E92mtm - Engineering 45 The Structure and Properties of...

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Engineering 45 The Structure and Properties of Materials Midterm Exam October 30, 1992 Problem 1: (a) Describe the ß-ZnS structure and show how it is related to the FCC structure. (b) The ß-ZnS structure is adopted by many covalently bonded compounds, like GaAs. Why would this structure be favored? [Ga has valence 3, As has valence 5.] (c) The ß-ZnS structure is also adopted by some ionic compounds, like AgI. What characteristic of these compounds would favor ∫-ZnS over other ionic structures? Problem 2: (a) Draw an edge dislocation in a simple cubic crystal. (b) Show how the glide of that dislocation produces plastic deformation. (c) Explain why it is difficult for an edge dislocation to climb unless the temperature is high. Problem 3: (a) A one-component material has three possible structures, α , ß and γ . At high T the system is α . If it is cooled slowly it transforms to ß at T αβ , and remains ß for all lower temperatures. If it is cooled quickly it transforms to γ at T
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E92mtm - Engineering 45 The Structure and Properties of...

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