The project will study quantification of enhanced diffusivity of hydrogen along dislocations (hydrogen pipe diffusion) in deformed Pd via quasi-elastic neutron scattering measurements and first-principles computations. The work scope takes advantage of protocols developed recently under NSF sponsorship to study hydrogen at very low concentration and as a function of temperature in deformed Pd. The experimental methodologies use advanced state-of-the-art quasi-elastic neutron spectrometers at the Spallation Neutron Source and the NIST Center for Neutron Research and employ computational protocols to simulate hydrogen within the local strain environment of a dislocation (relaxed into two partial dislocations) supercell. The combined experimental and computational efforts will provide the first direct quantification of hydrogen transport along dislocations over a temperature regime from classic transport (translational diffusion via site hopping) to quantum tunneling. Key objectives of the work are the direct measurement of pipe diffusion activation energy, the diffusion constant, extension to the quantum tunneling temperature regime, and the physical basis for enhanced diffusivity at dislocations from first-principles computations. A secondary objective is the study of grain boundary diffusion using the same experimental and computational methodologies. The work scope represents an extension of current advanced quasi-elastic neutron scattering capabilities in the U.S. to a regime of low hydrogen inventory and low temperature, thereby establishing new sensitivities for these instruments in the quantification of hydrogen.
NON-TECHNICAL SUMMARY: Impurities in metals influence many important properties related to performance and applications. Metals are crystalline in that the atoms form an ordered arrangement. Disruptions of this ordered arrangement are common and are called lattice defects. This work involves the study of hydrogen, an impurity, and one type of lattice defect, a dislocation. In particular, the interaction of hydrogen and dislocations in the metal palladium will be studied to better understand how the solute impurity moves or diffuses along the dislocation lattice defects. The project will employ advance computational tools and advanced neutron scattering instrumentation to achieve the stated goals. The work scope represents a significant extension of experimental and computational methodologies to new regimes of hydrogen diffusion behavior. It is anticipated that the research will promote an improved understanding of the influence of lattice defects on hydrogen transport in metals containing dislocations. The work will have an impact beyond a detailed study of hydrogen in palladium, potentially affecting areas of materials science ranging from embrittlement to energy storage in metal hydrides. Two graduate students will be educated and trained in two research methodologies, advanced neutron scattering techniques and first-principles computations, that are at the forefront of scientific inquiry. The breadth of first-principles computations 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.
