It has often been observed that solute hydrogen triggers a remarkable increase in flow stress in pure Ni. As reported in previous studies, this H-induced hardening has been attributed to the solid-solution strengthening and/or H-enhanced evolution of the dislocation structure. However, it remains unclear which of those two factors actually dominates such macroscopic hardening-behavior. Via two mechanical tests at different scales, i.e., slow strain-rate tensile (SSRT) and micro Vickers hardness (MVH) tests, the H-enhanced evolution of the dislocation structure and its contribution to H-induced hardening in pure Ni and in Cu–Ni alloy were investigated in this study. In pure Ni, the SSRT test at room temperature (RT) demonstrated an increase in flow stress with a greater work-hardening rate, indicating the H-enhanced evolution of the dislocation structure, as supported by the MVH test. Nevertheless, the SSRT and MVH tests revealed that the H-enhanced evolution of the dislocation structure was significantly suppressed at 77 K. On the other hand, with respect to Cu–Ni alloy, no H-enhanced evolution of dislocation structure was observed during the SSRT and MVH tests, regardless of the deformation temperatures. This difference in the evolution of the dislocation structure between pure Ni and Cu–Ni alloy was consistent with the experimental results obtained beforehand, using scanning transmission electron microscopy (STEM). A series of experimental results indicated that both the solid-solution strengthening and H-enhanced evolution of the dislocation structure could contribute to the increase in flow stress, with the degree of contribution dependent on the respective material and the deformation temperature.
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