Intergranular Oxidation Effects During Dwell-Time Fatigue of High-Strength Superalloys
The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crack propagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometer scale and had been termed ""dynamic embrittlement."" More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 µm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grain-boundary character. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys.