Revisiting mechanisms for hydrogen-assisted fracturing of Ni-Fe-Cr alloys

The susceptibility of polycrystalline Ni–Fe–Cr austenitic alloys to hydrogen-assisted fracturing was evaluated through tensile tests performed under in-situ hydrogen charging, and the dislocation structures affected by hydrogen were characterised. Hydrogen embrittlement in the stable fcc austenite region becomes more severe with an increase in the Ni content but is mitigated with an increase in the Cr content. Transmission electron microscopy images of the dislocations affected by hydrogen showed that hydrogen transforms the configuration of the dislocations from a planar one, with edge dislocations, to a screw-like one consisting of dipoles and loops. Meanwhile, the stacking fault energy values, determined based on the separation between the partial dislocations, suggested that hydrogen reduces the stacking fault energy by up to 30%. The mechanisms by which hydrogen alters the behaviour of the dislocations are elucidated in terms of hydrogen trapping within the dislocation cores. The energy of hydrogen trapping within the dislocation cores, which was calculated based on the difference between the stacking fault energies with and without hydrogen, increased with increasing Ni content and decreased with increasing Cr content. The compositional dependence of hydrogen embrittlement and the hydrogen trapping energy can be expressed as functions of the average density of the 3d electrons.