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# E93mtm - temperature T under two conditions(1 the solid has...

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Engineering 45 The Structure and Properties of Materials Midterm Exam October 20, 1993 Problem 1: (a) Materials that crystallize in the CsCl structure are ionic or metallic in their bonding, never covalent. Why would you expect this? (b) Materials that crystallize in the Cu 3 Au structure are never ionic. Why would you expect this? (c) Materials that crystallize in the ß-ZnS structure are covalent or ionic. Why would you expect this? Problem 2: (a) What chemical and geometric characteristics are desirable in a polymer that is to be used to make a good elastomer? (b) What chemical and geometric characteristics are desirable in a polymer that is to be used in a rigid, thermosetting plastic? Problem 3: (a) Plot the expected variation of the diffusivity with temperature for a substitutional solute in a polygranular solid, and explain the shape of the graph. (b) Let the diffusivity of a substitutional specie in a given solid be measured at

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Unformatted text preview: temperature T, under two conditions: (1) the solid has uniform temperature, T; (2) the solid has a temperature gradient, and T is an intermediate temperature. Would the two measurements differ? How and why? Problem 4: A binary system of atoms A and B has the phase diagram drawn below. (a) For given T and x, three pieces of information can be read from any phase diagram: (1) the phases present, (2) the compositions of the phases present, and (3) the fractions of the phases present. Briefly explain how this is done. (b) Only the one-phase fields are shown in the phase diagram above. Outline and label the two-phase fields. (c) Assume the composition indicated by the arrow, and describe how the system evolves if it is cooled from the liquid phase field to the lowest temperature on the plot slowly enough that equilibrium is always preserved. Page 1 Engineering 45: Fall, 1993 Midterm Exam ’ Page 2...
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E93mtm - temperature T under two conditions(1 the solid has...

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