Dynamic precipitation behavior and mechanical properties of hot-extruded Mg₈₉Y₄Zn₂Li₅ alloys with different extrusion ratio and speed

Dynamic recrystallization (DRX), dynamic precipitation and mechanical properties of the hot-extruded Mg₈₉Y₄Zn₂Li₅ (at. %) alloys with different extrusion ratio and speed are systematically investigated in this work. The results reveal the formation details of recrystallized grains in the presence of long-period stacking ordered (LPSO) phases during extrusion at 350 °C. In a low extrusion ratio of 4, the recrystallized grains start to form discontinuously along the fluctuant 18R/α-Mg interface and continuously along the 14H kink boundaries with high misorientation angles. As increasing of extrusion ratio from 4 to 16, besides discontinuous and continuous DRX, particle-stimulated nucleation (PSN) of recrystallized grains is accelerated by small 18R and 14H fragments. Moreover, various thin LPSO lamellae including 14H, 18R and 24H as well as γ′ phase are dynamically precipitated inside the DRX grains. Under extrusion with a high ratio of 25, the DRX is mainly produced by PSN mechanism, leading to an almost completely recrystallized microstructure. Also, most of the 18R blocks are broken down to small fragments while the 14H lamellae in the original α-Mg matrix are dissolved at the large deformation strain. In addition, decreasing extrusion speed from 1 to 0.1 mm/s at this extrusion ratio effectively limits the growth of DRX grains and facilitates the dynamic precipitation of thin 14H lamellae within the fine DRX grains. Consequently, the full and fine DRX microstructure, small 18R fragment and deformation kinking, abundant thin 14H lamellae in fine DRX grains result in the excellent strength-ductility balance through hot extrusion with ratio of 25 and speed of 0.1 mm/s.