Morphological evolution during the early stages of aluminide coating growth on a single-crystal nickel superalloy surface

The (100) surface of a single-crystal Ni alloy was aluminized as a function of time to study the development of the resulting coating microstructure. A chemical vapor deposition (CVD) reactor, which was specially configured for short-term aluminizing experiments, was used to prepare coating specimens at 1150°C. After 5 minutes, γ′-Ni₃Al particles ∼100 nm in size randomly nucleated on the alloy surface. Within 20 minutes, a coating layer consisting of preferentially oriented, columnar β-NiAl grains was formed with the segregation of refractory elements (i.e., Ta and W) from the alloy to the coating grain boundaries. The lateral growth of the columnar grains was observed to be relatively rapid for up to 45 minutes, but slowed considerably between 45 and 180 minutes. While the columnar nature of the coating did not change significantly after 20 minutes, the surface features continually evolved, with the appearance of a small amount of the γ′ phase, which coincided with the segregation of the refractory elements to the coating surface.