Dynamic recrystallization and strengthening mechanisms in a magnesium alloy processed by severe plastic deformation

An extruded Mg-Gd-Y-Ag alloy was subjected to simple shear extrusion (SSE) at 280 °C to obtain a refined microstructure, with a focus on examining microstructural evolutions through detailed EBSD analysis and TEM. The EBSD results revealed that the microstructures at the early stages of deformation contained large deformed grains with a significant fraction of low angle grain boundaries developed through dynamic recovery. Continuous dynamic recrystallization (CDRX) was dominant as deformation proceeded. Two kinds of CDRX grains were recognized considering their locations; “GB-type” grains, which formed in the vicinity of the pre-existing boundaries of the deformed grains, and “Core-type” grains, emerging within the interior areas. The EBSD exhibited pronounced misorientation gradients in the areas adjacent to pre-existing boundaries, and severe fragmentation into subgrains. At larger strains, the density of subgrain boundaries declined due to the massive progress of discontinuous dynamic recrystallization (DDRX). TEM investigations confirmed the emergence of globular Mg₅Gd-type nano-particles exclusively within the DRXed areas through dynamic precipitation, and the precipitation of nano-sized βʹ-phase, mainly within the larger deformed grains. Basal texture components of “type-I” and “type-II” were identified following 6 SSE passes. In the former case, the basal planes rearranged parallel to the shear planes with their poles aligned along the normal direction (ND) as a result of the activity of the basal slip system. In contrast, in the latter scenario, the basal poles were parallel to the transverse direction (TD) due to the 90° rotation of the workpiece between consecutive passes. The results of shear punch testing (SPT) indicated an increase in the shear strength, as the number of passes in SSE increased from 1 to 6. The improved mechanical response of the alloys after SSE was ascribed to various strengthening mechanisms, including the influence of low-angle grain boundaries (LAGBs), precipitation hardening and grain boundary strengthening.