Modeling of peritectic coupled growth in Cu–Sn alloys

In directional solidification experiments on hypoperitectic Cu-Sn alloys at low velocity and high thermal gradient, both lamellar and fibrous coupled peritectic growth patterns have been observed. Two phenomena that had not been observed in previous experiments on other alloy systems are investigated here with the help of different modeling approaches. The mean volume fraction of primary phase alpha,(g) over bar (alpha), as determined by X-ray microtomography, decreases with solidification distance over the entire length of the coupled zone, but is always much larger than that expected from the equilibrium phase diagram. Moreover, oscillations in (g) over bar (alpha), with a spatial periodicity approximately equal to the lamellar spacing are also observed. The first observation is explained semi-quantitatively by a simple ID diffusion model, which reveals that the onset of coupled growth occurs during the initial transient of the primary phase planar front growth. A two-dimensional phase-field model is used to monitor the subsequent microstructure evolution, and shows that the lamellar structure exhibits collective 1-lambda oscillations. In agreement with previous studies, it was found that these oscillations lead to stable coupled growth only for a limited range of the control parameters. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.