Grain size effect on twinning and annealing behaviors of rolled magnesium alloy with bimodal structure

Many wrought Mg alloys, which are manufactured by hot forming processes, have an inhomogeneous grain structure with locally different grain sizes. In the present study, variations in twinning and annealing behaviors caused by the difference in grain size within a material were investigated by subjecting a rolled Mg alloy with a bimodal grain structure to precompression and subsequent annealing. Activation of {10-12} twinning during precompression was found to be more pronounced in the coarse-grain region (CGR) than in the fine-grain region (FGR) owing to a lower twinning stress in the former. As a result, the twin structure formed in the FGR was distinctly different from that in the CGR; the CGR had a larger twinned area, stronger twin texture, fewer twin boundaries, and weaker grain refinement effect than the FGR. During the subsequent annealing, the microstructure of the FGR was greatly altered through grain growth driven by strain-induced boundary migration on account of its high internal strain energy, whereas that of the CGR remained nearly unchanged owing to its relatively lower internal strain energy. Consequently, the microstructural differences between the FGR and CGR induced by the precompression almost disappeared due to the subsequent annealing treatment, which resulted in a homogeneous microstructure.