Midterm 1 Solutions 11

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Unformatted text preview: ystalline iron. (Vacancies are plentiful at grain boundaries and the number of vacancies increases the rate of diffusion.) Substitutional species depends on the number of vacancies (present at grain boundaries) whereas interstitial species are independent of vacancies. (C) If an iron-nickel alloy is quenched from high temperature, the diffusivity of nickel has an anomalously high value for a short time after the quench, but eventually decreases to its expected value. Why? The high temperature vacancy concentration is preserved for some time; the excess vacancies must diffuse to boundaries, dislocations or other sinks before they can be eliminated. Thus, the diffusivity of substitutional species is high immediately after quench. But after some time, it decreases to its equilibrium value. 4. Grain boundaries are the result of uneven growth when the solid is crystallizing Grain boundaries are defects so they tend to decrease the electrical and thermal conductivity of the material. The free energy associated with forming a nucleusa=volume*free energy (which is negative for transformation to happen) and surface area formation (which is alwa...
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This note was uploaded on 01/26/2014 for the course ENG 45 taught by Professor Stephenmorris during the Fall '12 term at University of California, Berkeley.

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