Dendritic and eutectic solidification of undercooled CoSb alloys

Co-20 wt% Sb hypoeutectic, Co-41.4 wt% Sb eutectic, and Co-45 wt% Sb hypereutectic alloys were undercooled by up to 224, 225 and 177 K, respectively. The dendritic and eutectic growth velocities during rapid solidification were measured by an infrared technique. It is found that dendrite growth in the hypoeutectic alloy is still in the regime of solute diffusion-controlled growth even at an undercooling of 224K. A "dendritic-equiaxed" microstructure transition occurs if this hypoeutectic alloy undercooled by more than 90 K, resulting in an abrupt grain refinement effect. Two undercooling thresholds were observed for the eutectic alloy: below the lower limit of ΔT_el^* = 50 K lamellar eutectic is the unique growth morphology; above the upper limit of ΔT_e2^* = 150 K only anomalous eutectic can grow. Although anomalous eutectic is the product of rapid solidification, it shows much greater thermodynamic stability than the lamellar eutectic formed under slow solidification conditions. The hypereutectic alloy behaves like a eutectic alloy during the rapid solidification at sufficiently large undercoolings.