Modification of eutectic Si in hypoeutectic Al-Si alloy with novel Al-3Ti-4.35La master alloy

Abstract

Grain refinement and eutectic Si modification of hypoeutectic Al-Si alloys are crucial to improving their mechanical properties. In this paper, the effects of a novel Al-3Ti-4.35La master alloy on the morphology of eutectic Si and mechanical properties of hypoeutectic Al-7Si alloy were studied, and the modification mechanism on eutectic Si was revealed. The results show that the addition of 0.2 wt% Al-3Ti-4.35La master alloy can transform the eutectic Si phase from the coarse plate and needle to fine fiber and partial granular structure. After modification, the ultimate tensile strength and elongation of the Al-7Si alloy reached 178.3 MPa and 12.0%, which are enhanced by 15.6% and 106.9%, respectively, over the unmodified alloy. A detailed analysis revealed that, on one hand, the addition of Al-3Ti-4.35La master alloy can reduce the nucleation temperature of eutectic Si in Al-7Si, shorten the eutectic reaction time, and inhibit its growth. On the other hand, the morphology of eutectic Si can also be affected by altering its growth behavior. The twinning density in the eutectic Si after the modification increases significantly while the growth direction is anisotropic. The modification mechanism of the Al-3Ti-4.35La master alloy on eutectic Si is believed to be consistent with the impurity-induced twinning mechanism (IIT). This work provides important insights into alloy modifications by rare-earth master alloys.