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

Abstract

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 473–623K and from 2×10−2 to 1×10−4s−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 ∼3μm 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 573K with a strain rate of 1×10−4s−1. Moreover, its elongation to failure at 473K (0.51Tm) and 3×10−3s−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.