Bcc Tungsten Research Articles

First-principles investigation of possible clustering of noble gas atoms implanted in bcc tungsten

Using first-principles calculations, we have determined key properties of Ar and Ne in bcc W that relate to the possible bubble formation of implanted Ar and Ne atoms. The most stable interstitial site of Ar/Ne in bcc W is the tetrahedral site, as in the case of H and He. An interstitial atom causes the substantial strain of surrounding W atoms and the low electron-density region. The calculated migration energy of both Ar and Ne is very small, less than 0.2 eV of H. We calculated the binding energies of nearby interstitial atoms and found a strong dependence of atomic species. The attractive interaction between Ne atoms is the largest. Owing to such a small migration energy and a significant interaction between Ar/Ne atoms, interstitial atoms can act as traps for additional atoms and tend to form clusters in the absence of previous damage such as vacancies. The results presented here can explain the possible clustering of implanted atoms in bcc W via self-trapping.

{110} Slip with {112} slip traces in bcc Tungsten

While propagation of dislocations in body centered cubic metals at low temperature is understood in terms of elementary steps on {110} planes, slip traces correspond often with other crystallographic or non-crystallographic planes. In the past, characterization of slip was limited to post-mortem electron microscopy and slip trace analysis on the sample surface. Here with in-situ Laue diffraction experiments during micro-compression we demonstrate that when two {110} planes containing the same slip direction experience the same resolved shear stress, sharp slip traces are observed on a {112} plane. When however the {110} planes are slightly differently stressed, macroscopic strain is measured on the individual planes and collective cross-slip is used to fulfill mechanical boundary conditions, resulting in a zig-zag or broad slip trace on the sample surface. We anticipate that such dynamics can occur in polycrystalline metals due to local inhomogeneous stress distributions and can cause unusual slip transfer among grains.

Molecular Dynamics Study on the Diffusion Properties of Hydrogen Atoms in Bulk Tungsten

Molecular dynamics simulations were performed to study the diffusion behavior of hydrogen atoms in body-centered cubic(bcc) tungsten(W). The energy distribution of a single hydrogen atom in the (001) plane of tungsten lattice was computed. The values of diffusion barriers agree well with other theoretical and experimental results. The interaction between an H atom and a vacancy was simulated, which shows evidence of strong binding effect. The temperature effect on the diffusion behavior of hydrogen atoms was investigated. The critical temperature for an H atom to diffuse in bulk W with and without vacancies were calculated to be 950 K and 450 K, respectively, which is supported by several experimental results. In addition, the diffusion coefficient of hydrogen atoms in tungsten was evaluated and analyzed.

Coupled motion of grain boundaries in bcc tungsten as a possible radiation-damage healing mechanism under fusion reactor conditions

As a potential first-wall fusion reactor material, tungsten will be subjected to high radiation flux and extreme mechanical stress. We propose that under these conditions, coupled grain boundary (GB) motion, in some cases enhanced by interstitial loading, can lead to a radiation-damage healing mechanism, in which a large stress activates coupled GB motion, and the GB sweeps up the defects, such as voids and vacancies, as it passes through the material. The stress-induced mobility characteristics of a number of GBs in tungsten are examined to investigate the likelihood of this scenario.

Pressure Dependence of Phonon Dispersion in Bcc Tungsten

Combining the modified analytic embedded atom method with the theory of lattice dynamics, we reproduce the experimental results of the phonon dispersion in bcc metal tungsten at zero pressure along the major symmetry directions [00ζ], [0ζζ], and [ζζζ] and then predict the phonon dispersion relations of tungsten under the high pressures of 40, 50, 80, 100, and 120 GPa based on the equation of state of Rose and Vinet. Our simulated results at high pressure are in agreement with the finding of other bcc metals when the differences in masses, interatomic spacings, and melting temperatures of these metals are taken into account. This implies that the predicted phonon dispersion relations at different pressures are reasonable, showing that the modified analytic embedded atom method can describe the phonon dispersion relation of solids at zero pressure or at high pressure.

Comparison of molecular dynamics and binary collision approximation simulations for atom displacement analysis

Molecular dynamics (MD) and binary collision approximation (BCA) computer simulations are employed to study surface damage by single ion impacts. The predictions of BCA and MD simulations of displacement cascades in amorphous and crystalline silicon and BCC tungsten by 1keV Ar+ ion bombardment are compared. Single ion impacts are studied at angles of 50°, 60° and 80° from normal incidence. Four parameters for BCA simulations have been optimized to obtain the best agreement of the results with MD. For the conditions reported here, BCA agrees with MD simulation results at displacements larger than 5Å for amorphous Si, whereas at small displacements a difference between BCA and MD arises due to a material flow component observed in MD simulations but absent from a regular BCA approach due to the algorithm limitations. MD and BCA simulation results for crystalline W are found to be in a good agreement even at small displacements, while in crystalline Si there is some difference due to displacements in amorphous pockets.

Molecular dynamics simulation of low-energy hydrogen atoms bombarding tungsten (001) surface at different angles and their depth distribution

Molecular dynamics simulations are performed to study the diffusion behavior of low-energy hydrogen atoms in bcc tungsten (001). The simulation results show that when the energy of vertically incident hydrogen atoms is in a range of 0–20.0 eV, their retention probability increases rapidly; in the whole incident energy range 0.5–50.0 eV, the reflection probability gradually drops, but still exceeds 60%. By varying the incident angle, the retention probability may increase in some energy ranges compared with those in the case of vertical incidence, but the reflection probability still dominates. In this paper, we also obtain the depth distribution of energy deposition of incident hydrogen and its isotopes. It is found that tritium deposits more energies in the surface region than hydrogen.

Influence of point defects on grain boundary mobility in bcc tungsten

Atomistic computer simulations were performed to study the influence of radiation-induced damage on grain boundary (GB) sliding processes in bcc tungsten (W), the divertor material in the ITER tokamak and the leading candidate for the first wall material in future fusion reactors. In particular, we calculated the average sliding-friction force as a function of the number of point defects introduced into the GB for a number of symmetric tilt GBs. In all cases the average sliding-friction force at fixed shear strain rate depends on the number of point defects introduced into the GB, and in many cases introduction of these defects reduces the average sliding-friction force by roughly an order of magnitude. We have also observed that as the number of interstitials in the GB is varied, the direction of the coupled GB motion sometimes reverses, causing the GB to migrate in the opposite direction under the same applied shear stress. This could be important in the microstructural evolution of polycrystalline W under the harsh radiation environment in a fusion reactor, in which high internal stresses are present and frequent collision cascades generate interstitials and vacancies.

Melting curves and entropy of fusion of body-centered cubic tungsten under pressure

The melting curves and entropy of fusion of body-centered cubic (bcc) tungsten (W) under pressure are investigated via molecular dynamics (MD) simulations with extended Finnis-Sinclair (EFS) potential. The zero pressure melting point obtained is better than other theoretical results by MD simulations with the embedded-atom-method (EAM), Finnis-Sinclair (FS) and modified EAM potentials, and by ab initio MD simulations. Our radial distribution function and running coordination number analyses indicate that apart from the expected increase in disorder, the main change on going from solid to liquid is thus a slight decrease in coordination number. Our entropy of fusion of W during melting, ΔS, at zero pressure, 7.619 J/mol·K, is in good agreement with the experimental and other theoretical data. We found that, with the increasing pressure, the entropy of fusion ΔS decreases fast first and then oscillates with pressure; when the pressure is higher than 100 GPa, the entropy of fusion ΔS is about 6.575 ± 0.086 J/mol·K, which shows less pressure effect.

Synthesis, characterization and densification of W [sbnd]Cu nanocomposite powders

In the present investigation, W 20–40wt.% Cu nanocomposite powders with average sizes ranging between 25 and 30 nm were synthesized by a soft chemical approach using tungsten hexacarbonyl [W(CO) 6] and copper acetonyl acetonate [Cu(acac) 2] as metal precursors. Particle size, morphology and distribution were measured using transmission electron microscope (TEM) and small angle X-ray scattering (SAXS). Surfactant coating on W Cu composite powders was removed on heat treatment of powders at 450 °C in hydrogen atmosphere for 1 h. Elemental analyses of as-synthesized and annealed (at 450 °C) W Cu nanocomposite powders were carried out using inductively coupled plasma-optical emission spectrometer (ICP-OES) and Leco gas analyzers. X-ray diffraction studies showed that the tungsten phase is amorphous while the crystal structure of copper phase is fcc in as-synthesized W Cu nanocomposite powders. After annealing at 700 °C peaks corresponding to bcc tungsten are observed and peaks corresponding to fcc copper become sharper. Relative densities of 98.2%, 98.8% and 99.2% were achieved for W 20wt.% Cu, W 30wt.% Cu, and W 40wt.% Cu composite powders respectively when sintered at 1000 °C.

Facile Chemical Synthesis of W–Ag and W–Cu Nanocomposites at Low Temperature

W-Ag (80.2W-19.8Ag, 70.4W-29.6Ag and 60.5W-39.5Ag) and W-Cu (79.7W-20.3Cu, 70.5W-29.5Cu and 59.8W-40.2Cu) nanocomposites in the size range of 24-30 nm have been synthesized by thermal decomposition of W(CO)6/CH3COOAg and W(CO)6/Cu(acac)2 in diphenyl ether as solvent at 220 degrees C in presence of oleic acid and hexadecyl amine and characterized. FTIR spectra have been used to explain the role of oleic acid and hexadecyl amine in the synthesis of W-Ag and W-Cu composite powders. XRD studies show that the tungsten phase is amorphous, whereas both Ag and Cu crystallize in fcc for as-synthesized W-Ag and W-Cu nanocomposites. These composite powders when annealed at 700 degrees C results in the formation of bcc tungsten and peaks corresponding to fcc silver and copper still persists. The particle size, shape and distribution of these nanocomposites of various compositions have been studied by SAXS, ESEM and TEM and found to be nearly spherical with the average diameters below 30 nm.

Diffusion of hydrogen in bcc tungsten studied with first principle calculations

First principle calculations were used to study the hydrogen migration properties in bulk bcc tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion coefficient was evaluated using the harmonic transition state theory and was found to agree with the experimental results at temperatures above 1500 K. The height of the migration barrier between two adjacent tetrahedral sites was found to be 0.21 eV, which is lower than the value 0.39 eV obtained for the migration barrier from degassing measurements in the temperature range between 1100 and 2400 K. The tunneling correction to the diffusion rate provides much better agreement with the experimental result at 29 K than the extrapolated experimental D from high temperature measurements.

High-Efficiency Mechanical Energy Storage and Retrieval Using Interfaces in Nanowires

By molecular dynamics simulations, we demonstrate a new concept for mechanical energy storage and retrieval using surface energy as reservoir in body-centered cubic (bcc) tungsten nanowire, achieving a combination of unique features such as large and constant actuation stress (>3 GPa), exceptionally large actuation strain (>30%) and energy density, and >98% energy storage efficiency. The underlying mechanism is a shear-dominant diffusionless transformation akin to martensitic transformation, but driven by surface rather than bulk free energies, and enabled by motion of coherent twin boundary, whose migration has been shown to possess ultralow friction in bcc metals. Aside from energy storage, such surface-energy driven displacive transformations are important for phase transformation and energy-matter control at the nanoscale.

Bond-order potential for point and extended defect simulations in tungsten

A reactive interatomic bond-order potential for bcc tungsten is presented. Special attention in the potential development was given for obtaining accurate formation and migration energies for point defects, making the potential useful in atomic scale simulations of point and extended defects. The potential was used to calculate binding energies and trapping distances for vacancies in vacancy clusters and the recombination radius for self-interstitial atom and monovacancy.

Manufacture, microstructure and mechanical properties of Mo–W–N nanostructured hard films

Mo 1− x W x N y ( x = 0–0.67) hard films were fabricated on wafers of silicon and high speed steel by dc magnetron sputtering technique. The effect of tungsten concentration on the phase composition, microstructure, surface morphology, hardness, adhesion, and corrosion resistance of the films was studied by X-ray diffraction, scanning electron microscopy, nano-indentation, and scratch test. It was found that if the W concentration ( x) in the film is in the range of 0–0.52, the films exhibit fcc (Mo,W)N y single phase where larger W atoms substituted Mo atoms in fcc MoN y . At higher x values ( x > 0.52) the films exhibit a two-phase structure consisting of fcc (Mo,W)N y and pure bcc tungsten phase. The hardness of the Mo 1− x W x N y films increases at first with increasing x, and then decreases after passing a maximum. The maximum hardness of 47 GPa is obtained at x = 0.37 corresponding to an adhesion strength of 60 N. The Mo–W–N coated high speed steel has a lower corrosion current density and higher corrosion potential than the bare high speed steel substrates.

In situ TEM observation of the growth and decomposition of monoclinicW18O49 nanowires

The growth of monoclinic W18O49 nanowires by heat treatment of a tungsten filament at∼873 K and the decomposition of these nanowires under 200 keV electron irradiation at∼1023 K have been investigated using in situ transmission electron microscopy (TEM). In situ TEMobservation of the growth confirmed the vapor–solid growth mechanism of the monoclinicW18O49 nanowires. In situ irradiation experiments revealed the formation of metallic bcc tungsten from monoclinicW18O49 nanowires under 200 keV electron irradiation.

Atomistic simulations of nanometric dislocation loops in bcc tungsten

Small dislocation loops formed from self-interstitial atoms (SIAs) are commonly found in irradiated metals. These defects significantly influence the mechanical properties of the materials. Atomistic simulations are used to describe nanometric circular dislocation loops with Burger’s vectors 12a0〈111〉, a0〈100〉 and a0〈110〉 in bcc tungsten. Particular attention is paid to the habit plane of the 12a0〈111〉 loop. Two different embedded atom model (EAM) potentials are used. The energetics and geometry of the loops are studied as a function of their size.

The ideal tensile strength and deformation behavior of a tungsten single crystal

We employ first-principles total energy method based on the density functional theory with the generalized gradient approximation to investigate the ideal tensile strengths of a bcc tungsten (W) single crystal systemically. The ideal tensile strengths are shown to be 29.1, 49.2 and 37.6GPa for bcc W in the [001], [110] and [111] directions, respectively. The [001] direction is shown to be the weakest direction due to the occurrence of structure transition at the lower strain and the [110] direction is strongest. The results can provide a useful reference for W as a PFM in the nuclear fusion Tokamak.

Study on C–W interactions by molecular dynamics simulations

By means of molecular dynamics simulations using bond-order potential (BOP), we have investigated the interactions between carbon (C) atoms and bcc tungsten (W). At finite temperature (T=300K) with incident energy of C atoms ranging from 0.5 to 100eV at normal incidence, the projected range distribution as a function of incident energy and the average depth have been depicted. The properties of vacancy, vacancy migration, interstitial and substitutional C atoms in W have been determined. The most stable configuration for an interstitial C atom in W is in octahedral position and the lattice distortion around the C atom in octahedral interstitial configuration occurs along 〈100〉 and 〈110〉 directions. The mutual interaction between a vacancy and near interstitial C atom is also studied.

Molecular dynamics simulations of atomic carbon on tungsten surface

By means of molecular dynamics simulations using bond-order potential, we have investigated the behavior of atomic carbon on both bcc tungsten (0 0 1) surface, and amorphous tungsten surface that is formed by a simulated annealing process, at finite temperature ( T = 300 K) with incident energy ranging from 0.5 eV to 200 eV. The particle and energy reflection coefficients as well as mean range distribution as a function of incident energy at normal incidence have been calculated and the channeling effect in the energy range above 100 eV has also been discussed. The results are compared with those calculated by Eckstein using binary collision (BC) Code TRIM.SP in the energy range from 55 eV to 200 eV.