First-principles investigation of hydrogen interaction with metals
The presence of hydrogen (H) in metals has a deleterious effect on their mechanical properties. This phenomenon is called hydrogen embrittlement (HE). Since, the susceptibility of a metal to HE is strongly dependent on the rate and mode at which H can accumulate at a location of embrittlement, the transport of H is a critical aspect of HE. This thesis aims to advance the understanding of H trapping and diffusion in the microstructure of materials by means of first-principles methods. As first step, we considered the diffusion of H in bulk metals. The diffusion coefficients of interstitial hydrogen in bulk Fe and Ni crystals have been calculated over a wide range of temperatures. Quantum-mechanical effects have been included using the semi-classical transition state theory (SC-TST) and the small-polaron model of Flynn and Stoneham. Our results show that it is crucial to include such effects for a quantitative simulation of diffusion of H in bcc Fe even at room temperature, while in case of H in fcc Ni this is less important. Then we investigated the interaction of H interstitial with Ni grain boundaries and with TiC precipitates in Fe. In the former case two distinct types of GBs have been considered: the U+03A3 3(111)[-110] with a close-packed atomic interface structure and the U+03A3 5(210) with a less dense structure consisting of open structural units of atoms. Our calculations reveal that these two GBs have markedly different interaction behaviors with interstitial hydrogen atoms. The close-packed U+03A3 3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the U+03A3 5 GB provides numerous trapping sites for H within its open structural units as well as easy pathways for H migration along the GB plane that enhance the H diffusivity by about two orders of magnitude wot respect to bulk Ni. Additionally the maximum amount of H which can accumulate at the U+03A3 5 GB at finite temperature has been estimated by coupling density functional theory results with the Langmuir-McLean model. Finally we investigated the effects of H on the cohesion of the U+03A3 5 GB and observed two distinct effects. On one side the H lower the Ni-Ni bonds, but on the other side H forms H-Ni bonds which strengthen the GB cohesion. In case of H interacting with a TiC precipitate in Fe we investigated the energetics associated with the trapping of H at various interfaces as well as at C vacancies inside the TiC particles. Our results show that the interfaces offer moderately strong traps. The C vacancies provide strong traps, but their population is possible at high temperatures only. For all the cases studied, our results provide valuable insights into the understanding of H transport in metals and therefore into HE.
Freiburg/Brsg., Univ., Diss., 2016