e89mtm - Engineering 45 The Structure and Properties of...

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Engineering 45 The Structure and Properties of Materials Midterm Examination October 25, 1989 Problem 1: The element A crystallizes in the body-centered cubic structure. (a) Give the positions of the atoms in the unit cell (you may simply draw the cell). (b) Identify the closest-packed planes and closest-packed directions in the unit cell. (c) Identify the positions of the octahedral voids in the BCC structure. (d) If iron that contains a small amount of interstitial carbon is cooled slowly its high-T FCC structure transforms into a two-phase mixture of BCC iron and carbide (Fe 3 C). However, if it is quenched rapidly the FCC structure transforms to a body-cen- tered tetragonal (BCT) structure that is like BCC but has one cell edge longer than the other two. Why is the product BCT rather than BCC? (e) If the quenched Fe-C alloy of part (d) is heated to slightly above room temperature for some time (tempered) the tetragonal distortion of the unit cell is gradually lost and the iron becomes BCC. Why?
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e89mtm - Engineering 45 The Structure and Properties of...

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