The effect of hydrogen (H_2) gas on grain boundary fracture was evaluated with a novel micro-mechanical experimental method. Micro-specimens were fabricated from the same discrete grain boundary (GB) in a bulk poly-crystalline alloy. The specimens were then mechanically loaded in a special transmission electron microscope (TEM) equipped with an environmental cell (EC). It was shown that brittle GB fracture preceded by plasticity occurred only in a H_2-containing environment. The GB fracture tendency appeared to be related to the orientation difference between the constituent grains.
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