Abstract: We have studied the microstructure evolution and deformation behavior of Mg-5Gd-3Y-(1Sn)-0.5Zr alloys during hot compression (T = 350 °C, 400 °C, 450 °C and 500 °C, e ˙ = 0.002 s−1, 0.01 s−1, 0.1 s−1 and 1 s−1) by transmission electron microscopy and electron backscattering technology. Sn can promote dynamic precipitation to activate the particle-stimulated nucleation (PSN) mechanism induced by the cluster precipitates and promote and dynamic recrystallization (DRX); in addition, Sn can inhibit the formation of high-angle grain boundaries (HAGBs) by reducing the activation of pyramidal and slip and delaying DRX. The two processes are in a competitive relationship with each other in the hot deformation of Mg-Gd-Y-Sn-Zr alloys. At low temperatures (350 °C–400 °C) and high strain rates, the former dominates: DRX is promoted, accompanied by a decrease in flow stress. At high temperatures (450 °C–500 °C) and low strain rates, the latter is dominant due to the absence of dynamic precipitation: DRX is delayed, and flow stress is increased accordingly. Flow stress between the two extreme deformation conditions is determined by the competitive relationship between them. We also found that the addition of Sn could increase the thermal deformation activation energy of Mg-Gd-Y-Zr alloys, weaken the texture and inhibit twin growth. Finally, we constructed a schematic diagram of the DRX mechanism during the thermal deformation process to illustrate the effects of PSN, CDRX, and DDRX on the evolution of the microstructure in detail.
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