High Temperature Fatigue of Nickel-based Superalloys during High Frequency Testing

Microstructural features acting as stress raisers lead to localised and inhomogeneously distributed irreversible deformation in the very high cycle fatigue (VHCF) regime although load amplitudes are well below the classical fatigue limit. Hence, changes in the microstructure due to thermally activated processes can result in an overall change of damage mechanism compared to the fatigue behaviour at room temperature. The influence of isothermal testing at elevated temperatures up to 800 °C during high frequency testing was studied with regard to the VHCF behaviour of the nickel-based superalloys Nimonic 75 and 80A. This material combination allowed a distinction between the influence of the stability of the initial precipitation condition (precipitation-free, peak-aged, overaged) on the one hand and the formation of oxide layers on the other hand. High frequency tests were accompanied by extensive metallographic analysis by means of high resolution scanning and transmission electron microscopy. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers. In this respect, the relevance of high frequency testing regarding the true fatigue behaviour, which in the case of thermally activated microstructural changes implies a likely time dependence of damage mechanisms, will be critically reviewed.