Hydrogen embrittlement susceptibility of a Ni-16Mo-7Cr base superalloy

This study investigated the hydrogen embrittlement sensitivity of a Ni-16Mo-7Cr base superalloy under different hydrogen pre-charging states. Detailed electromicroscopy characterization has been employed on the hydrogen pre-charged alloy after tensile testing to understand the mechanism of hydrogen-assisted cracking. The results show that the tensile strength of the alloy is remarkably decreased with the increasing of hydrogen pre-charging time, whereas the yield strength remains stable all the time. The fractographic study combined with ECCI analysis by Scanning Electron Microscopy demonstrates that the cracks propagated predominantly along the grain boundary in the presence of hydrogen, and a few can propagate along planar dislocation slip bands (DSBs) especially along the intersections between nonparallel DSBs. The proposed hydrogen embrittlement mechanism for the Ni-16Mo-7Cr base superalloy is that the coalescence and widening of the nano-voids, which are resulted from the interaction between the hydrogen and dislocations, cause the crack initiation and propagation along the grain boundary, and further cause the alloy intergranular fracture.