Poisoning effect of bismuth on modification behaviour of strontium in LM25 alloy

Nucleation and growth, temperature measurements and microstructure observations of silicon phase are presented for strontium modified Al–7%Si (LM25) cast alloy treated with bismuth. The results show that addition of bismuth in strontium modified alloys may have a poisoning effect resulting in lost modification of the silicon phase. With increasing Bi/Sr ratio, thermal analysis measurements showed that the eutectic growth temperature increased remarkably to 573°C and recalescence decreased to 0·2°C and the morphology of silicon displayed the same flake-like structure as in the unmodified alloys. Microstructural observation showed that a minimum Bi/Sr ratio of 1·2 which is equivalent to a Sr/Bi ratio of 0·43 is required for effective strontium modification and neutralization of the poisoning effect of bismuth.