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The effect of hydrogen on the hardness and tensile properties of pure tungsten was examined using Vickers hardness and tensile tests. Samples were exposed to high-pressure hydrogen gas (5.8 MPa). The tensile behavior, tensile fracture surface, and hardness of as-received and stress-relieved tungsten did not change after hydrogen charging, owing to the low solubility of hydrogen. Therefore, to understand the effect of hydrogen on these materials, experiments must be performed to trap more hydrogen atoms at dislocations. In contrast, the hardness of electron-irradiated tungsten increased after hydrogen charging. Additionally, after a heat treatment at 473 K, hydrogen atoms dissociated from single vacancies, and the hardness decreased to the pre-charged value. Thus, single vacancies decorated with hydrogen atoms are expected to obstruct dislocation motion. Keywords: Tungsten, Hydrogen, Hardness, Tensile test, Defects, Electron irradiation

The change in the positron annihilation lifetime (PAL) of vacancies containing hydrogen atoms was investigated by taking PAL measurements in tungsten. The vacancies were introduced by electron irradiation, and hydrogen atoms were charged under a high-pressure hydrogen atmosphere (5.8 MPa). The PAL of single vacancies was measured to be approximately 175 ps, which decreased to approximately 155 ps after hydrogen charging. The PAL of single vacancies containing hydrogen atoms was calculated using the electron density obtained by a first principles calculation. The change in the PAL of vacancies containing hydrogen atoms was measured in experiments and calculated in simulations, and then, the two values were compared. It was found that one vacancy captured one or two hydrogen atoms (an average of 1.6 atoms). The binding energy of hydrogen to vacancies of 1.19 eV was obtained under the assumption of a thermal equilibrium state, which was slightly higher than the value determined by a previous study (1.06 eV). It is believed that the effect of impurities causes this discrepancy. The elucidation of the effect of impurities on the change in the PAL and the binding energy leads to greater accuracy in the quantitative evaluation of the hydrogen atoms trapped at single vacancies.