Microstructure, texture and mechanical properties evolution of extruded fine-grained Mg-Y sheets during annealing

The microstructure, texture and mechanical properties evolution of the extruded fine-grained Mg-(1, 5) Y (wt%) alloy sheets were investigated during annealing. The as-extruded sheets exhibited a fully dynamic recrystallized (DRXed) microstructure consisting of uniform and fine equiaxed grains (5.7-8.7 µm) and a small amount of YH2 second phase particles. Both sheets exhibited significant thermal stability at 300 °C annealing and remarkable grain growth at temperatures higher than 400 °C. The measured grain growth activation energy (Q) of Mg-1Y at all temperatures tested was 91 ± 3 kJ/mol, suggesting that the growth was controlled by grain boundary diffusion. Meanwhile, for Mg-5Y alloy at lower temperatures (300-400 °C), the Q value (59 kJ/mol) indicated that the grain growth was controlled by grain boundary diffusion while the Q value (175 kJ/mol) at higher temperatures (400-450 °C) implied that lattice self-diffusion controlled the process. A representative rare-earth texture was present for both sheets in the as-extruded condition, and remarkable basal texture weakening was observed with the grain growth during each isothermal annealing process. The texture weakening can be ascribed to the preferential growth of non-basal oriented grains based on the EBSD analysis, which was likely related to the segregation of Y atoms at grain boundaries. The microhardness and grain size relationship can be described well by the Hall-Petch relation for all the annealed sheets, while the yield stress was not the case indicating that the macroscopic strength was more sensitive to the texture than the localized microhardness.