Oxygen diffusion and crack growth for a nickel-based superalloy under fatigue-oxidation conditions

Abstract

Advanced microscopy characterisation and numerical modelling have been carried out to investigate oxygen diffusion and crack growth in a nickel-based superalloy under fatigue-oxidation conditions. Penetration of oxygen into the material and the associated internal oxidation, which leads to material embrittlement and failure, have been found from Focused Ion Beam (FIB) examinations. Applied fatigue loading tends to enhance the extent of internal oxidation for temperatures at 750°C and above. Using a submodelling technique, finite element analyses of oxygen penetration at grain level have been carried out to quantify the fatigue-oxidation damage and calibrate the diffusion parameters based on the measurements of maximum depth of internal oxidation. The grain microstructure was considered explicitly in the finite element model, where the grain boundary was taken as the primary path for oxygen diffusion. A sequentially coupled mechanical-diffusion analysis was adopted to account for the effects of deformation on diffusion during fatigue loading, for which the material constitutive behaviour was described by a crystal plasticity model at grain level. Prediction of oxidation-assisted crack growth has also been carried out at elevated temperature from the finite 2 element analyses of oxygen diffusion near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The predictions from the fatigue-oxidation failure curve compared well with the experimental results for triangular and dwell loading waveforms, with significant improvement achieved over those predicted from the viscoplastic model alone.

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

“…This appears to contradict much of the literature available (see for example, Refs. 27,36,37 ), which infers the greater penetration depths from stress-assisted grain boundary diffusion. The premise of this argument states that the hydrostatic stress increases the oxygen diffusivity.…”

“…The difference in oxide morphologies comes as a surprise given that most of the current literature only reports expedited kinetics. [27][28][29][30][31][32] The isotopic tracer experiments indicated that shot-peening and applied stresses have very little influence on the inward oxygen diffusion, thus eliminating enhancement due to a greater contribution from inward anionic growth. This suggests that the shot-peening and mechanical loads applied to polished samples act to influence the diffusivity of the oxidized solute species (i.e., chromium and titanium) in the parent and recrystallized alloy, [33][34][35] and this is proposed to result from defects induced from creep deformation.…”

“…The premise of this argument states that the hydrostatic stress increases the oxygen diffusivity. For example, modeling conducted by Karabela et al 27,28 described accelerated internal oxidation kinetics by first computing the hydrostatic stress gradient through crystal plasticity modeling and then the oxygen diffusion. The authors successfully rationalized the amplified oxygen penetration depths through this method; however, this analysis did not account for flux of the metallic species and the oxidation processes themselves, which inherently modify the media in which the oxygen diffuses through.…”

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

“…Recently, elevated levels of Co and Ti have been shown to be beneficial, providing improved high temperature strength and creep resistance [1,2]. However, in addition to their mechanical performance, the environmental resistance of these alloys is also critical, as surface degradation can accelerate fatigue crack initiation during high temperature exposure, leading to premature failure [3][4][5]. Yet, to date, the influence of elevated Co and Ti concentrations on the oxidation characteristics of Ni-based superalloys has not been systematically studied.…”

Embrittlement of an elasto-plastic medium by an inclusion

“…In the matrix surrounding the intrusion and crack, slip bands have been observed (Németh et al 2016;Hörnqvist et al 2014), indicating that the failure is not purely brittle. Karabela et al (2013) simulated using finite elements, fatigue crack growth and oxygen diffusion in the Ni-based superalloy RR1000. They treated the grain microstructure explicitly and adopted a power law relationship for the inelastic strain rate.…”