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 mu m/s) under a temperature gradient up to 40 K/mm. The peritectic coupled growth structure, containing rod dendrite primary beta phase plus peritectic alpha 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 alpha and beta phases also plays a critical role on the formation of banding structures. (C) 2011 Elsevier B.V. All rights reserved.