Hydrogen-Dislocation Interactions at Low Temperature in Deformed Pd: Spatial and Vibrational Characterization Using Neutron Scattering and Advanced Computational Techniques

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TECHNICAL: This project focuses on studying of hydrogen-dislocation trapping interaction at low temperature in deformed Pd using neutron scattering and advanced computational techniques. This project is a follow up of a recent SGER grant to PI (DMR-0634336) from the Metals program. This trapping interaction has been studied in the past by several groups, but never at low temperature (to about 4K), nor with a combination of inelastic neutron scattering (INS), small-angle neutron scattering (SANS), and density functional theory (DFT). The research effort will be based on recent INS and DFT work by the PIs. This work demonstrates 1) that hydrogen trapped at dislocations undergoes a bulk-like phase transformation during cooling from 295 K to 4 K, with a vibrational density of states (DOS) that may be a signature of local distortion and 2) that DFT can be used to relax an edge dislocation in Pd with flexible boundary conditions. This initial work provides a foundation for further experimental study of the behavior of hydrogen in the distorted environment of dislocations. The experimental characterization will include the measurements of the vibrational DOS with INS and quantification of the radial extent of the trapped hydrogen with SANS. The experimental parameter space will include temperature, hydrogen concentration, and control of the dislocation substructure. The computational component of the work will yield the hydrogen binding energy at different sites within and near the dislocation core and the vibrational DOS for trapped hydrogen. Of particular interest are perturbations of the measured vibrational DOS with the distorted dislocation environment. Although these perturbations are likely due to lattice distortion, the INS and SANS measurements alone cannot conclusively identify their origin. A direct, first-principles calculation of the trapped hydrogen vibrational DOS in a relaxed lattice is necessary. NON-TECHNICAL: The education merit of the research is related to the combined application of neutron scattering techniques and advanced computational methods to study these trapping interactions. The PIs have the expertise required to perform the work, and the preliminary INS and DFT work provides a reasonable basis for further investigation. It is anticipated that the research will promote an improved understanding of the behavior of hydrogen in distorted lattice environments. The work will have an impact beyond a detailed study of hydrogen trapping at dislocations in Pd. Two graduate students will be educated and trained in two research protocols, neutron scattering and advanced computational techniques that are at the forefront of scientific inquiry. The breadth of DFT in materials research is extensive, as are the recent investments in neutron scattering infrastructure at NIST, ORNL, and LANL. Graduate students trained in the use of these protocols will be well positioned for productive scientific careers.