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In this study, the microstructure evolution on hot deformation was investigated and a dynamic recrystallization (DRX) kinetic model was constructed based on the hot deformation behavior of the homogenized state Mg–7Gd–3Y–1Zn–0.5Zr (VW73B) alloy within 713–773 K and 0.01–0.1 s−1. The DRX critical strain reduced when the deformation temperature (T) increased and increased when the strain rate (ε˙) increased. The variation trend of the DRX volume fraction was inversely proportional to the critical DRX strain. The constructed DRX kinetic model was modified by introducing a compensation factor; the modified model showed a mean absolute deviation of the DRX volume fraction of 1.8%. Combined with the microstructure analysis, it can be found that the Long Period Stacking Ordered (LPSO) phase is able to promote DRX during the hot deformation, but its ability to promote DRX is less pronounced than that of grain boundaries. At the block-shaped LPSO phase interface, the grain boundary arched by strain induction provided a nucleation site for DRX, enabling discontinuous dynamic recrystallization (DDRX). At the lamellar LPSO phase interface, continuous dynamic recrystallization (CDRX) occurred predominantly through the continuous migration and merging of subgrains, resulting in the formation of DRX grains. The results of this study could guide future research on the DRX of the VW73B alloys with varied applications.

The effects of extrusion ratio (ER) on the microstructures, static mechanical properties, and tension-compression high cycle fatigue (HCF) behavior of the as-extruded Mg–5Zn–1Mn (ZM51) alloy was investigated systematically, and the HCF mechanism was discussed. The results show that texture weakening and grain refinement caused by increasing the ER can effectively enhance the static mechanical properties and alleviate the tensile-compressive asymmetry of the alloy. The fatigue strength at 107 cycles is 124 ± 5, 138 ± 4, and 142 ± 5 MPa for ER8, ER11.5, and ER23, respectively, and the corresponding fatigue ratio is approximately 0.45, 0.49, and 0.50, which means that the as-extruded ZM51 alloy possesses outstanding fatigue resistance. The failure analysis and microstructure evolution of post-fatigued samples demonstrated that the stress amplitudes and microstructure characteristics strongly influence the HCF mechanism of the alloy. At high-stress load, close to the tensile yield strength 160–180 MPa, abundant {101‾2} residual twin bands were observed near the fracture surface. Twinning/detwinning deformation is the dominant fatigue mechanism due to the plastic deformation incompatibility between the matrix and the residual twins. At low-stress load, close to the fatigue strength 125–145 MPa, the fatigue crack initiation mechanism transits from twinning/detwinning to dislocation slip. Grain refinement and texture weakening inhibit the activation of {101‾2} twins, alleviate the twin-dislocation interaction, and promote dislocation slip to dominate the fatigue deformation, thereby enhancing the fatigue life of the alloy. This work provides new insight into the design and development for enhancing the fatigue resistance of wrought Mg alloy to ensure the long-term service safety of structural materials.

The thermal compression deformation behavior of homogenized Mg–5Zn–1Mn (wt.%) alloy was systematically investigated over the temperature and strain rate ranges of 285–345 °C and 0.001–1 s−1, respectively. The constitutive model and thermal processing diagrams were established. The microstructure evolution process and dynamic recrystallization (DRX) mechanisms were analyzed. The results demonstrate that the flow stress curves of the ZM51 alloy exhibit prominent DRX characteristics. The peak stress can be improved by decreasing the deformation temperature or increasing the strain rate. A finite element simulation is performed, and the Arrhenius constitutive model can accurately depict the plastic flow process of the alloy with strain correction, and the average activation energy is about 143.821 kJ/mol. Both the percentage DRX and the DRX grain size are found to be sensitive to the applied deformation conditions, including temperature, strain rate, and true strain. The formation of DRX grains promotes the weakness of the texture and the refinement of the grains. Grain boundaries nucleation (including discontinuous and continuous DRX) and particle-stimulated nucleation (PSN) are the main DRX mechanisms. Based on the processing map, ZM51 alloy is suitable for large plastic deformation by multi-pass forging. The optimum processing temperature is 345 °C, and as the degree of plastic deformation increases, the strain rate can be suitably increased. Local plastic flow and microcracking at grain boundaries are the primary instability mechanisms of the alloy.