The phenomena of the leading and flake-fiber transitions of the eutectic Si phase in the eutectic solidification of hypo eutectic Al-Si alloy have yet to be explained clearly. In order to clarify these phenomena, the morphology of eutectic Si in Al-Si (-P) alloys was investigated. The results are as follows.

1) The leading transition of the Si phase was confirmed when quenching was performed during the eutectic solidification of Al-7%Si alloy containing P. The leading phase of eutectic solidification in Al-Si alloys was determined by the composition of the residual liquid phase which is the eutectic liquid/solid interface. The α-Al phase is thought to be the leading phase in the case of the hypo eutectic composition, and Si phase that in the case of the hyper eutectic composition.

2) The eutectic Si phase changes from the morphology of minimum eutectic α-Al/eutectic Si interfacial energy to the morphology of kinetics priority with increasing velocity of the solid/liquid interface. It is considered that the eutectic Si phase (non-faceted) transitions from plate-like to rod-like in order to decrease the high eutectic α-Al/eutectic Si interfacial energy.