Localized cyclic deformation and corresponding dislocation arrangements of polycrystalline Ni-base superalloys and pure Nickel in the VHCF regime

Knowledge about the fatigue behaviour of metallic materials in the range of very high cycle fatigue (VHCF, N > 107) is a key factor to improve the safety against failure of component parts. Even in the conventional field of application of nickel-base alloys (e.g. turbine blades) superimposed high frequency loading requires a safe life at very high number of cycles. Thus, the fatigue behaviour of structural materials in the very high cycle regime has become an important and active subject of research. In this paper, polycrystalline nickel-base alloys (Nimonic 80A and Nimonic 75) and pure Nickel (wavy slip character) were investigated in the VHCF regime by varying the precipitation conditions (peak-aged, overaged and precipitation-free), dislocation slip behaviour and test frequency. In addition, mechanisms of fatigue failure in the VHCF regime were compared to conventional damage evolution (in the LCF and HCF region) on the basis of load-controlled low frequency tests. Surprisingly the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak-aged condition. The results obtained document that fatigue failure can still occur beyond 107 cycles. Transmission electron microscopy (TEM) was carried out, in order to characterize the influence of microstructure, the resulting dislocation slip behaviour and the relevant dislocation particle interaction mechanism. Studies of the development of slip markings on surface grains were performed mainly using scanning electron microscopy (SEM).