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 γ′(L1₂)-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 ∼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 γ′-distribution to one-dimensional 〈0 0 1〉-strings to eventually clustered packs of γ′-precipitates in the less densely packed Ni-Al-Cr alloy.