The lattice defects, especially vacancies, formed during tensile deformation in a hydrogen environment have been evaluated by positron annihilation lifetime spectroscopy (PALS). The results from several such evaluations in previous studies in hydrogen-charged iron, steels, and Ni-based alloys are reviewed in this study with reference to hydrogen embrittlement models. A strong tendency to increase the positron lifetime for the vacancy cluster component, that is, the larger the vacancy cluster size, the lower the fracture strains, was found in many PALS studies on tensile-deformed metals. This suggests that plastic strain localization, a characteristic feature of hydrogen embrittlement, is consistent with hydrogen-enhanced vacancy clustering during plastic deformation. Early studies suggested that hydrogen precharging would result in a significant increase in the vacancy density, as inferred from the hydrogen content obtained from thermal desorption analysis (TDA). However, recent PALS studies have been negative, as no significant increase in vacancy density were observed.
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