Computational modeling of oxygen-assisted fracture in nickel-based superalloys

Sammanfattning: Nickel-based superalloys are a commonly used material in applications where high strength is required at high temperatures. A typical such example is jet engines and, in the case of polycrystalline nickel-based superalloys, components like turbine disks. Under severe loading conditions, such as cyclic loading combined with sustained dwell times at high temperatures, polycrystalline nickel-based superalloys are known to experience accelerated fatigue crack growth in oxygen-rich environments compared to vacuum. Environmentally assisted crack initiation could for example be identified as cause for turbine disk fracture in some cases of mechanical turbine failure of passenger airplanes. It has been shown by experimental work in the past that oxygen is the main deteriorating species in the case of intergranular fracture in nickel-based superalloys. In this work we develop a fully coupled chemo-mechanical cohesive zone model accounting for the interaction between oxygen transport into the grain boundaries and their stress state. The model is presented in a thermodynamic framework. Additionally, a chemo-mechanical cohesive finite element carrying both the displacement and the concentration field is suggested. The finite element formulation is complemented by a moving boundary condition for the concentration field, accounting for the increase in environment-exposed surface as edge cracks grow into the structure. Aside from handling edge cracks, the moving boundary condition yields physically meaningful results even in the case of crack initiation at interior grain boundaries. Numerical experiments are conducted on bi- and polycrystals. It is shown that the modeling framework can qualitatively reproduce experimentally observed phenomena, like the reduction of tensile strength in oxygen rich environments, the acceleration of crack growth rates upon increasing environmental oxygen concentration and saturation thereof for high environmental oxygen levels. It is also demonstrated that edge cracks can be propagated past preexisting cracks inside the polycrystal while maintaining realistic oxygen boundary conditions.