The thermodynamic and kinetic behavior of metal–vacancy–hydrogen systems Original Research Article

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.