Viscoplasticity and phase field modelling of the delayed cracking of oxide intrusions at grain boundaries

Corrosion resistant alloys can exhibit oxidation induced damage in the long term for high temperature applications in air ( > 700°C), e.g. in gas turbines, and also for elevated temperature applications in corrosive environments ( > 300°C), e.g. in nuclear power plants. Observations show delayed brittle damage associated with stress corrosion cracking. Oxidation and damage phenomena occur in the form of oxide intrusions along grain boundaries due to local enhancement of oxygen diffusion. This work presents a comprehensive phase field and mechanical modelling framework which is capable of simulating the growth of oxide intrusions along grain boundaries. Oxide formation is accompanied by the development of anisotropic eigenstrains. Viscoplasticity in the oxide layer and in the intrusion allows for stress relaxation while a crystal viscoplasticity framework is assumed in the metallic substrate composed of two misoriented grains separated by a grain boundary (GB) subjected to oxide intrusion. The finite element simulations of a growing oxide intrusion predict distinct kinetics of the oxide layer and intrusion growth, and chromium depletion in the metallic substrate around the intrusion. Complex stress relaxation and redistribution are observed, leading to the generation of delayed positive normal tractions along the ghost grain boundary at the free surface and at the tip of the oxide intrusion. The framework also includes a constitutive GB damage model active in the actual grain boundary and in the ghost GB in the oxide layer. A novel crack initiation mechanism ahead of the oxide intrusion tip is proposed. The GB fracture stress values assumed in the simulation are such that the oxide is much more brittle than the metallic grain boundary. This, combined with the traction sign reversal ahead of the intrusion, results in crack initiation from the free surface which further enhances oxygen diffusion in the GB. This mechanism is thought to play a significant role in crack initiation and propagation during oxidation of metallic components.