It was noted that knowledge concerning the fatigue behaviour of metallic materials in the range of very high cycle fatigue (N > 107) was a key factor in improving the non-failure of component. Even in the conventional field of application of nickel-base alloys, superposed high-frequency loading required a safe life at a very high number of cycles. Thus, the fatigue behaviour of structural materials in the very high cycle regime had become an important area of research. Here, polycrystalline nickel-base alloys and pure nickel (wavy slip character) were investigated in the very high cycle fatigue regime by varying the precipitation conditions (peak-aged, over-aged and precipitation-free), dislocation slip behaviour and test frequency. In addition, mechanisms of fatigue failure in the very high cycle fatigue regime were compared to conventional damage evolution (in the low-cycle and high-cycle fatigue regions) on the basis of load-controlled low frequency tests. Surprisingly the over-aged condition of Nimonic 80A showed a slightly higher fatigue strength in the very high cycle fatigue regime as compared to the peak-aged condition. The results obtained document that fatigue failure could still occur beyond 107 cycles. Transmission electron microscopy 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.

Localized Cyclic Deformation and Corresponding Dislocation Arrangements of Polycrystalline Ni-Base Superalloys and Pure Nickel in the VHCF Regime. C.Stöcker, M.Zimmermann, H.J.Christ: International Journal of Fatigue, 2011, 33[1], 2-9