Temperature dependence of vacancy concentration and void growth mechanism in Al with constant hydrogen concentration: A first-principles study

In this study, a formulation that yields the chemical potential of solute hydrogen and thermal equilibrium concentrations of various coexisting vacancies, including atomic vacancy and vacancy-hydrogen complexes under the conditions of constant solute hydrogen concentration and temperature, is first proposed. The chemical potential of solute hydrogen and vacancy concentrations in Al are then obtained by evaluating the temperature-dependent physical properties in the equations via density functional theory and lattice vibration analyses. We found that hydrogen disbanded from vacancy-hydrogen complexes to form empty vacancy and interstitial hydrogen at the critical temperature approximately 200-300 K, depending on the solute hydrogen concentration. Finally, we revealed the operation temperatures of two principal void growth mechanisms by using the hydrogen pressure in the voids that are estimated from the chemical potential of hydrogen, vacancy concentrations, and diffusion coefficient of atomic vacancy. The first mechanism is plastic deformation growth, which is caused by high pressure inside the void at lower temperature. The other one is vacancy absorption growth mechanism, which is caused by absorbing diffusive vacancy in the lattice at higher temperature.