MICROSTRUCTURES OF 2-STAGE ALUMINIZED COATINGS ON INCONEL-600

Two-stage aluminized coatings on a nickel-base alloy, Inconel 600, are prepared by aluminizing at 850 degrees C and then annealing at 1050 d egrees C. In the first-stage coating, a planar layer of sigma phase fo rms between the gamma substrate and the nickel-rich beta-NiAl the coat ing matrix near the interface. Being chromium-, iron-, and nickel-rich but aluminum-poor, the a phase layer can act as a diffusion barrier f or aluminizing, and the diffusion of aluminum can proceed across the a phase only through the grain boundaries. In the second-stage coating the interfacial sigma phase breaks down into a zone containing a beta phase and Cr23C6 carbide. An amorphous phase with CrC carbide is forme d near the interface in both stages. Chromium aluminides are found to precipitate in alpha-Cr in both stage coatings and in beta-NiAl in the second-stage coating. In the first-stage coating, fine Cr5Al8 particl es precipitate in alpha-Cr particles and the crystallographic relation ship between them is [001](Cr5Cl8)parallel to[001](alpha-Cr) and [110] (Cr5Al8)parallel to[110](alpha-Cr). In the second-stage coating, Cr2Al precipitates in alpha-Cr particles and bears an orientation relations hip of [010](Cr2Al8)parallel to[010](alpha-Cr) and [001](Cr2Al)paralle l to[001](alpha-Cr). Cr2Al also precipitates in the beta-NiAl matrix w ith an orientation relationship of [010](Cr2Al)parallel to[010](NiAl) and [001](Cr2Al)parallel to[001](NiAl).