Achieving superplasticity at room temperature in a Pb-Sn eutectic alloy through high-pressure torsion and self-annealing.

Abstract

A Pb-62% Sn eutectic alloy was processed by high-pressure torsion (HPT) for one turn and stored at room temperature (RT) up to 8 months to investigate the influence of selfannealing on the mechanical behaviour. The hardness decreased significantly immediately after HPT but there was a gradual increase in hardness as the storage time increased. Electron backscatter diffraction (EBSD) revealed a significant grain refinement immediately after HPT and grain growth during storage at RT. A high fraction of high-angle grain boundaries was achieved after HPT and grain boundary sliding (GBS) was observed in the processed alloy near the hardness indents. The HPT processed samples were tested in tension at RT after different storage times using strain rates from 1.0 × 10-4 to 1.0 × 10-1 s-1. The results showed that, irrespective of the storage times, all processed samples exhibited excellent RT superplasticity at 1.0 × 10-4 s-1 and a highest elongation of 630% was recorded after storage for 4 days at RT. A detailed investigation showed that the mechanical behaviour of the alloy was highly dependent on the strain rate. Specifically, there was a transition point at a strain rate of ~1.0 × 10-2 s-1 where strain softening was apparent with active GBS below the transition point and a higher strength due to grain boundary strengthening was observed at higher strain rates above the transition point.