Effect of Coupled Extrusion and Heat Treatment on the Microstructure and Properties of Magnesium Matrix Composites

In this work, 2.0 wt.% SiCp/AZ91D magnesium matrix composite was fabricated by stir casting, and its microstructure and properties were optimized through a coupled process of parallel equal-channel angular combined extrusion (PC-ECAP) and T6 heat treatment. The results indicate that the extrusion temperature has a significant influence on the microstructure and mechanical properties of the material. At an extrusion temperature of 350 °C followed by T6 heat treatment, the 2.0 wt.% SiCp/AZ91D composite exhibits a tensile strength of 221 MPa, an elongation of 19.2%, and a product of tensile strength and elongation (PSE) of 4.24 GPa%, which represent increases of 14.5%, 128.6%, and 161.7%, respectively, compared with the as-cast specimen. To elucidate the microscopic mechanism of the enhanced ductility, first-principles calculations were further performed. It is found that Al solute atoms can reduce the electron localization in the Mg–Mg bond region, causing a downward shift of the d-band center, thereby weakening the interatomic bonding strength on the slip plane. This effect is equivalent to reducing the stacking fault energy of non-basal slip. The finding provides a theoretical explanation for the activation of multiple slip systems and the suppression of twinning observed in the experiments. By combining experiments with calculations, this study systematically reveals the mechanism underlying the synergistic regulation of strength and ductility in the composite by PC-ECAP coupled with T6 heat treatment, offering a theoretical basis and process reference for the fabrication of high-performance magnesium matrix composites.