The interaction of applied stress and oxidation and their combined influence on the fatigue crack initiation behaviour of CG RR1000

Turbine discs manufactured from Ni-based superalloys experience high stresses and temperatures during operation. Whilst the stresses and oxidation of these components have been extensively characterised independently, their interaction has been rarely studied. It is imperative to understand the relationships between the two and the influence on crack initiation to improve component lives and their prediction. In this research, the oxidation under different stresses at different locations around double-edged notched specimens is examined. The crack initiation behaviour of fatigue samples is analysed, and the detection of microcracking using digital image correlation is assessed. Consistent with observations in literature, stress had little influence on the external oxidation of the alloy but increased the depth of internal intrusions. The stress in the intrusion tips is estimated using a recently developed numerical model. The diffusion of Ti is accelerated under stress by the presence of dislocations, expediting dissolution of tertiary γ′ by shearing, and allowing pipe diffusion of Ti to the surface. Etched samples revealed that machining characteristics influence the near-surface microstructure, which in turn influences the oxidation behaviour. Internal crack initiation yielded superior fatigue lives compared to surface crack initiation, and the level of oxidation lowers the threshold alternating stress required for multiple surface crack initiation, lowering the fatigue life. Oxidation produced an adequate speckle pattern for DIC analysis up to temperatures of 700°C. DIC detected microcracking of oxides, allowing estimations of the fracture stresses of the oxides. The calculated fracture stresses showed good agreement with the intrusion tip stresses estimated by the numerical model