Ultrasonication-assisted magnesiothermic synthesis of cast in-situ graphene-MgO reinforced AZ91 magnesium hybrid nanocomposites

A sustainable liquid metallurgy route was developed for the in-situ synthesis of graphene (Gr)@MgO hybrid reinforcements in AZ91 magnesium alloy via simultaneous CO 2 bubbling and ultrasonication. The magnesiothermic reduction of CO 2 enables the formation of multilayer Gr decorated with nanoscale (< 5 nm) MgO particles, as confirmed by Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The AZ91 composites exhibit significant microstructural refinement, with the average α-Mg grain size reduced by approximately 67%. This refinement, combined with the presence of Gr@MgO, leads to marked improvements in mechanical properties, with hardness, yield strength, and ultimate tensile strength (UTS) increasing by up to 20%, 96%, and 18%, respectively. Partial formation of nano-sized Al 4 C 3 was also observed, indicating interfacial reactions between Gr and the AZ91 matrix. The enhanced performance is attributed to the combined effects of grain refinement, load transfer, and dislocation pinning induced by the in-situ formed hybrid reinforcements. A four-stage mechanism is proposed to describe the formation and dispersion of Gr and MgO during CO 2 bubbling under ultrasonic treatment, involving bubble expansion, interfacial reaction, particle growth, and cavitation-assisted dispersion. These results demonstrate a potentially scalable approach for producing high-performance Mg matrix nanocomposites.