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The hydrogen embrittlement of metallic systems is a widespread problem, which causes ductile-to-brittle transition of fracture mode and accordingly the unpredictable lifetime of many structural materials. Although many mechanisms have been proposed to explain this phenomenon, the precise role of hydrogen in this process is not well understood. In this work, the effects of hydrogen on the deformation of Ni-based superalloy are investigated by performing the in-situ straining experiments inside a transmission electron microscope, coupled with ex-situ hydrogen processing. The microscopic features of the superalloy (such as different phases, grain boundaries and dislocations) and their responses to the tensile strain were carefully examined. The crack nucleation and propagation behaviors were studied by comparing their difference with and without H2 exposure. The fracture modes were directly correlated to the …