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 tu be. 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 t heories, which covers a composition range from 22.7% to 49%Sb. A lamellar-a nomalous' eutectic transition occurs to Co-41.3%Sb droplets with the increa se of undercooling. For Co-20%Sb hypoeutectic alloy, primary alpha-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 dro p tube conditions. The primary beta-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 tr apping takes place in primary alpha-Co and beta-CoSb dendrites. (C) 2000 El sevier Science S.A. All rights reserved.