Hydrogen Impurities and Dislocations in Transition Metals

Doktorsavhandling, 1997

Motion of impurities and dislocations is essential for many material properties of metals. In the present thesis we have made use of different atomistic simulation techniques to study the behavior of hydrogen and edge dislocations in transition metals. We have carried out molecular-dynamics (MD) simulations for hydrogen diffusion in niobium. The diffusion constant for hydrogen and its isotopes, obtained at high temperatures, deviates from the simplified classical expression, due to the more complicated diffusion motion. The quantum aspects of the hydrogen motion are elucidated using the path-integral Monte Carlo technique. We argue that the main quantum effects can be incorporated into a temperature- and isotope-dependent shift of the potential energy surface at high temperatures. Using MD-simulations we estimate the hydrogen induced lattice distortions and determine the quasi-elastic diffuse scattering function at room temperature. By comparing the obtained scattering function from the dynamic simulation with static defect models we argue that the hydrogen distribution around the interstitial sites gives important contributions to the scattering function. The motion and core-structure of the a/2[110] edge dislocation in palladium, have been studied using first principles electron-structure calculation and MD simulations. We use first-principles calculations to determine the generalized stacking fault (GSF) curve for Pd and Al, and calculate the core structure and the Peierls barrier for dislocation slip, using the Peierls-Nabarro (PN) model. We find that the PN-model gives a reasonable description of the slip-process, but the effect of atomic relaxation when calculating the GSF curve is found to be large. The relaxation of the dislocation core is studied in MD simulations. We find that for the core structure the difference between the full atomistic simulation and the PN model is substantial. In the atomistic case the core is much wider and extends over a larger region. The magnitude of the Peierls barrier is found to be more similar and the corresponding Peierls stress compares favorable with experimental data. Some studies of motion of dislocations with and without hydrogen present are also undertaken. We find that hydrogen reduces the velocities of dislocations and that the hydrogen distribution in the glide-plane is essentially unaffected by that motion.