Abstract: Superplastic mechanical properties of fine-grained AZ31 Mg alloy sheets in the temperature range of 250-450℃ and strain rate range of 0.7×10^-3-1.4×10^-1s^-1 were investigated by uniaxial tensile tests. The microstructure evolution during the superplastic deformation of AZ31 Mg alloy was examined by means of metallurgical microscope and transmission electronic microscope(TEM). It is shown that, fine-grained AZ31 Mg alloy starts to exhibit superplasticity at 300℃ and the maximum elongation of 362.5% is obtained at 400℃ and 0.7×10^-3s^-1. The predominate superplastic mechanism of AZ31 Mg alloy in the temperature range of 300-400℃ is grain boundary sliding(GBS). Twinning caused by pile-up of dislocations during the early stage of superplastic deformation is the hardening mechanism, and dynamic continuous recrystallization (DCRX) is the important softening mechanism and grain stability mechanism during the superplastic deformation of the alloy.

Key words:  AZ31 Mg alloy; superplasticity; microstructure evolution; dynamic recrystallization