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In this study, molecular dynamics (MD) simulations were used to systematically investigate the behaviour of hydrogen (H) and helium (He) around 1/2<111> screw dislocations (SDs) and 1/2<111>{110} edge dislocations (EDs) in tungsten (W). The binding energy distributions of H and He around these two dislocations were obtained by molecular statics methods. The results revealed that both types of dislocation cores strongly attracted H and He atoms, and capture strengths were stronger for EDs than SDs. The simulations predicted that two factors influenced the interaction: strain fields of dislocations (effects of dislocations on H/He atoms) and residual stresses of H/He atoms (influences of H/He atoms on dislocations), which were validated. In the selection of trapping sites, H was synergistically affected by hydrostatic compressive and shear stress fields, and He was influenced only by shear stress fields. Conversely, the insertion of H atoms reduced the local lattice distortion of the SDs and induced the EDs to glide. He atoms repelled neighbouring atoms to increase the free volume of the trapping site. The diffusion behaviour of H/He atoms around dislocation cores was studied through nudged elastic band (NEB) and MD methods. Both H and He atoms tended to diffuse along screw channels around SD lines and one-dimensional channels inclined to ED lines, preferentially migrating along the dislocation line. The results of this study provide new insight into the retention and diffusion of H and He atoms in W. However, we have not been able to determine the clear influence of dislocations on the retention amount and diffusion tendency of H or He atom. [ABSTRACT FROM AUTHOR]

Tungsten (W) and W alloys are regarded as the most promising candidates for plasma facing materials (PFMs), which will be widely used in the next generation of fusion reactors. However, blistering in W-PFM induced by extremely high fluxes of low-energy hydrogen (H) and helium (He) ions irradiation will seriously influence the plasma stability and limits the lifetime of PFM. We systematically investigate the interaction between H and He interaction in W using molecular dynamics (MD) calculations. The vacancy clusters can capture a large amount of H/He, and the H-He-vacancy clusters have much higher binding energy to He than to H. The binding energy of H-He-vacancy clusters to vacancy/W is positive, and H-He-vacancy clusters have strong trapping of W atoms and vacancies. He will strongly affect the H behavior in W, He atoms and He cluster will trap H atoms, thereby preventing the diffusion of H in W. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have been performed to investigate the diffusion behavior of helium (He) clusters in tungsten (W), because their diffusion properties provide basic knowledge in understanding the He bubble formation. The binding energy between He and He cluster is shown to be positive, and thus, He is easy to form bubbles by self-trapping. The mean squared displacements (MSDs) were employed to determine the diffusivities of He clusters with different sizes at different temperatures. The He bubble formation at different temperatures with 1% He was also investigated. It is revealed that the formation of He bubbles is strongly associated with the temperature and the diffusivities of the He clusters in W. The results demonstrate the initial stage of the He bubble formation and growth in W. [ABSTRACT FROM AUTHOR]

Abstract: Molecular dynamics simulations have been performed to investigate the interactions between helium (He) and defects in tungsten (W). The binding energy between He and He cluster is shown to be positive, which increases with increasing He cluster size. Both the W self-interstitial atoms (SIAs) and the vacancy can promote the He cluster formation. The binding energies of a He, a vacancy and an SIA W to a He–vacancy cluster (He nVm ) in W are also investigated, which depend on the n/m ratio. According to these results, we propose the formation and growth mechanism of He bubbles, which involves the procedures of He–vacancy cluster formation, the capturing of vacancies, then He atoms, and vacancies again. The mechanism provides a good reference to understand the initial stage of the He bubble formation and growth in W. [Copyright &y& Elsevier]

Abstract: We construct modified W–H and W–W analytical bond-order potentials for a W–H system. In combination with Brenner’s H–H potential, we demonstrate that such potentials can reproduce energetics and structural properties of W and W–H systems, including defect formation energies, surface energies and diffusion barriers as well as melting point determined from first-principles or experiments. The present potentials can be employed for modelling the behaviour of H in W containing defects such as vacancies and surfaces. [Copyright &y& Elsevier]

Abstract: Molecular dynamics simulations were performed to investigate the interaction between H and vacancy in W using an analytical bond-order potential to describe the interactions between W–W, W–H and H–H. The most stable configuration for H in W is the tetrahedron interstitial site. We calculated the binding energies of an H and a vacancy to an H-vacancy cluster (H n V m ) in W, respectively, where n and m ranged from 0 to 10. The binding energy was almost unchanged. The binding energy of a vacancy to H-vacancy cluster is about 0.4eV, which is higher than the binding energy of an H to H-vacancy cluster. Vacancy is much easier to bond with H-vacancy cluster than H. And H is easier to stay in the tetrahedron interstitial site or octahedron interstitial site in bcc W. [Copyright &y& Elsevier]

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. α-Al 2 O 3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk α-Al 2 O 3. The calculation results show that the most energetically stable form of hydrogen in a perfect α-Al 2 O 3 crystal under H 2 gas annealing treatment is the H 2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in α-Al 2 O 3 , these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in α-Al 2 O 3 is 2H i , with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H 2 molecule. We have derived the temperature-dependent diffusivity of the H 2 molecule in α-Al 2 O 3 as D (T) = (3.65 × 10 − 7 m 2 / s) e x p (− 2.27 e V / K T) , which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H 2 molecule are so high that dissolution and diffusion of the H 2 molecule in α-Al 2 O 3 are very difficult, resulting in low hydrogen permeability. [ABSTRACT FROM AUTHOR]

Abstract Atomic-level studies have been performed to investigate the hydrogen behavior near a 1/2<111>{110} edge dislocation and a 1/2<111> screw dislocation in α iron. Molecular statics analysis has been carried out to determine the stress distributions and hydrogen binding energy around the dislocation cores. The results reveal that the hydrogen binding is more sensitive to the hydrostatics stress than the shear stress around the edge dislocation, while the shear stress plays a leading role on the hydrogen binding around the screw dislocation. In addition, nudged elastic band (NEB) calculations have been applied to explore different migration paths and the corresponding migration energy barriers of a single hydrogen atom at the dislocation cores. It is of interest to note that both edge and screw dislocations are not able to offer fast pipe diffusion of hydrogen atoms along the dislocation line, as normally considered. Instead, hydrogen atoms prefer to diffuse slowly along oblique paths crossing the edge dislocation line on the slip plane and spiral paths surrounding the screw dislocation line. Furthermore, molecular dynamics simulations have been performed to study the diffusion behavior of hydrogen at the dislocation cores, which verifies the results of NEB calculations. Highlights • The distribution of the stress and hydrogen binding energies around the dislocations together with their correlations. • Neither the edge nor screw dislocations can provide fast diffusion pipes for hydrogen in α iron. • The hydrogen diffusion mechanism near the dislocation cores at different temperatures. [ABSTRACT FROM AUTHOR]

• A possible mechanism for the formation of vacancy dislocation loops in α-Fe is proposed. • Vacancy-NH clusters prefer to shape into 2D clusters, especially on (211) planes. • The presence of H can greatly promote the growth of vacancy dislocation loops. Genetic algorithm was applied to search the energy minimization configuration of vacancy clusters in α-Fe. Molecular statics calculations and dynamics annealing relaxation were employed to calculate the formation and binding energies of vacancies and 3D vacancy or vacancy-hydrogen (H) clusters, as well as 2D vacancy or vacancy-H clusters on (111), (011) and (211) planes. Our calculations show that vacancies prefer to form 3D clusters and vacancy dislocation loops are difficult to form, while vacancy-H clusters prefer to shape into 2D clusters, especially on (211) planes. Since H prefers the directional bonding, a vacancy cluster with H atoms tend to form a vacancy dislocation loop with its slip direction along the 〈100〉 direction and on the (211) habit plane. Our results are consistent with the experimental observations, and provide a possible mechanism for the formation of vacancy dislocation loops in α-Fe. Furthermore, we have also explored how dislocation loops trap self-interstitial atoms, vacancies and H atoms. It is of interest to note that H atoms strongly bound to a 〈100〉 vacancy dislocation loop, and are able to enhance the ability for the dislocation loop to further trap vacancies and reduce its ability to absorb self-interstitial atoms, thus promoting the growth of the 〈100〉 vacancy dislocation loops. [ABSTRACT FROM AUTHOR]