Effects of pre-annealing on microstructure and mechanical properties of as-extruded Mg-Gd-Y-Zn-Zr alloy

An as-extruded Mg-11.5Gd-4.5Y-1.5Zn-0.4Zr (wt %) alloy with an excellent strength-ductility balance has been successfully developed by pre-annealing and hot extrusion. The effects of pre-annealing on microstructure and mechanical properties have been studied. Results show that the Mg5RE (RE: rare earth) particles, which are generated by pre-annealing treatments, are cracked during hot extrusion, and resulting Mg5RE fragments not only enhance the recrystallization of particle simulated nucleation (PSN), but also improve the continuous dynamic recrystallization (C-DRX) by promoting the grain subdivision. After hot extrusion, the studied alloy exhibits a bimodal microstructure consisting of fine DRXed grains with relatively random orientations and coarse un-DRXed grains with strong basal texture. Increasing the pre-annealing duration raises both the quantity of Mg5RE particles and the fraction of DRX, thereby decreasing the strength but increasing the ductility. With pre-annealing for 1 h the studied alloy achieves the best strength-ductility balance with tensile yield strength (TYS) of 377 ± 1.2 MPa and elongation to failure (EL) of 10.8 ± 2.0%. Further pre-annealing degrades the ductility due to the excessive Mg5RE particles. The alloy strengthening is attributed to the bimodal microstructure, Mg5RE and long-period stacking ordered (LPSO) phases, solute segregated SFs, and texture.