Ductility enhancement mechanism of AZ31 magnesium alloy with through-thickness symmetrical gradient structure prepared by hard plate rolling

The optimal configuration of strength and ductility was the first choice in Mg-alloys plate forming. A rolling method with adding the hard plate and unique component structure design was proposed to achieve a symmetrical gradient structure from two upmost surfaces to the center of the AZ31 Mg-alloys plate during a single rolling pass. The microstructure evolution and mechanical properties of symmetrical gradient structure plates were studied. The results indicate that Mg-alloy plates' strength and tensile ductility reached 267 MPa and 26.1%, respectively. In the fine-grain region, the proportion of high-angle grain boundary was 56.4%, and the recrystallization structure was 57.3%. In the coarse-grain region, high-grain boundaries and recrystallization decreased to 42.9% and 42.1%, respectively. Both prismatic slip and pyramidal slip in the CG region were easier to initiate, which leads the interaction of multidirectional twins. The geometrically necessary dislocations mainly accumulate at the boundary of the different region. The fracture morphology showed a gradient distribution of dimples. The crack was deflected in the process of extension, which also leads high ductility of Mg-alloy. The design strategy provides a new idea for producing the high-performance plate with through-thickness symmetrical gradient structure.