CYCLIC DEFORMATION, CRACK GROWTH AND OXIDATION DAMAGE FOR NICKEL-BASED SUPERALLOYS

Viscoplasticity and crystal plasticity have been used to model cyclic deformation of nickel-based superalloys at elevated temperature. Model parameters were determined from strain-controlled cyclic test data, with consideration of strain rate effects. Model simulations are in good agreement with the experimental results for stress-strain loops, cyclic hardening behaviour and stress relaxation behaviour during the hold periods at the maximum and minimum strain levels. The models were also applied to study crack-tip deformation under fatigue, which showed the accumulation of permanent deformation at the crack tip due to plasticity-induced strain ratcheting. The strain accumulation was subsequently utilized as a criterion to predict crack propagation in a standard specimen using the finite element method, in comparison with experimental results. In addition, finite element analyses of oxygen penetration along grain boundaries have been carried out to quantify the fatigue-oxidation damage and calibrate the diffusion parameters based on FIB measurements of internal oxidation. A sequentially coupled mechanical-diffusion analysis was adopted to account for the effects of deformation on diffusion during fatigue loading. Prediction of oxidation-assisted crack growth has been carried out from finite element analyses of viscoplastic deformation and oxygen diffusion near a fatigue crack tip. The predictions compared well with the experimental results for triangular and dwell loading waveforms, with significant improvement achieved over those predicted from the mechanical model alone.