Mat-sci-mid2-2007-Junel-answer - Second Mid-term...

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Second Mid-term Examination of “Introduction to Materials Science and Engineering” National United University, Dept. of Materials Science and Engineering. 2007/06/01 Class: Name: ID#: 1. Define the followings terminologies. (20%) (a) What is the difference between a congruently melting compound and an incongruently melting one? A congruently melting compound maintains its composition right up to its melting point. Whereas an incongruently melting compound undergoes peritectic decomposition upon heating; the solid compound decomposes into a liquid and another solid solution. (b) Distinguish between the following three types of plain-carbon steels: (a) eutectoid, (b) hypoeutectoid, and (c) hypereutectoid. (a) Eutectoid plain-carbon steel contains 0.8 percent carbon. (b) Hypoeutectoid plain-carbon steel contains less than 0.8 percent carbon. (c) Hypereutectoid plain-carbon steel contains more than 0.8 percent carbon. (c) What are the three basic heat-treatment steps to strengthen a precipitation-hardenable alloy? And explain how a precipitation-hardenable alloy is strengthened by heat treatment. The three basic heat-treatment steps are solution heat treatment, quenching, and aging. Heat treatment strengthens a precipitation-hardenable alloy by causing the formation of a dense and fine dispersion of precipitated particles within a matrix of deformable metal. These particles provide strength by obstructing dislocation movement. (d) Briefly cite the differences between pearlite, bainite, and spheroidite relative to microstructure and mechanical properties. The microstructures of pearlite, bainite, and spheroidite all consist of α -ferrite and cementite phases. For pearlite, the two phases exist as layers which alternate with one another. Bainite consists of very fine and parallel needles of ferrite that are separated by elongated particles of cementite. For spheroidite, the matrix is ferrite, and the cementite phase is in the shape of spheroidal-shaped particles. Bainite is harder and stronger than pearlite, which, in turn, is harder and stronger than spheroidite.
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Mat-sci-mid2-2007-Junel-answer - Second Mid-term...

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