The eutectic growth behavior of unmodified and Sr modified Al-7 Si alloy was investigated. The eutectic Si in unmodified Al-7 Si alloy presents different morphologies with the increase of growth speed of directional solidification. At low growth speed of 2 μm/s

the flat plate eutectic Si sharing a center is interweaved into “star” morphology

and the lamellar eutectic Si has a “Wheatsheaf” morphology when the growth speed increased to a medium speed of 20 μm/s

while the quenched fibrous eutectic Si has a “Coral” morphology at high quenched speed. The different morphologies of eutectic Si at different growth speeds correspond to different growth and branching mechanisms. The Sr modified refined eutectic Si has a “Coral” morphology

which could be attributed to the synergy of the“Impurity-induced twinning mechanism”and the transition of {111}

Si

plane from smooth growth to rough growth.