Effect of overheating treatment on morphological evolution of highly undercooled Al-70%Si-0.5%RE alloys

The effect of overheating on the morphological evolution of primary silicon in undercooled Al-70%Si-0.5%RE alloys was investigated by observing and recording their growth processes and then analyzing the as-solidified microstructure. Investigations revealed that an increase in the overheating and undercooling temperatures, transforms the primary silicon's growth morphology from dendrite with several long and thin branches to a blocky shape and then to equiaxed grains as the silicon atom clusters disappear. In addition, we observed that, in contrast to previously reported and theoretically predicted critical undercooling promoting morphology transi-tion, the critical undercooling actually deceased the transformation from dendrite to blocky shape and then to equiaxed grains.