Superplastic behavior of a severely deformed Mg-6Gd-3Y-0.5Ag alloy

An extruded Mg-6Gd3Y-0.5Ag magnesium alloy was processed by the simple shear extrusion (SSE) technique at 553 K for 1, 2, 4 and 6 passes to refine the microstructure. The electron back scattered diffraction (EBSD) analysis was used to investigate the microstructural evolutions of the alloy after the SSE processing. The grain orientation spread (GOS) maps revealed that by increasing the SSE passes, the fraction of the fine dynamically recrystallized (DRXed) grains increased accordingly from 3% after 1 pass of SSE to about 81% after 6 passes. The fraction of low angle grain boundaries (LAGBs) was relatively high in the early stages of the SSE processing, due to occurrence of dynamic recovery (DRV), but it began to drop significantly after 4 and 6 SSE passes, as the DRX proceeded. Therefore, continuous dynamic recrystallization (CDRX) was identified as the governing recrystallization mechanism during SSE at 553 K. The shear punch testing (SPT) was carried out at different temperatures and under various shear strain rates to assess the superplastic behavior of the alloys. It was found that only the alloy processed by 6 SSE passes exhibited superplastic flow, for which a maximum strain rate sensitivity (SRS) index of 0.45 and an average activation energy of 112 kJ mol(-1) were obtained. Accordingly, grain boundary sliding (GBS) associated with the diffusion of grain boundaries was suggested to be the prevalent deformation mechanism during the superplastic flow of the 6-pass SSEed condition. Moreover, the SPT results revealed that as the SSE pass number increases the maximum SRS indices increase and shift to the lower temperatures indicating the rise in the contribution of the GBS mechanism. The kernel average misorientation (KAM) maps delineated that after SPT at 623 K the fine equi-axed DRXed grains were almost strain-free without any change in their shape for the 6-pass SSEed condition, while the one after 2 SSE passes was comprised of extremely deformed grains along the SPT loading direction.