Superior strength-ductility synergy and mechanical performance of sub-micron AZ31 magnesium alloy from 4K to 298K

AZ31 magnesium alloys with grain sizes of 600 nm and 800 nm were produced using a combination of melt-spinning and plastic consolidation (PC), achieved through cold compaction and hot extrusion. The alloys exhibit high strength and improved ductility, demonstrating that the processing approach effectively addresses the strength-ductility trade-off offering a novel technique for microstructural design of lightweight magnesium alloys. The mechanical properties of the alloys were examined under tensile and compressive loading at 4K, 78K, and 298K. The yield strength adheres to the Hall-Petch relationship, with grain boundary strengthening being the primary contributor to yield stress across all temperatures. The solid solution and particle strengthening mechanisms contribute an increasingly larger fraction of the yield stress at cryogenic temperatures. Compression deformation is governed by {101¯2} extension twinning, leading to unprecedentedly high work-hardening rates (Θ). The tensile deformation mainly occurs by dislocation slip at considerably lower Θ. The alloys exhibit low strain rate sensitivity, ranging from 0.015 to 0.03 at 298K and from 0.004 to 0.012 at 78K. Analysis of activation volume and activation distance supports the conclusion that plastic deformation is controlled by dislocation-mediated mechanisms rather than grain boundary sliding, even though the grain sizes are well below the 3 µm threshold for inverse Hall-Petch softening. The segregation of Zn solute at the grain boundaries were found responsible for the enhanced alloys’ resistance to intergranular deformation.