Effect of solute atom adsorption on heterogeneous nucleation by in-situ MgO particles: Experimental and theoretical studies

The introduction of oxide inclusions during the smelting process has a clear promising heterogeneous nucleation potency on Mg-based alloys, but the mechanism has not been explored clearly yet. In the present work, the grain refinement mechanism of MgO in pure Mg, Mg-3Al and AZ31 (Mg-2.9Al-0.9Zn-0.3Mn) alloys is investigated by combing first-principles calculations and experiments. The theoretical results show that solute atoms adsorption will affect the nucleation and the subsequent growth process, which is an important factor affecting the refinement efficiency. A contradiction between the experimental results and the grain growth restriction factor (GRF) theory is observed, that is the refinement ratio of AZ31 is worse than Mg-3Al alloy. This is explained by an adsorption model which reveals that Al promotes the adsorption of Mg on MgO surface so as to stimulate more particles available as nucleating sites. Meanwhile, Fe and Mn also have favorable effects on the adsorption of Mg, Zn may play the opposite role compared to Al. The theoretical analyzes provide a strong support to the experiments that Al benefits the initial nucleation of α-Mg on MgO to promote the grain refining effect of Mg-3Al prior to AZ31 alloy.