The influence of calcium on the primary Mg₂Si phase in the hypereutectic Mg-Si alloys

The microstructure of Mg-5Si alloy consists of the primary coarse Mg₂Si phase, α-Mg solid solution and eutectic α-Mg + Mg₂Si, in which eutectic Mg₂Si phase solidifies in the form of Chinese script particles. When 0.2 wt.% of Ca was added to the Mg-5Si alloy the size of primary Mg₂Si phase remained unchanged. The modification effect of calcium on the primary Mg₂Si phase was effective only in the Mg-5Si-0.5Ca alloy. The morphology of the primary Mg₂Si phase is changed from the coarse dendrite shape to polyhedral shape and the size of primary crystals is significantly reduced. The addition of 0.6 wt.% Ca to Mg-7Si alloy did not cause the modification of primary Mg₂Si phase.