E 45 - Spring 2002 - Morris - Midterm 1

# E 45 - Spring 2002 - Morris - Midterm 1 - phases are solid...

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Engineering 45: Mid-term Exams 1987-2005 Page 28 Engineering 45 The Structure and Properties of Materials Midterm Exam March 18, 2002 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 feature of the ß-ZnS structure makes it suitable for ionic materials? Problem 2: (a) Draw an edge dislocation in a simple cubic crystal. Indicate its “Burgers vector” and its “glide plane”. (b) Show how the glide of an edge dislocation causes plastic deformation. (c) Now let the dislocation have the form of a circular loop in its glide plane. Show how the expansion of the loop leads to plastic deformation. Problem 3: The elements A and B have a simple eutectic phase diagram. (a) Draw a possible form for the phase diagram and label the phase fields. Which

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Unformatted text preview: phases are solid solutions? (b) For any given point in the phase diagram (T,x) you can obtain three pieces of information: the phases present, the compositions of all phases present and the fractions of all phases present. How? (c) Given the phase diagram that you drew, what is the maximum solubility of B in A at one-half the eutectic temperature? Assuming your diagram is drawn correctly, what is the maximum solubility of B in A in the limit T → 0? Problem 4: (a) The principal contribution to the specific heat of a metal comes from the lattice vibrations. The valence electrons are, ordinarily, much less important. Why? (b) The entropy of a solid solution of A and B is always higher than the entropy of an ordered compound of the same elements. Why? Engineering 45: Mid-term Exams 1987-2005 Page 29 (c) While valence electrons do not contribute much to the specific heat of a metal, they are the primary source of its thermal conductivity....
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E 45 - Spring 2002 - Morris - Midterm 1 - phases are solid...

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