Oxygen enhanced crack growth in nickel-based superalloys and materials damage prognosis

This paper summarizes the results from a comprehensive multidisciplinary study to better understand the role of niobium and other strengthening elements in enhancing crack growth by oxygen in nickel-based superalloys at high temperatures, and considers its importance for materials damage prognosis and life cycle engineering in high temperature service. Three γ′ strengthened powder metallurgy (P/M) alloys, with 0, 2.5 and 5 wt pct Nb and comparable volume fractions (about 53 vol pct) of γ′′ precipitates, were specially designed for this study. Coordinated crack growth, microstructural and surface chemistry studies were conducted on the alloys. They were complemented by oxidation studies of Nb, Ni₃Nb, NbC, Ni₃Al and Ni₃Ti, and analyses of fracture surfaces of interrupted crack growth specimens by X-ray photoelectron spectroscopy (XPS). The findings taken in toto show that oxygen enhancement of crack growth is the result of the formation of a brittle film of surface oxides along grain boundaries and interfaces ahead of the crack tip by the preferential oxidation of Nb, Ti and Al in the Nb-rich carbides and Ni₃Al, Ni₃Ti and Ni₃Nb (in Inconel 718) precipitates. The results also showed that the oxidation of Nb-rich carbides alone can significantly enhance crack growth in oxygen. The findings are discussed in relation to the previously proposed crack growth mechanisms, and their applications.