Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on SEM images using secondary γ' themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain ɛ xy and transverse strain ɛ yy in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain ɛ yy in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain ɛ yy in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.

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The effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

The effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviours of a directionally superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than the specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young’s modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.