10_5

Materials Science and Engineering: An Introduction

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231 The activation energy, Q , is related to the slope of the line drawn through the data points as Q = - Slope(R) where R is the gas constant. The slope of this line is -1.126 x 10 4 K, and thus Q = - (-1.126 x 10 4 K)(8.31 J/mol-K) = 93,600 J/mol (c) At room temperature (20 ° C), 1/ T = 3.41 x 10 -3 K -1 . Extrapolation of the data in the plot to this 1/ T value gives ln (rate) -12.8 which leads to rate exp (- 12.8) = 2.76 x 10 -6 (min) -1 But since rate = 1 t 0.5 t 0.5 = 1 rate = 1 2.76 x 10 -6 (min) -1 = 3.62 x 10 5 min = 250 days 10.8 Two limitations of the iron-iron carbide phase diagram are:
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Unformatted text preview: 1) The nonequilibrium martensite does not appear on the diagram; and 2) The diagram provides no indication as to the time-temperature relationships for the formation of pearlite, bainite, and spheroidite, all of which are composed of the equilibrium ferrite and cementite phases. 10.9 (a) Superheating and supercooling correspond, respectively, to heating or cooling above or below a phase transition temperature without the occurrence of the transformation....
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This document was uploaded on 05/04/2010.

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