The effects of growth restriction and effective nucleant potency on grain size and morphology in Al-Si and Al-Cu alloys

The effect of increased solute content on dendrite morphology and grain size in both AI-Si and AI-Cu alloys has been investigated. The results show that a morphological transition occurs in both alloy systems as the solute level is increased towards the respective eutectic points. Although this change in morphology is accompanied by an increase in grain size in the Al-Si system, this effect is not observed in AI-Cu. Instead, the grain size decreases rapidly with initial solute additions and then levels out as the eutectic composition is approached. The behaviour in the AI-Cu system can be explained by the effect of increased growth restriction with increased solute content, which promotes a progressively decreasing grain size. The results for the AI-Si alloys indicate that the increase in growth restriction occurs while there is a concomitant decrease in apparent nucleant potency. Hence the grain size is first reduced to a minimum, then increased as the 'poisoning' effect of silicon begins to dominate. The effect of grain refinement additions and cooling rate on grain size was subsequently examined in the AI-Si system to investigate and confirm the mechanisms responsible for the transition in grain size.