Microstructural evolution in directional solidification of Nb-doped Co-Sn/Ni–Sn eutectic alloys

(Co 76 Sn 24 ) 99.5 Nb 0.5 and (Ni 81.3 Sn 18.7 ) 99.5 Nb 0.5 eutectic alloys were directionally solidified and the microstructural evolutions were investigated. Addition of minor Nb to the eutectic alloys destabilized the eutectic solidification interface. The (Co 76 Sn 24 ) 99.5 Nb 0.5 eutectic interface changed from planar to cellular, to dendritic and then to seaweed morphology with increasing withdrawal velocity at temperature gradient of 200 K/cm. At larger temperature gradient of 300 K/cm, however, dendritic growth did not occur. The cellular interface directly transited into seaweed pattern. In contrast, the (Ni 81.3 Sn 18.7 ) 99.5 Nb 0.5 eutectic interface with relatively larger interfacial energy anisotropy tended to grow in dendritic pattern. Compared with single β-Co 3 Sn 2 phase, the α-Co/β-Co 3 Sn 2 lamellar eutectic is conducive to growing into seaweed morphology. The critical growth velocity for transition from planar to cellular interface was calculated and the mechanism underlying the interface morphology selection was analyzed.