On the copious formation of vacancies in metals due to hydrogen dissolution

Abstract

Fukai and Okuma have recently given convincing evidence for the formation of extremely high vacancy concentrations ([approximately] 20% of the metal atom sites) when Ni and Pd are annealed for a few hours at high temperatures ([<=] 800 C) when under high H[sub 2] pressures ([approximately] 5GPa), i.e., at very high H concentrations. As indicated by Fukai and Okuma, the implications of this effect, especially through its possible influence on enhanced metal diffusion, could be profound. Fukai and Okuma discuss some other results which also seem to indicate large vacancy concentrations at very high H concentrations. These include the maximum observed H concentration exceeding that expected from structural considerations and an anomalous change in the apparent partial molar volume of H in Pd alloys at high H concentrations. Fukai and Okuma gave a tentative explanation for the formation of large vacancy concentrations in terms of vacancy-hydrogen complexes. In the following the authors develop a simple model which may explain the origin of such large vacancy concentrations in a more plausible way.