H. Zhou, C. Liu, Central South University, Changsha, China; Y. Wei, shanghai Jiao Tong University, shanghai, Colombia

Summary:

The superplasticity of coarsed grain AZ31 magnesium alloy was investigated in the temperature range of 623-723 K at strain rates from 10^-3 to 10^-1 s^-1. It was found that with the increase of strain rates, the elongation increased firstly, and then reached the maximum value of 300% nearly at the strain rate of 5×10^-2s^-1. After that, the elongation decreased rapidly to about the mean value of 180%. Throughout the high strain rate ranges, much higher elongations than 180% were obtained for most tested conditions, which was relatively higher than that at lower strain rate. The best superplascitic conditions were at 723K and at the strain rate of 5×10^-2s^-1. Under superplastic condition, the superplastic deformation active energy was about 185 J/mol which was higher than that deformation energy, while the mean strain rate sensitivity (m value) was 0.2 for the strain rates from 1×10^-3s^-1 to 1×10^-1s^-1. During superplascitic deformation, It was found that the mean grain size decreased from original 30μm to 18μm, indicating grain refinement was mainly process at an initial stage of the superplastic deformation, meanwhile grains became more homogenous and most of grains were still equiaxed. However, the size and distribution of deformation cavity varied with deformation conditions at the last stage of deformation. At best superplastic condition, the cavity exhibited homogenous and ball-like. It was indicated that the dominant superplastic deformation mechanism at high strain rate was both grain refinement and grain boundary sliding.