Thermodynamics of hydrogen and vacancies in metals

Abstract

This thesis studies metal-hydrogen systems. The interaction between hydrogen-atoms and vacancies in metals have been elucidated in Fermi-Dirac statistics. Calculations have been presented and compared for specific models in which H-atoms act both as simple interstitial species and form either decorated vacancies or substitutional defects. A model has been presented to explain the superabundant vacancy formation under high hydrogen pressures. The solutions based on these models apply to much lower temperatures and higher concentrations than the traditional ones. These results show abundant vacancies will be formed in the presence of hydrogen; the vacancy concentration is many orders of magnitude larger than those in the H-free lattice. A study of the diffusion of hydrogen in the crack tip area has been provided. The slow diffusivity of H-atoms at low temperatures and the interaction between H-atoms and vacancies in the crack tip plastic zone give an explanation of the experimental data which show a maximum crack growth rate at room temperature. This work is associated with the embrittlement of steel by hydrogen.