The thermodynamic and kinetic behavior of metal–vacancy–hydrogen systems

The thermodynamics of metal-hydrogen solid solutions have been studied using a Fermi-Dirac distribution function in order to describe the formation of H-vacancy clusters. The H-vacancy interaction energies for clusters (decorated vacancies) containing up to six H-atoms have been taken from effective medium calculations. The solute metals Pd, Ni, Fe, Mo and Nb have been considered. The calculations enable the relative fractions of clusters of a given order to be calculated as a function of H-concentration and temperature. A mathematical model for the simultaneous diffusion of H-atoms, vacancies and H-VAC clusters has been developed. The numerical evaluation of the basic kinetic equations is based upon finite difference methods. Kinetic calculations have been made based upon Ni-H solutions as a model system.