Fundamental Study of Hydrogen Segregation at Vacancy and Grain Boundary in Palladium (original) (raw)
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Hydrogen Embrittlement in Pd Crystals: Critical Hydrogen Binding at Vacancy and Grain Boundary
We studied the fundamental process of hydrogen binding and embrittlement at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. Hydrogen prefers to occupy octahedral interstitial in Pd bulk, however the stable H-vacancy complex with most H occupations will contain eight hydrogen surrounding the vacancy at its tetrahedral sites. Furthermore, H presence assists the pairing or formation of vacancies, which in agreement with other experimental and theoretical studies. Also, this observation implies about an hydrogen embrittle-ment (HE) mechanism through the connections of microvoid and cracks. Segregation of hydrogen at grain boundary, however, results in another way of possible ruptures. At GB, H-Pd bond length is the same as that in tetrahedral interstitial site and H atoms prefer locations of threefold bonding with Pd. High H accumulation results in grain boundary extension, which supports HE mechanism of grain decohesion observed by experiments. This is the first time high H occupation at grain boundary was studied and the critical H concentration at GB was reported, by means of first-principles calculations.
2015
Atomistic calculations were carried out to investigate the mechanical properties of Pd crystals as a combined function of structural defects, hydrogen concentration and high temperature. These factors are found to individually induce degradation in the mechanical strength of Pd in a monotonous manner. In addition, defects such as vacancies and grain boundaries could provide a driving force for hydrogen segregation, thus enhance the tendency for their trapping. The simulations show that hydrogen maintains the highest localization at grain boundaries at ambient temperatures. This finding correlates well with the experimental observation that hydrogen embrittlement is more frequently observed around room temperature. The strength-limiting mechanism of mechanical failures induced by hydrogen is also discussed, which supports the hydrogen-enhanced localized plasticity theorem.
The effect of lattice defects on hydrogen solubility in palladium
Journal of the Less Common Metals, 1976
It is proposed that the stress-field of the dislocation array is the principal cause of the solubility enhancement observed in the low hydrogen content, a-phase of cold-worked palladium. With an assumed uniform dislocation density of 9 X 1011 cmh2, the observed solubility enhancement of 1.65 (298 K) for heavily cold-worked palladium can be reproduced if the core radius is assumed to be 2 X Burgers vector. The temperature dependence of the solubility enhancement is also reasonably well predicted. The significance of measured relative partial molar enthalpies and entropies of absorption of hydrogen into cold-worked palladium is examined.
First-principles study of vacancy-hydrogen interaction in Pd
Physical Review B, 2009
Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy ͑up to six hydrogen atoms͒ of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.
Stabilization of Lattice Defects in HPT-Deformed Palladium Hydride
Materials Science Forum, 2010
Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.
Barriers for diffusion and interactions with hydrogen in palladium
Physica B: Condensed Matter, 1996
We investigated theoretically relaxations of a few surface layers of palladium (001) by a calculation of the total energy based on the real-space tight-binding framework. We discussed the behaviors of H atoms in vacuum, on the (001) surface and in the bulk. The diffusion barrier height of H atoms in Pd depends on the H concentration. For H occupations ofl3 and ½ on surface layers and on subsurface layers, respectively, the change of activation energy barriers is the most insensitive. We obtained metal-H binding energies and diffusion energies as x increases in PdHx, which are in agreement with experimental data of Nernst and Harada. We also obtained electronic structures of adsorbed H and absorbed H by the recursion method.
The effect of hydrogenation/dehydrogenation cycles on palladium physical properties
Physics Letters A, 2009
A series of hydrogenation/dehydrogenation cycles have been performed on palladium wire samples, stressed by a constant mechanical tension, in order to investigate the changes in electrical and mechanical properties. A large increase of palladium electrical resistivity has been reported due to the combined effects of the production of defects linked to hydrogen insertion into the host lattice and the stress applied to the sample. An increase of the palladium sample strain due to hydrogenation/dehydrogenation cycles in α → β → α phase transitions is observed compared to the sample subjected to mechanical tension only. The loss of initial metallurgical properties of the sample occurs already after the first hydrogen cycle, i.e. a displacement from the initial metallic behavior (increase of the resistivity and decrease of thermal coefficient of resistivity) to a worse one occurs already after the first hydrogen cycle. A linear correlation between palladium resistivity and strain, according to Matthiessen's rule, has been found.