Superplastic behaviour of AZ91 magnesium alloy processed by high-pressure torsion

Abstract

An investigation has been conducted on the tensile properties of a fine-grained AZ91 magnesium alloy processed at room temperature by high pressure torsion (HPT). Tensile testing was carried out at 423K, 473K and 573K using strain rates from 1×10−1s−1 to 1×10−4s−1 for samples processed in HPT for N=1, 3, 5 and 10 turns. After testing was completed, the microstructures were investigated by scanning electron microscopy and energy dispersive spectroscopy. The alloy processed at room temperature in HPT exhibited excellent superplastic behaviour with elongations higher than elongations reported previously for fine-grained AZ91 alloy produced by other severe plastic deformation processes, e.g. HPT, ECAP and EX-ECAP. A maximum elongation of 1308% was achieved at a testing temperature of 573K using a strain rate of 1×10−4s−1, which is the highest value of elongation reported to date in this alloy. Excellent high-strain rate superplasticity (HSRSP) was achieved with maximum elongations of 590% and 860% at temperatures of 473K and 573K, respectively, using a strain rate of 1×10−2s−1. The alloy exhibited low-temperature superplasticity (LTSP) with maximum elongations of 660% and 760% at a temperature of 423K and using strain rates of 1×10−3s−1 and 1×10−4s−1, respectively. Grain-boundary sliding (GBS) was identified as the deformation mechanism during HSRSP, and the glide-dislocation creep accommodated by GBS dominated during LTSP. Grain-boundary sliding accommodated with diffusion creep was the deformation mechanism at high test temperature and slow strain rates. An enhanced thermal stability of the microstructure consisting of fine equiaxed grains during deformation at elevated temperature was attributed to the extremely fine grains produced in HPT at room temperature, a high volume fraction of nano β-particles, and the formation of β-phase filaments.