A New Perspective on the Nucleation, Growth Morphology and Modification of the Silicon Phase During the Formation of Eutectic Al-Si Grains

Abstract

Modification of the Al-Si eutectic has two effects: converting the unmodified flake-like silicon to refined fibrous silicon and reducing the number of nucleation events, resulting in large eutectic grains. The first effect is beneficial by increasing strength and toughness, while the second may influence the formation of porosity and casting defects. These two effects are interconnected as they are both affected by the degree of undercooling required to nucleate silicon. In this analysis, we consider the role of constitutional supercooling in controlling the eutectic grain size in response to differences in the nucleation undercooling of the silicon phase due to strontium addition, and the constitutional effect of ternary copper additions. It is shown that increased nucleation undercooling simultaneously increases the eutectic grain size and its growth rate, while the addition of copper decreases the eutectic grain size in accordance with the Interdependence model.