Crystal Tungsten Research Articles

The evolutionary process of W-V mixed dumbbell in tungsten crystals: A study about W-V alloy as a plasma-facing material in fusion devices

Alloying is widely used to improve the radiation resistance of plasma-facing materials. The first-principles method based on density function theory was used in this work to study the stability and mobility properties of the W-V mixed dumbbell pair. The diffusion of monovacancy and recovery of mixed dumbbell pairs were also studied. The rotations and diffusion results indicated that the W-V mixed dumbbell pair diffusions are favored in two/three-dimensional motion because of low energy barriers. And the results are also suggested the mixed dumbbell lowers the diffusion energy barriers of the monovacancy. New vacancy diffusion modes were also observed along the 〈110〉 direction during the simulation. Besides, the addition of V may also moderate the diffusion of W SIA in 〈111〉 direction.

Features of the Structural Formation of Tungsten Single Crystals in the Shape of Hollow Rotational Bodies

The results of the development of the technology for growing super-large single crystals of refractory metals, which were developed at the E.O. Paton Electric Welding Institute of the NAS of Ukraine. Based on proven technology and acquired skills, a new generation of equipment was created that allows the growth of single crystals of refractory alloys in the form of bodies of rotation. Experiments were conducted on growing a single crystal of tungsten in the form of a hollow cylinder, from which it is possible to make such a product as a crucible. Technological parameters and energy regimes were established, which allowed for control of the thickness of the wall to be welded. As a result of the experiments, an ingot with a welded wall height of 68 mm, a thickness of 20–22 mm, and an outer diameter of 85 mm was grown. The structure of the obtained samples was studied.

EFFECT OF VACANCIES AND VOID DEFECTS ON THE STRUCTURAL AND MECHANICAL BEHAVIOR OF TUNGSTEN UNDER HARSH TEMPERATURE AND PRESSURE CONDITIONS

The body-centered cubic transition metal tungsten is frequently used as a pressure calibration material at high temperatures and pressures due to its outstanding mechanical and thermal properties. In this study, molecular dynamics simulations were performed to investigate the behavior of tungsten under harsh temperature and pressure conditions and the impact of fundamental defects, particularly vacancies, and voids, on its physical, structural, and mechanical properties through their correlation with elastic constants. The study also covers mechanical stability, elastic properties, brittleness and ductility, and hardness. The simulations utilized two different embedded atom methods and one modified embedded atom method interatomic potentials. The results show that the fundamental structural characteristics and properties of pure tungsten crystal, including lattice constant, density, cohesive and vacancy formation energies, elastic constants, and moduli in the ground state for all three potentials, are in good agreement with previous experimental and theoretical calculations and results. The calculated results demonstrate that the elastic constants-related properties for defective structures also have the same trend as the perfect crystal. The presence of defects in the crystal causes a decrease in properties at all temperatures and pressures, directly correlated to the fraction of crystal defects. As the percentage of vacancies increases, a further reduction in the elastic constants is observed. Likewise, these findings reveal that the presence of scattered vacancies in the crystal structure causes a more significant decrease in the substance's properties than a void in the center of the crystal (with the same percentage). The presence of any vacancy weakens the interatomic bonds of the atoms around the vacancy, while the existence of a void in the center has less effect on the interatomic bonds of atoms further away from the center of the crystal.

INVESTIGATION OF THERMODYNAMIC PROPERTIES AND ANISOTROPIC FACTORS OF PERFECT AND DEFECTIVE TUNGSTEN THROUGH CORRELATION WITH ELASTIC CONSTANTS: A MOLECULAR DYNAMICS STUDY

In this study, the thermodynamic properties and anisotropic factors of perfect and defective tungsten were investigated through their correlation with elastic constants. The study examined sound velocities, Debye temperature, minimum thermal conductivity, melting point, and elastic anisotropy factors at various temperatures and pressures. The utilized elastic constants were calculated by molecular dynamics simulations. We used three different interatomic potentials in the simulations involving two embedded atoms and one modified embedded-atom method. The findings indicated that temperature and pressure were positively correlated with anisotropic factors, with increased values leading to an increase in metal anisotropy. Also, defects were found to cause an increase in anisotropy, with a single vacancy having a greater impact on elastic anisotropy compared to a central void in the crystal structure. The study also found that the fundamental thermodynamic characteristics of pure tungsten crystal including density, sound velocities, Debye temperature, and Grüneisen parameter in the ambient conditions for all three potentials were in good agreement with previous experimental and theoretical calculations. The results showed that defective structures displayed the same trend as perfect crystals for elastic constant-related properties. The presence of defects in the crystal caused a decrease in thermodynamic properties at all temperatures and pressures, with the degree of decrease directly correlated with the fraction of crystal defects. The study also found that the minimum thermal conductivity as a key parameter of tungsten showed a downward trend with temperature and upward with pressure.

Effect of lattice defects, hydrogen impurity and temperature on electronic thermal conductivity in first wall tungsten materials

Significant changes in the microstructure of the first wall tungsten (W) materials caused by intense particle irradiation from the plasma in fusion reactors should affect the thermal property of the first wall materials. Although it has been established that for metals including W, the electrons contained therein play a major role in heat transport, few theoretical studies have focused on the heat transport performance by electrons of the defective tungsten. In this work, we have intensively investigated the effect of irradiated defects and temperature on the electronic thermal conductivity of tungsten by the tight-binding (TB) potential model. It is found that the electronic transport performance of tungsten crystals with different defects, such as point defects, voids, or hydrogen (H) impurities, degrades with increasing defect concentration, where the greatest reduction is caused by the randomly-distributed point defects and the smallest by H at the same defect concentration. Importantly, filling H in the void is beneficial to reduce the scattering of the void against electrons and improve electronic transport performance. As the temperature rises, the electrical conductivity decreases, but the electronic thermal conductivity increases. The behavior in the electronic thermal conductivity is attributed to the additional ‘gain effect’ of temperature on the electronic thermal conductivity compared to the electrical conductivity. Besides, it is inferred that the weaker the electron scattering caused by defects, the slighter the response of electronic thermal conductivity to temperature. For all concerned systems, the changes in their electronic transport performance can be understood through our calculated electron mean free path (MFP).

Morphological and structural evolutions of tungsten-tungsten oxide powders after a novel vacuum-reduction thermo-chemical treatment

In the current paper, morphological, chemical and structural evolution of WO3 particles under an interesting thermo-chemical treatment was investigated. This proposed treatment is consisting of partial oxidation of initial tungsten particles at 800 °C under air atmosphere, vacuum heat treatment at 1100 °C and hydrogen reduction at 800 °C, subsequently. The morphological and microstructural investigations of the material during different stages carried out by SEM and EDS analysis. The obtained results confirmed the sever size reduction of initial tungsten powders due to the formation of needle-like tungsten oxide particles after vacuum heat treatment. Formation of cauliflower-like tungsten oxide, the phase transformation from yellow tungsten oxide into brown one after vacuum treatment, and growth of tungsten crystal cluster during hydrogen reduction is the main microstructural observation in this study.

Strong enhancement of electromagnetic shower development induced by high-energy photons in a thick oriented tungsten crystal

We have observed a significant enhancement in the energy deposition by 25–100~textrm{GeV} photons in a 1~textrm{cm} thick tungsten crystal oriented along its langle 111 rangle lattice axes. At 100~textrm{GeV}, this enhancement, with respect to the value observed without axial alignment, is more than twofold. This effect, together with the measured huge increase in secondary particle generation is ascribed to the acceleration of the electromagnetic shower development by the strong axial electric field. The experimental results have been critically compared with a newly developed Monte Carlo adapted for use with crystals of multi-X_0 thickness. The results presented in this paper may prove to be of significant interest for the development of high-performance photon absorbers and highly compact electromagnetic calorimeters and beam dumps for use at the energy and intensity frontiers.

Two-temperature molecular dynamics simulations of crystal growth in a tungsten supercooled melt

In this work we use the two-temperature model (TTM) coupled to molecular dynamics (MD) with sinks at the boundaries of the electronic subsystem to study crystal-growth rate in a quasi-one-dimensional tungsten crystal into a supercooled melt. The possibility of varying the extension of the electronic grid along with the sinks allows a more realistic description of the electronic thermal transport away from the system, providing a considerable heat dissipation from the crystallization front. Based on this approach, our results regarding crystal-growth rates are not affected even if the size of the system is changed. Moreover, comparisons are established with respect to MD and standard TTM simulations. For these comparisons between models, something remarkable is found, and it is that the temperature and the value of the maximum growth rate are the same. In contrast, the inclusion of sinks has a great impact with respect to the standard approaches specially reflected at low temperatures, where a frustration of the liquid–crystal interface dynamics is seen until a state of zero crystal growth is reached, which is not possible to characterize quantitatively since a kind of stochastic behavior is present.

Выбор межатомных потенциалов для моделирования экстремальных воздействий в решетке вольфрама

Simulation of crystal lattices under conditions far from equilibrium is an increasingly important subject of research and requires confidence in the validity of the applied interatomic potentials in a wide range of atom deviations from the balanced condition. To make such an assessment for modeling tungsten as an advanced material for various nuclear applications, the authors analyzed the nonlinear behavior of the lattice using several interatomic potentials. In a bcc tungsten crystal, oscillations were simulated according to the laws of several delocalized nonlinear vibrational modes – exact solutions to the equations of motion of atoms, the geometry of which is determined by the lattice symmetry at any amplitudes and does not depend on the type of interaction between the nodes. The authors considered two-dimensional cases of oscillations in one of the close-packed planes and three-dimensional cases when the motions of atoms have three components in space for a tungsten cell consisting of 2000 atoms and 31.6×31.6×31.6 Å in size. The amplitude-frequency characteristics of these modes were calculated for several interatomic potentials available in the LAMMPS library. The study identified that several interatomic potentials, namely eam.fs, set, Olsson, and Zhou show practically identical results, which is an indirect confirmation of their validity and the possibility of their use for modeling extreme impacts in the considered lattice. The authors calculated such characteristics of the system as kinetic energy, heat capacity, and pressure. Based on the results obtained, one can assume that mode 15, due to the modulation instability, will lead to the energy localization on individual atoms.

Pressure Dependence of Structural and Mechanical Properties of Single-Crystal Tungsten: A Molecular Dynamics Study

In the current study, molecular dynamics (MD) simulations were performed to study the pressure dependence of the structural and mechanical properties of single-crystal tungsten. The results show that single-crystal tungsten possesses noteworthy high-pressure stability and exhibits linear lattice contraction with increasing external pressure. Consistent with the results of the performed experiments, the predicted elastic moduli, including Young’s modulus, shear modulus, and bulk modulus, as well as Poisson’s ratio and Pugh’s modulus ratio, show a clear increasing trend with the increase in pressure. Under uniaxial tensile loading, the single-crystal tungsten at high pressures experiences a phase transition from BCC to FCC and other disordered structures, which results in a stripe-like morphology in the tungsten crystal. These results are expected to deepen our understanding of the high-pressure structural and mechanical behaviors of tungsten materials.

Nano-Indentation Hardness and Strain Hardening of Silicon, Sodium Chloride and Tungsten Crystals

BackgroundA previous review of micro- and nano-indentation hardness tests and their analyses gave emphasis to obtaining measurements of continuous nano-indentation load, P, versus, depth, h, recordings that monitor the full elastic–plastic deformation behavior of a localized crystal volume [1].ObjectiveAttention is given to determining the complete, indentation-based, elastic–plastic deformation properties of the local volume, including the initial crystal elastic deformation behavior and, especially to evaluation of post pop-in plastic strain hardening.MethodStress–strain calculations are presented for an initial Hertzian elastic loading and follow-on crystal micro- and nano-scale plastic deformation responses [2].ResultsApplied load, P, dependencies on contact diameters, di, of silicon crystals are compiled on the basis of elastic, plastic and cracking predictions, giving indication at the lowest P values of an indentation size effect (ISE) for the crystal hardness. Elastic–plastic stress–strain curves are presented for sodium chloride and tungsten crystals. The hardness and strain hardening calculations also demonstrate an influence of the ISE.ConclusionsThe exceptional plastic strain hardening behaviors scale dimensionally with corresponding dislocation interactions and sessile reactions within the very localized plastic indentation zones. There is usefulness in determining elastic modulus values from the initial loading record. Micro- and nano-scale dislocation interactions/reactions account for the high stress and strain hardening levels as well as the occurrence of an ISE.

Fracture toughness of crystalline solids.

This paper describes an attempt to construct a first-principles theory of the fracture toughness of crystalline solids. It is based on the thermodynamic dislocation theory (TDT), which starts with the assertion that dislocations in solids must obey the second law of thermodynamics. A second starting assumption is that fracture is initiated when the tip of a notch is driven to undergo a dynamic shape instability. The results of this analysis are developed in comparison with measurements by Gumbsch and colleagues of the notch toughness of both predeformed and non-predeformed tungsten crystals. The theory includes a simple ad hoc conjecture regarding tip dynamics at small dislocation densities. Nevertheless, its predictions agree quantitatively with the experimental data, including both brittle and ductile fracture, over a wide range of temperatures, loading rates, and initial conditions.

Coupling crystal plasticity and stochastic cluster dynamics models of irradiation damage in tungsten

Irradiation damage is known to alter a material’s microstructure due to the accumulation of high densities of defect clusters. Such irradiated microstructures change the mechanical response of the material due to dislocation-defect interactions, which leads to a host of issues such as hardening, swelling, irradiation creep, embrittlement, etc. Traditionally, the effect of irradiation on the mechanical response of materials is evaluated via tensile tests of pre-irradiated specimens at different doses and temperatures. From a modeling perspective, methods exist that simulate irradiation and deformation as separate processes, with the former based on kinetic transport theory and the latter on crystal plasticity (CP). Generally, these are connected by a state variable, usually in the form of a characteristic length scale, that represents the defect concentration and strength and its effect on dislocation-mediated slip. However, cases where deformation takes place during irradiation are also important despite being less common. In this paper, we develop a coupled CP and stochastic cluster dynamics (SCD) approach capable of treating all instances of irradiation/deformation in irradiated materials. We apply the methodology to tungsten crystals due to its importance as a high-temperature candidate structural material and to its extensive defect and mechanical data base. SCD evolves the defect microstructure stochastically, providing a statistically-averaged defect cluster spacing parameter that informs CP calculations of the material’s mechanical deformation. The coupling is bi-directional in the sense that the SCD method updates the obstacle density and furnishes a resistance stress to the CP model, while CP feeds updated dislocation densities that act as defect sinks in the SCD calculation cycles. The coupling can be done sequentially, as in standard tensile tests of pre-irradiated materials, or concurrently, as in in situ straining tests during irradiation. We carry out simulations of realistic irradiation/deformation scenarios and highlight the differences between the present method and past works considering similar situations.

Gradient of Residual Stress and Lattice Parameter in Mechanically Polished Tungsten Measured Using Classical X-rays and Synchrotron Radiation

In this work, the stress gradient in mechanically polished tungsten sample was studied using X-ray diffraction methods. To determine in-depth stress evolution in the very shallow subsurface region (up to 10 μm), special methods based on reflection geometry were applied. The subsurface stresses (depth up to 1 μm) were measured using the multiple-reflection grazing incidence X-ray diffraction method with classical characteristic X-rays, while the deeper volumes (depth up to 10 μm) were investigated using energy-dispersive diffraction with white high energy synchrotron beam. Both complementary methods allowed for determining in-depth stress profile and the evolution of stress-free lattice parameter. It was confirmed that the crystals of tungsten are elastically isotropic, which simplifies the stress analysis and makes tungsten a suitable material for testing stress measurement methods. Furthermore, it was found that an important compressive stress of about − 1000 MPa was generated on the surface of the mechanically polished sample, and this stress decreases to zero value at the depth of about 9 μm. On the other hand, the strain-free lattice parameter does not change significantly in the examined subsurface region.

MANUFACTURE OF SUPER LARGE TUNGSTEN SINGLE CRYSTALS IN THE FORM OF ROTATION BODIES

The results of researches made at creation of technology of production of super-large single crystals of tungsten in the form of plates and bodies of rotation are presented. In E.O. Paton Electric Welding Institute NAS of Ukraine created a unique technology for the production of single crystals of refractory metals in the form of plates 170x160x20 mm, worked out the technology and conducted material studies, which confirmed their quality and gave the basis for further development in this direction. In recent years, the technology of growing super-large single-crystal tungsten in the form of rotating bodies with a diameter of 85 mm has been created.

Testing Strong Field QED Close to the Fully Nonperturbative Regime Using Aligned Crystals.

Processes occurring in the strong field regime of QED are characterized by background electromagnetic fields of the order of the critical field F_{cr}=m^{2}c^{3}/ℏ|e| in the rest frame of participating charges. It has been conjectured that if in their rest frame electrons and positrons experience field strengths of the order of F_{cr}/α^{3/2}≈1600F_{cr}, with α≈1/137 being the fine-structure constant, their effective coupling with radiation becomes of the order of unity. Here we show that channeling radiation by ultrarelativistic electrons with energies of the order of a few TeV on thin tungsten crystals allows us to test the predictions of QED close to this fully nonperturbative regime by measuring the angularly resolved single photon intensity spectrum. The proposed setup features the unique characteristics that essentially all electrons (1)undergo at most a single photon emission and (2)experience at the moment of emission and in the angular region of interest the maximum allowed value of the field strength, which at 2TeV exceeds F_{cr} by more than 2 orders of magnitude in their rest frame.

On the Reflection of Hydrogen and Deuterium Atoms from a Tungsten Crystal in the Surface Channeling Mode

The reflection coefficients and angular distributions of scattered particles under the bombardment of amorphous and crystalline tungsten targets by hydrogen and deuterium atoms are calculated. A reflection coefficient that is close to 100% is observed in the case of grazing angles of incidence on the crystalline targets. It is planned that tungsten in the form of polycrystalline plates will be used as the material for the ITER tokamak divertor. This phenomenon should be taken into account when calculating the energy input into the divertor.

Spin rotation of the planar channeled muons

In the present paper we obtain exact solution of the BMT equation for the spin rotation of relativistic negatively charged particles passing through a thin crystal in the regime of planar channeling. The derived equations are applied for investigation of spin rotation of relativistic muons channeled in the tungsten crystal.

Структура крупных профилированных монокристаллов вольфрама, полученных аддитивным плазменно-индукционным наплавлением

Структура крупных профилированных монокристаллов вольфрама, полученных аддитивным плазменно-индукционным наплавлением

Prediction of the Critical Energy Release Rate of Nanostructured Solids Using the Laplacian Version of the Strain Gradient Elasticity Theory

The aim of the paper is quantify the material length scale parameter of the simplified form of the strain gradient elasticity theory (SGET) using first principles density-functional theory (DFT). The single material length scale parameterlis extracted from phonon-dispersions generated by DFT calculations and, for comparison, by adjusting the analytical SGET solution for the displacement field near the screw dislocation with the DFT calculations of this field. The obtained results are further used in the SGET modeling of cracked nanopanel formed by the single tungsten crystal where due to size effects and nonlocal material point interactions the classical fracture mechanics breaks down.