Significant grain refinement and mechanical property enhancement of AZ91 alloy via synergistic regulation of α-Mg nucleation and growth using a novel GNP@MgO nanoparticles

Grain refinement is widely recognized as an effective strategy for simultaneously improving the strength and plasticity of Mg alloys. In this study, a novel GNP@MgO nanoparticles with a unique composite structure were synthesized by utilizing the in-situ reaction between CO₂ and Mg. The results showed that the confinement effect of MgO particles on the GNP surface enables GNP@MgO to achieve uniform dispersion in the Mg matrix. Simultaneously, the GNP@MgO nanoparticles also significantly refined the grain structure and improved the mechanical properties of the AZ91 alloy. The average grain size of the AZ91 alloy was reduced markedly from 535 to 77 μm, corresponding to a grain refinement efficiency of 86%. The significant grain refinement was mainly attributed to the synergistic regulatory effect of graphene and magnesium oxide, specifically: the Al₄C₃ phase formed through the in-situ conversion of graphene promoted the heterogeneous nucleation of α-Mg grains; MgO nanoparticles located near grain boundaries effectively restricted grain growth. Furthermore, HRTEM analysis of Al₄C₃/Mg interface confirmed a specific crystallographic orientation relationship between Al₄C₃ and α-Mg, described as 13 (003)_Al4C3//(002)_Mg, and [1-210]_Al4C3//[11-20]_Mg. Additionally, the formation and growth process of the Al₄C₃ phase in the Mg-Al-C system is discussed in detail, and the interfacial diffusion model demonstrates that rod-like Al₄C₃ morphology results from the markedly increased lateral growth relative to the longitudinal growth of the Al₄C₃ phase. This work provides new design ideas and theoretical guidance for the microstructure control and performance improvement of cast magnesium alloys.