GRAIN REFINEMENT IN UNDERCOOLED Ni-50%Cu ALLOY

With undercooling increasing, the undercooled Ni-50%Cu alloy underwent two grain refinement processes. One occurred in the undercooling range of 50 -100 K and the other one at the undercoolings higher than the critical value 220K where the refined crystal possesses the dendritic substructures. It is suggested that the tendency to remelting of the primarily formed dendrite can be evaluated with the dimensionless superheating of the earliest frozen part in the dendrite stems during recalescence. The dimensionless superheating increases first and then decreases with increasing undercooling, and its highest value corresponds to the undercooling range in which the first grain refinement takes place. Both the thermodynamic analysis and the crystal cotheuration indicate that the grain refinement at low undercoolings is caused by the remelting of solid and that at higher undercoolings by stress which leads to the disintegration and recrystallization of the primary dendrite.