Morphologies of primary silicon in hypereutectic Al–Si alloys with melt overheating temperature and cooling rate

The morphologies of primary silicon were studied in detail with different cooling rates and melt overheating temperatures. According to the present results, the morphologies of primary silicon are a strong function of the solidification conditions such as cooling rate and melt overheating temperature. With the elevation of melt overheating temperature, the morphologies of primary silicon will change from star-like and other irregular primary silicon to octahedral primary silicon and the size of primary silicon will gradually decrease. On the other hand, it can be safe to deduce that the Si–Si cluster of larger size should be beneficial for the formation of the nucleus of star-like and other irregular primary silicon and the Si–Si cluster of smaller size should be beneficial for the formation of the nuclei of octahedral primary silicon. With increasing cooling rate, the size of primary silicon will gradually decrease. If the cooling rate is lower than a certain (or critical) cooling rate, the cooling rate will have hardly effect on the morphologies of primary silicon in the solid state. If the cooling rate is higher than a certain (or critical) cooling rate, the cooling rate will play an important role in determining the morphologies of primary silicon in the solid state.