Achieving superplasticity in a Bi-Sn alloy processed by high-pressure torsion

A Bi-Sn alloy was processed by high-pressure torsion (HPT) at room temperature for 10 turns to achieve a homogeneous fine microstructure. Microhardness observations showed that the refined microstructure experienced self-annealing during storage at room temperature. The microhardness of HPT processed samples was increased after storage at room temperature but the grain size of the samples was increased. Following HPT processing and storage at room temperature for 35 days, a sample had excellent superplastic properties when testing at room temperature. The HPT processed sample had an elongation to failure of > 1200 % when testing at an initial tensile strain rate of 1.0x10(-4) s(-1), while a coarse-grained sample only exhibited an elongation to failure of 110 % at the same testing condition. An HPT-processed tensile sample was polished carefully and pulled at an initial tensile strain rate of 1.0x10(-4) s(-1) to a strain of 0.3, and observations of the microstructure showed that Bi-Sn and Sn-Sn boundaries had excellent grain boundary sliding behavior whereas cavities were formed on Bi-Bi boundaries. The enhanced superplastic flow behavior of the fine-grained Bi-Sn alloy was attributed to the homogeneously distributed Bi-Sn and Sn-Sn boundaries after HPT processing since these are preferred sites for grain boundary sliding.