Tungsten Single Crystals Research Articles

Molecular dynamics studies of sticking and reflection of low-energy deuterium on single crystal tungsten

Molecular dynamics simulations have been performed to study deuterium sticking and reflection properties of single crystal tungsten surfaces using two different Tersoff-type tungsten–hydrogen potentials. Single crystal tungsten surfaces of (001) and (110) orientations were bombarded with deuterium atoms up to 100 eV energy at 300 K sample temperature. The potentials show differences in the nature of sticking as well as in the sticking coefficient. In order to understand the variation in the observed sticking coefficient, detailed potential energy analysis has been carried out using both the potentials. The analysis is able to explain the nature of the sticking for various surfaces as well as the observed minima in sticking coefficient in both the potentials. The variation in the sticking and reflection coefficients with energy can be explained from the local variation of the repulsive and attractive potential energy in the near-surface region which are considerably different in both the potentials.

The materials irradiation experiment for testing plasma facing materials at fusion relevant conditions.

The Materials Irradiation Experiment (MITE-E) was constructed at the University of Wisconsin-Madison Inertial Electrostatic Confinement Laboratory to test materials for potential use as plasma-facing materials (PFMs) in fusion reactors. PFMs in fusion reactors will be bombarded with x-rays, neutrons, and ions of hydrogen and helium. More needs to be understood about the interactions between the plasma and the materials to validate their use for fusion reactors. The MITE-E simulates some of the fusion reactor conditions by holding samples at temperatures up to 1000 °C while irradiating them with helium or deuterium ions with energies from 10 to 150 keV. The ion gun can irradiate the samples with ion currents of 20 μA-500 μA; the typical current used is 72 μA, which is an average flux of 9 × 10(14) ions/(cm(2) s). The ion gun uses electrostatic lenses to extract and shape the ion beam. A variable power (1-20 W), steady-state, Nd:YAG laser provides additional heating to maintain a constant sample temperature during irradiations. The ion beam current reaching the sample is directly measured and monitored in real-time during irradiations. The ion beam profile has been investigated using a copper sample sputtering experiment. The MITE-E has successfully been used to irradiate polycrystalline and single crystal tungsten samples with helium ions and will continue to be a source of important data for plasma interactions with materials.

Neutron energy spectrum influence on irradiation hardening and microstructural development of tungsten

Neutron irradiation to single crystal pure tungsten was performed in the mixed spectrum High Flux Isotope Reactor (HFIR). To investigate the influences of neutron energy spectrum, the microstructure and irradiation hardening were compared with previous data obtained from the irradiation campaigns in the mixed spectrum Japan Material Testing Reactor (JMTR) and the sodium-cooled fast reactor Joyo. The irradiation temperatures were in the range of ∼90–∼800 °C and fast neutron fluences were 0.02–9.00 × 1025 n/m2 (E > 0.1 MeV). Post irradiation evaluation included Vickers hardness measurements and transmission electron microscopy. The hardness and microstructure changes exhibited a clear dependence on the neutron energy spectrum. The hardness appeared to increase with increasing thermal neutron flux when fast fluence exceeds 1 × 1025 n/m2 (E > 0.1 MeV). Irradiation induced precipitates considered to be χ- and σ-phases were observed in samples irradiated to >1 × 1025 n/m2 (E > 0.1 MeV), which were pronounced at high dose and due to the very high thermal neutron flux of HFIR. Although the irradiation hardening mainly caused by defects clusters in a low dose regime, the transmutation-induced precipitation appeared to impose additional significant hardening of the tungsten.

Применение аддитивных технологий для выращивания крупных профилированных монокристаллов вольфрама и молибдена

Применение аддитивных технологий для выращивания крупных профилированных монокристаллов вольфрама и молибдена

Application of additive technologies for growing large profiled single crystals of tungsten and molybdenum

Application of additive technologies for growing large profiled single crystals of tungsten and molybdenum

Surface morphologies of He-implanted tungsten

Surface morphologies of tungsten surfaces, both polycrystalline and single-crystal [110], were investigated using SEM and FIB/SEM techniques after implantations at elevated surfaces temperatures (1200–1300K) using well-characterized, mono-energetic He ion beams with a wide range of ion energies (218eV–250keV). Nanofuzz was observed on polycrystalline tungsten (PCW) following implantation of 100-keV He ions at a flux threshold of 0.9×1016cm−2s−1, but not following 200-keV implantations with similar fluxes. No nanofuzz formation was observed on single-crystal [110] tungsten (SCW), despite fluxes exceeding those demonstrated previously to produce nanofuzz on polycrystalline tungsten. Pre-damaging the single-crystal tungsten with implanted C impurity interstitials did not significantly affect the surface morphologies resulting from the high-flux He ion implantations. The main factor leading to the different observed surface structures for the pristine and C-implanted single-crystal W samples appeared to be the peak He ion flux characterizing the different exposures. It was speculated that nanofuzz formation was not observed for any SCW target exposures because of increased incubation fluences required for such targets.

Use of tungsten single crystals to enhance nuclear reactors structural elements properties

This paper demonstrates advantages of using tungsten single crystal as structural material in science absorbing industry in comparison with polycrystalline material. High-temperature short-term static strength and creep resistance characteristics of polycrystalline tungsten and single crystal alloy were obtained and compared. The results of comparative studies of the mechanical properties of polycrystalline and monocrystalline tungsten – 4% tantalum alloy are presented.It is shown that monocrystalline tungsten based materials have both high-temperature strength and plasticity which makes them most suitable for use under high temperatures and high mechanical stresses. Use of monocrystalline tungsten as a structural material makes it possible to prolong the service life and avoid the failures resulting from embrittlement and strain. Moreover, thermo mechanical treatment of monocrystalline tungsten enhances its high-temperature strength. However, it is necessary to carry out additional experimental research to choose the appropriate thermo mechanical treatment conditions.

Microstructure of Chemical Vapor Transport Deposition Single Crystal Tungsten Coating Working at 1600°C

Thermionic energy conversion is a process by which thermal energy is transformed into electrical energy directly without the intermediate steps. Microstructure of Chemical Vapor Transport Deposited (CVTD) single crystal tungsten coating working at 1600°C for 1000 h was investigated using optical microscopy and electron backscatter diffraction (EBSD) technique. The experimental results showed that the etching morphology of single crystal tungsten coating was not clear compared to the etching morphology before working. The electro-etched surface of single crystal tungsten coating mainly consist of {110} crystal planes and {112} crystal planes before working at 1600°C. The surface of the single crystal tungsten coating mainly consists of near {110} crystal planes and near {112} crystal planes instead after working at 1600°C.

Molecular dynamics simulation of temperature effect on tensile mechanical properties of single crystal tungsten nanowire

High-purity single crystal tungsten nanowire was prepared by the metal-catalyzed vapor-phase reaction method, which was firstly proposed by our research group. Its tensile stress–strain curves and microscopic deformed structures at different temperatures were simulated by molecular dynamics method, in order to study the effects of temperature on its tensile mechanical properties and failure mechanisms. Research results show that the stress–strain curves can be all divided into five stages: elastic, damage, phase transition (dominant), hardening and failure stages. Elastic modulus, tensile strength and strain of phase transition are decreased with increase of temperature, while hardening peak stress is first increased and then decreased. There exists a critical temperature Tc for its phase transition mechanism. When T⩽Tc, the phase transition occurs in one direction and leads to less damage and random failure in its damage zone. When T>Tc, the phase transition occurs in two directions and leads to more damage (twin plane) and failure in its twin plane. That is quite different from FCC metal, where there is only one direction of phase transition.

Proportion quantitative analysis and etching of {110} planes on tungsten single crystal coating surface

Tungsten single crystal and poly crystal were treated by electrolytic etching in a 3% by weight solution of NaOH in distilled water. The method for determining the proportion of {110} planes and characteristic morphology on the coating surface after electrolytic etching were investigated using EBSD and auto-focusing microscope. Then the optimization of process parameters for electrolytic etching is achieved. In order to compare the effect of process parameters, three process parameters were selected for the tungsten single crystal electrolytic etching. Through analyzing the change of {110} planes' proportion, we found that when the coatings are etched with 1.4 amp/cm2 and 3 min, {110} planes can be exposed in the greatest degree that can reach 61.4% on tubular surfaces. The proposed approach greatly improves the proportion of {110} planes relative to the original surface.

Study of hydrogen isotopes behavior in tungsten by a multi trapping macroscopic rate equation model

Density functional theory (DFT) studies show that in tungsten a mono vacancy can contain up to six hydrogen isotopes (HIs) at 300 K with detrapping energies varying with the number of HIs in the vacancy. Using these predictions, a multi trapping rate equation model has been built and used to model thermal desorption spectrometry (TDS) experiments performed on single crystal tungsten after deuterium ions implantation. Detrapping energies obtained from the model to adjust temperature of TDS spectrum observed experimentally are in good agreement with DFT values within a deviation below 10%. The desorption spectrum as well as the diffusion of deuterium in the bulk are rationalized in light of the model results.

Single- and few-layer WTe2 and their suspended nanostructures: Raman signatures and nanomechanical resonances.

Single crystal tungsten ditelluride (WTe2) has recently been discovered to exhibit non-saturating extreme magnetoresistance in bulk; it has also emerged as a new layered material from which atomic layer crystals can be extracted. While atomically thin WTe2 is attractive for its unique properties, little research has been conducted on single- and few-layer WTe2. Here we report the isolation of single- and few-layer WTe2, as well as the fabrication and characterization of the first WTe2 suspended nanostructures. We have observed new Raman signatures of single- and few-layer WTe2 that have been theoretically predicted but have not been reported to date, in both on-substrate and suspended WTe2 flakes. We have further probed the nanomechanical properties of suspended WTe2 structures by measuring their flexural resonances, and obtain a Young's modulus of E(Y) ≈ 80 GPa for the suspended WTe2 flakes. This study paves the way for future investigations and utilizations of the multiple new Raman fingerprints of single- and few-layer WTe2, and for explorations of mechanical control of WTe2 atomic layers.

Defect evolution in single crystalline tungsten following low temperature and low dose neutron irradiation

The tungsten plasma-facing components of fusion reactors will experience an extreme environment including high temperature, intense particle fluxes of gas atoms, high-energy neutron irradiation, and significant cyclic stress loading. Irradiation-induced defect accumulation resulting in severe thermo-mechanical property degradation is expected. For this reason, and because of the lack of relevant fusion neutron sources, the fundamentals of tungsten radiation damage must be understood through coordinated mixed-spectrum fission reactor irradiation experiments and modeling. In this study, high-purity (110) single-crystal tungsten was examined by positron annihilation spectroscopy and transmission electron microscopy following low-temperature (∼90 °C) and low-dose (0.006 and 0.03 dpa) mixed-spectrum neutron irradiation and subsequent isochronal annealing at 400, 500, 650, 800, 1000, 1150, and 1300 °C. The results provide insights into microstructural and defect evolution, thus identifying the mechanisms of different annealing behavior. Following 1 h annealing, ex situ characterization of vacancy defects using positron lifetime spectroscopy and coincidence Doppler broadening was performed. The vacancy cluster size distributions indicated intense vacancy clustering at 400 °C with significant damage recovery around 1000 °C. Coincidence Doppler broadening measurements confirm the trend of the vacancy defect evolution, and the S–W plots indicate that only a single type of vacancy cluster is present. Furthermore, transmission electron microscopy observations at selected annealing conditions provide supplemental information on dislocation loop populations and visible void formation. This microstructural information is consistent with the measured irradiation-induced hardening at each annealing stage, providing insight into tungsten hardening and embrittlement due to irradiation-induced matrix defects.

Performance of a fast response miniature Adiabatic Demagnetisation Refrigerator using a single crystal tungsten magnetoresistive heat switch

The performance of a fast thermal response miniature Adiabatic Demagnetisation Refrigerator (ADR) is presented. The miniature ADR is comprised of a fast thermal response Chromium Potassium Alum (CPA) salt pill, two superconducting magnets and unconventionally, a single crystal tungsten magnetoresistive (MR) heat switch. The development of this ADR is a result of the ongoing development of a continuously operating millikelvin cryocooler (mKCC), which will use only magnetoresistive heat switches. The design and performance of the MR heat switch developed for the mKCC and used in the miniature ADR is presented in this paper; the heat switch has a measured Residual Resistivity Ratio of 32,000±3000 and an estimated switching ratio (on thermal conductivity divided by the off thermal conductivity) of 15,200 at 3.6K and 38,800 at 0.2K when using a 3T magnetic field. The performance of the miniature ADR operating from a 3.6K bath is presented, demonstrating that a complete cycle (magnetisation, cooling to the bath and demagnetisation) can be accomplished in 82s. A magnet current step test, conducted when the ADR is cold and fully demagnetised, has shown the thermal response of the ADR to be sub-second. The measured hold times of the ADR with just parasitic heat load are given, ranging from 3min at 0.2K with 13.14μW of parasitics, to 924min at 3K with 4.55μW of parasitics. The cooling power has been measured for operating temperatures in the range 0.25–3K by applying an additional heat load to the ADR via a heater, in order to reduce the hold time to 3min (i.e. approximately double the recycle time); the maximum cooling power of the miniature ADR (in addition to parasitic load) when operating at 250mK is 20μW, which increases to 45μW at 300mK and continues to increase linearly to nearly 1.1mW at 3K. To conclude, the predicted performance of a tandem continuous ADR utilising two of the miniature ADRs is presented.

Extreme nanomechanics: vacuum nanoindentation and nanotribology to 950 °C

Elevated temperature mechanical and tribological properties can be more relevant for practical wear situations than corresponding measurements at room temperature. However, high temperature nanomechanics and nanotribology is highly challenging experimentally. To overcome these challenges the NanoTest*** has been developed with active heating of the indenter and sample with resistive heaters, horizontal loading, patented thermal control method and stage design. By separately actively heating*** and controlling the temperatures of indenter and sample their temperatures can be precisely matched so that there is no heat flow and minimal/no thermal drift during the high temperature indentation,*** and measurements can be performed as reliably as at room temperature. Above 500 °C it is beneficial to use a cubic Boron Nitride indenter with gas purging to limit oxidation of samples. To achieve higher temperatures without indenter or sample oxidation an ultra-low drift high temperature vacuum nanomechanics/tribology system capable of testing to*** much higher temperatures has been recently developed (NanoTest Xtreme). The influence of time-dependent deformation on elevated temperature nanomechanical behaviour is discussed, using published results in Argon on glass-ceramic solid oxide fuel cell seal materials and previously unpublished nanoindentation measurements on single crystal silicon and polycrystalline tungsten using the NanoTest Xtreme in vacuum at temperatures up to 950 °C. Studies of the elevated temperature nano-/micro-tribological*** behaviour of wear-resistant*** nitride-based and MAX-phase coatings are also briefly reviewed.

Orientation-dependent responses of tungsten single crystal under shock compression via molecular dynamics simulations

The orientation-dependent mechanical responses of shock loading on tungsten single crystal are investigated via molecular dynamics simulations. All the simulated Hugoniot relations are comparable with the available experimental data. By linear fitting our us–up data, we produced the relation us=(3.975±0.207)+(1.331±0.113)up, which compare relatively well with the experimental results. Combined Hugoniot P–T curve and radial distribution function (RDF), we detected that the solid–liquid transition occurs at the (P, T) points of (311GPa, 9166K)–(335GPa, 9938K) along [100], (295GPa, 7762K)–(321GPa, 8590K) along [110], and (211GPa, 5396K)–(237GPa, 5972K) along [111] orientation. The shock profiles, i.e. unshocked, shock front and post shocked regions, are investigated in detail by tracing the evolutions of stresses, particle velocities, temperatures, local-order parameters and RDFs from one- or two-dimensional. Moreover, the time-dependent stresses profiles are also further analyzed.

Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models.

Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single‐crystal tungsten strength

AbstractUnderstanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body‐centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse‐scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum‐level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation‐based CP framework. The complete model is used to perform material point simulations of single‐crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non‐Schmid effects are inlcuded in the model by considering the twinning‐antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of 〈111〉{110} and 〈111〉{112} slip systems in the homologous temperature range from 0.08Tm to 0.33Tm, where Tm =3680 K is the melting point in tungsten. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation-mediated strain hardening in tungsten: Thermo-mechanical plasticity theory and experimental validation

A self-consistent thermo-mechanical model to study the strain-hardening behavior of polycrystalline tungsten was developed and validated by a dedicated experimental route. Dislocation–dislocation multiplication and storage, as well dislocation-grain boundary (GB) pinning were the major mechanisms underlying the evolution of plastic deformation, thus providing a link between the strain hardening behavior and material's microstructure. The microstructure of the polycrystalline tungsten samples has been thoroughly investigated by scanning and electron microscopy. The model was applied to compute stress–strain loading curves of commercial tungsten grades, in the as-received and as-annealed states, in the temperature range of 500–1000°C. Fitting the model to the independent experimental results obtained using a single crystal and as-received polycrystalline tungsten, the model demonstrated its capability to predict the deformation behavior of as-annealed samples in a wide temperature range and applied strain. The relevance of the dislocation-mediated plasticity mechanisms used in the model have been validated using transmission electron microscopy examination of the samples deformed up to different amounts of strain. On the basis of the experimental validation, the limitations of the model are determined and discussed.

The effects of tungsten's pre-irradiation surface condition on helium-irradiated morphology

Erosion is a serious concern associated with the use of tungsten as a plasma-facing component in fusion reactors. To compare the damage progression, polycrystalline tungsten (PCW) and (110) single crystal tungsten (SCW) samples were prepared with (1) a mechanical polish (MP) with roughness values in the range of 0.018–0.020 μm and (2) an MP and electropolish (MPEP) resulting in roughness values of 0.010–0.020 μm for PCW and 0.003–0.005 μm for SCW samples. Samples were irradiated with 30 keV He+ at 1173 K to fluences between 3 × 1021 and 6 × 1022 He/m2. The morphologies that developed after low-fluence bombardment were different for each type of sample—MP SCW, MPEP SCW, MP PCW, and MPEP PCW. At the highest fluence, the SCW MPEP sample lost significantly more mass and developed a different morphology than the MP SCW sample. The PCW samples developed a similar morphology and had similar mass loss at the highest fluence. Surface preparation can have a significant effect on post-irradiation morphology that should be considered for the design of future fusion reactors such as ITER and DEMO.