Microstructure evolution and grain refinement mechanism of fine-grained Mg-Gd-Y-Zn-Zr alloy during multi-directional forging

Fine-grained Mg-8.59Gd-3.85Y-1.14Zn-0.49Zr alloy was subjected to multi-directional forging (MDF). The corresponding grain refinement mechanism was analyzed by observing the microstructure before and after MDF, and the evolution mechanism of various phases and texture states was discussed. The results showed that the 9 passes MDF with deformation temperature of 350 ? and strain rates of 0.01 s(-1) and 0.1 s(-1) can further refine the grain structure. The average grain size decreases to 2.95 um at the strain rate of 0.1 s(-1). The refinement of the grain structure is dominated by dynamic recrystallization (DRX), while the beta phase maintains the refinement results through the pinning effect. The main refinement mechanism of the original equiaxed grains is discontinuous dynamic recrystallization (DDRX), and the growth of the original equiaxed grains and new grains is hindered by the beta phases. The dissolution of lamellar structures and continuous dynamic recrystallization (CDRX) at the deflected lamellar long-period stacking ordered (LPSO) lead to the fracture and refinement of large-sized deformed grains, while the beta phases precipitated precede CDRX can hinder the growth of new grains. The effect of the deformation temperature on the grain refinement is more notable than that of the strain rate owing to the significant difference in the number of beta phases at different temperatures. Because of the low nucleation rate and high dissolution rate associated with the high temperature sensitivity, the number of beta phases is low at 400 ? and can not effectively maintain the refinement results. The random orientation of DRX grains and change in the load direction of each pass promote the weakening of the basal texture, making the MDFed microstructure exhibits multi-textured components. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

The grain refinement mechanism of a fine-grained Mg-8.59Gd-3.85Y-1.14Zn-0.49Zr alloy during multi-directional forging was investigated. It was found that dynamic recrystallization and the pinning effect of beta phases were the main mechanisms for grain refinement, while discontinuous dynamic recrystallization and continuous dynamic recrystallization had different effects on the growth of original and new grains. The deformation temperature had a greater impact on grain refinement compared to the strain rate. Multi-directional forging also weakened the basal texture, resulting in a microstructure with multiple textured components.