Microstructural characterization of cobalt–antimony eutectic alloy droplets solidified in drop tube

Co–Sb alloy droplets have been solidified during free fall in a 3 m drop tube. The cooling rate and undercooling are estimated versus droplet diameter, both of which play a dominant part in dendritic and eutectic growth. The eutectic coupled zone has been calculated on the basis of TMK and LKT/BCT theories, which covers a composition range from 22.7% to 49%Sb. A ‘lamellar-anomalous’ eutectic transition occurs to Co-41.4%Sb droplets with the increase of undercooling. For Co-20%Sb hypoeutectic alloy, primary α-Co phase experiences a ‘dendritic to equiaxed’ structural morphology transition and an abrupt grain refinement at a critical undercooling of about 88 K under drop tube conditions. The remaining interdendritic liquid does not solidify in a mode of eutectic growth but forms into a divorced structure under drop tube conditions. The primary β-CoSb intermetallic compound of Co-45%Sb hypereutectic alloy grows in a dendritic way inside droplet and forms as a spherical shell on droplet surface. Most Co-45%Sb droplets undercooled by 80∼130 K exhibit complete eutectic microstructures. Significant solute trapping takes place in primary α-Co and β-CoSb dendrites.