Achieving significant grain refinement efficiency in Mg-Al alloys via a GNP@MgO composite refiner

Master alloys containing Zirconium (Zr) serve as effective grain refiners in most commercial cast magnesium (Mg) alloys. However, their efficacy is diminished in Mg-Al series alloys due to the formation of Al-Zr compounds. In this work, a novel Mg-GNP@MgO master alloy was prepared utilizing the in-situ reaction between CO2 and Mg. The grain refinement efficiency of this composite inoculation on Mg-9Al alloys was systematically investigated using SEM-EBSD, HRTEM observations, Density Functional Theory (DFT), and Sharp Interface Model (SIM) calculations. The findings suggest that the composite grain refiner (GNP@MgO) exerted a notable refining influence on Mg-9Al alloy. Through the conventional casting process, the average grain size was refined to approximately 13 mu m upon the addition of 3 vol% GNP@MgO, achieving a grain refinement efficiency of 90 %. The enhanced refining efficiency arises from the synergistic control over the nucleation and growth processes of alpha-Mg grains, facilitated by GNP@MgO particles. This process includes improved heterogeneous nucleation, triggered by the newly formed Al4C3 phases through the interaction between GNPs and Al element, followed by the limited expansion of alpha-Mg grains induced by the nano-sized MgO particles. The MgO particles predominantly gather around grain boundaries to establish stable nano-coating layers, thereby exerting a substantial Zener pinning effect. The pinning role played by the MgO nanoparticles was also verified by a sharp interface model. The findings in this work not only carve out a new avenue for the grain refinement of cast Mg-Al alloys but also inspire fresh perspectives for the development of novel grain refiners.