Microstructures of two-stage aluminized coatings on pure Ni

Two-stage aluminized coatings on pure Ni (gamma phase) were prepared by alu minizing at 850 degrees C and then annealing at 1050 degrees C. Transmissio n electron microscopy coupled with energy dispersive X-ray analysis indicat ed the first-stage coating consisted of gamma, gamma', beta and delta layer s from substrate to the surface. The beta-NiAl zone was found to be clearly divided into Ni-rich and Al-rich beta-NiAl layers by an interface of appro ximately stoichiometric beta-NiAl composition. No crystallographic relation ship was found at the gamma/gamma', gamma'/beta and beta/delta interfaces. The gamma and gamma' phases near the interface consist of very fine grains with some scattered amorphous regions and containing a high density of disl ocations. After the second-stage treatment, epitaxial gamma' precipitates w ere formed in the gamma substrate near the gamma/gamma' interface. (C) 1999 Elsevier Science S.A. All rights reserved.