Superplasticity of the fine-grained friction stir processed Mg–3Gd–1Zn sheets

Abstract Friction stir processing (FSP) was conducted on the hot-rolled Mg–3Gd–1Zn (GZ31) sheets for 1 and 4 passes to create a fine-grained structure. The superplastic behavior of the upper surface layer in the center of the stirred zone (SZ) was evaluated by the localized shear punch test (SPT) in the temperature range of 673–748 K. Microstructural analysis showed that FSP transformed the coarse-grained structure of the hot-rolled alloy with an average grain size of 49.5 μm to the fine and equiaxed grains of 4.6 and 2.7 μm after 1 and 4 passes, respectively. The maximum strain rate sensitivity (SRS) indices (m-values) of 0.23, 0.31, and 0.48 were achieved for the hot-rolled, 1 pass, and 4 passes FSPed conditions at 698 K, respectively. The corresponding activation energies were found to be 321, 212, and 119 kJ mol−1 for these conditions. The shear stress-strain rate behavior of the alloy after 4 FSP passes conformed to regions I, II and III, which is characteristic of superplastic behavior. This was manifested by the high m-value of 0.48 and the activation energy of 119 kJ mol−1, which are indicative of superplastic behavior, caused by the grain boundary sliding (GBS) of the fine equiaxed grains having a large fraction of high angle grain boundaries (HAGBs) of about 87%.