using samples deformed at the slowest strain rate. (c) What would you see in the microstructure that would indicate the sample had undergone dynamic recrystallization instead of dynamic recovery? 9 (a) Define, using words and any figures you find appropriate, the following qualifying words as applied to continuum material behavior. For example, a material model may be called “rigid plastic.” For each word of this type, show the meaning clearly. rigid visco-elastic visco-plastic anelastic elastic plastic ideal plastic (b) Discuss which kinds of laws listed in part (a) would be most appropriate for each of the following continuum analysis problems and why. Include in your material description only the most important effect: that is, the ones that are essential to a reasonably accurate solution. 1) Forming of an automobile fender from plain-carbon steel at room temperature. 2) Hot rolling of aluminum slabs. 3) Analysis of buckling initiation in vertical support columns. 4) Interaction energies between separated dislocations in a single crystal

5)Evaluation of the work done in the dynamic advance of a real crack in a pure metal polycrystal. (c)Discuss the following yield function in terms of isotropy,pressure-dependence, and Baushinger effect. f=AlG,-G2+Bla:-03+Clo:-0; 10.Many creep theories are based on the concept of pure vacancy diffusion.Yet the models can fit power law dependences of laws dependences of creep rate on stress with exponent n varying from 1 to 5.Describe how this can come about and provide a detailed derivation for n=5.Does this type of model apply for polycrystals Solid-solutions?Complex engineering alloys?Why or why not? 11.(a)Starting from first principles and definitions,prove that the stress tensor has only six independent components.Illustrate your arguments carefully. (b)Starting from first principles,derive the mechanical equilibrium equations for an infinitesimal element of material.Assume no body forces. 12.Describe how each of the following kinds of experiments can be used to understand creep mechanisms.Specifically,also state how these tests may be used to differentiate between Class I (solid solution)and Class II (pure metal)creep.Be specific in regard to the kinds of information you expect to get from each kind or series of test(s). (a)A series of creep tests done at fixed temperature,but various stresses. (b)A series of creep tests done at fixed stress but various temperatures.(Some people believe these kinds of tests should be done at fixed a/E.What difference does it make?) (c)Creep tests with discontinuous stress changes. (d)High temperature tests with discontinuous changes in strain rate imposed. (e)Creep tests followed up with characterization with transmission electron microscopy. 13.Porosity or voids are a common,if not the most prevalent,casting defect in cast alloys.Describe the various forms of porosity which occur in alloy castings and identify the mechanism(s)and phenomena which are responsible for each type of porosity.Where possible,give criteria which indicate the solidification conditions for which each type porosity should not occur.Discuss how the various forms of porosity are controlled as part of the foundry practices. 14.(a)Discuss the models which have been proposed for the nucleation of new grains during static recrystallization.Use diagrams to supplement your discussion. (b)Discuss the microstructures which develop hot working to large strains. 15.The lattice parameters of platinum(fcc)and Pt3Al(cubic,ordered)have been reported as 0.39239nm.and 0.3876nm.,respectively.Terminal solid solutions of

5) Evaluation of the work done in the dynamic advance of a real crack in a pure metal polycrystal.. (c) Discuss the following yield function in terms of isotropy, pressure-dependence, and Baushinger effect. 12 13 23 M N P fA B C = − + −+ − σ σ σσ σσ 10. Many creep theories are based on the concept of pure vacancy diffusion. Yet the models can fit power law dependences of laws dependences of creep rate on stress with exponent n varying from 1 to 5. Describe how this can come about and provide a detailed derivation for n=5. Does this type of model apply for polycrystals ? Solid-solutions? Complex engineering alloys? Why or why not? 11. (a) Starting from first principles and definitions, prove that the stress tensor has only six independent components. Illustrate your arguments carefully. (b) Starting from first principles, derive the mechanical equilibrium equations for an infinitesimal element of material. Assume no body forces. 12. Describe how each of the following kinds of experiments can be used to understand creep mechanisms. Specifically, also state how these tests may be used to differentiate between Class I (solid solution) and Class II (pure metal) creep. Be specific in regard to the kinds of information you expect to get from each kind or series of test(s). (a) A series of creep tests done at fixed temperature, but various stresses. (b) A series of creep tests done at fixed stress but various temperatures. (Some people believe these kinds of tests should be done at fixed a/E. What difference does it make?) (c) Creep tests with discontinuous stress changes. (d) High temperature tests with discontinuous changes in strain rate imposed. (e) Creep tests followed up with characterization with transmission electron microscopy. 13. Porosity or voids are a common, if not the most prevalent, casting defect in cast alloys. Describe the various forms of porosity which occur in alloy castings and identify the mechanism(s) and phenomena which are responsible for each type of porosity. Where possible, give criteria which indicate the solidification conditions for which each type porosity should not occur. Discuss how the various forms of porosity are controlled as part of the foundry practices. 14. (a) Discuss the models which have been proposed for the nucleation of new grains during static recrystallization. Use diagrams to supplement your discussion. (b) Discuss the microstructures which develop hot working to large strains. 15. The lattice parameters of platinum (fcc) and Pt3Al (cubic, ordered) have been reported as 0.39239nm. and 0.3876nm., respectively. Terminal solid solutions of

Pt(Al)can be rendered supersaturated and aged to cause continuous precipitation of the stable phase Pt3Al.In binary Al(Cu)alloys continuous precipitation of the stable phase CuAl2 can occur.In this case typical lattice parameters are 0.404nm. for die aluminum solid solution and 0.607nm,0.607nm,0.487nm for me complex tetragonal unit cell (12 atoms)of CuAl2. Using the concepts of classical nucleation theory discuss,in detail,the transformations you would predict to occur when terminal solutions of these alloys are slowly cooled to room temperature. 16.Misruns,cold shuts,and incomplete fills are casting defects frequently attributed to inadequate fluidity. (a)Define fluidity relevant to cold mold casting processes such as the sand or permanent mold casting methods.Identify the casting variables which control fluidity and indicate the expected interrelation between the casting variables and fluidity (b)Define fluidity relevant to hot mold casting processes such as the investment casting processes.Identify the casting variables which control fluidity and indicate the expected interrelations between the casting variables and fluidity. Where possible,provide quantitative relations between fluidity and the casting variables in responding to both parts(a)and(b). 17.Consider two ferrous alloys which are deformed at 800C(0.60 Tm).One is an annealed,coarse grained,fully ferritic (bcc)iron with a fairly uniform grain size which has an average grain diameter of 70 4m.The other is a microduplex alloy which has an average grain diameter of2m and consists of 30 volume percent austenite and 70 volume percent ferrite. (a)Both of these materials are crept at constant stress such that the steady-state strain rate is approximately 10-5 s-1.Sketch the resulting time-elongation plots.Use the same set of axes for both curves. (b)Sketch the stress versus steady-state strain rate relationship for each material.Use the same set of axes for both curves.Be precise and describe the physical origins of the behavior you show. (c)Imagine now that you will make a 5-layer laminated composite (alternating sheets)of these two steels,with greater than 80 volume-percent of the duplex alloy.Draw the tensile stress versus steady-state strain rate relationship that you would expect for this composite material loaded in a tensile direction which is parallel to the interfaces.What assumptions must you make in making this graph? (d)Using the plot above,sketch the relationship that you expect to see between tensile elongation and creep stress for the laminated composite:(i)if 80% duplex alloy,(ii)if 60%duplex alloy.State what assumptions you must make (assume constant stress creep)

Pt(Al) can be rendered supersaturated and aged to cause continuous precipitation of the stable phase Pt3Al. In binary Al(Cu) alloys continuous precipitation of the stable phase CuAl2 can occur. In this case typical lattice parameters are 0.404nm. for die aluminum solid solution and 0.607nm, 0.607nm, 0.487nm for me complex tetragonal unit cell (12 atoms) of CuAl2. Using the concepts of classical nucleation theory discuss, in detail, the transformations you would predict to occur when terminal solutions of these alloys are slowly cooled to room temperature. 16. Misruns, cold shuts, and incomplete fills are casting defects frequently attributed to inadequate fluidity. (a) Define fluidity relevant to cold mold casting processes such as the sand or permanent mold casting methods. Identify the casting variables which control fluidity and indicate the expected interrelation between the casting variables and fluidity. (b) Define fluidity relevant to hot mold casting processes such as the investment casting processes. Identify the casting variables which control fluidity and indicate the expected interrelations between the casting variables and fluidity. Where possible, provide quantitative relations between fluidity and the casting variables in responding to both parts (a) and (b). 17. Consider two ferrous alloys which are deformed at 800°C (0.60 Tm). One is an annealed, coarse grained, fully ferritic (bcc) iron with a fairly uniform grain size μ which has an average grain diameter of 70 m. The other is a microduplex alloy μ which has an average grain diameter of 2 m and consists of 30 volume percent austenite and 70 volume percent ferrite. (a) Both of these materials are crept at constant stress such that the steady-state strain rate is approximately 10-5 s-1. Sketch the resulting time-elongation plots. Use the same set of axes for both curves. (b) Sketch the stress versus steady-state strain rate relationship for each material. Use the same set of axes for both curves. Be precise and describe the physical origins of the behavior you show. (c) Imagine now that you will make a 5-layer laminated composite (alternating sheets) of these two steels, with greater than 80 volume-percent of the duplex alloy. Draw the tensile stress versus steady-state strain rate relationship that you would expect for this composite material loaded in a tensile direction which is parallel to the interfaces. What assumptions must you make in making this graph? (d) Using the plot above, sketch the relationship that you expect to see between tensile elongation and creep stress for the laminated composite: (i) if 80% duplex alloy, (ii) if 60% duplex alloy. State what assumptions you must make (assume constant stress creep)

18.What are the physical mechanisms for "nucleation"in recrystallization?How does the nucleation rate enter the expression for the rate of recrystallization?Give a detailed expression for the rate of grain growth during recrystallization. 19.Aluminum alloys are noted for their tendencies to show precipitation sequences involving zones and other metastable transition precipitates.Select such an alloy (e.g.Al-Cu,Al-Ag,Al-Mg-Si,Al-Cu-Mg,etc.),describe the characteristics of the precipitation processes and discuss them in terms of the concepts of nucleation theory. 20.The microstructures resulting from solidification of alloys depend primarily on the alloy composition and the three solidification variables: (a)thermal gradient in the liquid at the liquid-solid interface,GL (b)isothermal advance rate or solidification velocity,R,and (c)local quench rate,T. The occurrence of many casting defects,such as shrinkage porosity and macrosegregation patterns,including freckles,also depend on the same variables given above. Draw two GL versus R plots,one for a binary alloy which solidifies as a single phase solid solution,the other for a binary alloy which solidifies as a eutectic.On each GL-R graph,indicate the various regions of expected morphologies and state any quantitative relations for the boundaries or transitions between the various regions.Also indicate those GL-R-T conditions for which various casting defects are anticipated. 21.Dynamic recovery and dynamic recrystallization are associated with the hot working,e.g.rolling,of metals and alloys. (a)What is the most common method used to investigate these processes? (b)What kind of problem or difficulty might be associated with this experimental method? (c)Discuss the results of experiments of this kind with an emphasis on the microstructures which are produced during hot working. 22.Suppose you were asked to design an experiment to quantitatively examine the predictions of classical nucleation theory for homogeneous nucleation during the initial stages of the decomposition of a solid solution.First describe what you would like your experiment to achieve and then discuss what problems you would expect to encounter. 23.(a)Discuss the relationship between the apparent density of a powder and the particle size of the powder. (b)Describe the morphological characteristics of powder particles prepared by the electrolyte,carbonyl,oxide reduction and atomization processes. (c)Discuss the relative rates of sintering of a coarse powder and a fine powder and

18. What are the physical mechanisms for "nucleation" in recrystallization? How does the nucleation rate enter the expression for the rate of recrystallization? Give a detailed expression for the rate of grain growth during recrystallization. 19. Aluminum alloys are noted for their tendencies to show precipitation sequences involving zones and other metastable transition precipitates. Select such an alloy (e.g. Al-Cu, Al-Ag, Al-Mg-Si, Al-Cu-Mg, etc.), describe the characteristics of the precipitation processes and discuss them in terms of the concepts of nucleation theory. 20. The microstructures resulting from solidification of alloys depend primarily on the alloy composition and the three solidification variables: (a) thermal gradient in the liquid at the liquid-solid interface, GL (b) isothermal advance rate or solidification velocity, R , and (c) local quench rate, T . The occurrence of many casting defects, such as shrinkage porosity and macrosegregation patterns, including freckles, also depend on the same variables given above. Draw two GL versus R plots, one for a binary alloy which solidifies as a single phase solid solution, the other for a binary alloy which solidifies as a eutectic. On each GL -R graph, indicate the various regions of expected morphologies and state any quantitative relations for the boundaries or transitions between the various regions. Also indicate those GL-R-T conditions for which various casting defects are anticipated. 21. Dynamic recovery and dynamic recrystallization are associated with the hot working, e.g. rolling, of metals and alloys. (a) What is the most common method used to investigate these processes? (b) What kind of problem or difficulty might be associated with this experimental method? (c) Discuss the results of experiments of this kind with an emphasis on the microstructures which are produced during hot working. 22. Suppose you were asked to design an experiment to quantitatively examine the predictions of classical nucleation theory for homogeneous nucleation during the initial stages of the decomposition of a solid solution. First describe what you would like your experiment to achieve and then discuss what problems you would expect to encounter. 23. (a) Discuss the relationship between the apparent density of a powder and the particle size of the powder. (b) Describe the morphological characteristics of powder particles prepared by the electrolyte, carbonyl, oxide reduction and atomization processes. (c) Discuss the relative rates of sintering of a coarse powder and a fine powder and

the factors which determine the rate of sintering. 24.Show how a constitutive relation can be derived from a stress relaxation test.If the test is performed at elevated temperature (0.75 Tm),is it likely that one mechanism will be applicable for the entire test?Why or why not,and what mechanism(s)should apply? 25.Discuss the deformation pattern in a single grain of a polycrystal of a fcc metal being deformed in tension at strains of 0.001,0.01,0.1 and 0.5. 26.Draw a typical deformation map for a pure metal.Indicate how a change in strain rate influences one of the boundary lines in the creep region of the diagram. Present typical constitutive equations for each region of the diagram. 27.Discuss how a large decrease in stacking fault energy would influence the following at room temperature: (a)Stress strain curve for a single phase material. (b)Flow of a dispersion-hardened system. (c)A precipitation hardened system with the effect in the matrix. (d)As in(c)but with the effect in the precipitates. 28.Samples for elevated temperature tensile testing were all made from a single heat of an annealed single phase metal.These samples were tested under controlled true-strain-rate conditions,at three temperatures and three strain rates. In all cases,after a few percent strain,a well-defined,steady-state flow stress was observed.This flow stress as at each temperature and strain rate are shown below. Flow Stress (MPa) Temperature (C) Strain Rate 400 450 500 1E-4 20.3 14.6 11.0 5E-4 34.7 25.0 18.7 1E-3 43.7 31.4 23.6 Based on this data: (a)What is the activation energy for creep in this metal? (b)What can you infer to be the mechanism of deformation?Back up your statements and be as precise as possible. (c)What can you say about the metal based on this? (d)After each of the tests the samples are immediately quenched and examined.In the 450C,1E strain rate,test the dislocation density was measured at 1.0 x 10 cm.What do you expect the dislocation density will be at the same temperature at a strain rate of 1E3? 29.The six major types of porosity in castings are: 1.Macroshrinkage porosity

the factors which determine the rate of sintering. 24. Show how a constitutive relation can be derived from a stress relaxation test. If the test is performed at elevated temperature (0.75 Tm), is it likely that one mechanism will be applicable for the entire test? Why or why not, and what mechanism(s) should apply? 25. Discuss the deformation pattern in a single grain of a polycrystal of a fcc metal being deformed in tension at strains of 0.001, 0.01, 0.1 and 0.5. 26. Draw a typical deformation map for a pure metal. Indicate how a change in strain rate influences one of the boundary lines in the creep region of the diagram. Present typical constitutive equations for each region of the diagram. 27. Discuss how a large decrease in stacking fault energy would influence the following at room temperature: (a) Stress strain curve for a single phase material. (b) Flow of a dispersion-hardened system. (c) A precipitation hardened system with the effect in the matrix. (d) As in (c) but with the effect in the precipitates. 28. Samples for elevated temperature tensile testing were all made from a single heat of an annealed single phase metal. These samples were tested under controlled true-strain-rate conditions, at three temperatures and three strain rates. In all cases, after a few percent strain, a well-defined, steady-state flow stress was observed. This flow stress as at each temperature and strain rate are shown below. Flow Stress (MPa) Temperature (°C) Strain Rate 400 450 500 1E-4 20.3 14.6 11.0 5E-4 34.7 25.0 18.7 1E-3 43.7 31.4 23.6 Based on this data: (a) What is the activation energy for creep in this metal? (b) What can you infer to be the mechanism of deformation? Back up your statements and be as precise as possible. (c) What can you say about the metal based on this? (d) After each of the tests the samples are immediately quenched and examined. In the 450°C, 1E-4 strain rate, test the dislocation density was measured at 1.0 x 107 cm -1. What do you expect the dislocation density will be at the same temperature at a strain rate of 1E-3? 29. The six major types of porosity in castings are: 1. Macroshrinkage porosity

found that the dissolved carbon content first decreases,reach a minimum value, then increase to a saturation limit.Explain this behavior,discuss the suitability of magnesia as a container,and suggest alternative approaches. (b)What factors determine the minimum coke rate(KgC per Kg Fe)required in blast furnace?Present a simplified model of the system(consider Fe,C and O only)and show the importance o f temperature profiles and relative coke reactivity. 34.Illustrate the phenomenon of latent hardening (overshoot)on a unit stereographic triangle for a tensile specimen of a fec single crystal. 35. Sketch stress-strain curves for brittle and ductile materials.Identify the elastic and plastic zones and briefly discuss the mechanisms by which fractures can occur in these regions.Your answer should discuss the meaning of terms such as "yield strength","ultimate tensile strength",ETC,which are frequently used to characterize the mechanical properties of materials. 36.List and discuss the various parameters that can influence the degree of strengthening attainable by precipitation hardening.What combination of parameters would you desire for optimum strengthening? 37.(a)Define hardness (b)Describe how one obtains hardness values using Rockwell,Brinell,and Vickers hardness testing equipment. (c)How does one determine which of the preceding methods to use? (d)What other methods of hardness testing are commonly used,and what are their advantages or disadvantages? 38.List some of the factors leading to a concern for brittle fracture of a metal engineering structure.What tests would you conduct to assure that the alloy had sufficient toughness?What are some of the limitations of these methods? 39.Give an explanation for aging and overaging for a precipitation-hardening alloy where the precipitate is never coherent with the matrix.Sketch a schematic phase diagram and indicate solution treating and aging ranges.Plot temperature versus time for the full heat treatment process.Sketch hardness versus time for a variety of different aging temperatures and explain the shape of curves. 40.a)What is the purpose of the following constituents of alloy steels? (i)Nickel (ii)Manganese (iii)Carbon (iv)Aluminum

found that the dissolved carbon content first decreases, reach a minimum value, then increase to a saturation limit. Explain this behavior, discuss the suitability of magnesia as a container, and suggest alternative approaches. (b) What factors determine the minimum coke rate (KgC per Kg Fe) required in blast furnace? Present a simplified model of the system (consider Fe, C and O only) and show the importance o f temperature profiles and relative coke reactivity. 34. Illustrate the phenomenon of latent hardening (overshoot) on a unit stereographic triangle for a tensile specimen of a fcc single crystal. 35. Sketch stress-strain curves for brittle and ductile materials. Identify the elastic and plastic zones and briefly discuss the mechanisms by which fractures can occur in these regions. Your answer should discuss the meaning of terms such as “yield strength”, ”ultimate tensile strength”, ETC, which are frequently used to characterize the mechanical properties of materials. 36. List and discuss the various parameters that can influence the degree of strengthening attainable by precipitation hardening. What combination of parameters would you desire for optimum strengthening? 37. (a) Define hardness (b) Describe how one obtains hardness values using Rockwell, Brinell, and Vickers hardness testing equipment. (c) How does one determine which of the preceding methods to use? (d) What other methods of hardness testing are commonly used, and what are their advantages or disadvantages? 38. List some of the factors leading to a concern for brittle fracture of a metal engineering structure. What tests would you conduct to assure that the alloy had sufficient toughness? What are some of the limitations of these methods? 39. Give an explanation for aging and overaging for a precipitation-hardening alloy where the precipitate is never coherent with the matrix. Sketch a schematic phase diagram and indicate solution treating and aging ranges. Plot temperature versus time for the full heat treatment process. Sketch hardness versus time for a variety of different aging temperatures and explain the shape of curves. 40. a) What is the purpose of the following constituents of alloy steels? ( ) Nickel ⅰ ( ) Manganese ⅱ ( ) Carbon ⅲ ( ) Aluminum ⅳ

b)What is the structure of lower bainite,how is it formed,and what properties does it produce in a steel? 41 a)What is the Considerate condition for instability in a tensile test? b)Define the Schmidt Factor for resolved shear stress. 42 Describe four methods for increasing the flow stress of metals.In each case, discuss the reasons for the rise in the flow stress. 43.Identically oriented monocrystalline specimens of Cu,Al and Fe are extended in tension at room temperature at a strain rate of 10sec".Sketch possible stress-strain curves,noting carefully any differences among them,and explain the curves you sketch 44 Define (or give an expression for)and explain the following: a.Von Mises criterion for deformation of an arbitrary grain in a polycrystal. b.Resolved shear stress for an arbitrary slip system in a crystal strained in simple tension. c.Relation between tensile strength and tensile strain for a case o=0,other stresses zero,eyw=0,shear strain=-O,ea≠0，E=≠0 d.Mohr circle 45.(a)What is the Portevin-LeShatelier effect and what is the mechanism causing it? (b)Why does an aluminum-copper alloy aged to contain O precipitate have a lower ductility than one aged to contain G-P I zones? (c)What is the influence of grain boundaries on plastic flow?Why does it differ in aluminum and copper? 46 Discuss the phenomena which can lead to serrated stress curves in a tensile test. 47 a)Discuss in detail the process of intersection of two like-sign,perpendicular dislocations.What influence does this process have on metal deformation at high temperature?At low temperature?On recovery processes? b)What is at stacking fault tetrahedron,how does it form,and what effect does it have on mechanical properties? 48. The age hardening of Cu-2%Be alloys develops an exceptionally large increase in yield strength over that of pure copper.As overaging occurs a very fine pearlitic structure develops along the grain boundaries and grow into the grains transforming them to a new structure

b) What is the structure of lower bainite, how is it formed, and what properties does it produce in a steel? 41 a) What is the Considerate condition for instability in a tensile test? b) Define the Schmidt Factor for resolved shear stress. 42 Describe four methods for increasing the flow stress of metals. In each case, discuss the reasons for the rise in the flow stress. 43. Identically oriented monocrystalline specimens of Cu, Al and Fe are extended in tension at room temperature at a strain rate of 10-4sec-1. Sketch possible stress-strain curves, noting carefully any differences among them, and explain the curves you sketch 44 Define (or give an expression for) and explain the following: a. Von Mises criterion for deformation of an arbitrary grain in a polycrystal. b. Resolved shear stress for an arbitrary slip system in a crystal strained in simple tension. c. Relation between tensile strength and tensile strain for a case 0 σ xx = , other stresses zero, , shear strain=0, 0, 0 xx zz ε yy = 0 ε ≠ ε ≠ d. Mohr circle 45. (a) What is the Portevin-LeShatelier effect and what is the mechanism causing it? (b) Why does an aluminum-copper alloy aged to contain O precipitate have a lower ductility than one aged to contain G-P I zones? (c) What is the influence of grain boundaries on plastic flow? Why does it differ in aluminum and copper? 46 Discuss the phenomena which can lead to serrated stress curves in a tensile test. 47 a) Discuss in detail the process of intersection of two like-sign, perpendicular dislocations. What influence does this process have on metal deformation at high temperature? At low temperature? On recovery processes? b) What is at stacking fault tetrahedron, how does it form, and what effect does it have on mechanical properties? 48. The age hardening of Cu-2% Be alloys develops an exceptionally large increase in yield strength over that of pure copper. As overaging occurs a very fine pearlitic structure develops along the grain boundaries and grow into the grains transforming them to a new structure