Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718
Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.
Fatigue crack growth mechanisms in superalloys: overview
Abstract Fatigue studies on disc and blade nickel based superalloys by the author and co-workers are reviewed. Crack initiation in single crystal turbine blade alloys is dominated by interdendritic porosity with oxidation processes affecting initiation position. Lifetime trends can be modelled using a multipart Paris type lifing approach. Orientation, loading state, temperature and environment determine stage I/II crack growth mechanisms and the resulting crack path and should be considered in lifing. Mechanistic insights on how complex stress states, subsurface failures and different temperatures/environments affect fatigue processes can thus improve turbine blade lifing, and direct alloy development programmes. In polycrystalline disc alloys cracks at high temperature may initiate at oxidised subsurface carbides or porosity. Grain size controls cycle and time dependent crack growth: the benefits of increased grain size in resisting grain boundary attack mechanisms predominate over those of γ′ distribution variation. Optimising grain boundary character and distribution should yield the best alloy design strategy for high temperature fatigue performance in turbine discs.
Effects of grain size and precipitate size on the fatigue crack growth behavior of alloy 718 at 427 °C
The fatigue crack growth rates of four Alloy 718 microstructures comprising a two-by-two matrix of grain size and γ" precipitate size were determined in air at 427 °C and 0.33 Hz. For a stress ratio of 0.05, slower Region II rates were obtained for coarse-grained microstructures, independent of γ" size, and microstructures with large γ", independent of grain size. In the near-threshold regime, the coarse-grained microstructures again showed slower growth rates (higher ΔKth), whereas the effect of γ" size was mixed. Deformation modes were studied using scanning and transmission electron microscopy. Fatigue deformation resulted in the formation of slip bands which were longer for coarse-grained microstructures and typically spaced farther apart and more planar for large γ micro-structures. The concepts of dislocation reversibility and slip band strain localization were used to explain the microstructural effects. Fatigue morphologies and cyclic constitutive behavior were con-sistent with the observed deformation modes. For a stress ratio of 0.75, the effects of grain size and γ" size were essentially identical to those observed for a stress ratio of 0.05. This indicated that roughness-induced closure had a minimal influence on the differences that were observed in fatigue crack growth behavior.