Achieving high strength via significantly enhanced precipitation strengthening in Mg-5Bi-3Al-xSn alloys

High strength is a primary objective in developing low-cost Mg-Bi-Al alloys. This study fabricates new Mg-5Bi-3Al-xSn (wt%, x = 0, 1, 3 and 5) alloys possessing high mechanical strength after extrusion. In as-extruded alloys, grains are gradually refined with increasing Sn content. Trace Sn (1 wt%) promotes dynamic recrystallization, but a further increase in Sn content inhibits this process, and subsequently leads to weakening and strengthening of texture. Experimental results and density-functional theory calculations suggest that Bi atoms replace Sn atoms in Mg₂Sn phase and promote precipitation of Mg₂Sn phase, and Mg₂Sn gradually becomes one of the main strengthening phases. Tensile tests demonstrate that yield strength and ultimate tensile strength increase progressively with Sn content. As-extruded Mg-5Bi-3Al-5Sn alloy exhibits increments of 72 MPa in yield strength and 63 MPa in ultimate tensile strength compared to as-extruded Mg-5Bi-3Al alloy, which reaches 384 and 400 MPa, respectively. Fine-grained structure and massive uniformly distributed nano precipitates are mainly responsible for high performance. This work offers new insights for optimizing Mg-Bi-Al alloy composition to develop high-strength Mg alloys.