mtm04 - Engineering 45 The Structure and Properties of...

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Engineering 45 The Structure and Properties of Materials Midterm Examination October 26, 1987 Problem 1: (a) The compound CsCl is an ordered arrangement of Cs and Cl over the sites of a BCC lattice. Draw the unit cell. (b) Show that each atom in the CsCl structure has eight nearest neighbors. (c) Most materials that have the CsCl structure are metals or insulators. Why would you expect this? Problem 2: (a) Define and distinguish between a glass and a crystal. (b) SiO 2 can be made into a glass by rapid cooling. However, almost all com- mercial glasses have ionic impurities added to the SiO 2 base material, in part to make the glass easier to process. Why? Problem 3: (a) A binary system of atoms of types A and B has a simple eutectic phase diagram. The A-rich solid solution ( α ) has a maximum solubility of 8 atom percent B at the eutectic temperature (500 K), but dissolves much less than 1 atom percent B at room temperature (273 K). The B-rich solid solution (ß) has a maximum solubility of 5 atom percent A at the eutectic temperature, and dissolves 2 atom percent A at room temperature. Show the qualitative form of the phase diagram and label the phase fields. (b) A liquid solution of 6 atom percent B solidifies into a chemically inhomoge- neous solid when it is solidified at any practical cooling rate. Why? (c) You wish to process it into a homogeneous (supersaturated) a solid solution at room temperature. Describe a thermal processing sequence that might achieve this, and explain why it might work.
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Engineering 45: Mid-term Exams 1987-2004 Problem 4: A pipe of essentially pure copper is joined to a steel water line to supply water to a house. (a) If the junction is buried in the ground, the steel pipe corrodes rapidly near the junction, while if the junction is made inside the house the rate of corrosion is very slow. Why? (b) Assuming that the junction must be in the soil, propose two ways to protect the junction from corrosion. Problem 5: Explain the following observations concerning the electrical conductivity of metals: (a) the current density, j, increases linearly with the magnitude of the applied electric field; (b) the resistivity of a metal increases with temperature; (c) the resistivity of a metal increases if a small amount of solute is added to it, irrespective of the solute species. Page 2
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Engineering 45: Mid-term Exams 1987-2004 Engineering 45 The Structure and Properties of Materials Midterm Examination October 26, 1988 Problem 1: The element A crystallizes in the diamond cubic structure. (a) Give the positions of the atoms in the unit cell. (b) Show that each atom in the diamond cubic structure has four nearest neighbors. (c) All elements that crystallize in the diamond cubic structure are either semicon- ductors or insulators. Why are they one or the other and why can they be either? (d) If A is cooled very quickly it forms an amorphous structure.
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mtm04 - Engineering 45 The Structure and Properties of...

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