This dissertation expands upon the current knowledge of plastic deformation in nickel-based superalloys, with a focus on dislocation mechanisms and the effect of microstructure. The two main topics treated are the evolution of the dislocation structure during low cycle fatigue and the effect of loading orientation on the propagation of stacking faults. A series of experimental and modelling techniques are employed throughout the thesis.

Low cycle fatigue is investigated with the motivation of understanding how slip bands form and develop prior to crack initiation. Electron microscopy techniques are used in alloy RR1000 to characterise these dislocation structures at different length scales in interrupted tests at room temperature and 700°C. The true slip line spacings and shear step lengths at the precipitate interfaces are measured for the first time ever with a new methodology, allowing for quantitative comparisons. Non-coplanar stacking faults are observed to be the main obstacles against which dislocations in a slip band pile up and accumulate in the form of mutipoles. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.

The characteristic response of these alloys, i.e. cyclic hardening followed by cyclic softening, is linked with the underlying evolution of the microstructure. A study on alloy 718Plus with different ageing heat treatments looks particularly at the influence from precipitate size and volume fraction. Microstructural characterisation is used as an input to a new physics-based mesoscale model that accounts for the prolonged precipitate shearing observed. Local accumulation of precipitate shearing near surface cracks is identified and regarded as an additional factor for fatigue failure.

Alternatively, the effect of loading orientation on the plastic deformation behaviour is investigated here in a more theoretical way. This variable is often oversimplified due to the complexity of some mechanisms, in particular those driven by dislocation reactions or splitting into Shockley partials. A novel orientation analysis framework is introduced as an alternative to simultaneously scrutinise the behaviour of multiple slip systems in a graphical and intuitive way. Due to the generality of this approach, this is developed for fcc crystals. Its application in the analysis of two simplistic twin nucleation models is performed to exemplify the use of the orientation maps generated. This framework opens the door to the study of many more deformation mechanisms in a new and more in-depth manner.

The orientation analysis framework is then expanded to the specific case of stacking faults in nickel-based superalloys, accounting for the presence of glide obstacles. The regions where stacking fault propagation is promoted are developed analytically and validated with simulations from the literature for different precipitate morphologies and distributions. This theory is then extended to include thermally assisted superlattice stacking faults and microtwinning in a comprehensive way. Mechanistic maps that simultaneously consider stress, orientation and microstructure are developed for low and intermediate temperatures. The results found are in full agreement with data from the literature.