Microstructure Evolution Mechanism and Mechanical Properties of Mg-RE Alloy at a Critical Transition Temperature of Material Performance

The present study aimed at investigating the microstructure evolution mechanism and mechanical properties of the rare earth magnesium alloy (Mg-RE alloy). The compression tests over a temperature range of 250-450 °C as well as the interrupted compression tests at a temperature of 350 °C and a strain rate of 0.1 s⁻¹ to strains of 0.02, 0.39, 0.79 and 1.48 were conducted by using Gleeble-3800. The results show that there exists a critical transition temperature of material performance. When the temperature is lower than 350 °C, the fracture occurs at the strain of 0.31 and there was poor toughness; on the contrary, when the temperature is higher than 350 °C, there is no crack even if the strain is up to 1.49. At the same time, the material exhibited pseudo-super plastic behavior. Furtherly, the microstructure tends to be uniform and fine grains exist around some large and long grains. And, many particles emerge in the grains and at the grain boundaries. These results imply that dynamic recrystallization occurs in advance during deformation. It is found that the second-phase particles play an important role in particle stimulated nucleation effect and promote dynamic recrystallization. With further increasing strain, θ becomes negative and then tends to zero. This is because that the work hardening and strain softening compete with each other and finally reach a dynamic equilibrium. Moreover, fine grains were formed at the particles, which provided a potential path for localized shear zones. During hot deformation process, the continuous dynamic recrystallization mechanism and grain boundary strengthening were dominated. The grain growth and the particles appearance became the primary mechanism of microstructure evolution. Besides, the optimal initial reduction of 32.3% is determined. These results can help guide the development of feasible processes to enhance strength and ductility.