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.
The goal of our project was to better understand the interaction of hydrogen with a certain type of defect in metals called a dislocation. Hydrogen is a common element in our environment that can adversely affect the mechanical properties of metals, primarily through interations with dislocations. Even though hydrogen is common in our enviroment and the adverse influence of this element on metals has been known for many decades, the application of advanced experimental and computational techniques to study hydrogen-dislocation interactions is limited. We used two neutron scattering techniques, small-angle neutron scattering (SANS) and incoherent inelastic neutron scattering (IINS), at two of the strongest neutron sources in the US, the NIST Center for Neutron Research and the Spallation Neutron Source. In addition, first principle computations (density functional theory, DFT) were preformed to quantify the characteristics of hydrogen trapped at dislocation cores, thereby providing a computational analog to the neutron scattering measurements. The findings the work sponsored by the NSF fall under two main areas of research, experimental investigation of the hydrogen trapped at dislocations at low temperature and first principle calculations of the perturbed environment of hydrogen near a dislocation . The objective of the computational aspect of the work is to provide a foundation of interpretation of the experimental findings, especially certain features of the neutron scattering measurements. One of the key findings of the research sponsored by the NSF with respect to intellectual merit is shown in Figure 1. This figure shows the comparison of a series of IINS results and the DFT computational analog. A related series of SANS results is shown in Figure 2. This figure shows the interaction radius between hydrogen and the dislocation as a function of temperature. Based on the DFT and neutron scattering results sponsored by the NSF we have the following findings: 1. The lattice perturbations associated with dislocations alter the behavior of hydrogen, leading to a loss of degeneracy. This finding represents the use of DFT to provide interpretation of the IINS measurements. 2. The phase transformation expected for hydrogen in bulk Pd also occurs at dislocations, but with a loss of degeneracy. This finding stems from the IINS measurements. 3. The temperature dependence of the interaction radius is consistent with a depopulation of hydrogen from dislocations at higher temperature. This finding is a direct result of the SANS work. With respect to broader impact of the work sponsored by the NSF: 1. Our work is the first to directly confirm that hydrogen does in fact undergo a phase transformation at dislocations. The confirmation is the combined result of the computational and experimental activities. 2. Given that hydrogen is common in our environment, that dislocations are a common defect in metals, and that the introduction of hydrogen into metals adversely affects mechanical properties, the direct confirmation of the hydrogen phase transformation at dislocations can be extrapolated to the response of many metal-hydrogen systems at reduced temperature. 3. Our work may help explain certain solute-dislocation phenomenon related to the so-called Cottrell atmosphere.
