Experimental and theoretical investigation on refining mechanism of Mg2Si by Y addition in the Mg-Al-Si alloys

Aiming at development of new lightweight and high-strength magnesium alloys, a combined experimental and theoretical study on refining mechanism of Mg ₂Si by Y addition in the Mg-Al-Si alloys was performed. Two alloys, Mg _63.32Al _22.83Si _13.85 and Mg _62.56Al _22.55Si _14.36Y _0.52 (wt.%), were prepared and investigated by combining experimental characterization with quantum-mechanical and CALPHAD calculations. The experimental results indicated that Y can refine the Mg ₂Si phase in Mg-Al-Si alloys but it is rather sensitive to the preparation procedure of the alloys. It was observed that Al ₄MgY is surrounded by Mg ₂ Si in the as-cast alloys with Y, but it undergoes separation after homogenization annealing. First-principles calculations revealed that the mechanism of Y refining the Mg ₂Si phase is direct adsorption on the Mg ₂Si surface rather than indirect adsorption in the form of the elements atomic substitution or entering the center interstitial sites of Mg ₂Si. The significant refinement of the Mg ₂Si phase after annealing is attributed to recrystallization within the grains, eliminating the original grain boundaries, and the surface activity is reduced with Y adsorption, resulting in the formation of new and smaller Mg ₂Si phases. The mechanism behind the Al ₄MgY phase being surrounded by Mg ₂Si in the as-cast state but subsequent separation after homogenization annealing is not heterogeneous nucleation, but maybe due to liquid entrapment.