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 f or short-term aluminizing experiments, was used to prepare coating specimen s at 1150 degreesC. After 5 minutes, gamma'-Ni3Al particles similar to 100 nm in size randomly nucleated on the alloy surface. Within 20 minutes, a co ating layer consisting of preferentially oriented, columnar beta -NiAl grai ns 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 c olumnar grains was observed to be relatively rapid for up to 45 minutes, bu t slowed considerably between 45 and 180 minutes. While the columnar nature of the coating did not change significantly after 20 minutes, the sm-face features continually evolved, with the appearance of a small amount of the gamma' phase, which coincided with the segregation of the refractory elemen ts to the coating surface.