Microstructures of two-stage aluminized coatings on pure Ni

Two-stage aluminized coatings on pure Ni (γ phase) were prepared by aluminizing at 850°C and then annealing at 1050°C. Transmission electron microscopy coupled with energy dispersive X-ray analysis indicated the first-stage coating consisted of γ, γ′, β and δ layers from substrate to the surface. The β-NiAl zone was found to be clearly divided into Ni-rich and Al-rich β-NiAl layers by an interface of approximately stoichiometric β-NiAl composition. No crystallographic relationship was found at the γ/γ′, γ′/β and β/δ interfaces. The γ and γ′ phases near the interface consist of very fine grains with some scattered amorphous regions and containing a high density of dislocations. After the second-stage treatment, epitaxial γ′ precipitates were formed in the γ substrate near the γ/γ′ interface.