Anomalous eutectic formation in the solidification of undercooled Co–Sn alloys

Three Co–Sn alloys with compositions around the eutectic point were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification structure. It is revealed that the primary phase during rapid solidification changes complexly with the increasing undercooling in the off-eutectic alloys, while coupled eutectic growth takes place at all undercoolings in the eutectic alloy. Two types of anomalous eutectics form in the alloys: one evolving from coupled eutectics and the other from single phase dendrites or seaweeds. The crystallographic orientation of eutectic phases in the anomalous eutectic is dependent on which type their precursors belong to. ► Transition from decoupled to coupled growth happens above a critical undercooling for Co–Sn off-eutectic alloys. ► Both decoupled and coupled crystal growth can produce anomalous eutectic. ► Crystallographic orientations of eutectic phases in anomalous eutectics depend on the formation mechanism.