A possible critical temperature mechanism for H blistering nucleation/dissociation in metals

We present a possible physical origin underlying the relationship between temperature and hydrogen (H) blistering nucleation/dissociation in metals according to systematic first-principles calculations in junction with statistical model and diffusion theory employing W and Mo as examples. By calculating the concentrations of total vacancies including thermal (empty) vacancy and Hn-vacancy (HnV) clusters and analyzing their diffusion behaviors, we demonstrate that H and vacancy will separate from the HnV cluster to form an empty vacancy and interstitial H atoms at T=900 K in W and T=1100 K in Mo, respectively. This implies that HnV clusters cease to exist except in name (i.e., H will never stay at vacancy) when the temperature is beyond the critical upper value. More importantly, the two critical temperature values are in quantitative agreement with experiments. Our investigating could solve a long-standing puzzle on why H blistering can only nucleate when the temperature is below a certain critical value in metals.