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.

This study presents the results of systematic variation of essential processing parameters with regard to thermal debinding and sintering of components fabricated by MIM using Ti-6Al-4V powder. The investigation aims at the understanding of the particular influence these parameters have on the mechanical properties of the sintered parts. This study shows that the debinding parameters appear to be rather uncritical, whereas sintering and cooling rates as well as maximum temperature are important in terms of their effect on tensile strength. Contrary to the strength, the ductility remains nearly unaffected. Based on these results, samples displaying a yield strength of 757 MPa, UTS of 861 MPa and a plastic elongation of more than 14% were produced. These values meet the requirements of the ASTM B 348-02 for titanium alloy grade 23. * 3. Manuscript Click here to view linked References

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