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

摘要

The effect of adding 2 at. percent 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 approx 20 percent to 30 percent), reduction in coarsening kinetics by one-third and a larger number density (N_v) of smaller mean radii () precipitates. The kinetics of 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 inter-precipitate distance distributions give insight into two-dimensional microstructural evolution, including the elastically driven transition from a uniform gamma'-distribution to one-dimensional (00 l)-strings to eventually clustered packs of gamma'-precipitates in the less densely packed Ni-Al-Cr alloy.