Statistical mechanics and kinetics of hydrogen vacancy interaction of metals
McLellan, Rex B.
Doctor of Philosophy
Several important aspects of hydrogen vacancy interaction are discussed with a physical model as well as numerical methods for the model presented. A statistical mechanical model was first introduced to calculate the solubility and diffusivity of H in Ni based upon the concept of H-atoms possessing translational mobility over path lengths of several lattice parameters. It has been shown to be compatible with the solubility behavior but not compatible with the Arrhenius behavior of H-diffusivity above 300K. Statistical mechanical calculations were carried out to determine the vacancy concentration in Ni crystals containing dissolved hydrogen, the surface situation was discussed in particular using modified energy level spectra taken from effective medium calculations. The results showed that the presence of H-atoms results in an enhancement of the vacancy concentrations, and the effect is even more obvious for surface. Finite Difference Method (FDM) were employed to treat the "upquenching" process, ie vacancy formation due to rapid hydrogenation. A nickel crystal was studied and it was shown that the atomic fraction of vacancies was substantially increased by the presence of a tensile stress fields and that the relative magnitude of the effect decreases with increasing temperature. Finally using Ni as example again, the hydrogen diffusion under high fugacity conditions was computed considering the H-H and H-lattice interaction. Results show much greater H-concentration are produced than the previously estimated.
Chemical engineering; Engineering; Materials science