The significance of strain weakening and self-annealing in a superplastic Bi–Sn eutectic alloy processed by high-pressure torsion

Abstract

A cast Bi–Sn eutectic alloy was processed by high-pressure torsion (HPT) at room temperature and stored at room temperature for durations of up to 91 days in order to investigate the effect of self-annealing. The HPT processing produces grain refinement but hardness measurements show there is strain softening with lower microhardness values than in the initial as-cast material and with hardness values that decrease with increasing amounts of imposed torsional strain. This softening is attributed to a loss of precipitates within the Sn and Bi phases during the processing operation. In self-annealing at room temperature, the microhardness increases significantly due to reprecipitation and there is also a minor increase in grain size with increasing time of storage. It is demonstrated by tensile testing that the HPT-processed Bi–Sn alloy exhibits superplastic behavior with elongations of up to >1000% after storage for 35 days and with an associated strain rate sensitivity close to ∼0.5. Grain boundary sliding plays an important role in superplastic flow and it is shown that maximum sliding occurs on the Bi–Bi interfaces where this is consistent with estimates of the coefficients for grain boundary diffusion in the Bi–Sn alloy.