On the effects of particle strengthening and temperature on the VHCF behavior at high frequency

The fatigue properties in the high-temperature range at very high number of cycles of the precipitation-hardened nickel-base alloy Nimonic 80A were investigated. With the applied global strain amplitude being well below the elastic limit, cyclic life is dominated by heterogeneously distributed and localized plastic deformation in the microstructure. The material studied was fatigued in the peak-aged and overaged condition applying high frequency testing methods. Microstructural analysis by means of scanning and transmission electron microscopy was performed to determine the dominating damage mechanism. At room temperature planar single slip and at elevated temperatures partially wavy dislocation arrangements were found, with the wavy arrangement being accompanied by dislocation climbing processes as well as Orowan loops. At room temperature the overaged Nimonic 80A showed a slightly superior fatigue behavior compared to the peak-aged condition in the VHCF range, whereas at elevated temperatures the difference in fatigue behavior at high number of cycles seemed to be of less significance. The overaged condition showed a slight increase in fatigue life at 600 °C accompanied by a tendency towards a more pronounced homogeneous dislocation movement. Isothermal cyclic deformation at 800 °C led to a pronounced decrease of cyclic life with both heat treatments exhibiting a very similar fatigue behavior. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers.