Primary Si phase was finally evolved into spheroidal structure in the modified hypereutectic Al-Si alloy

which was attributed to the coupled twins induced by modifier atom in silicon crystal greatly decreasing anisotropy of primary Si phase growth based on the theory of coupled twins induced by impurity atom.Na-enriched region can be observed in primary Si phase as a result of Na atom absorbed at sub-boundary in coupled twin.Crack of primary Si phase is attributed to sub-boundary decreasing strength of primary Si and Na-enriched region deteriorating further strength of sub-boundary as well as existing thermal stress resulting from difference of thermal expansion coefficient between α-Al phase and Si phase.Crack in spheroidal primary Si appears in solid contract after the hypereutectic Al-Si alloys solidification.