Hydrogen-induced acceleration of fatigue crack growth (FCG) in steels obstructs the safe design of pressure vessels and pipelines for the utilization of hydrogen gas as an energy carrier. Extensive investigations have been carried out to uncover the rationales of the phenomenon in ferritic steels, while none of the proposed models can thoroughly explain the overall crack growth acceleration behavior and its dependences on mechanistic/environmental variables. Pure iron was selected in our study as a model system of ferritic steels to avoid any microstructural complexity and simplify the interpretation of fracture mechanisms. FCG experiments were performed in 0.2-90 MPa hydrogen gas, followed by post-mortem observation of the deformation microstructures around the crack-wake by a combination work of electron backscattered diffraction, electron channeling contrast imaging, and transmission electron microscopy.

   The FCG in hydrogen gas exhibited two-stage behavior, in which the FCG rate was almost equivalent to that in the air at a relatively lower ΔK regime (Stage I), while the higher ΔK resulted in substantial acceleration up to 30 times (Stage II). The fracture mode was intergranular (IG) in Stage I and quasi-cleavage (QC) in Stage II. The detailed observations revealed well-evolved dislocation cell structures beneath the IG fracture surface in Stage I, generating small-sized voids along the peripheral un-fractured grain boundaries (GB) wherein the IG potentially commenced in virtue of the linkage of these GB micro-voids. At the other extreme, the QC was characterized by only discrete dislocations with the fracture surface microscopically lying along {001} cleavage plane. The essential process for IG fracture was enhanced nucleation of GB damages via hydrogen-dislocation-GBs interactions, while the dislocations pinning by hydrogen and concomitant suppression of plastic relaxation at the crack tip triggered microscopic cleavage and resulted in an accelerated FCG in Stage II.