Grain refinement mechanism in gradient nanostructured Mg-Gd-Y-Zn-Zr alloy prepared by severe shear deformation

Abstract

This study investigated the effects of severe shear (SS) deformation with different revolutions on the microstructures and hardness of Mg-12.12Gd-4.2Y-2.28Zn-0.36Zr (wt%) alloy. Gradient nanostructures formed naturally after SS deformation, and the thickness and number of each gradient layer increased considerably as the number of rotations increased. After deformation, the average grain size of the alloy gradually decreased from 126.3 μm to 500 nm with the increase in cumulative strain, and the size of the secondary phase also decreased. Consequently, the hardness of the alloy increased considerably after SS deformation and reached 121.3 HV, which is nearly 36.4% higher than that of the as-cast alloy. The achievement of fine and ultrafine grains mainly depends on the rotational dynamic recrystallization (RDRX) behavior. Low-angle grain boundaries (LAGBs) rapidly expanded in soft-oriented grains and were accompanied by gradual rotation of subgrains, while expansion in hard-oriented grains was more difficult. Moreover, the origins of LAGBs included not only grain boundaries with a high distortion but also lamellar structures that are characteristic of SS conditions. In addition, all oversized coarse grains were transformed into equiaxed fine grains under the synergistic effect of continuous dynamic recrystallization (CDRX) and particle-stimulated nucleation mechanisms. However, the formation of nanocrystallines differed greatly from that observed for fine and ultrafine grains, in which nanocrystallines were mainly formed by the rotation of substructures composed of high-density dislocation cells. Currently, grain growth is inhibited by pinning a large number of the nano-precipitated phase at nanocrystalline boundaries.