  The effect of adding 2 at.% W to a model Ni-Al-Cr superalloy on the
morphological evolution, spatial correlations and temporal evolution of
g'(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 (Nv) of smaller mean radii (<R>) precipitates. The
kinetics of <R> and interfacial area per unit volume obey t1/3 and t-1/3
relationships, respectively, which is consistent with coarsening driven by
interfacial energy reduction. The Nv 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 gives
insight into 2D microstructural evolution, including the elastically driven
transition from a uniform g'-distribution to one-dimensional <001>-strings to
eventually clustered packs of g'-precipitates in the less densely packed
Ni-Al-Cr alloy.
