An investigation of hydrogen/microstructure interaction in complex nickel-based alloys : Multi-scale detection and embrittlement mechanisms

Precipitation-hardened nickel-based alloys are established as standard materials for key applications in oil and gas industry. Alloy 718 (UNS N07718) is one of the most commonly used commercial alloys in this field. Despite the superior corrosion resistance and mechanical properties, high strength Ni-based alloys show susceptibility to hydrogen embrittlement. To improve material’s resistance to hydrogen embrittlement, it is important to have a better understanding of the mechanisms/factors, which affect the hydrogen-related degradation of mechanical properties. Hydrogen embrittlement of alloy 718 was investigated by in situ tensile testing under hydrogen charging, placing the focus on the underlying hydrogen-related deformation mechanisms and cracking. Hydrogen-induced cracking was studied with the correlative use of electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) analyses to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. The material was investigated at different strength levels: after solution annealing (as quenched state) and after age-hardening at 780 °C, to elucidate the contributions of stress or strain to the cracking mechanisms. It was found that the deformation modes affecting the stress and/or strain localization play an important role in hydrogen-related cracking mechanisms. Material in over-aged state (aged at 870 °C) with different precipitation conditions of the δ-phase was also investigated to check the effect of the grain boundary δ-phase in crack propagation. To further elucidate the role of the δ-Ni3Nb intermetallic precipitates in hydrogen embrittlement, a binary Ni-Nb model alloy was used. The approach of conducting correlative measurements using thermal desorption spectroscopy (TDS), silver decoration, scanning kelvin probe force microscopy (SKPFM), and secondary ion mass spectrometry (SIMS), enabled performing multiscale mapping of hydrogen in the microstructure, ranging from the macroscale down to few-nanometers. With this joint and spatially resolving approach the microstructure- and time-dependent distribution and release rate of hydrogen were observed with high spatial and temporal resolution. By correlating the obtained results with mechanical testing the hydrogen-assisted failure mechanism was discussed