Critical Assessment 21: oxygen-assisted fatigue crack propagation in turbine disc superalloys

Ni-based superalloys in turbine disc applications face increasing susceptibility to oxygen-assisted fatigue crack propagation due to increased turbine entry temperatures. The continued lack of understanding of the interplay between the factors operating during oxygen-assisted fatigue crack propagation limits: (1) development of lifing methodologies to accurately predict the fatigue performance of disc alloys/components and (2) associated disc alloy developments. An underpinning requirement to better understand the role of oxygen is to characterise the process of oxygen diffusion in the localised stress/strain state at the crack tip, which is related closely to microstructural features. The link between three-dimensional crack tomography, crack propagation rate and oxygen-related attack needs to be established. Quantitative models which include the interaction between fatigue-creep-oxygen attack need further development.