Microstructural aspects of aluminized coatings on nickel-base alloys

Abstract

The influence of base-alloy composition on the microstructure of aluminide coatings has been studied for a series of nickel-base alloys aluminized by the pack-cementation process. The microstructural features of the coatings formed on Nickel 200. Brightray Alloy C, Nimonic 80A, and Inconel 718 were evaluated by optical metallographic examination, together with microhardness surveys and scanning electron microscopy. The coating composition was determined from electron microprobe traverses of the coating layer and the aluminide phases were identified by X-ray diffraction examination of the bulk sample. The coatings were found to consist primarily of the nickel aluminide NiAI. although some Ni₂Al₃ was found to a lesser extent as a Widmanstätten structure in the coating on the Brightray alloy. A discontinuous, lamellar-type layer, enriched in chromium and other base-alloying elements with low solubility in the NiAI, was also observed at the coating/substrate interface. The coating could be precipitate-rich (as on Inconel 718 and Nimonic 80A) or precipitate-rich, depending on the aluminium deposition rate and the rate at which elements segregate at the interface. There appeared to be substantial outward diffusion of nickel from the substrate alloy, but little inward diffusion of aluminium. This diffusion process tends to stabilize NiAl, rather than Ni₂Al₃. The aluminide coatings had very high hardnesses and should provide surfaces with good wear resistance.