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.

In order to make full use of the potential fatigue crack growth resistance provided by layered architectures, a validated crack path simulation algorithm for crack propagation through different elements of the layered architectures was established. The crack path approaching a material interface was predicted by using the maximum tangential strain (MTSN) criterion and the crack behaviour at the interface was simulated by a developed two-step method (a modified stress-and-energy-based cohesive zone method considering the change in direction of an interface penetrating crack). The crack path simulation by using this algorithm in layered example architectures indicates: 1) there are two criteria zones for the transition between crack deflection and penetration in terms of the relationship between interfacial strength and toughness; 2) the likelihood of a crack deflecting out of the interface will increase with the propagation of an interfacial crack and 3) the architecture difference which affects shielding or anti-shielding behaviour has a significant effect on crack deflection or penetration events.

A multilayer overlay coating system containing an intermediate intermetallic layer (designated 2IML) is an architecture expected to show good fatigue resistance. Experimental characterisation and modelling simulations were carried out to classify the different crack initiation mechanisms occurring during fatigue of this coating system and to reveal how changes in the layer architecture lead to fatigue improvement. Fatigue improvement is achieved by decreasing the IML-Top layer thickness due to the increased surface crack initiation resistance. However subsurface initiation mechanisms inhibit the improvement (dominated by surface initiation mechanism) achieved by locating the IML-Top layer closer to the top surface.