Tungsten Single Crystals Research Articles

WOx-driven growth of 2H- and 3R-WS2 multilayers by physical vapor deposition

Transition metal dichalcogenides (TMDs), including MoS2 and WS2, exhibit distinct optical and electrical properties that are highly dependent on their thickness and stacking order. However, controlling these parameters during synthesis remains a challenge. Here, we present a synthesis technique for multilayer single-crystal tungsten disulfide (WS2) utilizing a liquid-phase tungsten oxide (WOx)-driven physical vapor deposition (PVD) method. Our approach successfully addresses the challenges of manipulating the stacking order in TMD multilayers, which is vital for harnessing their unique properties for advanced technological applications. By employing a molten WOx intermediate, we achieved controlled growth of WS2 single crystals with varied layer numbers and stacking configurations of 2H and 3R. This method not only facilitates the fabrication of TMD layers beyond monolayers but also provides the ability to tailor their stacking orders. Our findings offer a pathway for the growth of multilayer TMDs, underscoring the potential for scalable production of high-quality single-crystal TMDs for industrial use.

Multi-physics modeling of tungsten collector probe samples during the WEST C4 He campaign

We describe the results of a multi-scale, multi-physics modeling assessment of SOLPS-ITER, hPIC2, RustBCA and Xolotl, in which five single-crystal tungsten (W) samples were placed in a reciprocating collector probe and exposed to helium (He) plasma in the WEST fusion device. In our models, we considered a pure (100 %) He plasma, as well as one with oxygen (O) present (95% He 5% O) corresponding to the impurity concentration estimated during the C4 He campaign in WEST. Our SOLPS simulations approximately match experimental reciprocating Langmuir probe plasma measurements of plasma density and temperature. Using these plasma parameters as input, hPIC2 and RustBCA predict that the presence of oxygen impurities lead to a 15%–20% decrease in ion and heat fluxes to the surface, and an order of magnitude higher sputtering yields (compared with a pure He plasma). Xolotl predictions for the response of tungsten to plasma surface interactions (PSIs) agree with experimental LAMS analysis, and indicate large near-surface He concentrations, which quickly decay with depth. Our model also shows an increasing role of erosion—in removing the near-surface He—with time. Overall, slightly higher retention is predicted for tungsten exposed to a pure He plasma, with the largest differences in the near-surface gas content caused by the large oxygen-induced erosion. This highlights the important role that impurities play in PSI. Therefore, future work will focus on providing a fully self-consistent description of oxygen (and oxides, etc.) in our models, through multi-species implementation in GITR and inclusion of oxygen and tungsten oxide formation in Xolotl.

Determining the density and spatial descriptors of atomic scale defects of 2H–WSe2 with ensemble deep learning

We have demonstrated atomic-scale defect characterization in scanning tunneling microscopy images of single crystal tungsten diselenide using an ensemble of U-Net-like convolutional neural networks. Coordinates, counts, densities, and spatial extents were determined from almost 16 000 defect detections, leading to the rapid identification of defect types and their densities. Our results show that analysis aided by machine learning can be used to rapidly determine the quality of transition metal dichalcogenides and provide much needed quantitative input, which may improve the synthesis process.

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.

Plastic deformation mechanisms in BCC single crystals and equiatomic alloys: Insights from nanoindentation

Deformation plasticity mechanisms in alloys and compounds may reveal the material's capacity towards optimal mechanical properties. We conducted a series of molecular dynamics (MD) simulations to investigate plasticity mechanisms due to nanoindentation in pure tungsten, molybdenum, and vanadium body-centered cubic single crystals, as well as the body-centered cubic, equiatomic, random solid solutions (RSS) of tungsten–molybdenum and tungsten–vanadium alloys. Our analysis focuses on a thorough, side-by-side comparison of dynamic deformation processes, defect nucleation, and evolution, along with corresponding stress–strain curves. We also checked the surface morphology of indented samples through atomic shear strain mapping. As expected, the presence of Mo and V atoms in W matrices introduces lattice strain and distortion, increasing material resistance to deformation and slowing down dislocation mobility of dislocation loops with a Burgers vector of 1/2 (111). Our side-by-side comparison displays a remarkable suppression of the plastic zone size in equiatomic W–V RSS, but not in equiatomic W–Mo RSS alloys, displaying a clear prediction for optimal hardening response of equiatomic W–V RSS alloys. If the small-depth nanoindentation plastic response is indicative of overall mechanical performance, it is possible to conceive a novel MD-based pathway towards material design for mechanical applications in complex, multi-component alloys.

Unveiling the radiation-induced defect production and damage evolution in tungsten using multi-energy Rutherford backscattering spectroscopy in channeling configuration

Radiation-induced defect production in tungsten was studied by a combination of experimental and simulation methods. The analysis of structural defects was performed using multi-energy Rutherford backscattering spectroscopy in channeling configuration (multi-energy C-RBS). To create different microstructures, (111) tungsten (W) single crystals were irradiated with W ions at two different doses (0.02 and 0.2 dpa) at 290 K. Detailed transmission electron microscopy (TEM) analysis of the samples revealed the presence of dislocation lines and loops of different sizes. The RBSADEC code was used to simulate the measured C-RBS spectra, recorded with four different He beam energies along the 〈111〉 direction. For the first time for tungsten, molecular dynamics (MD) simulations of overlapping cascades were used as input. The well-known method of randomly displaced atoms (RDA) was applied for comparison. RDA does not provide a satisfactory understanding of the nature of the induced defect structure. With MD, a very good agreement between the simulated and experimental spectra was obtained for the sample prepared at a lower dose, despite the fact that the absolute defect densities are two orders of magnitude higher than those found with TEM. A discrepancy is observed for the high-dose-irradiated sample, which is ascribed to the presence of extended defects such as dislocation lines, which are clearly observed by TEM, but cannot be formed in finite size MD cells. RBSADEC with MD cells as input can describe correctly the response of the RBS signal with analysing beam energy while RDA as input gives the wrong trend.

Confluent effects of initial dislocation microstructures on yield behavior in single-crystal tungsten at high temperature

Though it is well known that initial dislocation microstructures play an important role in the deformation behavior of materials, how dislocation microstructures affect the macroscopic properties is still subjected to intensive research due to their complexity. In this work, we use discrete dislocation dynamics (DDD) to understand the effect of initial dislocation microstructures on the dynamic yield behavior of single-crystal tungsten above the critical temperature (800 K). DDD results suggest that the dislocation source length and the dislocation density are responsible for the initial yield stress and the flow stress of tungsten under dynamic loading, respectively. As and increase, the plastic yield mechanism transforms from dislocation-source activation into dislocation-dislocation interactions, resulting in the initial yield stress decreasing and the flow stress increasing. The confluent effects of and on the steady-state flow stress can be unified by a linear relationship between and Our results could be a valuable piece that connects dynamic yield behavior to the initial dislocation microstructures.

Microstructure evolution mechanism of tungsten induced by ultrasonic elliptical vibration cutting at atomic/nano scale

Ultrasonic elliptical vibration cutting (UEVC) technology has been utilized for ultra-precision machining of difficult-to-machine metal materials such as tungsten. Nevertheless, the microstructure evolution mechanism of tungsten under the synergistic effect of ultrasonic and mechanical loads remains unclear, particularly at the atomic/nano scale. Additionally, the plastic deformation mechanism of tungsten differs from that of other metallic materials due to its low dislocation mobility (brittle at room temperature). Hence, the molecular dynamics simulation of UEVC for single crystal tungsten was established to study its mechanisms in plastic deformation and microstructure evolution under stress induction in this study. The results indicated that the main plastic deformation mechanisms including dislocation slip, amorphous phase transformation and nanocrystal were found during the tungsten removal, and accompanied by some extent of lattice distortion. The instantaneous shear stress of UEVC reached 16.88 GPa. Compared with common cutting (CC), the formation of nanocrystals mainly occurred in UEVC because the instantaneous shear stress exceeded the critical shear stress of multiple slip systems during cutting. Similarly, the high dislocation density and high plastic deformation degree of the machined zone in UEVC were also attributed to the high shear stress. The dynamic recrystallization of tungsten induced by UEVC was realized from dislocation slip to the formation of dense dislocation walls, followed by the formation of sub-grain boundaries, and finally to the formation of nanocrystals.

Enhanced oxidation resistance of single crystal tungsten powder by forming tungsten carbide on the surface

The tungsten-type delay composition is composed of tungsten powder, oxidizing agent and binder, but the tungsten powder will be pre-oxidized after long-term storage, which deteriorates the combustion performance of the tungsten-type delay composition. Therefore, it is necessary to improve the oxidation resistance of tungsten powder. Single crystal tungsten powder (SCTP) is successfully prepared through hydrogen reduction at high temperature and short time, and anti-oxidative single crystal tungsten powder (ASCTP) is obtained by adopting carbonization with a carbon source of gas phase (Carbon monoxide), which enables fuller and more uniform reactions. The TEM results show that the surface of the SCTP is coated by the tungsten carbide, which forms synaptic structures on the surface of the SCTP to protect the SCTP from oxidation by surface modification. As a result, it has been confirmed through TG-DSC tests, oxidation experiments and related electrochemical tests that the oxidation resistance performance of the ASCTP is substantially improved compared with that of the SCTP.

Post neutron irradiation annealing and defect evolution in single crystal tungsten

Post neutron irradiation annealing and defect evolution in single crystal tungsten

Lattice and electronic structure of ScN observed by angle-resolved photoemission spectroscopy measurements

Scandium nitride (ScN) has recently attracted much attention for its potential applications in thermoelectric energy conversion, as a semiconductor in epitaxial metal/semiconductor superlattices, as a substrate for GaN growth, and alloying it with AlN for 5G technology. This study was undertaken to better understand its stoichiometry and electronic structure. ScN (100) single crystals 2 mm thick were grown on a single crystal tungsten (100) substrate by a physical vapor transport method over a temperature range of 1900–2000 °C and a pressure of 20 Torr. The core level spectra of Sc 2p3/2,1/2 and N 1s were obtained by x-ray photoelectron spectroscopy (XPS). The XPS core levels were shifted by 1.1 eV toward higher values as the [Sc]:[N] ratio varied from 1.4 at 1900 °C to ∼1.0 at 2000 °C due to the higher binding energies in stoichiometric ScN. Angle-resolved photoemission spectroscopy measurements confirmed that ScN has an indirect bandgap of ∼1.2 eV.

The first millikelvin cryocooler (mKCC): Design and performance

The design and performance of the first millikelvin cryocooler (mKCC) is presented. The mKCC is based upon a tandem Adiabatic Demagnetisation Refrigerator (ADR) that uses two single ADRs operated out of phase and connected to a common cold stage to provide continuous cooling. Development of this mKCC is part of an on-going research program to ultimately achieve sub-100 mK continuous cooling and builds upon our previous research, with each single ADR in the mKCC being a fast thermal response miniature ADR using a single crystal tungsten magnetoresistive heat switch as detailed in our 2015 publication [1]. With the mKCC operating from a 3.6 K bath temperature, the goal is to achieve continuous cooling at 250 mK (sub-100 mK is not possible without additional pre-cooling). The mKCC has dimensions of 120 × 56 × 228 mm and a mass of 4.67 kg. It can operate on very fast timescales – each superconducting magnet can be ramped to 2 Tesla in 30 s and the Chromium Potassium Alum pills have a measured sub-second thermal response, resulting in each miniature ADR being recycled in minutes. Unconventionally, the mKCC uses single crystal tungsten magnetoresistive heat switches.We present the performance of the first version of the fully automated mKCC (from a 3.6 K bath temperature), which has been determined by undertaking a range of tests analysing the cool down from 3.6 K to the operating temperature, the baseline performance, the thermal stability at the continuous stage, the reliability and repeatability in performance and the cooling power at a range of operating temperatures. The base temperature has been measured to be 750 mK and we have demonstrated that the mKCC can be operated at any temperature between 750 mK and 3 K, with the program-controlled transition between operating temperatures taking approximately 60 s. The cooling power of the mKCC (in addition to parasitic load) has been measured at a range of temperatures between 800 mK and 3 K by applying a heat load to the continuous stage via a heater; the maximum cooling power at 800 mK is 6 µW, increasing to 32 µW at 1 K and 412 µW at 3 K. In addition we conducted a six-week continuous test during which each ADR undertook 5,498 5.5 min cycles with no significant variation in performance detected. To conclude, we compare the measured performance of the mKCC to the expected performance based on mathematical thermal modelling and the performance of the miniature ADR. Whilst the measured performance does not meet the expected performance in terms of base temperature or cooling power, we have identified the limiting factors and discuss them here.

Tensile deformation and failure of tungsten single crystals

The past century has seen extensive research into the deformation behavior of tungsten and tungsten alloys. Research has focused on understanding the flow behavior, work hardening, and mechanisms of brittle fracture to improve the performance of tungsten alloys at low temperatures. While the plastic behavior of tungsten is inherently linked to the mobility of individual dislocations, the collective motion of dislocations has implications for plasticity and the ultimate failure mechanisms observed at low temperatures. This study provides a detailed analysis of deformation mechanisms observed in tungsten single crystals. Microscopy techniques were utilized to explain significant differences in the deformation behavior of tungsten single crystals as a function of orientation. Electron backscatter diffraction was used to study plasticity with orientation imaging techniques. This work reinforces and expands our understanding of the deformation behavior of tungsten by investigating primary causes for failure in single crystals tested along the [001] and [011] tensile axes.

Evolution of bubble in tungsten irradiated by deuterium of low energy and high flux by molecular dynamics simulations

Using molecular dynamics simulations, the Tersoff-potential was used to investigate the evolution of deuterium (D) bubbles in tungsten (W). This work documents the entire evolution process of D bubbles: formation, growth and burst. The effects of flux, temperature and incident energy of D on the evolution and position of D bubbles were discussed. Temperature and flux together affect the time required for D bubble formation, and energy has a significant effect on depth. At 300 K, single-crystal tungsten surfaces (001), (110), (111), and (112) were bombarded by D atoms with 80 eV energy and 6.24 × 1028 m−2s−1 flux. Although all D bubbles go through three stages, the pressure, volume, and the ratio of (D in D2)/D inside bubbles can tell the difference between the four surfaces. Under the same simulation conditions, the evolution of D bubbles under a surface with a higher crystal face density was found to be harder.

Effects of Cutting Force on Formation of Subsurface Damage During Nano-Cutting of Single-Crystal Tungsten

Abstract Single-crystal tungsten is widely utilized in various fields, benefiting from its outstanding properties. Nano-cutting, as an ultra-precision machining method, can realize high efficiency and low damage. However, from the atomic perspective, the formation mechanism of subsurface damage during the nano-cutting of tungsten is still unclear. Herein, the molecular dynamics (MD) simulation of nano-cutting single-crystal tungsten was established to elucidate the evolution of subsurface damage and the effects of cutting force on subsurface damage. The corresponding results showed the existence of damage including atomic cluster, vacancy defect, “V-shaped” dislocation, stair-rod dislocation, and dislocation ring on the subsurface during the cutting. There were dislocation lines in 1/2<111>, <100>, <110>, and other directions due to plastic deformation dominated by dislocation slip, and the 1/2<111> dislocation lines could be merged into stable <100> dislocation lines under certain circumstances during the cutting. The variation of cutting force and cutting force fluctuation induced by changing cutting parameters had a great influence on the subsurface damage of tungsten, including the number of surface defect atoms, dislocation density, and thickness of the subsurface damage layer. In nano-cutting of single-crystal tungsten, a smaller cutting depth and appropriate cutting speed should be selected to reduce subsurface damage. This study provides an insight into the evolution mechanism of subsurface damage of tungsten and is high of significance for achieving low-damage machining of tungsten components.

Thermal diffusivity in ion-irradiated single-crystal iron, chromium, vanadium, and tungsten measured using transient grating spectroscopy

The speed-up of radiation science development with the advent of ion-irradiation experiments has, until recently, been omitted in the post-irradiation examination technique. This paper reports the results of transient grating spectroscopy—a rapid, non-destructive, in situ photothermal surface technique—of ion-irradiated single-crystals of iron, chromium, vanadium, and tungsten at room temperature. Thermal diffusivity was used to track damage development throughout irradiation, with 5 MeV self-ion irradiated iron, chromium, and vanadium showing little to no change up to damages of the order of 1 dpa. 5 MeV Si3+-ion irradiated tungsten exhibits a reduction of thermal diffusivity from 0.78(7) to 0.29(2) cm2 s−1 with logarithmically increasing dose over a similar damage range. A comparison to literature of transient grating spectroscopy thermal diffusivity values past and present shows good agreement; radiation-induced change can be clearly distinguished from differences between mono- and poly-crystalline tungsten.

A coupled crystal-plasticity and phase-field model for understanding fracture behaviors of single crystal tungsten

The brittle-ductile transition (BDT) of tungsten depends heavily on temperature-dependent dislocation mobility. To well understand the underlying mechanism of tungsten fracture behavior, the dislocation activities need to be well described, but the related model remains limited. In this work, a coupled crystal-plasticity and phase-field model (CP-PFM) is developed based on a unified thermodynamic framework, which considers the thermal activated kink-pair mechanism of screw dislocation motion, the evolution of dislocation density, and the coupling effects between the plasticity and the crack propagation. The developed model can well predict the experimental results and is used to study the BDT process of tungsten. Four typical fracture processes are disclosed depending on the temperature, including the brittle fracture, semi-brittle fracture, micro-ductile fracture and ductile fracture. They exhibit different micro-cleavage crack features, induced by the competition between the shielding effect of the plastic zone and the crack propagation near the crack tip zone. In addition, the fracture process of tungsten is found to be significantly influenced by non-Schmid effect in the low and medium temperature regimes.

Thermal research of a single crystal tungsten target positron source for the STCF project in China

A new generation electron–positron collider, the Super Tau-Charm Facility (STCF), has been proposed in China. As a critical part of the STCF Linac, the positron source will use a 10 nC/50 Hz 1.5 GeV electron beam to bombard a single crystal tungsten target. To obtain a stable and reliable positron source with a long lifetime, this paper presents a molecular dynamics model to simulate the anisotropy of thermal conduction for single crystal tungsten. A series of electron beam bombardment experiments were performed, which confirmed that the anisotropy of thermal conductivity and the highest thermal conductivity for single-crystal tungsten is the close-packed crystallographic orientation of [111]. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to study the recrystallization of the single crystal tungsten target. Finally, a new oscillation motional target system was designed for the STCF, and thermal simulation and moving speed optimization were performed.

Decoupling indentation size and strain rate effects during nanoindentation: A case study in tungsten

Materials indented at small scales may simultaneously exhibit indentation size and strain rate effects which complicate the identification of the mechanisms that control deformation and strength. Here, we explore the possibility that indentation size and rate effects in some materials can be decoupled in a simple way. Nanoindentation tests with various load-time histories were carried out to measure the hardness of a tungsten single crystal over a wide range of indentation depths (∼500–3600 nm) and indentation strain rates (∼5 × 10–5–2 × 10–1 s–1). Under these conditions, this material exhibits significant indentation size and rate effects, but the size effect is, to a good approximation, independent of strain rate. It is shown that this behavior can be understood by the Nix-Gao model for the indentation size effect modified to include the effects of a strain rate dependent friction stress. As a consequence, the size and rate dependencies of the hardness can be expressed as the sum of two independent terms:H(ε˙i,hc)=Hf(ε˙i)+(H0−Hf)1+h*hc,where H(ε˙i,hc) is the hardness at given indentation strain rate (ε˙i) and contact depth (hc), Hf(ε˙i) is the hardness contributed by the rate dependent friction stress, and (H0−Hf) and h* are size and rate independent constants that follow from the Nix-Gao analysis. This formula, together with an expression for the rate dependence of Hf, was successfully applied to decouple the indentation size and rate effects observed in tungsten. In addition, the physics underlying the rate independence of indentation size effect is discussed, which provides guidance for application of the proposed approach to other materials.

Blister growth model in proton-irradiated metals - application to tungsten irradiated by MeV protons

Tungsten and tungsten alloys are utilized as structural materials in nuclear facilities due to their excellent mechanical and thermal properties. Blistering is a primary failure mechanism of proton-irradiated tungsten, as well as in other metals, that may shorten the erosion lifetime of components. Furthermore, mature blisters with high internal gas pressure are likely to burst, leading to exfoliation of the surface. Recent MeV proton irradiation studies have found significantly larger blisters formed deeper below the surface than blisters formed from keV irradiation. Understanding these differences and predicting blistering phenomena require a model of blister growth. We develop a model of blister growth under irradiation that combines the mechanisms of plate bending and crack propagation through strain energy release. The process is assumed to be quasi-static and proceeds between equilibrium states. Closed-form expressions were obtained for limiting cases. The model's validity for blister growth under hydrogen pressure is evaluated by comparison with experimental blister results formed during MeV proton irradiation on single-crystal and polycrystalline tungsten at various temperatures. A blister aspect ratio (height to the area) was found to be a characteristic feature of blistering mechanics and strongly dependent on the ratio between plate-bending strain energy and material fracture toughness. The effect of irradiation on material properties is discussed in the context of our model and the experimental results.