Optimizing superplasticity of AZ91−xSn magnesium alloys with competitive grain growth and boundary sliding

To minimize the room-temperature brittleness of AZ magnesium alloys (Mg-Al-Zn) and to inhibit grain growth at elevated temperatures for superplasticity optimization, AZ91−xSn alloys (x=0, 1, 2 and 3 wt%) were developed, processed by high-ratio hot extrusion for microstructure refinement. The mechanistic correlations of their superplasticity, from the perspectives of grain growth and boundary sliding, with the Sn content and the testing parameters were examined. With a higher Sn content, the activation energy of grain growth and diffusion-mediated boundary sliding, QG and QS, increased from 29.7 to 35.2 kJ/mol and from 114.1 to 126.5 kJ/mol, respectively. With an adequate Sn addition (2 wt%) and at a relevant strain rate and temperature, a compromise between competitive grain growth and boundary sliding was reached, and the superplasticity was optimized (elongation >1000%), following a proposed superplasticity parameter (pQG−QS)/2.3nGpRT+(nGp−1)logε̇.