Eutectic grain size and strontium concentration in hypoeutectic aluminium–silicon alloys

Recently it has been shown that modification with strontium causes an increase in the size of eutectic grains. The eutectic grain size increases because there are fewer nucleation events, possibly due to the poisoning of phosphorus-based nuclei that are active in the unmodified alloy. The current paper investigates the effect of strontium concentration on the eutectic grain size. In the aluminium–10 wt.% silicon alloy used in this research, for fixed casting conditions, the eutectic grain size increases as the strontium concentration increases up to approximately 150 ppm, beyond which the grain size is relatively stable. This critical strontium concentration is likely to differ depending on the composition of the base alloy, including the concentration of minor elements and impurities. It is concluded that processing and in-service properties of strontium modified aluminium–silicon castings are likely to be more stable if a minimum critical strontium concentration is exceeded. If operating below this critical strontium concentration exceptional control over composition and casting conditions is required.