Grain refinement in the solidification of undercooled Ni–Pd alloys

Two bulk Ni–Pd alloys Ni₇₅Pd₂₅ and Ni_54.8Pd_45.2 have been undercooled beyond their hypercooling limits. As undercooling increases, two grain refinements, occurring at low and high undercooling, respectively, were observed in the solidification of both alloys, even though the equilibrium crystallization temperature range of the latter is only 5 K. Reaching of the hypercooling limit did not change the microstructural morphology abruptly, and dendritic substructure could still be found in the refined grains. Such experimental results cannot be interpreted satisfactorily by the theory that the break-up of dendrites during solidification is dominated by capillary force. It is proposed that the grain refinement at low undercooling results from the remelting associated with chemical superheating, but that at high undercooling is due to the post-solidification recrystallization.