Effects of Mg17Al12 phase on microstructure evolution and ductility in the AZ91 magnesium alloy during the continuous rheo-squeeze casting-extrusion process

In this study, a high-ductility AZ91 magnesium alloy was fabricated by the novel continuous rheo-squeeze casting-extrusion (CRSCE) process. The semi-solid slurry was prepared by ultrasonic vibration (UV) treatment, then solidified under pressure, and finally hot extruded. UV treatment can reduce the Al element content in primary grains and increase it in secondary grains. The refined Mg17Al12 phase was scattered along secondary grain boundaries and then stretched into narrow, fibrous bands during the hot extrusion. The fibrous bands with proper separation distances can accelerate the dynamic recrystallization (DRX) process and suppress the growth of DRXed grains. Microcracks were initiated inside the brittle Mg17Al12 phase and tended to propagate along the continuously distributed Mg17Al12 phase during the tensile test. Thus, the tiny Mg17Al12 phase in the billet and narrow, fibrous bands in as-extruded rods can prevent cracks from spreading and enhance ductility. Therefore, excellent comprehensive mechanical properties were obtained, with an ultimate tensile strength of 326.3 MPa and an elongation of 16.46%. The CRSCE method offers a novel way to fabricate high-ductility and high-alloyed magnesium alloys without homogenization. Microstructure regulation mechanisms of CRSCE, microstructural hereditary laws, and the effect of the Mg17Al12 phase on mechanical properties were further discussed.