Acta Materialia 51 (2003) 599–611 www.actamat-journals.com Two-phase microstructure selection in peritectic solidification: from island banding to coupled growth T.S. Lo a,b , S. Dobler c , M. Plapp a,d , A. Karma a , W. Kurz c,* a Physics Department and Center for Interdisciplinary Research on Complex Systems Northeastern University, Boston MA 02115, USA b Courant Institute of Mathematical Sciences, New York University, New York NY 10012, USA c Department of Materials, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland d Laboratoire de Physique de la Matie `re Condense ´e, CNRS/Ecole Polytechnique, 91128 Palaiseau, France Received 2 January 2002; accepted 8 August 2002 Abstract We report the first experimental observation in directionally solidified peritectic Fe–Ni alloys of two-phase island banding microstructures that consist of rows of islands of one solid phase (either peritectic or primary) inside the continuous matrix of the other phase. These microstructures form under predominantly diffusion-limited growth con- ditions and when both phases are morphologically stable, as recently predicted by numerical simulations of a phase- field model. They are observed either as transients that seed the formation of coupled growth structures, or as the final microstructure. Phase-field simulations are reported that shed light on the relationship of island banding and coupled growth as well as on the growth conditions and nucleation parameters that control the dynamical selection of these two basic microstructures in peritectic alloys.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Peritectic solidification; Directional solidification; Phase transformation kinetics; Phase field models; Iron alloys 1. Introduction The past few years have witnessed rapid pro- gress in the understanding of two-phase micro- structures formed by solidification of peritectic alloys [1]. This important class of microstructures is characterized by the growth competition between a primary (α) and a peritectic (β) phase, which can both coexist with the liquid at the per- * Corresponding author. Fax: +41-21-693-5890. E-mail address: wilfried.kurz@epfl.ch (W. Kurz). 1359-6454/03/$30.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00440-8 itectic temperature T p . Above this temperature, α- liquid equilibrium is stable and β-liquid equilib- rium is metastable, whereas the opposite is true below T p . Under growth conditions where both solid phases are morphologically stable, and thus do not form cells or dendrites, the microstructure is con- trolled by the interplay of solute transport and nucleation kinetics. In this regime, banded struc- tures consisting of alternate layers of the two phases stacked perpendicularly to the growth direc- tion have been traditionally observed [2–6]. Tri- vedi has proposed a conceptual model of cyclic