Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression at 450 °C and 0.001 s−1 was investigated in detail. Based on microstructure analysis, Mg-Gd-Y-Zn-Zr alloys with bimodal grain size distribution and bimodal texture distribution were obtained by uniaxial isothermal compression. There were more heavily bimodal levels in grain size and texture at true strain greater than 0.8. There was a significant inhomogeneous plastic deformation between and within the grains in Mg-Gd-Y-Zn-Zr alloy during the deformation, which is attributed to the difference the grain orientation and a large number of intergranular block-shaped LPSO phases. Further studies revealed that particle-stimulated nucleation (PSN) caused by a large intergranular LPSO phases and continuous dynamic recrystallization (CDRX) developed layer by layer toward the grain interior were the main grain refinement mechanisms. This PSN and CDRX mechanism contributed to the fine grains in bimodal grain size distribution and the random texture in bimodal texture distribution. However, the original coarse grains with basal orientation were preserved during deformation due to smaller plastic deformation and energy storage. Meanwhile, grain boundary segregation and intragranular lamellar LPSO precipitate promoted CDRX by retarding DDRX. In addition, the grain boundary pinning effect dominated by grain boundary segregation and intragranular lamellar LPSO precipitate hindered the coarsening of fine DRXed grains, which was the ultimate guarantee for the formation of bimodal microstructure. Ultimately, inhomogeneous plastic deformation, DRX, and grain boundary pinning effect were demonstrated as the necessary conditions for the formation of heterogeneous bimodal microstructure during thermal deformation of Mg-Gd-Y-Zn-Zr alloy.