Enhanced microstructural homogeneity and tensile properties of bimodal AZ31 alloy sheet via combined ultrasonic nanocrystal surface modification and electropulsing treatment

Rolled AZ31 (Mg–3.0Al–0.8Zn–0.3Mn, wt.%) alloys often exhibit a bimodal microstructure consisting of coarse and fine grains, which promotes strain localization and limits mechanical performance. In this study, ultrasonic nanocrystal surface modification (UNSM) followed by electropulsing treatment (EPT) was employed to transform the bimodal surface structure into a uniform fine-grained layer, thereby enhancing both strength and ductility. UNSM introduced severe plastic deformation at the surface, generating ultrafine grains, high dislocation density, and numerous twins. This significantly increased the strength of the AZ31 alloy sheet but reduced its elongation. Subsequent short-duration annealing by EPT at 300 °C for 2 min induced rapid static recrystallization, producing fine equiaxed surface grains and reducing the dislocation density. As a result, while the strength of the UNSM-treated alloy decreased after EPT, its ductility improved markedly. The UNSM–EPT alloy exhibited superior strength and ductility compared with the as-rolled alloy, with yield and ultimate strengths of 158 and 252 MPa, respectively, and an elongation of 21.9%. Digital image correlation (DIC) revealed strong strain localization from the early stage of deformation in the as-rolled specimen, whereas the UNSM–EPT specimen displayed a more uniform strain distribution throughout tensile loading. These results demonstrate that combining UNSM with EPT effectively eliminates detrimental surface coarse grains and provides an efficient strategy for improving microstructural homogeneity and mechanical properties of rolled Mg alloys with a surface bimodal structure.