To understand the mechanism for hydrogen-induced embrittlement in a nickel-based superalloy, detailed electronmicroscopy characterisation has been employed on the UNS N07718 (Alloy 718) after hydrogen charging and slow strain rate testing to investigate the strain localisation and damage accumulation caused by hydrogen. Transmission Electron Microscopy analysis demonstrates that the microstructure of the material after tension is characterised by planar dislocation slip bands (DSBs) along {111}γ planes. Consistent results from Electron Channeling Contrast Imaging (ECCI) reveals that cracks always propagate along planar DSBs in the presence of hydrogen. This phenomenon is rationalised by the evident nucleation of nanoscale voids along the DSBs, especially at the intersections between nonparallel DSBs. The proposed mechanism, confirmed by both the ECCI analysis and fractographic study by Scanning Electron Microscopy, indicates that the interaction between the hydrogen and dislocations along the DSBs leads to void nucleation. Furthermore, the results suggest that coalescence and widening of voids via the dislocation process promote the crack propagation along the DSBs in hydrogen charged Alloy 718.

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Twin boundaries (TBs) in Ni-based superalloys are vulnerable sites for failure in demanding environments, and a current lack of mechanistic understanding hampers the reliable lifetime prediction and performance optimisation of these alloys. Here we report the discovery of an unexpected γ″ precipitation mechanism at TBs that takes the responsibility for alloy failure in demanding environments. Using multiscale microstructural and mechanical characterisations (from millimetre down to atomic level) and DFT calculations, we demonstrate that abnormal γ″ precipitation along TBs accounts for the premature dislocation activities and pronounced strain localisation associated with TBs during mechanical loading, which serves as a precursor for crack initiation. We clarify the physical origin of the TBs-related cracking at the atomic level of γ″-strengthened Ni-based superalloys in a hydrogen containing environment, and provide practical methods to mitigate the adverse effect of TBs on the performance of these alloys.

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