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 and18. 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