Superplasticity of a finegrained Mg–1.5 wt% Gd alloy after severe plastic deformation

The strain rate sensitivity (SRS) of Mg–1.5 wt% Gd processed by conventional extrusion and 2 passes of simple shear extrusion (SSE) was investigated by shear punch testing. Shear punch tests were conducted at initial shear strain rates in the range of 0.003–0.3 s1 and at temperatures in the range of 573–773 K. A finegrained microstructure with an average grain size of about 2.5 µm, obtained after 2 passes of SSE, resulted in high SRS index (mvalue) of 0.4 at 723 K. The calculated activation energy for 2 passes deformed alloy is 116 kJ/mol which is higher than the activation energy of grain boundary diffusion in magnesium (75 kJ/mol). This higher amount of activation energy can be attributed to the presence of gadolinium in this alloy. This SRS index together with an activation energy of 116 kJ/mol are indicative of a superplastic deformation behavior dominated by grain boundary sliding accommodated by grain boundary diffusion at 723 K.