Ultrafine-grained AZ61 alloy produced by high-pressure torsion: Advent of superplasticity and effect of anisotropy

Abstract

In this study, an AZ61 Mg alloy was processed by high-pressure torsion (HPT) for grain refinement. The HPT process was carried out using the facility with an upscaled capacity of 500 ton so that 30 mm diameter disks were processed under 6 GPa. Tensile tests were carried out at elevated temperatures and microstructures were observed by transmission electron microscopy (TEM). The grain size was well reduced to ∼320 nm after HPT processing at 423 K for 10 turns. This reduction in the grain size led to superplastic elongation more than 400% at 473 K with an initial strain rate of 1.0 × 10−3 s−1. The results were then compared with those obtained using conventional 10 mm diameter disks. The total elongation is invariably higher for the 30 mm disks than the 10 mm disks. This difference is discussed based on the anisotropy due to material flow.