This PhD project is concerned with fatigue crack nucleation in a range of superalloys, including single, oligo, and polycrystalline. An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out on these alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation.
The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the mechanical tests. In addition, the stored energy is also able to correctly identify where secondary fatigue cracks are observed to nucleate in experiments.
Further, experimental and crystal plasticity modelling studies have also been carried out to investigate non-proportionality and stress state effects in fatigue in a 316 stainless steel and Ni-based alloy RR1000 which have been shown to have substantial effects on fatigue life. Stored energy density has provided a consistent and unifying explanation for the experimental observations of fatigue life in differing loading regimes (both proportional and non-proportional). A single fatigue property (the critical stored energy density, equating to new surface energy) determined from axial fatigue data alone has been shown to provide good qualitative and reasonable quantitative prediction of the experimental observations of the complex loading, providing a mechanistic explanation for the fatigue behaviour.