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

The modification effects of Ge on Mg 2 Si 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, Mg 2 (Si x Ge 1- 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 Mg 2 Si crystal, the morphology of primary Mg 2 Si 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 Mg 2 Si phase, but a small amount of dispersed solute segregation with a size of 10 - 20 nm can be observed. In Mg-2.5Si1Ge alloy, many Mg 2 Ge precipitates with a small amount of Si dissolved having a size of 20 - 50 nm and a lattice constant a of 0.644 nm were uniformly distributed in primary Mg 2 Si, and only one orientation relationship (OR) in form of [00 1 ] Mg2Ge || [00 1 ] Mg2Si , (001) Mg2Ge || (00 1 ) Mg2Si with a tiny rotational angle of 0.39 degrees was found. The stiffness and hardness of Mg 2 Si phase decreased with the increase of Ge content. Despite Mg 2 (Si x Ge 1- 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 Mg 2 Ge precipitates and Mg 2 Si phase.