Effects of hydrogen on macroscopic and microscopic features of crack growth in thin specimens were investigated using a thin sheet of single-crystal Fe-3wt%Si alloy. Center-cracked specimens were tested under a sustained load in a hydrogen environment, and under continuous stretching in an air environment. The fracture features were compared to elucidate the role of hydrogen in hydrogen-induced delayed crack growth. In both air and hydrogen environments, the crack growth mode of the thin specimens was the same as that of the thick specimens, despite the significantly reduced thickness. Surprisingly, the crack grew discontinuously, and left striations on the fracture surface, in which shorter striation spacing was observed in hydrogen. In addition, there was a similarity in the deformation microstructures beneath the fracture surface, that is, both microstructures were composed of three distinct layers characterized by different plastic strain gradients and dislocation densities. In the hydrogen environment, the hydrogen-enhanced localized plasticity (HELP) mechanism is believed to be relevant to the crack growth process. Also, HELP was supposed to cause different characteristics (the magnitude of plastic strain, the plastic strain gradient, and dislocation structure) of the three layers in the hydrogen compared to those in the air. Reverse plastic deformation occurred in the regions behind the crack front during crack growth, which is speculated to contribute not only to enlarge the crack tip opening angle (CTOA) but also to blunt the crack tip.
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