Processing of a two-phase alloy by severe plastic deformation

This thesis presents a systematic study on evolutions of microstructure, microhardness and superplasticity of a Pb-62% Sn alloy processed by both equal-channel angler pressing (ECAP) and high-pressure torsion (HPT) and the subsequent self-annealing process at room temperature (RT). The Pb-Sn alloy exhibits characteristics with significant grain refinement after processing by ECAP and HPT but with a reduction in the hardness values by comparison with the initial as-cast condition. For HPT processing, it is shown that there are generally smaller grains at the edges of the discs by comparison with the disc centres. The hardness results are different from those generally reported for conventional single-phase materials where a hardening trend was commonly observed after HPT processing. The significance of this difference is examined. The microstructures of the alloy after HPT were repeatedly investigated during the course of self-annealing by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and scanning electron microscopy (TEM). A significant grain growth combined with increase of microhardness was observed. It was demonstrated that there was a large fraction of twin boundaries with a twin relationship of 62.8° in the microstructure for the as-cast condition. Owing to the presence of high pressure, the mobility of ?21 boundaries at 71° was greatly favoured during processing by HPT. But the mobility of the dislocation-twin boundary near 62.8° was favoured during self-annealing at RT once the high pressure was removed. The HPT processing significantly increased the solubility of Sn in Pb phase. This supersaturated state of Sn in Pb is, however, not stable at RT during self-annealing and therefore a decomposition of Sn from Pb-rich phase was observed after 16 days of storage. Lattice diffusion should be considerable as the main mechanism for the decomposition. Moreover, abnormal grain growth was observed to be greatly favoured during self-annealing when the introduced strain was relatively low, i.e. 2 passes by ECAP and the centre region of a HPT-processed disc after one turn. Consequently, a series of HPT-processed samples with different storage time was tested in tension at RT and at 1.0 × 10-4 - 1.0 × 10-1 s-1. The results demonstrated that, despite the storage time, all processed alloy exhibited excellent RT superplasticity at 1.0 × 10-4 s-1 and the highest elongation of 630% was recorded in the processed alloy after storage for 4 days at RT. The detailed investigation showed, due to the high strain rate sensitivity of the processed alloy, a transition strain rate of ~1.0 × 10-2 s-1 was observed in which stain softening with ductile behaviour is apparent due to active GBS below the transition point but high strength is observed because of grain boundary strengthening above the transition during plastic deformation at RT in the Pb-Sn alloy after HPT. Nanoindentation tests were then performed applying both indentation depth-time (h-t) relationship at holding stage and the hardness, H, at various loading rates to explore the evolution of strain rate sensitivity (SRS), m. The results obtained by both tensile test and nanoindentation show that the relatively fast self-annealing of the HPT-processed Pb-62% Sn eutectic alloy is occupying by an unambiguous changing-tendency of strain rate sensitivity. The results confirm the validity of using nanoindentation for measuring strain rate sensitivity.