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High strength–ductility synergy is difficult to achieve in Mg alloys. Although high strength has been achieved through considerable alloying addition and low-temperature extrusion, these techniques result in low ductility (2%–5%). In this work, a novel low-alloy Mg–Ca-based alloy that overcomes this strength–ductility trade-off is designed. The alloy has an excellent tensile yield strength (∼425 MPa) and exhibits a reasonably high elongation capacity (∼11%). A microstructure examination reveals that a high density of submicron grains and nano-precipitates provides the alloy high strength, and the leaner alloy additions and higher extrusion temperatures initially improve ductility. As a result, the density of residual dislocations is reduced, and the formation of low-angle grain boundaries (LAGBs) is enhanced. With fewer residue dislocations, it becomes less probable for the newly activated mobile dislocations to be impeded and transformed into an immobile type during the subsequent tensile test. The LAGBs function as potential sites to emit new dislocations, thus enhancing the dislocation–multiplication capability. More importantly, they can induce evident sub-grain refinement hardening and guarantee that the alloy achieves high strength. The findings lead to a controllable Mg alloy design strategy that can simultaneously afford high strength and ductility.

Exceptionally high strength Mg 3Al 1Zn-0.3Mn wt.% (AZ31) is demonstrated herein, revealing a yield strength of 380 MPa in both tension and compression, for extrusions prepared at 175 °C. Extruded AZ31 nominally has a low-to-moderate yield strength amongst typical magnesium (Mg) extrusion alloys, generally less than 250 MPa. The low strength is predominantly due to a large grain size and the absence of effective precipitation strengthening. In this study, AZ31 was extruded at different temperatures to reveal an exceptionally high strength with ultrafine grains of 0.65 μm in diameter. To reveal the origin of high strength in this AZ31 alloy, the microstructure of AZ31 was compared with those of Mg 1Zn alloy and pure Mg with similar grain size and textures. The solute atoms were identified to be the key to alloy strengthening (∼210 MPa). In contrast to grain boundary sliding, grain boundary migration and grain rotation that is observed submicron-grained pure Mg; the solute in submicron-grained AZ31 suppressed such intergranular deformation modes, with the grain boundaries in submicron-grained AZ31 providing significant strengthening (via the Hall-Petch relationship). In the high-strength AZ31 presented, Al reacts with Mn to form uniformly distributed particles, whilst Zn solute atoms segregated to grain boundaries, the latter posited to stabilize the boundaries of submicron grains by reducing grain boundary energy and thus suppressing the intergranular deformation. The results herein reveal high strength Mg-alloys are readily achievable, the related concepts of which have implications for numerous Mg alloy systems.

Texture evolution in cold-rolled Mg-0.3Zn-0.1Ca, Mg-0.4Zn and Mg-0.1Ca (at.%) alloys during static recrystallization is monitored using a quasi-in-situ electron backscatter diffraction (EBSD) method. The quasi-in-situ EBSD results show that most of recrystallized grains formed in the early stage of recrystallization have randomised orientations in the ternary alloy and they grow uniformly during the recrystallization process. The formation and uniform growth of these recrystallized grains with randomised orientations give rise to a weak texture in fully recrystallized samples of the ternary alloy. A weak recrystallization texture also forms in the early stage of recrystallization in the two binary alloys, but it is gradually replaced by a strong basal texture via the preferential growth of recrystallized grains with specific orientations. The grain size in the ternary alloy is smaller than those in the two binary alloys at each stage of recrystallization, and the grain size distribution in the ternary alloy is significantly narrower than those in the two binary alloys after full recrystallization. Solute segregation to grain boundaries is observed in all three alloys in the fully recrystallized state. It is hypothesised that Zn and Ca atoms in the ternary alloy segregate strongly to high-energy boundaries of the recrystallized grains that would otherwise grow preferentially in the counterpart binary alloys, and that this co-segregation would significantly reduce the boundary mobility, by reducing grain boundary energy and enhancing solute dragging effect, and therefore lead to a more uniform growth of recrystallized grains with randomised orientations.