Effects of initial undercooling on microstructure formation and recrystallisation of undercooled melts

The critical undercoolings for the two grain refinement events and the onset of recrystallisation event are determined by detailed analysis of the microstructure evolution of bulk undercooled Ni–20 at.-%Cu alloy melts. The first grain refinement event occurred in the low undercooling range was explained by dendrite remelting. The second grain refinement event occurred in the high undercooling range was due to the combined effects of dendrite remelting stress-induced dendrite breakup during recalescence and recrystallisation during the near-equilibrium solidification stage after recalescence. The micro-stress induced by the solidification contraction during recalescence in the so called ‘first mushy zone’ would lead to distortion and breakup of primary dendrites. The stress-induced broken-up dendrites have sufficient driving force for recrystallisation.