Coarsening of c (Ni–Al solid solution) precipitates in a c 0 (Ni 3 Al) matrix Yong Ma, Alan J. Ardell * Henry Samueli School of Engineering and Applied Science Los Angeles, CA, USA Received 28 January 2007; received in revised form 31 March 2007; accepted 8 April 2007 Available online 7 June 2007 Abstract The coarsening behavior of Ni–Al solid–solution precipitates in an Ni 3 Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as pre- dicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol 1 , agrees very well with results from conven- tional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate– matrix interface, controls the kinetics. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coarsening; Nickel-based superalloys; Ostwald ripening 1. Introduction Precipitation of the disordered Ni–Al solid solution (the c phase) from the supersaturated, ordered off-stoichiome- tric c 0 phase (Ni 3 Al, with the L1 2 or Cu 3 Au crystal struc- ture) is well documented [1–9]. Coherent precipitates of disordered c were first observed inside large precipitates of the ordered c 0 phase by Ham et al. [1] in an as-solidified ternary Ni–Al–Ti alloy. Merrick [2] shortly thereafter definitively identified the presence of very small precipitates of c inside very coarse particles of c 0 in the Ni-base super- alloy Udimet 700. Since then there have been many obser- vations, in compositionally complex alloys, of the precipitation of c particles from a supersaturated c 0 phase based on Ni 3 Al [3–8], as well as in binary off-stoichiometric Ni 3 Al [9]. Cornwell and Purdy [9] reported that c precipi- tates in binary Ni–Al alloys were coherent and plate- shaped, with interfaces parallel to {1 0 0}. Ham et al. [1] claimed that the c precipitates in a ternary Ni–Al–Ti alloy nucleated as platelets and retained this shape as they grew with increasing aging time. However, Liu et al. [6] noted that the precipitates in the ternary alloy were initially spherical and evolved with aging time to cuboidal shapes, becoming plate-like only at much longer aging times. Putt- ing aside for the moment the question of whether the reported plate-shaped c precipitates are really rods rather than plates, since the necessary tilting experiments in the transmission electron microscope (TEM) were not done, an obvious question is why should the morphology of indi- vidual c precipitates in a c 0 matrix evolve differently from the morphology of individual precipitates of c 0 in c? After all, the interfacial free energies, magnitudes of the lattice misfit strain and magnitudes of the mismatches of the elas- tic stiffness constants are identical. In the case of the latter two types of mismatch, only the signs are different. To address this question, as well as others, we initiated research on the precipitation of disordered Ni–Al precipi- tates from supersaturated Ni 3 Al. Among the important issues are the evolution of the morphology of c precipitates and the kinetics of coarsening and its dependence on vol- ume fraction. Our initial investigations of the precipitation of the c phase from supersaturated c 0 [10–12] revealed two major 1359-6454/$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.04.008 * Corresponding author. E-mail address: aardell@ucla.edu (A.J. Ardell). www.elsevier.com/locate/actamat Acta Materialia 55 (2007) 4419–4427