Effect of trace bismuth on the solidification structure of hypereutectic Al-22Si alloys

The effect of trace Bi on the solidification structure of the Al-22Si alloy was studied by optical microscopy, scanning electron microscopy, field emission transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry. Trace amounts of Bi were found to have an inhibitory effect on the growth of primary silicon in the Al-22Si alloy. The average size and area fraction of primary silicon reached a minimum when 0.05 wt% Bi was added. As the bismuth content gradually increased, the average size and area fraction of primary silicon continued to increase. The addition of Bi improved the morphology of eutectic silicon in the Al-22Si alloy, as it gradually changed from long needle fiakes to short needle fiakes. Stacking faults formed in eutectic silicon may have resulted in its morphogenesis.