Competition of the primary and peritectic phases in hypoperitectic Cu–Sn alloys solidified at low speed in a diffusive regime

Directional solidification experiments on hypoperitectic Cu–Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 μm to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary α- and peritectic β-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (α + β) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid–liquid interface.