Sustained macroscopic deflected fatigue crack growth in nickel based superalloy 720Li

Sustained deflected crack growth is a concern as we need to understand both the factors which cause it and when it may happen in a component to ensure we have appropriate lifing strategies in place. This behaviour has been reported in a number of fine-grained Ni-based turbine disc alloys at intermediate temperatures (200-600°C). Such fracture surfaces show a competition between opening and shear modes of crack growth, with macroscopic deflection being initiated from the free surface. The interaction of the crack and surrounding microstructure have been studied via serial sectioning using a focused ion beam, allowing a reconstruction based on combined 3D electron back scatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) to be produced. This showed both intergranular and transgranular crack growth modes occurring in the deflected crack growth and no obvious influence of grain orientation or texture to explain the crack deflection. Visualisation and representation of such data-rich images is challenging - the additional compositional (phase identification), orientation and deformation/strain mapping features of this approach together with the spatial information in 3D may require new approaches to data presentation.