Effect of Hot Extrusion and Annealing Treatment on Microstructure and Mechanical Properties of In Situ Mg2Si and TiB2 Hybrid Particulates Reinforced Magnesium Matrix Composites

In situ Mg2Si and TiB2 nanoparticle-reinforced magnesium matrix composites were prepared using semi-solid stirring casting followed by ultrasonic treatment. The obtained composites were then subjected to hot extrusion and annealing treatment to investigate the effects on Mg2Si morphology, alpha-Mg grain size, and mechanical properties. The results showed that after hot extrusion, the eutectic Mg2Si phases transformed from Chinese-script shapes to shorter rod-like or particle-like forms, while the primary Mg2Si phases became finer. In addition, incomplete dynamic recrystallization and strong basal texture occurred during the extrusion process. The extruded composites exhibited significant improvements in Vickers hardness, tensile strength, and yield strength, with increases of 24.4%, 95.4%, and 211.5%, respectively, compared to the as-cast composites. However, the elongation dropped from 9.2% to 4.5%. After annealing at 250 degrees C for 1 h, static recrystallization occurred in the uncrystallized region, resulting in the formation of equiaxed alpha-Mg grains and decreased texture intensity. This led to a return of the elongation to 8%, despite a slight decrease in strength and hardness. Overall, the composites exhibited good comprehensive mechanical properties after hot extrusion followed by annealing.