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Unformatted text preview: ME 382 Lecture 31 1 C REEP M ECHANISMS Diffusional (linear) creep • At low stresses creep occurs by diffusion provided T ≥ 0.4 T M • Gibbs free energy is proportional to normal stresses on grain boundaries • Driving force for diffusion is gradient across grains of normal stresses on boundaries • Consider a case of pure shear • Normal tensile stress on “B” > normal tensile stress on “A” ∴ Energy of atoms on grain boundary “B” < energy on grain boundary “A” ∴ Atoms diffuse from “A” → “B” • Creep is a shear driven process Requires a difference in normal stress on boundaries of different orientations • No creep under hydrostatic stress (same stress at “A” and “B”) • Two diffusion routes between grain boundaries (i) Lattice (bulk) diffusion creep • Also known as “Nabarro-Herring” creep (1948/1950) • Atoms move through the grain by bulk (lattice) diffusion Width of diffusion path = grain size ME 382 Lecture 31 2 (ii) Grain-boundary creep • Also known as “Coble” creep (1963) • Atoms move along the grain boundaries Width of diffusion path = region of disorder of width...
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This note was uploaded on 09/08/2011 for the course MECHENG 382 taught by Professor Thouless during the Fall '08 term at University of Michigan.
- Fall '08