Microstructure evolution during superplastic deformation process and its impact on superplastic behavior of a Mg-Gd-Y-Zn-Zr alloy

Abstract

In this work, the microstructure evolution during superplastic deformation process and its impact on superplastic behavior of a peak-aged wrought Mg-10Gd-3Y-1.5Zn-1Zr (wt%) alloy were investigated by comparing the microstructure before and after high temperature tensile test (HTTT). The results show that except for the sample deformed at 400 °C with the strain rate of 1 × 10 −3 s−1, all the other samples exhibit superplasticity when deformed at temperatures between 400 °C and 475 °C with the strain rate from 1 × 10−3 s−1 to 5 × 10−3 s−1. The dominant superplastic deformation mechanism of the alloy is grain boundary sliding (GBS) controlled by grain boundary (GB) diffusion. In addition, three main changes of the microstructure are confirmed after HTTT compared with the peak-aged alloy, i.e., the disappearance of the Mg3Gd phase, the emergence of the Mg24Y5 phase, and the various volume fraction of 14H long period stacking ordered (LPSO) phase. Moreover, when deformed at 450 °C with the strain rate of 5 × 10−3 s−1, this alloy obtains the highest superplastic elongation (972%). This is due to the largest number of precipitated 14H LPSO and Mg24Y5 phases, as well as the fragmentation of the Mg24Y5 particles, which can delay the grain boundaries separation and improve the ability of the microstructure to accommodate more dislocations.