Acta Materialia 50 (2002) 4511–4526 www.actamat-journals.com On the fcc→D0 19 transformation in Co–W alloys P.A. Carvalho, P.M. Bronsveld, B.J. Kooi, J.Th.M. De Hosson * Department of Applied Physics, Materials Science Center and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received 24 October 2001; received in revised form 2 July 2002; accepted 2 July 2002 Abstract The continuous precipitation of D0 19 Co 3 W from a highly supersaturated Co-rich fcc matrix has been investigated. A detailed analysis of planar defects generated in the precipitates has been carried out using conventional and high- resolution transmission electron microscopy. The spatial arrangement of stacking faults and antiphase boundaries revealed the actual defect formation mechanisms and disclosed important aspects of the phase transformation at an atomic level. The results point to a displacive/diffusional Widmansta ¨tten type of transformation, which is not simply based on a common ledge growth and, furthermore, follows the path: fcc→hcp→D0 19 . The α'+ Co 7 W 6 →Co 3 W peritec- toid transformation has also been studied.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Intermetallic compounds; Microstructure; Transmission electron microscopy (TEM); Phase transformations; Lattice defects 1. Introduction The purpose of the work described in this paper is to investigate defect structures generated in D0 19 CO 3 W during its continuous precipitation (CP), aiming at a better understanding of the phase transformation processes. The general atomic mechanisms associated with the CP of an hcp- based phase from a supersaturated fcc matrix (α') have been established a long time ago [1–7]. In particular, much attention was given to the precipi- tation of hP2 Ag 2 Al from Al-rich fcc solid sol- utions [2–7], and this system served as a model to * Corresponding author. Tel.: +31-50-363-4898; fax: +31- 50-363-4881. E-mail address: hossonj@phys.rug.nl (J.T.M. De Hosson). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(02)00269-0 test the theory of growth for (semi)coherent plates by a ledge mechanism [1]. An fcc→D0 19 trans- formation occurring in a (Ni,Co) superalloy has been reported [8], and accounts of L1 2 →D0 19 transformations have also been given [9,10]. Fur- thermore, a nearly reverse transformation, D0 19 →L1 0 has been scrutininized in detail, because it occurs during aging in the well-known Ti–Al alloy system [11–13]. Under certain composition/aging conditions, an L1 0 →D0 19 trans- formation can also take place in the Ti–Al system, and some effort has been dedicated to its charac- terization [7,14–18]. However, the previous studies on D0 19 phase precipitations have not been focused on the particular planar defects generated in the plates. Furthermore, to the best of our knowledge, the CP occurring in the Co–W system has not yet been studied.