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Deformation enhanced grain growth in a superplastic Sn - 1% Bi alloy Clark, Malcolm Arthur

Abstract

A Sn - 1% Bi alloy has been studied to determine the effects of superplastic deformation on the grain growth kinetics. Using both constant crosshead speed and creep tests, the grain size was measured as a function of deformation time and strain over a wide range of strain rates. It was found that during deformation, considerable increases in the grain growth rates occurred when compared to static annealing. The effect was most pronounced at intermediate strain rates (≃10⁻²/minute) in the high strain rate sensitivity region. However, the grain growth rates on annealing after deformation were found to be less than static rates. To aid in understanding the mechanism of the enhanced growth, alternating tension-compression tests were performed. The amount of grain elongation and the changes in preferred orientation with deformation were also measured. Grain type and grain size distributions after deformation and after annealing were established and analyzed in terms of a grain coalescence mechanism. However, the most favourable mechanism appears to involve the production of excess vacancies in the grain boundary region leading to increased boundary mobility.

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