Directional solidification of Cu-20Sn alloy at low speed: From peritectic coupled growth to banding

Bridgman-type directional solidification experiments have been carried out in Cu-20Sn peritectic alloy. Peritectic coupled growth and banding structures have been observed at low growth rates (1.5 and 2 μm/s) under a temperature gradient up to 40 K/mm. The peritectic coupled growth structure, containing rod dendrite primary α phase plus peritectic β phase, forms initially. As solidification proceeds, peritectic coupled growth is overgrown by banding or island banding structures. The formation of banding structure from coupled growth is explained by a model involving Sn concentration change at nucleation of the secondary phase ahead of the solid/liquid interface. It is found that the competitive growth between the α and β phases also plays a critical role on the formation of banding structures.