Solidification behavior of Co-Sn eutectic alloy with Nb addition

Abstract

(Co76Sn24)100-xNbx (x = 0, 0.5, 0.8, 1.0) eutectic alloy melts were solidified at small undercooling for investigating the effect of Nb addition on microstructural development. With increasing Nb content, the solidification interface transits from eutectic seaweed (x = 0) to eutectic dendrite (x = 0.5) as a response to the change in the anisotropy of interfacial energy. Coupled eutectic growth can no longer be maintained within the main stems at x = 0.8 because the enrichment of Nb in the liquid ahead of the interface causes a significant difference in growth velocity between the α-Co and β-Co3Sn2 phases and, as such, Co3Sn2 doublons form. For x = 1.0, the difference in growth kinetics between the two eutectic phases is so large that divorced eutectic growth takes place on a large scale. Letting the undercooling prior to solidification increase, the eutectic interface morphology of (Co76Sn24)99.5Nb0.5 eutectic alloys returns from dendritic pattern back to factual seaweed pattern at more than 38 K undercooling and then transits to compact seaweed pattern at more than 181 K undercooling. The eutectic growth velocity slightly increases at low and intermediate undercooling but obviously decreases at large undercooling due with Nb addition.