Chemical aspects of environmentally enhanced crack growth in nickel-based superalloys

The research presented in this dissertation is a surface chemistry study of the chemical aspects of environmentally enhanced crack growth (EECG) in commercial, Ni-based superalloys. Previous studies have shown that oxygen increases the crack growth rates in superalloys at high temperatures, compared to those observed in an inert environment. Proposed mechanisms have attributed this enhancement to (a) the preferential oxidation of Ni and Fe at the crack tip to form an oxide “wedge” and (b) the oxidation of carbon and metallic carbides at grain boundaries to form high, internal pressures of CO and CO₂. These mechanisms, however, cannot explain the observed differences in fracture surface morphology and crack growth kinetics for several superalloys. An alternative mechanism, therefore, was proposed which attributes EECG to oxygen penetration ahead of the crack tip during crack growth and the subsequent oxidation of NbC (and possibly Ni₃Nb) on grain boundary surfaces. To assess these mechanisms, a surface chemistry study was undertaken to determine the relative reactivity of several superalloys and grain boundary phases (such as NbC, Ni₃Nb, Ni₃Ti and Ni₃Al) with oxygen at elevated temperatures, using X-ray photoelectron spectroscopy (XPS). XPS analyses were also made of fracture surfaces that were produced during crack growth studies in oxygen. The results showed the first direct confirmation of oxygen penetration ahead of the crack tip, leading to the oxidation of Cr, NbC, Ni₃Nb, Ni₃Ti and Ni 3Al on grain boundary surfaces. These observations are consistent with the high temperature oxidation studies for each alloy and pure phase, and support crack growth enhancement mechanisms that involve the oxidation of alloying phases ahead of the crack tip. Qualitative models were then developed, based on the results of this dissertation research, that considered the role of internal oxidation in oxygen enhanced crack growth. From these models, it was concluded that oxygen atoms penetrate readily ahead of the crack tip and along grain boundaries during crack growth. This oxygen reacts with Cr, NbC, Ni₃Nb, Ni₃Ti and Ni₃Al to form an adherent film of brittle oxides at the interfaces between the grain boundary phases and matrix, thus causing EECG.