1.Use the concepts of classical nucleation theory to explain the following: (a)Homogeneous nucleation rarely occurs in solid-state transformations (b)Liquid metals can be easily supercooled but solid metals are difficult to super-heat at atomspheric pressure. (c)Metastable phases are often formed in solid state phase transformations. (d)Solid nuclei on substrates exposed to vapors can show epitaxial relationships (e)Homogeneous nucleation of liquid droplets from water vapor is easily accomplished in a cloud chamber whereas this is not true for the case of copper. 2.Describe with specific examples,the characteristics of the athermal martensitic transformation. 3.Suppose that you are attempting to device an experiment with the objective of testing homogeneous nucleation for continuous precipitation in a binary alloy.Discuss the problems you would expect to encounter in creating a successful experiment according to the possible observation techniques that might be used. 4. Describe,with the aid of diagrams,what you understand by a pure twist boundary and a pure tilt boundary.Emphasize in your answer what parameters can be used to define the boundary in each case,and what type of dislocations are they? 5.Write an essay entitled "Effects Upon Solid State Phase Transformations Due to Grain Boundaries." 6. A single phase solid solution alloy is to be solidified to obtain the following final microstructure: (a)a single crystal, (b) large equiaxed grains. (c)columnar grains, (d)extremely small equiaxed grains. Describe how you would control the solidification processing in order to produce each of the above four microstructures in a given nominal composition solid solution alloy.Clearly identify the solidification variables which you would control and,where possible,the quantitative values you would need for those variables to produce the various structure. 7 The theory of nucleation and growth kinetics(Johnson-Mehl-Avrami)is often applied to an a>B allotropic transformation by assuming nuclei to form randomly within a unit volume of bulk material and to grow as spheres at constant rates.How would you modify the theory to describe a similar transformation taking place in a very thin sheet?How would the time dependence change?
1. Use the concepts of classical nucleation theory to explain the following: (a) Homogeneous nucleation rarely occurs in solid-state transformations. (b) Liquid metals can be easily supercooled but solid metals are difficult to super-heat at atomspheric pressure. (c) Metastable phases are often formed in solid state phase transformations. (d) Solid nuclei on substrates exposed to vapors can show epitaxial relationships (e) Homogeneous nucleation of liquid droplets from water vapor is easily accomplished in a cloud chamber whereas this is not true for the case of copper. 2. Describe with specific examples, the characteristics of the athermal martensitic transformation. 3. Suppose that you are attempting to device an experiment with the objective of testing homogeneous nucleation for continuous precipitation in a binary alloy. Discuss the problems you would expect to encounter in creating a successful experiment according to the possible observation techniques that might be used. 4. Describe, with the aid of diagrams, what you understand by a pure twist boundary and a pure tilt boundary. Emphasize in your answer what parameters can be used to define the boundary in each case, and what type of dislocations are they? 5. Write an essay entitled “Effects Upon Solid State Phase Transformations Due to Grain Boundaries.” 6. A single phase solid solution alloy is to be solidified to obtain the following final microstructure: (a) a single crystal, (b) large equiaxed grains, (c) columnar grains, (d) extremely small equiaxed grains. Describe how you would control the solidification processing in order to produce each of the above four microstructures in a given nominal composition solid solution alloy. Clearly identify the solidification variables which you would control and, where possible, the quantitative values you would need for those variables to produce the various structure. 7. The theory of nucleation and growth kinetics (Johnson-Mehl-Avrami) is often applied to an α → β allotropic transformation by assuming nuclei to form randomly within a unit volume of bulk material and to grow as spheres at constant rates. How would you modify the theory to describe a similar transformation taking place in a very thin sheet? How would the time dependence change?
8 Consider the transformationy>+y taking place in an Fe-C alloy such that nuclei of a form only on y/y grain boundaries;nucleation is complete in a very short time;the a particles thicken and also grow along the boundaries;eventually all the prior boundaries become slabs of a that continue to thicken.How would you predict the fraction transformed with time in this case? 9. The local curvature of the liquid-solid interface has a direct thermodynamic effect (e.g.,through the Gibbs-Thomson coefficient)on the thermal field associated with the solidifying system.In many theoretical and experimental solidification cases it is desirable to work with a planar or flat liquid-solid interface. (a)Describe and discuss the solidification conditions required to obtain a planar liquid-solid interface with an alloy which solidifies as a single phase binary solid solution.If you use equations to formulate the criteria for planar liquid-solid interfaces,define all the symbols used in the equations. (b)Discuss how the conditions for the formation of a planar liquid-solid interface are similar different with respect to your answer in part (a) for the solidification of a binary eutectic alloy. 10. Considering both the initiation and the growth stages,describe the principal characteristics of discontinuous precipitation reactions and discuss their significance with respect to theoretical descriptions of the reactions. 11.For the growth of NiO scale upon the oxidation of nickel: (a)Why have investigators claimed that grain boundary Ni diffusion is significant? (b)By what mechanism(s)can cation vacancies be eliminated at the metal/scale interface? (c)Why should Li impurities reduce the scaling rate,while Cr impurities increase the rate? (d)What data do you need to predict the high temperature parabolic rate constant by the Wagner theory? (e)How would the oxidation characteristics change for an alloy of 80Au-20Ni? (f)The scale NiO is very susceptible to acid fluxing in hot corrosion by a Na2SC4 condensate.Explain the problem/mechanism. 12.(a)What are ternary eutectic and peritectic reactions? (b)Consider the hypothetical ternary system A-B-C shown below and
8. Consider the transformationγ → + α γ taking place in an Fe-C alloy such that nuclei of a form only on γ / γ grain boundaries; nucleation is complete in a very short time; the a particles thicken and also grow along the boundaries; eventually all the prior boundaries become slabs of a that continue to thicken. How would you predict the fraction transformed with time in this case? 9. The local curvature of the liquid-solid interface has a direct thermodynamic effect (e.g., through the Gibbs-Thomson coefficient) on the thermal field associated with the solidifying system. In many theoretical and experimental solidification cases it is desirable to work with a planar or flat liquid-solid interface. (a) Describe and discuss the solidification conditions required to obtain a planar liquid-solid interface with an alloy which solidifies as a single phase binary solid solution. If you use equations to formulate the criteria for planar liquid-solid interfaces, define all the symbols used in the equations. (b) Discuss how the conditions for the formation of a planar liquid-solid interface are similar / different with respect to your answer in part (a) for the solidification of a binary eutectic alloy. 10. Considering both the initiation and the growth stages, describe the principal characteristics of discontinuous precipitation reactions and discuss their significance with respect to theoretical descriptions of the reactions. 11. For the growth of NiO scale upon the oxidation of nickel: (a) Why have investigators claimed that grain boundary Ni diffusion is significant? (b) By what mechanism(s) can cation vacancies be eliminated at the metal/scale interface? (c) Why should Li impurities reduce the scaling rate, while Cr impurities increase the rate? (d) What data do you need to predict the high temperature parabolic rate constant by the Wagner theory? (e) How would the oxidation characteristics change for an alloy of 80Au-20Ni? (f) The scale NiO is very susceptible to acid fluxing in hot corrosion by a Na2SC4 condensate. Explain the problem/mechanism. 12. (a) What are ternary eutectic and peritectic reactions? (b) Consider the hypothetical ternary system A-B-C shown below and
explain the equilibrium cooling of melts of compositions 1,2,and 3 as indicated. ⊙人 中2 AB号 S AB 13.(a)Briefly explain the distinction between O in solution and combined oxygen.Explain how the use of deoxidizers in combination is beneficial both with respect to lowering o and promoting the separation of deoxidation products. (b)A melt of steel contains 0.02%C and 0.005%O.At the freezing point the solubility product of CO at Pco=1 atm is [C][%O]=0.002 Estimate the fraction of steel solidified before the melt becomes saturated with CO(Pco=1atm).The segregation coefficients for carbon and oxygen are approximately 0.3 and 0.0,respectively.Maximum segregation conditions apply. 14. Consider a group of binary systems for which the internal energy of both the liquid phase and the solid solution phase may be described adequately in terms of the regular solution model.The nearest-neighbor pair exchange energy is given as follows: 1 W=64B26u+5s） In general,the values of W for the liquid and solid solutions are different.Let Wi and Ws represent the values for the liquid and solid solutions,respectively. (a)If both Wi and Ws are zero,sketch the T-X diagram expected. (b)What conditions of Wi and Ws are necessary to create a eutectic system.Be quantitative.Use Gibbs energy-composition diagrams to illustrate your analysis. (c)Sketch a T-X diagram for Wi>0 and Ws 0. (d)If an intermetallic compound,AB,may be stable,what additional information is needed to predict the stability of a peritectic transformation?Sketch Gibbs energy composition diagrams
explain the equilibrium cooling of melts of compositions 1,2, and 3 as indicated. 13. (a) Briefly explain the distinction between O in solution and combined oxygen. Explain how the use of deoxidizers in combination is beneficial both with respect to lowering O and promoting the separation of deoxidation products. (b) A melt of steel contains 0.02% C and 0.005% O. At the freezing point the solubility product of CO at Pco = 1 atm is [% C] [% O] = 0.002. Estimate the fraction of steel solidified before the melt becomes saturated with CO (Pco=1atm). The segregation coefficients for carbon and oxygen are approximately 0.3 and 0.0, respectively. Maximum segregation conditions apply. 14. Consider a group of binary systems for which the internal energy of both the liquid phase and the solid solution phase may be described adequately in terms of the regular solution model. The nearest-neighbor pair exchange energy is given as follows: 1 ( ) 2 W AB AA BB =⋅ + ε ε ε In general, the values of W for the liquid and solid solutions are different. Let Wl and Ws represent the values for the liquid and solid solutions, respectively. (a) If both Wl and Ws are zero, sketch the T-X diagram expected. (b) What conditions of W1 and Ws are necessary to create a eutectic system. Be quantitative. Use Gibbs energy-composition diagrams to illustrate your analysis. (c) Sketch a T-X diagram for Wl > 0 and Ws < 0. (d) If an intermetallic compound, AB, may be stable, what additional information is needed to predict the stability of a peritectic transformation? Sketch Gibbs energy composition diagrams
15.(a)Use the diagram below to construct a schematic ternary isotherm for the Fe-Ni-O system at 1000C.What is the relevance of this diagram to the oxidation of an Fe-Ni alloy? -0 spinel NiO T Nio spinel - Nio+y -12 spinel oustenite ov -13 8 wustite -14 wustite +oustenite FelFe austenite(y） -16 01020.3040.50.60.70.8090 N(NN Equilibrfum Oxygen Activity-Composition Dlagram for the Fe-N1-0 System at 10000c Alloy that show the memory (mar-mem)effect are currently being used to make solder-less pipe-joints and also as devices that respond mechanically to heat changes.Describe the memory effect and discuss,quoting experimental observations,a possible mechanism to explain it. 16 a)Define the following:(1)iron,(2)commercial iron,(3)steel,(4)cast iron. What is the basis for differentiation between steel and cast iron? b)List all the factors which make steel such a versatile alloy from a metallurgical viewpoint. c)What factors control the appearance of graphite in iron-carbon alloys? 17.Describe the motion as a function of time of the a-p in each of the three couples shown below
15. (a) Use the diagram below to construct a schematic ternary isotherm for the Fe-Ni-O system at 1000°C. What is the relevance of this diagram to the oxidation of an Fe-Ni alloy? Alloy that show the memory (mar-mem) effect are currently being used to make solder-less pipe-joints and also as devices that respond mechanically to heat changes. Describe the memory effect and discuss, quoting experimental observations, a possible mechanism to explain it. 16 a) Define the following: (1) iron, (2) commercial iron, (3) steel, (4) cast iron. What is the basis for differentiation between steel and cast iron? b) List all the factors which make steel such a versatile alloy from a metallurgical viewpoint. c) What factors control the appearance of graphite in iron-carbon alloys? 17. Describe the motion as a function of time of the α − β in each of the three couples shown below
Alloy II Alloy I Alloy I Alloy II Alley I Alloy II B(bee) (fee) 4 I compoeition 37 You are faced with the task of choosing a binary alloy in which precipitation from solid solution is to be studied with the objective of comparing the results with the predictions of homogeneous nucleation theory.Describe the requirements that should be met by a suitable alloy and discuss the general problem of achieving the objective
37 You are faced with the task of choosing a binary alloy in which precipitation from solid solution is to be studied with the objective of comparing the results with the predictions of homogeneous nucleation theory. Describe the requirements that should be met by a suitable alloy and discuss the general problem of achieving the objective
