Effects of a tungsten addition on the morphological evolution, spatial correlations and temporal evolution of a model Ni-Al-Cr superalloy

The effect of adding 2 at.% W to a model Ni-Al-Cr superalloy on the morphological evolution, spatial correlations and temporal evolution of gamma'(L1(2))-precipitates at 1073 K is studied with scanning electron microscopy and atomic force microscopy. Adding W yields a larger microhardness, earlier onset of spheroidal-to-cuboidal precipitate morphological transition, larger volume fraction (from similar to 20% to 30%), reduction in coarsening kinetics by one-third and a larger number density (N-v) of smaller mean radii (< R >) precipitates. The kinetics of (R) and interfacial area per unit volume obey t(1/3) and t-(1/3) relationships, respectively, which is consistent with coarsening driven by interfacial energy reduction. The N-v power-law dependencies deviate, however, from model predictions, indicating that a stationary state is not achieved. Quantitative analyses with precipitate size distributions, pair correlation functions and edge-to-edge interprecipitate distance distributions give insight into two-dimensional microstructural evolution, including the elastically driven transition from a uniform gamma'-distribution to one-dimensional < 001 >-strings to eventually clustered packs Of gamma'-precipitates in the less densely packed Ni-Al-Cr alloy. (c) 2007 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.