A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the 2 unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to "less plasticity" rather than "localized plasticity" around the crack tip.

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In order to study the influence of hydrogen on plastic deformation behavior in the vicinity of the fatigue crack-tip in a pure iron, a multi-scale observation technique was employed, comprising electron channeling contrast imaging (ECCI), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The analyses successfully demonstrated that hydrogen greatly suppresses the dislocation structure evolution around the fracture path and localizes the plastic flow in the crack-tip 2 region. Such clear evidence can reinforce the existing model in which this type of localized plasticity contributes to crack-growth acceleration in metals in hydrogen atmosphere, which has not yet been experimentally elucidated.

The scope of this thesis is to explore the applications of electron microscopy in the eld of hydrogen embrittlement (HE). The fundamental understanding of HE is still lacking and requires some advanced approaches to improve it. Electron energy-loss spectroscopy (EELS) was tested as a potentially viable technique for indirect hydrogen detection at nano-scale with transmission electron microscope (TEM). Hydrogen trapped at the grain boundaries or dislocations was expected to change electronic structure of local iron atoms in pure Fe specimens as predicted by density functional theory (DFT). Unfortunately, no changes in electronic structure were detected experimentally and the hypothesis was not con rmed due to several possible reasons. Namely, high dose of electrons inducing radiolysis and knock-on damage, and too high background signal. Recently developed direct-detection cameras are suggested as a possible solution for overcoming both of these limitations. This PhD project was quite a journey. Hydrogen embrittlement eld is like a forest-the deeper you go the darker it gets. I am very happy that in the end I found the exit from this forest and managed to reach the goal of the four year project-nishing this thesis. A more exciting part of the journey was the opportunity to stay in Japan for more than six months. There I met some great experts in the eld of hydrogen embrittlement, but more importantly great people who became not only my colleagues, but also friends. I would therefore like to thank my main supervisor, Annett Thøgersen, for providing me with the opportunity to explore the eld of hydrogen embrittlement, for guiding me through the pitfalls and encouraging when I was trapped in one. I also want to acknowledge her organizational skills, which made the trip to Japan possible. My co-supervisors, Øystein Prytz and Amin S. Azar, made an invaluable contribution to my development as a researcher. Their clear explanations and thorough discussions on di cult technical topics helped me a lot to proceed forward and taught me to think and work in a more methodical way, which is an indispensable skill in academia. I would also like to thank the people in the Structural Physics group who were together all along these past four years. In particular for their time spent on assisting me with the microscopes, sharing the knowledge about di erent techniques, doing workouts at the ping-pong table and having quick, two hour long tea/co ee breaks. My only regret for these years is that I didn't spend enough time with you. I am extremely grateful to have met my collaborates from Japan, Hisao Matsunaga, Yuhei Ogawa, Osamu Takakuwa, and Junichiro Yamabe. I felt like by simply being among them I was getting smarter and more disciplined. Without their contributions, this thesis would not be possible. My family deserves a special thanks for supporting me during all these years. They are the ones who taught me determination and that everything is possible if you want it enough. I am extremely proud of having such encouraging, but al...

Abstract. The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.