EFFECT OF STRONTIUM ON GROWTH OF α DENDRITE IN NEAR-EUTECTIC Al-Si ALLOYS

The infleunce of adding Sr in Al-11.6%Si and Al-11.6%Si-0.4%Mg alloys on the mor-phology of dendrite α phase was investigated and the amount of dendrite αphase and the characteristic parameters, the primary dendrite spacing and secondary dendrite arm spacing, were also measured. It was found that Sr addition can promote the growth of columnar dendrite and result in a consider-able increase of the amount of dendrite αphase; the primary dendrite spacing and secondary dendrite spacing decrease with increasing Sr content in modified near-eutectic Al-Si alloys. It was thought that the addition of Sr into Al-Si alloys caused considerable changes of properties of the alloy melts, that is, Sr could lead to a reduction of the solid-liquid interfacial energy of dendrite a. It is rational to conclude that the present of Sr in Al-Si alloy melts results in a decrease of the growth undercooling of dendrite tips and the nucleation of equaixed grains in the liquid in front of columnar dendrite tips is restrained, so the Sr addition in Al-Si alloys promotes the growth of columnar dendrite. The reduction of the solid-liquid interfacial energy also leads to the decreases of the primary dendrite spacing and the secondary dendrite arm spacing.