Microstructures of two-stage aluminized coatings on Inconel 600

Two-stage aluminized coatings on a nickel-base alloy, Inconel 600, are prepared by aluminizing at 850°C and then annealing at 1050°C. In the first-stage coating, a planar layer of σ phase forms between the γ substrate and the nickel-rich β-NiAl, the coating matrix near the interface. Being chromium-, iron-, and nickel-rich but aluminum-poor, the σ phase layer can act as a diffusion barrier for aluminizing, and the diffusion of aluminum can proceed across the σ phase only through the grain boundaries. In the second-stage coating the interfacial σ phase breaks down into a zone containing a β phase and Cr23C6 carbide. An amorphous phase with CrC carbide is formed near the interface in both stages. Chromium aluminides are found to precipitate in α-Cr in both stage coatings and in β-NiAl in the second-stage coating. In the first-stage coating, fine Cr5Al8 particles precipitate in α-Cr particles and the crystallographic relationship between them is [001]Cr5Al8‖[001]α-Cr and [110]Cr5Al8‖[110]α-Cr. In the second-stage coating, Cr2Al precipitates in α-Cr particles and bears an orientation relationship of [010]Cr2Al‖[010]α-Cr and [001]Cr2Al‖[001]α-Cr. Cr2Al also precipitates in the β-NiAl matrix with an orientation relationship of [010]Cr2Al‖[010]NiAl and [001]Cr2Al‖[001]NiAl.