Magnesium (Mg) alloys are the lightest metallic structural materials, holding significant potential for automotive, aerospace, electronic, and biomedical applications. However, their broader adoption is impeded by inherent drawbacks, including low strength, limited ductility, and poor corrosion resistance. High-pressure torsion (HPT) has proven effective in generating ultrafine-grained (UFG) Mg alloys, resulting in substantial property enhancements. This review critically assesses the microstructure evolution of HPT-processed Mg alloys covering not only grain refinement but also solute segregation, texture evolution, dissolution and precipitation of second phases, allotropic transformation, crystal-to-amorphous transition and nanocrystallization. In particular, it elucidates the impact of these microstructures’ evolutions on mechanical properties, including yield strength, hardness and superplasticity. Additionally, the review discusses the improvements in the addresses the functional augmentation of HPT-processed Mg alloys, specifically corrosion behavior, hydrogen storage capabilities, and biomedical performance.

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg₅RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.