Towards understanding the strong trapping effects of noble gas elements on hydrogen in tungsten

We have investigated the effects of noble gas elements (helium, neon, argon and krypton) on the dissolution and accumulation behavior of hydrogen (H) in tungsten (W) using a first-principles method, as well as the behaviors of themselves. Noble gas atoms energetically prefer to occupy the tetrahedral interstitial site (TIS) in W, as the same of H. The TIS → TIS is their optimal diffusion path, and the diffusion energies increase with the increasing of atomic radius. All of them are energetically favorable clustering by self-trapping. It is found that the presence of the noble gas elements have significant effect on H behavior in W. Both interstitial noble gas atoms and their complex with vacancy can serve as strong trapping centers of H, which can be attributed to the redistribution of electron density and lattice distortion induced by noble gas. Most importantly, we have demonstrated that H trapping capability of noble gas clusters will increase with the increasing of the number of noble gas atoms in both interstitial and vacancy-complex cases. The H trapping energy surrounding the noble gas clusters with four atoms is comparable to that in noble gas-free vacancy. Further, the formation of noble gas clusters is much easier than that of vacancy due to the self-trapping interaction between noble gas atoms (>1 eV for interstitial case and >4 eV for vacancy-complex case), and thus the H trapping centers will rapidly increase after noble gas pre-irradiation/implantation. Consequently, the self-trapping character of noble gas induces their strong trapping effect on H.     

Unified mechanism for hydrogen trapping at metal vacancies

Interaction between hydrogen (H) and metals is central to many materials problems of scientific and technological importance. H segregation or trapping at lattice defects plays a crucial role in determining the properties of these materials. Through first-principles simulations, we propose a unified mechanism involving charge transfer associated strain destabilization to understand H segregation behavior at vacancies. We discover that H prefers to occupy interstitials with high pre-existing charge densities and the availability of such interstitials sets the limit on H trapping capacity at a vacancy. Once the maximum H capacity is reached, the dominant charge donors switch from the nearest-neighbor (NN) to the next-nearest-neighbor (NNN) metal atoms. Accompanying with this long-range charge transfer, the sharply increased reorganization energy would occur, leading to the instability of the H-vacancy complex. The physical picture unveiled here appears universal across the BCC series and is believed to be relevant to other metals/defects as well.     

Finite element modeling of hydrogen atom diffusion and distribution at corrosion defect on aged pipelines transporting hydrogen

Repurposing existing natural gas pipelines for hydrogen transport has attracted wide interests. However, the corrosion defect present on these aged pipelines can affect hydrogen (H) atom accumulation, potentially causing hydrogen embrittlement. In this work, a finite element-based model was developed by coupling solid mechanics and H atom diffusion to investigate the distribution of H atoms at a corrosion defect on a steel pipe segment under applied longitudinal tensile strains. The applied strain causes local stress (both Mises stress and hydrostatic stress) and strain concentrations at the corrosion defect, affecting the H atom diffusion and distribution. In the absence of the tensile strain, the H atoms, once entering the interior of pipe, diffuse uniformly into the pipe body along the radial direction driven by a concentration gradient. When a strain is applied on the pipe, the H atom diffusion is driven by hydrostatic stress. The maximum H atom concentration exceeds the initial concentration of H atoms entering the steel pipe, indicating the H atom accumulation at the corrosion defect. The applied tensile strain also affects the location where the H atoms accumulate. For both internal and external corrosion defects, more H atoms will be concentrated at the defect center when the defect length reduces and the depth increases.     

Molecular dynamics studies of hydrogen diffusion in tungsten at elevated temperature: Concentration dependence and defect effects

Influence of hydrogen concentration and defects introduced by neutron irradiation on hydrogen diffusion in tungsten has been investigated by molecular dynamics simulation at elevated temperatures. Hydrogen diffusion is shown to be significantly restrained at high concentrations due to spontaneous formation of platelet-like hydrogen clusters. For neutron irradiation defects, self-interstitials, mono-vacancies and vacancy clusters are considered. By clustering and acting as dislocation loops, self-interstitials show considerable trapping effects on hydrogen, leading to the suppression of hydrogen effective diffusion and the change of diffusion model in which hydrogen mainly diffuses along dislocation lines instead of hopping between tetrahedral interstitial sites. Moreover, an equation connecting hydrogen diffusion parameters and the total length of dislocation loops is empirically established. Different influences of mono-vacancies and vacancy clusters on hydrogen diffusion have been carefully identified. With the same vacancy concentration, hydrogen diffusivity is lower with mono-vacancies than that with vacancy clusters because more isolated trapping sites are provided by mono-vacancies. This work is not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for larger scale simulations as input data.     

Comparative study of H-atom location,electronic and chemical bonding in ideal and vacancy containing-FCC iron

We studied one of the aspects of iron embrittlement in an FCC lattice with a vacancy in the presence of hydrogen as an impurity. The energy calculations were performed using the ASED method (Atom Superposition and Electron Delocalization). The electronic structures were analyzed by the YAeHMOP program (extended Hückel Molecular Orbital Package). H in bulk FCC iron prefers octahedral sites while in the presence of a vacancy it is located near the defect in a position shifted from the center of the hole. The formed Fe–H bond makes the Fe–Fe bonds first neighbors to the vacancy 28% weaker with respect to the H-free structure.     

Role of solute atoms and vacancy in hydrogen embrittlement mechanism of aluminum: A first-principles study

Hydrogen trapping performances of Al with solute atoms X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn) and X-vacancy defects are investigated using First-principles method. For X-doped Al supercells, most structures show strong alloying ability. Among the solute atoms studied, Cr is the most useful element to trap H due to its lowest H trapping energy. For vacancy@X-doped Al supercells, the strong interactions of X-vacancy are explored. All vacancy@X-doped Al supercells are more favorable to capture H than X-doped Al supercells. In addition, both elastic and chemical interactions should comparably contribute to H-X or H-X-vacancy interactions in Al. Solute atoms and vacancy may regulate electron distribution of Al to enhance the ability of capturing H. Overall, our insights present the quantitative role of solute atoms and vacancy in H trapping for Al, and guide the design of new alloys with high resistance to hydrogen embrittlement.     

Effect of hydrogen on Fe and Pd alloying and physical properties

Metastable Fe–Pd powder samples were synthesized by mechanically activated solid-state diffusion using high-energy ball milling. The Fe and Pd alloying and the hydrogen effect on this process were followed by preparation of two samples: the A-sample was a mixture of Fe powder and of Pd powder pre-charged with hydrogen (PdH) and milled under Ar atmosphere, the B-sample was a mixture of the Fe and Pd powders milled under hydrogen atmosphere.The fundamental properties, i.e., chemical and phase composition, lattice parameters, microstructure, morphology, grain size, defect structure, and macro- and micro-magnetic properties, were monitored after several steps of the alloying at room and appropriately at elevated temperatures.The alloying of Fe and Pd in both samples begins already after 5 h of milling and two phases are formed, the dominating bcc-Fe(Pd) phase and a minor fraction of the fcc-Pd(Fe) phase. The occurrence of the fcc phase, not observed previously by solid-state diffusion under argon atmosphere, is ascribed to mainly a positive effect of hydrogen reducing the formation energy of lattice defects and facilitating their formation. Consequently, the moving defects during mechanical alloying make the solid-state diffusion of Pd into bcc-Fe lattice and Fe into fcc-Pd lattice easier. On the other hand, hydrogen used as atmosphere in the milling procedure is adsorbed on the particle surfaces and after the vial opening hydrogen atoms form water molecules with oxygen from air. This exothermic reaction causes a removal of hydrogen atoms from the particle surface which thus becomes more sensitive to oxidation. Nothing similar was observed after mechanical alloying under argon atmosphere having positive impact on the particle surface stability.     

Molecular dynamics studies of the grain-size dependent hydrogen diffusion coefficient of nanograined Fe

Nanograined materials have much denser grain boundary (GB) networks than their coarse-grained counterparts, thereby the hydrogen (H) diffusion and trapping behaviors in nanograined materials, which are strongly influenced by GBs, may differ greatly from those in coarse-grained materials. In the present research, the grain-size dependent hydrogen diffusion coefficient in nanograined Fe is studied by theoretical analysis and molecular dynamics (MD) simulations. A theoretical model based on thermodynamics is developed. The GB-related material parameters required by the model are then obtained by fitting the MD simulation results. Finally, the grain-size dependent diffusion coefficients are compared with model predictions to evaluate the validity of the model. It is found that the trapping effect of triple junctions that usually ignored in coarse-grained materials becomes increasing important as grain size and temperature decrease. Due to the strong trapping effect of GBs in nanograined Fe, H diffusion is slowed down by the GBs.     

Diffusion,permeation and solubility of hydrogen,deuterium and tritium in crystalline tungsten: First principles DFT simulations

Tungsten (W) has shown promise to be the most favored plasma facing material in nuclear fusion with deuterium (D) and tritium (T) as fuel. In order to identify a better material, atomistic understanding of the behavior of D/T with W is highly desirable. Here, we report the mechanistic pathways of diffusion of H, D and T in bcc W employing density functional theory (DFT) using PBE and PBE-D3 functional and harmonic transition state theory. The activation diffusion barrier was determined using nudge elastic band techniques. The zero point energy (ZPE) was incorporated by phonon calculations to include the isotope effects. The surface adsorption of H, D and T is predicted to be exothermic, whereas surface to sub-surface and bulk absorption is endothermic but the vacancy induced absorption in bulk W was predicted to be exothermic. H, D and T atoms were observed to be diffused preferably from tetrahedral site to the nearest tetrahedral site. The calculated diffusion coefficients, rate constants, permeability constants and solubility are found to be higher for H compared to its heavier isotopes D and T. The trends of diffusion, permeation and solubility for H, D and T are well in agreement with the experimental results. The diffusion values predicted for H isotopes in bcc W from PBE-D3 method is quite close to the experimental values compared to PBE method. But, the calculated and experimental ratio of diffusivity, permeability and solubility of any two H isotopes follow the similar trend and no considerable change is noticed between PBE and PBE-D3 methods. Further, the calculated diffusion coefficients are shown to be one order of magnitude lower than Cr and two orders than Fe and thus might be the basis for considering W as plasma facing materials. The present study might be useful in modeling the behavior of tritium in W metal for which data is either scattered or least available.     

Hydrogen diffusion coefficient in monoclinic zirconia in presence of oxygen vacancies

To better understand the hydrogen diffusion mechanisms in monoclinic zirconia that take place in fuel rod cladding during reactor operation, we calculate the diffusion paths of different defects involving hydrogen and oxygen vacancies using Density Functional Theory with hybrid functionals, and use them to obtain the hydrogen and oxygen diffusion coefficients. We find a hydrogen diffusion coefficient varying between 10^?10 to 10^?20 cm²?s^?1 at 600 K, strongly depending on the hydrogen to oxygen vacancy ratio. We find that the interstitial hydrogen atoms are the main diffusing species even though they are not the dominant configuration of hydrogen atoms. We confirm the existence of different huge trapping effects, which slow the hydrogen diffusion. The main mechanism is either the trapping of hydrogen atoms in oxygen vacancies or the formation of interstitial dihydrogen molecules depending on the hydrogen to oxygen vacancy ratio.     

A first principles study on H-atom interaction with bcc metals

Solute hydrogen trapping has long been proposed as one of the mechanisms for hydrogen embrittlement in steel. It has been reported that the maximum hydrogen trapping energy of metallic solutes ranged from ?0.7 eV to ?0.9 eV. In this work, the mechanism of metal-H interaction in Cr-Mo steels was investigated with first principles calculations by modelling the binary alloy Fe-X (X = C, Si, Mn, Cr, Mo) system with reference to the chemical composition of Cr-Mo steels. The formation of hydrogen bonds in the case of H atoms located at different sites in Fe-X crystals was analyzed. Results indicated that various atomic doping had different roles in hydrogen effect in the steel, with C, Si and Mo doping making the solid solution of hydrogen in Fe crystals easier, while Mn and Cr doping was rather more difficult. In Fe-Mn and Fe-Cr crystals, the repulsion between Fe lattices was insignificant when H atoms were located in tetrahedral sites, which considerably reduced the binding energy in the crystal. When H atoms were dissolved into the crystal, the interatomic bonding interactions in Fe-X crystals were weakened, resulting in higher charge density fluctuations. The current work extends the understanding of H-atom diffusion and migration in steel from the microscopic scale to the atomic and electronic scales, which underpins the physics for tailoring chemical elements of bcc metals towards higher resistance to hydrogen embrittlement.     

Texture dependence of hydrogen diffusion in nanocrystalline nickel by atomistic simulations

Atomistic simulations were performed to highlight the importance of the texture on the diffusion of hydrogen atoms in nanocrystalline nickel. Significant anisotropic diffusion is observed in longitudinal and through thickness directions. Our results show that the diffusion coefficient of hydrogen atoms through thickness in [001] textured nickel is larger than those values obtained for [011] and [111]. The diffusivity along longitudinal and transverse directions in [111] textured samples is found to be higher than that along thickness direction. Additionally, it is determined that the presence of hydrogen atoms changes the vacancy formation of the substrate and the vacancy defects are responsible for the anisotropy of hydrogen diffusion. These findings improve our understanding of hydrogen diffusivity at the atomistic level for hydrogen storage in the materials.     

Insight into the enhancement effect of rhenium-rich precipitation on hydrogen retention in tungsten by investigating the behaviors of hydrogen in tungsten-rhenium sigma phase

We have investigated the dissolution, diffusion, and retention of hydrogen (H) in tungsten-rhenium (W-Re) sigma (σ) phase using a first-principles method and thermodynamic models, in order to explore the influence of Re-rich precipitation on H behaviors in W. Taking the most stable W-Re σ phase (W-33.3 at.% Re) as an example, it is found that the solution energy of H at most interstitial sites (>80%) in W-Re σ phase is much lower than that in pure W. Specifically, the H solution energy at most stable interstitial site in W-Re σ phase is only 0.47 eV, ~54% lower than that in pure W. This can be attributed to that W-Re σ phase provides the larger available volume for interstitial H than the pure W, weakening the W-H repulsive interaction. Moreover, it has been demonstrated that this trend is almost independent on the Re concentration in W-Re σ phase. We have further determined the interaction between H atoms and mono-vacancy in W-Re σ phase based on the calculation of H trapping energies and the Polanyi-Wigner equation. The W-type and Re-type vacancy can accommodate 10 and 5 H atoms at room temperature (RT) in σ phase, respectively, and thus the average trapping capability of vacancy for H in σ phase is stronger than that in pure W (~6 H atoms at RT). Consequently, our calculations reveal that the Re-rich precipitation (both interstitial and vacancy sites) can serve as the strong trapping centers for H in W, significantly enhancing the H retention, which is entirely different from the negative effect of dispersed-Re/small Re clusters.     

Comparison of hydrogen transport through pre-deformed synthetic polycrystals and homogeneous samples by finite element analysis

This study aims at comparing the hydrogen transport in polycrystals and in equivalent homogeneous material, with 3D FE simulations accounting for stress-assisted diffusion and trapping due to plastic strain, in order to examine the hydrogen concentration fields consistency in multi-scale simulations. The effective diffusion features are compared for various sizes of iron polycrystals. For trapping free diffusion, it is shown that hydrogen concentration fields are consistent between scales. When trapping is accounted for, effective diffusion in polycrystals and in homogeneous materials are different, underlying the importance of the trap density function formulation at different scales.     

Hydrogen diffusion and vacancy clusterization in iron

Molecular statics and molecular dynamics simulations were performed to study hydrogen diffusion and vacancy clustering in alpha iron. In particular, it was found that hydrogen atom binds very strongly with vacancies, rather than other hydrogen atoms. The monovacancies were inclined to form the VH4, VH3, VH2 and VH1 complexes, rather than VH6 in the range of our simulated temperatures. The rate of hydrogen diffusion was apparently reduced in the presence of vacancies, while the vacancy trap effect was gradually weakened with increasing temperature. The presence of vacancies changes the diffusion mechanism of H atoms. Moreover, we found that vacancy clusters tended to be formed at the moderate range of temperatures, and fewer clusters were observed at either low or high temperatures. The number of vacancy clusters reduced, while hydrogen-vacancy clusters were gradually created with the increase of hydrogen concentration.     

Abnormal trapping of hydrogen in the elastic stress field of dislocations in body-centered cubic iron

It has been widely accepted that the tetrahedral interstitial site is the normal lattice trapping site for H atoms in the body-centered cubic iron. In this work, we found H trapped in a tetrahedral site is unstable in the dislocation elastic stress field. By contrast, the octahedral site is energetically preferred. This abnormal trapping phenomenon was rooted in the H-dislocation elastic interaction and zero-point quantum vibration of the H atom. At the abnormal trapping state, the interaction between H-edge dislocation and H-screw dislocation was identified to be at a similar level. As a result, the diffusivity of H around dislocation is enhanced by the abnormal trapping, and the local H content is much higher than the prediction with the assumption of normal trapping. Our results suggest that the dislocation motion can shuffle the trapping position of H atoms and assist the immature failure in the hydrogen environment.     

Investigation of hydrogen embrittlement properties of Ni-based alloy 718 fabricated via laser powder bed fusion

The hydrogen embrittlement behavior of heat-treated Alloy 718 fabricated by laser powder bed fusion was fundamentally investigated under electrochemical hydrogen charging. The H transitioned the fracture mode from ductile dimpled to transgranular fracture with a flat fracture surface. Crystallographic analysis showed that H promoted the dislocation slip band, and the resulting concentrated strain and H along the slip planes caused cracking regardless of the distribution of additive manufacturing (AM) microstructural features such as sub-grain boundaries. In addition, thermal desorption spectroscopy and H-permeation tests indicated that the AM microstructural features after heat treatment only slightly influenced the H trapping and diffusion.     

The effect of rapid solidification on the microstructure and hydrogen storage properties of V35Ti25Cr40 hydrogen storage alloy

The microstructures and hydrogen storage properties of as-cast and rapidly solidified V₃₅Ti₂₅Cr₄₀ alloys have been investigated in this paper. The results showed that the rapid solidification refined the dendritic microstructure and altered the element distribution of the alloy. And through the positron annihilation measurements of the vacancy trapping rate (K_d1) and vacancy-trapped positron annihilation lifetime (τ₂), it was found that the rapid solidification increased the vacancy concentration and at the same time decreased the vacancy size in the alloy. The XRD results showed that the rapid solidification also significantly enlarged the alloy's lattice parameter. As a result of the microstructure change, the hydrogen absorption capacity and hydrogen absorption rate were increased; and the kinetic mechanism of hydrogen absorption was changed from 3-D diffusion control in the as-cast alloy to chemical reaction control in the rapidly solidified alloy; but the activation property was to some extent weakened after the rapid solidification.     

First principle study of hydrogen diffusion in equilibrium rutile, rutile with deformation twins and fluorite polymorph of Mg hydride

The transport properties of hydrogen are crucial to the kinetics of hydrogen storage in MgH₂. We use first principle calculations to identify the hydrogen diffusion paths and barriers and diffusion rates in three different MgH₂ structures: equilibrium rutile, rutile with ball-milling-induced deformation twins and fluorite polymorph. Hydrogen vacancy mediated mechanism was applied when hydrogen diffusion was studied. We observed that both hydrogen diffusion barriers in deformation twins and fluorite structure are lower compared to that in the equilibrium rutile. This is because the hydrogen diffusion is facilitated by new interstitial sites in the Mg lattice: a new hexahedral site formed by the reconstruction of Mg lattice at the twinning interface in the deformation twins and the octahedral sites in the fluorite structure. Furthermore, the hydrogen vacancy density effect on the diffusion barrier was estimated. The general trend is the higher the density of hydrogen vacancies, the lower the hydrogen diffusion barrier, the higher the diffusion rate. Our results demonstrate how the hydrogen kinetics is altered by controlling the structure of the hydrides.