An exceptional ductility of AZ31 magnesium alloy sheet achieved by consecutive multi-pass cooperative lowered-temperature rolling

In this study, a novel technique of consecutive multi-pass cooperative lowered-temperature rolling (CMCLR) was proposed for the first time, with the purpose of preparing high performance wrought magnesium alloys. Based on this, a highly ductile magnesium alloy sheet could be effectively produced from ingot. The corresponding microstructure could directly evolve from coarse dendrites to equiaxed grains with an average size of similar to 5 mu m, which was attributed to the intense dynamic recrystallization (DRX) during isothermal deformation. The equivalent strain gradually decreased from the surface to the interior and edge of the rolled sheet due to the anisotropic plastic deformation. Instead, the temperature of the core of the sheet was slightly higher than that of the surface due to the deformation heat effect. The basal texture in the RD-TD plane generally weakened in the form of the splitting of basal texture toward the RD and the broadening of basal plane away from the pole center due to texture randomization dominated by DRXed grains. The exceptional ductility of more than 28.7% together with acceptable ultimate tensile strength of not less than 267 MPa was ascribed to the progressive work hardening as a result of seamless switching of multiple deformation modes, i.e., basal dislocation-slip-mediated deformation was preferentially operated and then was substituted by twinning-controlled deformation and non-basal dislocation-slip-dominated deformation. The selection of {10 (1) over bar2} twin variants agreed well with geometrical compatibility factor (m') criterion instead of Schmid's law. The activated twin variants nearly engulfed the whole parent grains for the sake of preserving compatible deformation continuity at ambient temperature, giving rise to the formation of TD-split texture. Comparatively, the {10 (1) over bar2} twinning could randomize the basal texture via reorienting grain orientations which in turn facilitated the reactivation of dislocation slip systems in the case of high temperature deformation.