Achieving exceptional ultrahigh-strength in low-alloy Mg-1Al-1Ca-0.4Mn (wt%) alloy by regulating bimodal microstructure

A Mg-1Al-1Ca-0.4Mn (AXM1104, wt%) alloy extruded at 250 ℃ with a ram speed of 0.1 mm/s obtains a unique bimodal microstructure. As-extruded alloy exhibits a high tensile yield strength (YS) of 474 MPa but a low ductility of 2.1%, in which the YS exceeds conventionally extruded rare-earth-free Mg alloys reported so far. After annealing at 300 ℃ for 10 min, the annealed alloy achieves a good strength-ductility balance, with a YS of 426 MPa and an elongation of 5.4%. The ultrahigh YS of the as-extruded alloy is attributed to the combined strengthening efect of grain boundary, dislocation and texture, while the as-annealed alloy is mainly strengthened by both grain boundary strengthening and precipitation strengthening from planar Al₂Ca nano phase.