Morphology and properties of Mg2Si phase modified by Ge in Mg-Si alloys

The modification effects of Ge on Mg2Si phase in a Mg-2.5Si alloy were studied by computational simulation and experimental investigation. Based on first-principles structural prediction and electronic structure calculation, Mg2(SixGe1-x) phases were structurally stable and had sufficient hardness and inherent toughness as a reinforcement phase. With the increase of Ge content, due to the preferential adsorption of Ge atoms on the {100} planes of Mg2Si crystal, the morphology of primary Mg2Si crystal changed from equilateral-dendrite to polygonal, and finally to quadrilateral. In Mg-2.5Si-0.5Ge alloy, Ge was almost completely dissolved in the Mg2Si phase, but a small amount of dispersed solute segregation with a size of 10-20nm can be observed. In Mg-2.5Si-1Ge alloy, many Mg2Ge precipitates with a small amount of Si dissolved having a size of 20-50nm and a lattice constant a of 0.644nm were uniformly distributed in primary Mg2Si, and only one orientation relationship (OR) in form of [001]Mg2Ge || [001]Mg2Si, (001) Mg2Ge || (001)Mg2Si with a tiny rotational angle of 0.39° was found. The stiffness and hardness of Mg2Si phase decreased with the increase of Ge content. Despite Mg2(SixGe1-x) crystal models only considering the solid solution of Ge and neglecting segregation and precipitation, the theoretical predictions of mechanical properties remained consistent with experimental results. This agreement was primarily due to the unique OR between Mg2Ge precipitates and Mg2Si phase.