Superplastic tensile behavior of a fine-grained AZ91 magnesium alloy prepared by friction stir processing

Friction stir processing (FSP) was used to refine the microstructure of cast AZ91 magnesium alloy. Superplastic tensile behavior of the fine-grained AZ91 was investigated by means of hot tensile tests at the temperature and strain rate ranges of 473–623 K and from 2×10−2 to 1×10−4 s−1, respectively. Microstructure of the as-cast AZ91 alloy was mainly composed of coarse α dendrites and network-like eutectic β-Mg17Al12 phase. After FSP, α grains were greatly refined to equiaxed grains with an average grain size of ∼3 μm due to dynamic recrystallization, and β-Mg17Al12 networks were broken into small particles. It was found that the FSP AZ91 alloy exhibits excellent superplasticity, and a maximum elongation of 1604% is achieved at 573 K with a strain rate of 1×10−4 s−1. Moreover, its elongation to failure at 473 K (0.51Tm) and 3×10−3 s−1 is 204.4%, indicating superplasticity can be attained under low temperature and high strain rate. In this multi-phase fine-grained alloy, cavities nucleated around the second phase particles during superplastic deformation, and connected to each other due to the grain boundary sliding.