Oxygen influenced intergranular crack propagation: analysing microstructure and chemistry in the crack tip region

Ni-base superalloys have for decades been studied with regard to environmentally influenced intergranular crack propagation. For high temperature fatigue frequencies <0.1 Hz, it has been shown that an oxygen-rich environment promotes time-dependent crack growth while at >0.1 Hz and/or in inert environments (e.g. vacuum) crack growth is cycle dependent. Oxygen interaction at, or ahead of, the crack tip has been pointed out as the reason for the degraded mechanical properties. While many aspects of this type of crack growth have been previously investigated there is still no consensus about the detailed mechanisms, mainly due to the lack of in-detail investigations of the crack-tip region. Here, crack tip regions in the Ni-base superalloy Alloy 718 were studied. Specimens were subjected to 90 s hold-times at 550 degrees C and 650 degrees C. Crack growth was arrested before final fracture, allowing cross-sectional analyses of the crack-tip region using scanning electron microscopy (SEM). Detailed studies of the crack-tip region were performed using transmission electron microscopy (TEM) and atom probe tomography (APT). For both APT and TEM samples, site-specific focussed ion beam (FIB) sample preparation was performed in a combined FIB-SEM system. The methodology of accessing and analysing the crack tip region is shown. Initial results on oxidation, oxygen penetration and plastic deformation are shown and discussed