113_2009_MT2_solns

113_2009_MT2_solns - MSE C113 Mechanical Behavior of...

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MSE C113 Mechanical Behavior of Materials Professor Ritchie Midterm Exam 2 Solutions Problem 1 (35 Points) a) Creep Deformation mechanisms: Dislocation glide barrier surmounted by thermal activation Dislocation creep barrier surmounted by diffusional processes Diffusional creep Coble and Nabarro-Herring (boundary vs bulk diffusion) Grain Boundary Sliding Obstacles which inhibit dislocation movement will also reduce the creep rate at a given temperature. Thus precipitation/dispersion hardening, and solid solution strengthening will have an effect on creep properties. For creep purposes, grain size effects preclude use of grain boundary hardening. At low grain sizes, grain sliding is enhanced. Additionally, small grain sizes increase grain boundary diffusion rates in the Coble creep regime. A good creep resistant alloy will have the following properties: Solid-solution hardening Low diffusional rates Small stable precipitates or dispersoids High melting point Large grain size
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113_2009_MT2_solns - MSE C113 Mechanical Behavior of...

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