Effect of dynamic recrystallization, LPSO phase, and grain boundary segregation on heterogeneous bimodal microstructure in Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy

The formation conditions and influence mechanisms of the heterogeneous bimodal microstructure of Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy were systematically studied by thermal compression experiments in the range of 0.001–1.0 s−1 and 350–500 °C. Combined with the microstructural characterization, the inherent relationships between the grain structure and the deformation parameters are deeply analyzed. The results indicate that heterogeneous bimodal microstructures composed of fine dynamically recrystallized (DRXed) grains with random orientation and coarsely deformed grains with basal orientation can only be formed under deformation conditions of 40.51 < lnZ < 46.33. This mainly stems from the dominant continuous dynamic recrystallization (CDRX) and particle stimulated nucleation (PSN) mechanisms under this deformation condition. However, dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX) dominate isothermal compression process with lnZ > 47.41 and lnZ < 39.74, which eventually lead to unimodal distributions of deformed grains and DRXed grains, respectively. Furthermore, results reveal that deformation parameters have a strong influence on the long-period stacking order (LPSO) phase and grain boundary (GB) segregation. The lamellar LPSO phases and grain boundary co-segregation of Zn, Y, and Gd atoms jointly promote CDRX and inhibit DDRX by preventing GB migration under the condition of 40.51 < lnZ < 46.33, which is ultimately beneficial to the development of heterogeneous bimodal Mg-Gd-Y-Zn-Zr alloy.