Microstructure evolution in undercooled Co₈₀Pd₂₀ alloys

High undercooling has been achieved in Co₈₀Pd₂₀ melts by employing the method of molten glass denucleating combined with cyclic superheating, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0-415 K, two kinds of grain refinements have been observed in the solidification microstructures. The three critical undercoolings are 72, 95, and 142 K, respectively. When undercooling is less than 72 K, the coarse dendritic morphology is formed, which is similar to the conventional as-cast microstructure. The first grain refinement occured in the range of undercooling, 72-95 K can be attributed to the breakup of dendrite-skeleton owing to remelting. When undercooling locates within 95-142 K, highly developed directional fine dendrite can be obtained because the severe solute trapping weakens the effect of solute diffusion during the dendrite growth. The second grain refinement occurred when undercooling exceeds the critical undercooling (ΠT* = 142 K), the formation of fined equiaxed microstructure can be ascribed to the stress that originates from the extremely rapid solidification process, which resulted in the dendrite fragmentation finally.