The modification effects of sodium salt and rare earth lanthanum as well as compound modification of Na salt+La on microstructures and hardness of hypoeutectic Al-10 Si alloy were investigated.Meanwhile

the modification mechanism of sodium salt and rare earth lanthanum was analyzed.The results show that the grain exhibits fine equiaxed structure after Na-La compound modification

and the coarsensess of grain after annealing can be ignored.Eutectic silicon appears branch and finer to be fiberlike.The primary dendrite tends to fusing and presents equiaxed

and the interface between eutectic and primary phase exhibits clear and smooth.The effect of annealing on microstructure and morphology of alloy after sodium salt and rare earth lanthanum compound modification is slight.The hardness of the modified alloy is increased.The mechanism of sodium salt modification is that it can trigger twin crystal of eutectic silicon and continuously block the groove of twin crystal

while the mechanism of rare earth lanthanum modification dependent on its gathering in the front of the solid-liquid interface changing the growing characteristic of eutectic silicon.