High strain rate superplasticity in friction stir processed ultrafine grained Mg–Al–Zn alloys

Friction stir processing of a rolled AZ31 alloy and a high pressure die cast AZ91 alloy produced ultrafine grained materials with average grain size of 0.8 μm and 0.5 μm, respectively. Tensile testing was done in the temperature range of 210–360 °C and strain rate range of 1×10−4–3×10−2 s−1. Higher amounts of β-precipitates in AZ91 thermally stabilized the microstructure up to 330 °C. Friction processed ultrafine grain AZ91 exhibited high strain rate superplasticity at all tested temperatures. The best elongation of 1251% was achieved at a strain rate of 1×10−2 s−1 and temperature of 330 °C. The kinetic analysis of the superplastic data revealed a large variation in the kinetic parameter due to difficulty in using proper grain size values because of concurrent grain growth.