In this work, we have extended our earlier work on the concept of ratchetting strain as a crack driving force (Tong et al., 2013), to examine the crack growth of a nickel-based superalloy at selected temperatures in vacuum under both fatigue and fatigue-creep loading conditions. The parameters of a unified constitutive model were calibrated against the material data obtained at selected temperatures from 550 to 775°C, and a finite element model was developed to simulate the near-tip stress–strain responses under fatigue and creep-fatigue loading conditions at the experimental temperatures. Both ratchetting strain and accumulated inelastic strain near the crack tip were utilised in the prediction of the crack growth rates collected in vacuum. It seems that, although both ratchetting strain and accumulated inelastic strain correlate with the crack growth rates obtained under fatigue and fatigue-creep loading conditions, the predictions based on accumulated plastic strain are particularly close to the experimental results at all temperatures and loading conditions examined.This is the first time the concept of ratchetting strain has been used to predict the crack growth rates of an engineering alloy at elevated temperature in vacuum, where the influence of oxidation on crack growth is removed.

Serrations known as Portevin Le-Chatelier effect have been observed in a coarse grained nickel based superalloy RR1000 in the temperature range from 300 to 750 degrees C. These serrations may be due to dynamic strain aging (DSA), which affects the stress relaxation behaviour of the material. Further experiments have been carried out over a wide range of temperature and under selected loading conditions to interrogate the DSA phenomenon. The results show that, in addition to serrations, the alloy also exhibits unusual strain rate insensitivity and increased strain hardening in the DSA temperature regime. Possible mechanisms of DSA have been explored using transmission electron microscopy (TEM) to examine typical samples tested under selected and well defined loading conditions.

Cyclic response of a nickel-based superalloy, Alloy RR1000, has been studied at 650°C using both experimental and numerical approaches. Experimental results from uniaxial strain-controlled tests showed both hardening and softening behaviour for the material. A viscoplastic-damage model has been developed to simulate the material behaviour, where a damage variable, based on plastic strain development, was introduced in the framework of viscoplasticity. A good agreement between the model simulations and the experimental results has been obtained for the full history of the cyclic response under simple cyclic and dwell loading conditions. The model was utilised to investigate the damage evolution near a crack tip for a single edge notch tension specimen (SENT), where the damage accumulation rate with respect to cycles was found to be linear once an inelastic strain threshold was reached. The damage development seems to be localised to the crack tip and becomes more significant at lower frequencies and longer dwell periods.

Simulation of both damage development and strain ratchetting in uniaxial loading conditions has been presented for a nickel-based superalloy at 650°C using the unified Chaboche viscoplastic model. A third kinematic hardening component was employed to simulate strain ratchetting; and a damage variable, based on plastic strain development, was also incorporated to simulate the damage evolution behaviour. Good agreement between the model predictions and the experimental results was obtained for both damage evolution and strain ratchetting. The model was then utilised to investigate the cyclic deformation behaviour near a crack tip for a single edge notch tension (SENT) specimen. Finite element analyses showed that strain ratchetting is seems to be a characteristic of the fatigue crack tip, which has been used as a criterion to predict the crack growth rates.

Mechanical behaviour of a nickel-based superalloy, RR1000, has been investigated at 650°C under cyclic and dwell loading conditions. The microstructural characteristics of the alloy have been studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the distribution patterns of the dislocations and slip planes have been compared between samples tested under fatigue and creep–fatigue loading conditions. Constitutive behaviour of the alloy was described by a unified constitutive model, where both cyclic plastic and viscoplastic strains were represented by one inelastic strain. The results show that the precipitation state is very stable at 650°C and only minor differences exist in the dislocation arrangements formed under pure fatigue and combined creep and fatigue conditions. Hence, a unified constitutive model seems to be justified in describing and predicting the constitutive behaviour in both cases.

Progressive increase in tensile strains near a crack tip has been observed from finite element studies of stationary and growing cracks (Zhao, 2004, 2008) [1], [2] under cyclic loading conditions. In this work, the significance of such a phenomenon was further explored. In particular, stress-controlled experiments were carried out to evaluate the uniaxial ratchetting response of a nickel-based superalloy, and the material parameters were re-calibrated using both strain-controlled and stress-controlled experimental data. An additional kinematic hardening term was introduced in the viscoplastic constitutive model and the models were utilised via a user-defined subroutine to study near crack tip ratchetting behaviour of a single edge notch tension (SENT) model geometry at elevated temperature. Loading modes near the crack tip were examined, together with the influence of particular constitutive models on the mechanistic response of the crack tip. The crack tip deformation was found to be predominantly strain-controlled, where the mean ratchetting strain seems to be more relevant to crack growth than the strain range. The former was used as a measure of crack tip damage to correlate crack growth rates at selected loading conditions.