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A high hydrogen concentration at the sub-surface layers of tungsten plasma facing materials has been previously observed by experiments. We have performed the first-principles calculations to understand this experimental phenomenon. The layer-dependent hydrogen dissolution energies, vacancy formation energies and hydrogen trapping energies are calculated for the W(110) surface. Our results show that hydrogen dissolving into the vacancy-free sub-surfaces is energetically unfavorable. A surface tungsten vacancy is easier to form than in the bulk and has a large trapping effect on hydrogen dissolving. The present work demonstrates the important role of surface vacancies on the hydrogen retention in the tungsten sub-surfaces. [ABSTRACT FROM AUTHOR]

The effects of hydrogen induced vacancy formation in tungsten have been investigated with a first-principles method based on the density functional theory. Hydrogen is demonstrated to assist the formation of vacancy and such effect is enhanced with increasing of hydrogen concentration. Using the energy of a single tungsten atom for the bulk as the reference energy, a spontaneous vacancy formation in tungsten is observed when more than two hydrogen atoms are introduced. These results provide a good explanation for creation of super-abundant vacancies in tungsten and can contribute to understand the mechanism of hydrogen bubble formation. [ABSTRACT FROM AUTHOR]

We have performed a first-principles investigation with a zero point energy correction on dissolution and diffusion of hydrogen (H) in a Σ5 (3 1 0)/[0 0 1] molybdenum (Mo) symmetrical tilt grain boundary (STGB). H prefers to stay at the interstitial site in the vacant space of the Mo GB with a negative solution energy (−0.42 eV) and segregation energy (−1.16 eV), which are decreased in the presence of vacancy in the GB. Furthermore, the H solution energy at the vacancy in the Mo GB is lower than that in the Mo bulk, suggesting a larger trapping capability for H of the vacancy in the Mo GB. The dissolving stability of H in the Mo GB can be explained by the low charge density that the GB provides for H. Kinetically, H prefers to easily diffuse within the vacant space along the Mo GB with a small diffusion barrier (0.04 eV), and to migrate to the GB from the bulk. These results provide a systematically exploration and analysis of H behaviors in the Mo GB at an atomic scale, which suggest that the Mo GB can serve as a trapping center for H, similar to the vacancy. In particular, a detailed comparison of H dissolution and diffusion behaviors in a Mo GB with those in a tungsten GB is performed. This can be helpful in understanding the experimentally observed H bubble formation in the Mo GB from the microscopic view. [ABSTRACT FROM AUTHOR]

Abstract: A first-principles computational tensile test has been preformed to investigate the effects of hydrogen on a tungsten grain boundary. It has been found that the maximum ideal tensile strength of the tungsten grain boundary with hydrogen atom segregation was 32.85 GPa, which was about 9% lower than that of the clean tungsten grain boundary (36.23 GPa). This indicated that the theoretical strength of the tungsten grain boundary became weaker in the presence of the hydrogen atom. Atomic configuration analysis showed that the grain boundary fracture was caused by the interfacial bond breaking. The Griffith fracture energy was calculated to be 161 meV/Å2 (2.58 J/m2) and 155 meV/Å2 (2.48 J/m2) for the tungsten grain boundary without and with the hydrogen atom segregation, respectively. The solution energy of the hydrogen atom in a fracture free surface was −0.31 eV, which was 0.08 eV lower than that of the hydrogen atom in a tungsten grain boundary. This indicated that hydrogen was a grain boundary embrittler according to the Rice-Wang thermodynamic theory. The Bader charge analysis suggested that the physical origin for hydrogen-induced embrittlement was the charge transfer induced by hydrogen in the tungsten grain boundary. [Copyright &y& Elsevier]

Helium (He) has a synergistic effect on hydrogen (H) retention in metallic plasma-facing materials for nuclear fusion devices, which further makes a potential impact on hydrogen bubble formation in the materials. Using first-principles calculations, we investigated the interactions between H and He in molybdenum. The calculated results show that the binding energies of H are decreased at the pre-formed He-vacancy complex compared to those at the monovacancy without He. This indicates H trapping at the He-vacancy complex is energetically less favorable and the plasma-facing material will have less H retention. Most importantly, we revealed the critical characteristics of hydrogen concentration with the presence of He based on a thermodynamic model and the calculated energies of H m HeV complexes. The existence of He could apparently enhance the critical hydrogen concentration beyond which an initial stage of hydrogen bubble formation will occur. Our findings provide explicit evidences for the suppressing effect of helium on hydrogen bubble formation in molybdenum. [ABSTRACT FROM AUTHOR]

Abstract: The effects of carbon (C) on hydrogen (H) trapping in tungsten (W) have been investigated by using a first-principles method, C exhibits a weak attractive interaction with H at a distance of ∼2.5Å in intrinsic W. H is energetically favorable to be far away from C, and prefers to bind onto an isosurface of an optimal charge density of 0.10electrons/Å3 at the vacancy with C. Such optimal charge density region shrinks when more H atoms are added, and disappears with the addition of a sixth H, which bonds strongly with C characterized by a much higher C–H charge density. The presence of C increases the trapping energy of H at the vacancy, indicating a strong effect of C on H trapping in W. [Copyright &y& Elsevier]

• The crack tip is blunted and the crack propagation rate is reduced in W at high temperatures. • The crack propagation rate decreases as the strain rate increased due to the emission of dislocations from the crack tip in W, regardless of the presence of H. • H atoms in the vicinity of the crack tip can hinder the propagation of the crack at low temperatures, while they can promote the propagation of the crack at high temperatures. Tungsten (W) is regarded as a viable choice for plasma-facing materials in nuclear fusion reactors. However, its mechanical properties are significantly degraded by hydrogen (H) atoms during irradiation, of which the mechanism is still elusive. In this study, we conduct molecular dynamics (MD) simulations to study the impact of H atoms on the propagation of a crack in single crystal W. The results show that the propagation rate of the crack slows down with increasing temperature. This is due to the enhanced plastic deformation, leading to blunting of the crack tip. A pre-existing crack in W is then considered at various temperatures and uniaxial applied tensile strain conditions. The propagation rate of the crack decreases with the increase of the applied tensile strain rate. This phenomenon occurs due to the relaxation of the stress around the crack tip following the emission of the dislocation at high strain rates. After introducing H atoms, it can be observed that at low temperatures, H impedes the propagation of the crack, while at high temperatures, H promotes it. This is primarily due to the formation of voids at the slip traces of dislocations and the reduction in surface energy. Additionally, the crack tip becomes blunted and its propagation rate decreases with increasing strain rate. These results indicate that providing sufficient time for H atoms to migrate is a key factor affecting the mechanical properties of W. The current results provide valuable insights into understanding the interaction mechanism of a crack and H atoms in W. [ABSTRACT FROM AUTHOR]

Abstract: We perform first-principles calculations to investigate the stress effects on stability and diffusion of H in W. Both the solution energy and the diffusion barrier of H decrease with increasing tensile stress. We show that the stress will not alter the site preference and the relative stability, with ∼0.4eV stress-independent energy difference between the TIS and the OIS. We demonstrate the solution and the diffusion of H in W becomes more difficult under the applied compressive stress, which theoretically provides a possible way to suppress the H retention in W. [Copyright &y& Elsevier]

In this work, numerical calculations have been employed to investigate the adsorption equilibrium state of hydrogen (H) atoms on tungsten (W) surface. The surface energy and relevant surface atomic density of different W surfaces are obtained through the empirical-potential model. The calculations show that the higher surface atomic densities correspond to the more stable surfaces, of which the most stable surface is the W (110) surface. The H adsorption energies are systematically analyzed by density functional theory. The equilibrium concentration dependence for adsorption upon H coverage, a temperature range from 300 K to 1100 K, and a pressure range from 10−6 Pa to 106 Pa is examined by employing a Monte Carlo model. Our results suggest that most of H atoms desorb from 700 K to 1100 K when the pressure region varies from 10−6 Pa to 102 Pa. The monolayer-adsorbed H density is estimated from ∼1.39 × 1018 m−2 to ∼1.40 × 1019 m−2 at the equilibrium state. • Surface energies of various W surface are determined. • Most of H desorb from the W (110) surface at 700 K–1100 K and 10−6 Pa–102 Pa. • The monolayer H fluence on the W (110) surface at the equilibrium state is estimated. [ABSTRACT FROM AUTHOR]

We apply the Object Kinetic Monte Carlo method to simulate the hydrogen (H) clustering behaviour with large vacancy (V) clusters in tungsten. The main advantage of this method is to consider the temperature effects, large V clusters, and long-time evolution. It is thus a very useful tool to study the microstructure evolution of defects in irradiated materials covering from the atomic scale to mesoscale. Simulation results show that the number of trapped H atoms by V and V clusters decreases with the increase of temperature. This is expected because the de-trapping ability of H atoms increases when the temperature is increased. With the correction of the zero-point-energy in calculating binding energies, the present results show that the number of H atoms trapped by monovacancy decreases for all studied temperatures. We also found that the trapping ability of V clusters for H decreases with the increase of the sizes of V clusters, independent on temperature. The H concentration can largely affect the clustering behaviour of H m V n. When the H saturation concentration is reached, the clustering behaviour of H m V n is mainly determined by the sizes of V clusters. [ABSTRACT FROM AUTHOR]