Hot Tear formation and coalescence observations in organic alloys

Hot tear formation has been observed during the solidification of a succinonitrile–acetone (SCN-acetone) alloy by pulling the columnar dendrites in the transverse direction with a stick. It is observed that the cracking of the mushy zone (hot tears) always occurs at grain boundaries. At low volume fraction of solid, the opening can be compensated for by leanersolute interdendritic liquid (i.e., “healed” hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores or air bubbles induced by solidification shrinkage. Moreover, coalescence/bridging of dendrite arms has been carefully observed and the temperature at which this occurs has been measured. This allowed us to determine the corresponding solid fraction. It is observed that coalescence between columnar dendrites inside a grain (intragranular coalescence) occurs at a higher temperature/lower solid fraction than coalescence of dendrites. Located across a grain boundary (intergranular coalescence), these results bring a new light on the formation of hot tears in metallic alloys.