Rapid solidification of undercooled Co₈₅Cu₁₅ hypoperitectic alloy

Co₈₅Cu₁₅ hypoperitectic alloy melt was undercooled by 351 K (0.21T_L) with glass fluxing technique. The hypercooling limit of this alloy was determined to be 379 K. With increasing undercooling temperature, the microstructure transformed from dendrites into equiaxed grains and was refined drastically. Significant solute trapping has taken place during the rapid solidification of highly undercooled Co₈₅Cu₁₅ alloy, and it resulted in the remarkable extension of solubility of Cu in Co phase and much weaker intragranular segregation. It was also revealed by X-ray diffraction that the microstructures were comprised mainly by metastable supersaturated α-Co and Cu solid solutions together with a small amount of ∈-Co phase. In addition, the theoretical calculation for nucleation rate indicates that heterogeneous nucleation has still taken place at an undercooling of 351 K. Calculations of dendrite growth velocity, solubility of Cu in α-Co, partial undercoolings, and dendrite tip radius were also performed on the basis of LKT/BCT theory.