Annealing-induced hardening in an ultrafine-grained Mg–Zn–Ca alloy processed by high pressure torsion

The microstructural evolution and hardness behavior of an Mg–1Zn–0.2Ca alloy processed by high-pressure torsion (HPT) and subsequent annealing were systematically investigated. The initial grain size of ∼105 μm was refined to the ultrafine regime after HPT processing and subsequent annealing, with average grain sizes of 1.62–1.97 μm depending on annealing time. Post-deformation annealing at 300–400 °C revealed an unusual non-monotonic hardness response characterized by an initial decrease followed by a pronounced increase with increasing annealing time. Quantitative optical microscopy confirmed progressive grain coarsening during annealing, indicating that the secondary hardening cannot be attributed to further grain refinement. Electron backscatter diffraction (EBSD) analysis revealed a reduction in local misorientation and grain orientation spread at intermediate annealing times, indicating heterogeneous recovery rate within the ultrafine-grained structure. With increasing annealing time, a decrease in basal texture intensity was observed, accompanied by crystallographic reorientation. These microstructural and textural changes modify the deformation geometry under indentation and are responsible for the abnormal hardness increase observed during annealing. These findings highlight the critical role of microstructural recovery and crystallographic orientation in governing the post-annealing mechanical response of HPT-processed Mg alloys.