Development of Mg-6Al-4Sn-1Zn alloy sheets with ultra-high strength by combining extrusion and high-speed rolling

Achievement of ultra-high yield strength of above 400 MPa for rare-earth-free magnesium (Mg) alloys is quite difficult via traditional thermal-mechanical processing. In this study, Mg-6Al-4Sn-1Zn alloy (ATZ641) sheet with the yield strength of 410 MPa, ultimate tensile strength of 442 MPa, and elongation of 5.1 %, was fabricated by combining extrusion and high-speed rolling (HSR). The results showed significant refinement in the grains of high-speed rolled ATZ641 alloy due to dynamic recrystallization (DRX) effectively promoted by particlestimulated nucleation (PSN) of pre-existing relatively coarse Mg17Al12 precipitates uniformly distributed along grain boundaries, formed during extrusion prior to HSR. Meanwhile, the Mg17Al12 nano-precipitates with high number density, mainly due to dynamic precipitation during HSR and submicron Mg2Sn precipitates formed during both extrusion and HSR, suppress DRX and thus promote the formation of a certain amount of subgrains via particle pinning. The excellent mechanical properties are mainly attributed to fine-grain strengthening, high density residual dislocations and presence of numerous hybrid particles such as bimodal size (micron and nanometer) Mg17Al12 precipitates and submicron Mg2Sn precipitates as well as a large amount of subgrains. This study offers an important notion for developing rare-earth-free Mg alloys with ultra-high strength.