In situ observation of hot tearing formation in succinonitrile-acetone

Hot tears have been induced during the solidification of a succinonitrile-acetone alloy by pulling the columnar dendrites in the transverse direction with a pulling stick. The opening of the mushy zone (hot tears) always occurred at grain boundaries. At low volume fraction of solid, the: opening can be compensated by leaner-solute interdendritic liquid (i.e., healed hot tears). At higher volume fraction of solid, hut tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores induced by solidification shrinkage. Their surface (edge) is made of secondary dendrite arms, which have not yet bridged, but a few spikes have also been observed. These later spikes formed either by the necking of solid bridges established across the grain boundaries prior to pulling, or by the sudden break-up of the liquid Blm during pulling. Similar spikes have been found by SEM on the hut lear surface of an aluminium-copper alloy. (C) 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.