Mechanism for oxygen enhanced crack growth in inconel 718

Nickel based superalloys are used extensively in high temperature applications, for example, turbine disks in jet engines. Since turbine discs operate in air at 700 to 973 K, these alloys have received considerable attention with respect to their sensitivity to environmentally enhanced crack growth (EECG). A crack mechanism involving Nb in the intergranular embrittlement of Inconel 718 has been explored. The enhancement in crack growth of this alloy was tentatively attributed to the oxidation of niobium-rich carbides (e.g. NbC) at grain boundaries to form a brittle film of niobium oxides. The oxidation of intermetallic gamma '' (Ni sub 3 Nb) precipitates along these boundaries, at a much higher concentration (approx =10:1, Ni sub 3 Nb:NbC), was also recognized as a potential source for intergranular failure. It was proposed that NbC and Ni sub 3 Nb at grain boundaries oxidize ahead of the crack tip during crack growth, increasing the niobium surface concentration by up to eight fold. This niobium was believed to be in the form of a brittle, niobium oxide, Nb sub 2 O sub 5 , which promoted grain boundary cracking in the alloy. The observed CO evolution during exposure of the alloy surface to oxygen at high temperatures was taken as evidence for the oxidation of NbC. Ni sub 3 Nb was also heated with oxygen to assess the role of gamma '' (Ni sub 3 Nb) precipitates in EECG. This study demonstrated the preferential oxidation of Ni sub 3 Nb to Nb sub 2 O sub 5 , showing the likelihood of Ni sub 3 Nb oxidation along grain boundaries. The enrichment and oxidation of niobium on Inconel 718 and Ni sub 3 Nb were confirmed by X-ray Photoelectron Spectroscopy (XPS).