SURFACE ENRICHMENT OF NIOBIUM ON INCONEL-718 (100) SINGLE-CRYSTALS

The surface composition of an Inconel 718 (100) single crystal was inv estigated by X-ray photoelectron spectroscopy over the temperature ran ge 300 to 1000 K in ultra-high vacuum (UHV). The surface composition o f an ion etched surface closely resembled the bulk alloy composition d etermined by atomic absorption spectroscopy. The composition of the io n etched surface remained close to the bulk composition until the temp erature was raised above 800 K. At temperatures above 800 K, a dramati c enrichment of surface niobium (Nb) occurred. The extent of Nb enrich ment was highly influenced by the presence of trace amounts of surface or subsurface oxygen. Numerous cycles of ion etching and annealing le d to nearly complete removal of oxygen from the surface and subsurface region. Surface enrichment of Nb at high temperatures on these oxygen -free surfaces was greatly reduced. However, extensive Nb enrichment c ould be easily reintroduced by pre-exposure of the surface to oxygen p rior to high-temperature vacuum annealing. The evolution of CO during the high-temperature anneal suggested that oxygen attack of the Nb car bide particles played a role in the mechanism of surface enrichment.