Modification of Al-Si eutectic alloys with Na

Abstract

Quenching experiments have been carried out on cast Al-Si eutectic alloys to show that additions of Na affect both the nucleation and growth of the Si phase. The Na not only changes the growth morphology of the Si from the platelike to the fibrous form but also prevents Si nucleating ahead of the eutectic growth front. It is the latter effect, resulting in a change in the average growth velocity, which is responsible for the much finer spacings and larger undercoolings found in modified cast structures. The mechanism for the change in Si growth morphology is discussed and it is suggested that the change may be the result of non-faceted Si growth at high velocities or in the presence of Na.