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

Abstract

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 µm at the strain rate of 0.1 s−1. The refinement of the grain structure is dominated by dynamic recrystallization (DRX), while the β 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 β 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 β 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 β phases at different temperatures. Because of the low nucleation rate and high dissolution rate associated with the high temperature sensitivity, the number of β 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 multitextured components.