The Role of Surfaces in Superplasticity
The rate of neck growth in superplastic materials depends directly on surface irregularities and inversely on the strain rate sensitivity, m = (d lnσ/d lnε). High values of m reflect a substantial contribution of diffusional creep, which is inversely related to a power function of the grain size. The characteristic maxima in the m versus log ε curves is explained by the extension of the diffusional creep model to include a back stress, σo, which increases with impurity and inclusion content, and is greater in the rolling than in the transverse direction.
The Coble model of grain boundary diffusional creep is shown to be appropriate for the viscous component in lead-tin.
KeywordsHigh Strain Rate Rate Sensitivity Strain Rate Sensitivity Back Stress Diffusional Creep
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