Achieving low tension-compression yield asymmetry and ultrahigh strength in Mg–1Al–1Ca–0.2Mn (wt%) alloy via sub-micron grain refinement

Traditional low-alloyed Mg–Al–Ca–Mn extrusion alloys always show a low strength and a high tension-compression yield asymmetry. In the present work, Mg–1Al–1Ca–0.2Mn (AXM1102, wt.%) alloys were extruded at 150–350 °C to produce different grain structure and mechanical properties. It is shown that AXM1102-150 (extruded at 150 °C) sample exhibits superhigh strength in both tension and compression, which exhibited a yield strength (YS) of 428 MPa in tension and 416 MPa in compression. Namely, ultrahigh strength and low tension-compression yield asymmetry were both obtained. The ultrahigh strength was found to be ascribed to the ultra-fine dynamically recrystallized (DRXed) grains (0.47 μm) together with grain boundary co-segregation of Ca and Al atoms. Submicron DRXed grains accounts for the improved tension-compression yield asymmetry through suppressing {10-12} twining during compression. Additionally, the discontinuous yielding was detected during tension of AXM1102-150 alloy, which is potentially related with the high energy barrier required for dislocation emission caused by grain boundary co-segregation of Al and Ca atoms. Once the tensile stress reaches the peak value, the mobile dislocation density increases immediately, thus the tensile stress-strain behavior of the AXM1102-150 alloy is dominated by strain softening. The results indicate that low-alloyed Mg–Al–Ca–Mn alloys demonstrate tremendous potential as next generation ultrahigh-strength and low tension-compression yield asymmetry wrought Mg alloys.