Narrow Results

The dissolution and diffusion behaviors of H in the four low-Miller-index W surfaces ((110), (112), (100) and (111)) are systematically studied by the density functional theory approach to understand the orientation dependence of the H bubble distribution on surface. The results show that H accumulation on surface is influenced by H diffusion barrier as well as vacancy and H formation. The barriers of diffusion towards surfaces are larger than that in bulk. It indicates that H is prone to diffuse into the deep in bulk once H dissolves in surface. H is preferred to accumulate on the W(111) surface due to the lower formation energies of vacancy and H comparing to that in bulk. However, W(110) is the resistant surface for forming H bubble due to the higher formation energies of vacancy and H. The results are helpful for understanding the orientation dependence of surface damages on W surface and designing new plasma-facing materials.

First-principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) have been carried out to study the migration of helium in tungsten. The results show that helium interstitial jump directly between two tetrahedral sites with very low migration energy. The migration manner of interstitial He is related to the configuration of the host metal. Furthermore, the dissociation energy of a substitutional helium atom from a vacancy has been calculated from first-principles.

Tungsten as the most effective carbide-forming element was added in the Fe–Cr13–C hardfacing alloy to precipitate WC particles. Optical microscope (OM), scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS) were used to investigate the microstructures of the hardfacing alloy. The wear resistance was tested through a slurry rubber wheel abrasion test machine, and the wear behavior was also studied. The results indicate that the microstructures of the hardfacing alloy consist of lath martensite, residual austenite and WC particles. The wear resistance can be significantly improved through the addition of tungsten element being provided by the precipitation of WC particles. And the predominant wear mechanism was microcutting with shallow grooves and spalling.

With the promotion of fuel economy policy and automobile lightweight concept, ferritic stainless steels applied in vehicles’ exhaust hot end systems have been developed. This paper simulated the high-temperature environment at which the automobile exhaust system serviced in for high-temperature corrosion. Kinetic curves were conducted in isothermal environments at 1000${}^{\circ }$C. X-ray diffraction, scanning electron microscope and energy dispersive spectrometer were used to study the oxidation behavior of ferritic stainless steels and the effects of tungsten (W) addition. The results show that, with increasing oxidation time, the rate of weight gains increase and the main failure is spalling of surface oxide layer. The addition of W has a complicated effect on the oxidation behavior of ferritic stainless steel weldment.

Brazing experiments between tungsten and MA956 ODS steel were conducted at $41050^{\circ }\mathrm{\text{C}}$ with different holding time to investigate the interface microstructure evolution and mechanical properties of the brazed joints. The interface microstructures were analyzed by the scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), and the mechanical properties of the joints were measured to find the optimal brazing parameter. The results show that tungsten and MA956 steel has been successfully joined by brazing. The intermetallic compound of Ni–W formed at the W/BNi-2 interface, which was detrimental to the strength of the joint. Due to the excellent mutual solubility between Fe and Ni, no new formed intermetallic compound was found at MA956-brazing filler interface. The shear strength of the brazed joints with BNi-2 filler reached 246 MPa at the optimal brazing parameter $1050^{\circ }\mathrm{\text{C}}/20\mathrm{\text{min}}$.

Diffusion bonding is an effective method for joining dissimilar materials. In this study, dissimilar metals of MA956 steel and tungsten (W) were diffusion bonded with Ni/Nb composite interlayer. The experiments were carried out at $1050^{\circ }\mathrm{\text{C}}$, 20 MPa for 20 min in vacuum by spark plasma sintering (SPS) technique. The microstructure and mechanical properties of the bonded joints were evaluated. SEM images and the results of elementary composition indicate that no intermetallics formed at Ni/MA956 steel and Nb/W interfaces, but ${\mathrm{\text{Ni}}}_6{\mathrm{\text{Nb}}}_7$ and ${\mathrm{\text{Ni}}}_3\mathrm{\text{Nb}}$ formed at the Nb/Ni interface. Compared with the directly bonded joint between W and MA956 steel, the average shear strength of the joint with Nb/Ni composite interlayer significantly increased to 270 MPa. Although the result of joint residual stresses simulation shows that the maximum residual stress was near the W substrate, the joints with composite interlayer in shear experiments fractured at Nb/Ni interface. The hardness changes along joint interfaces indicate the formation of intermetallic compounds and solid solution phases in the diffusion layers.

Field-ion microscopy and molecular dynamics simulation were used for the characterization of the translation states at special grain boundaries in tungsten. It is revealed that rigid-body displacement had the component of displacement parallel to the [110] direction equal to 0 or 1 interplanar spacing. Full vectors of lateral shifts are described by broad statistical distribution. The mathematical simulation revealed two mirror-symmetry-breaking lateral translations along the direction of the Σ9 grain boundary corresponding to different atomic structures. It is shown that the partial disconnection with the Burgers vector can be considered as the elemental carriers of grain-boundary polymorphic transformation.

Ti-48%Al-2%Cr-2%Nb-(0, 0.5, 1) at.%W alloys were synthesized via the powder metallurgical route, and cyclically oxidized at 800, 900, or 1000°C in air for up to 100 h in order to find the effects of W on their oxidation characteristics. At 800°C, they oxidized relatively slowly, and the scales were thin and adherent. At 900°C, the scales began to spall locally. At 1000°C, they spalled repetitively during oxidation. Cr, Nb, and W improved the cyclic oxidation resistance of TiAl alloys. The oxides formed were TiO₂ and Al₂O₃, which contained the alloying elements of Cr, Nb, and W. Nitrides formed were TiN and Ti₂AlN.

Tungsten thin films were prepared by pulsed laser deposition (PLD) technique on glass substrates placed at the angles of 0${}^{\circ }$ to 70${}^{\circ }$ with respect to the target surface normal. Rutherford backscattering Spectrometry (RBS) analysis of the films indicated that about 90% of tungsten material flux is distributed in a cone of 40${}^{\circ }$ solid angle while about 54% of it lies even in a narrower cone of 10${}^{\circ }$ solid angle. Significant diffusion of tungsten in glass substrate has been observed in the films deposited at smaller angles with respect to target surface normal. Time-of-flight (TOF) measurements performed using Langmuir probe indicated that the most probable ion energy decreases from about 600 to 91eV for variation of $\theta$ from 0${}^{\circ }$ to 70${}^{\circ }$. In general ion energy spread is quite large at all angles investigated here. The enhanced tungsten diffusion in glass substrate observed at smaller angles is most probably due to the higher ion energy and ion assisted recoil implantation of already deposited tungsten.

We have investigated the adsorption sites and the electronic structure correlated with the magnetic properties of ultrathin Fe films on W(110) system using spin-polarized calculations within the density-functional approach with generalized gradient approximation by the pseudopotential plane-wave code. For one Fe monolayer (ML) on W(110) system the Fe atoms prefer to bind on the bridge adsorption sites of the W(110) surface, with an inward relaxation of $-$12.68%. The top and diagonal bridge sites investigated are energetically less favorable. We have shown that intermixing between Fe and W is unlikely: the surface ordered Fe–W alloy is unstable against the 1-ML Fe on W(110). While the control of oxygen element is known to be an important key to a perfect growth of Fe on W(110), its possible contamination is checked. Performing spin polarized calculations with the optimized geometry, the induced magnetic moments on W subsurface are obtained: the W atoms are always antiferromagnetically coupled to the Fe atoms, one exception being the case of the antiferromagnetic Fe surface where, due to frustration, the induced polarization on the W atoms is zero. The bridge site is the lower adsorption energy one for O₂ molecular bonding perpendicular to the surface. In the case of O₂ bonding parallel and oblique to the surface, it is always dissociated into two O atoms on Fe/W(110) surface through geometry optimization, for all considered sites.

The defect structure of four pure tungsten samples is investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques before and after annealing. The results of the SEM and XRD analyses show that the grain size is increased up to 43.3$\mu$m, and their growth in the (2 0 0) orientation occurs at 1673K. Also, the smooth of the peaks at 1673K in the PL test demonstrates the increase of the size of defects of the sample. Moreover, the results of the PAS indicate a decrease in the concentration of defects and an increase in their size at 1673K, which confirms the results obtained from the PL measurement. The collected results from the four examination techniques are in close agreement with each other and reveal that w ith the increase in the annealing temperature of tungsten samples, a gradual coalescence of the defects would happen to lead to an increase in their size and hence a reduction in their number as well as preferred grain growth in the (2 0 0) orientation and virtually perfect recrystallization of the samples at higher temperatures.

The interaction of accelerated helium atoms with the atomic-smooth surface of tun+gsten single crystals was investigated using the low-temperature field-ion microscope, equipped with the source of helium atoms with an energy of 5 keV together with the techniques of molecular dynamics. It was observed the effect of the surface channeling of fast atoms of a target along the surface steps which occurs as a result of the collision cascade induced by the incident projectile. A substantial part of these displaced atoms in the target gains momentum oriented along the < 111 > close-packed crystallographic directions. The fine morphology of the trajectory of an excited tungsten atom reveals the transverse oscillations of the W atom normally to the < 111 > surface atomic step. The rate of the kinetic energy loss during the surface channeling of fast tungsten atoms does not exceed 0.4 eV/Å. This provides relatively large displaced surface atom ranges along the close-packed atomic steps. The found out results can be regarded as a special case of the correlation mechanism of the radiation-induced surface mass transfer.

Fine-scale field evaporation of stepped (001) and (112) surfaces of tungsten nanotips was studied by field ion microscopy. It was shown that some atoms at kinks and steps are anomalously stable against field evaporation. This effect is responsible for the observed alternate field evaporation near the kink and step sites. The phenomenon of alternate field evaporation could be used to determine an atomic relaxation at kinks on nanotip surface. Using the geometric method of analysis of field ion images, the normal to surface differential displacements of the kink-site atoms were estimated.

Tungsten incorporated diamond like carbon nanocomposite films were deposited onto Si substrate by using target biased ion beam assisted deposition. The effect of W target bias voltage on the chemical bonding, structure, surface morphology and mechanical properties of DLC films were investigated by means of XPS, Raman spectroscopy, AFM and Nanoindentation. It was found that the content of W in the films increased from 6 at.% to 13.7 at.% due to the increase in target bias voltage from -300 V to -700 V. XPS analysis revealed that most of the tungsten starts to react with carbon to form WC nanoparticles. Raman analysis shows that with the increase of W fraction in the DLC matrix, the intensity ratio I_D/I_G increases and the G band shifts to higher wavenumber. Thus it proves that the incorporation of tungsten leads to increase in sp² hybridized carbon content, and hence decrease in the hardness of W-DLC films compared to that of the pure DLC films. The result of AFM indicates that the surface roughness of the DLC gets modified with the incorporation of tungsten.

With the emergence of e-commerce and social networking systems, the use of recommendation systems gained popularity to predict the user ratings of an item. Since the large volume of data is generated from various sources at high speed, predicting the ratings accurately in real-time adds enormous benefit to the users while choosing the correct item. So a recommendation system must be capable enough to predict the rating accurately when the data are large. Apache Spark is a distributed framework well suited for processing large datasets and real-time data streams. In this paper, we propose an efficient matrix factorisation algorithm based on Spark MLlib alternating least squares (ALS) for collaborative filtering. The optimisations used for the proposed algorithm using Tungsten improved the performance of the algorithm significantly while doing the predictions. The experimental results prove that the proposed work is significantly faster for top-N recommendations and rating predictions compared with the existing works.

The first example of a tungsten(V) corrole complex, (Mes₂(p-OMePh)corrole)WCl₂, has been prepared through a metathesis reaction of a lithium corrole (Mes₂(p-OMePh)corrole)Li₃ ⋅ 6THF and WCl₆. The product constitutes the first example of a tungsten(V) corrole complex synthesized under mild conditions and only the second example of a tungsten corrole complex.

Interaction of tungsten surface with plasma beam has tremendous importance for development of materials for fusion applications. Trapping and release of Helium (He) and Deuterium (D) in tungsten exposed to high flux plasma with varying He and D content are studied here. Recrystallized and plastically deformed tungsten samples were used to clarify the impact of the material microstructure on trapping and release of plasma components. Thermal desorption spectroscopy measurements were performed to reveal the release stages and quantify the integral desorption of both types of molecules. Comparison of reference and plastically deformed samples was used to clarify the impact of plastic deformation (expressed in the high dislocation density and dislocation networks) on the trapping and release processes. It has been demonstrated that in the mixed beam exposures, the integral He release weakly depends on the He/D ratio, but suddenly grows as the fraction of He/D increases from 80/20 to 100/0. The integral release of D is remarkably enhanced by He seeding in both reference and plastically deformed samples. It is concluded that under the mixed beam exposure conditions, the D release is primary controlled by the He seeding and not by the deformation-induced microstructure.

The microstructure and properties of super martensitic stainless steel (SMSS) microalloyed with tungsten and copper were studied by means of optical microscopy, dilatometer, X-ray diffraction, and tensile tests. The results showed that the microstructure of SMSS, after quenching and tempering, was a typical biphase structure with tempered martensite and reversed austenite dispersedly distributed in the martensite matrix. W and Cu were added into the SMSS to reduce the transformation temperature (Ms) and improve the strength and hardness of the matrix by grain refining and solid solution strengthening. Thermocalc calculations confirmed that M₂₃C₆ compound and Laves phase were precipitated during tempering in the investigated steel. Compared with the traditional SMSS, the steel microalloyed with W and Cu performed better mechanical properties.

The ECRH system at ASDEX Upgrade is currently extended from 1.6 MW to 5 MW. The extension so far consists of 2-frequency units, which use single diamond-disk vacuum-windows to transmit power at the natural resonances of these disks (105 & 140 GHz). For the last unit of this extension two additional intermediate non-resonant frequencies are foreseen, requiring new window concepts. For the torus a polarisation-independent double-disk window has been developed. For the gyrotron a grooved diamond disk is actually favoured, for which the grooved surfaces act as anti-reflective coating. Since ASDEX Upgrade operates with completely W-covered plasma facing components, central ECRH is often applied to suppresses W-accumulation in the plasma center. In order to extend the operational range for central ECRH, X3- and O2-heating schemes were developed. Both are characterized by incomplete single-path absorption. For X3 heating, the X2 resonance at the pedestal on the high field side is used as a 'beam-dump', for the O2 scheme a specific reflector tile on the inner heat shield enforces a second path through the plasma center. The geometry for NTM control had to be modified to allow simultaneous central heating. In real-time the ECRH position can be determined either by ray-tracing based on real-time equilibria and density profiles or from ECE for modulated ECRH power. Fast real-time ECE also allows to determine the NTM position. Further major physics applications of the system are summarized.

The time structure of hadronic showers is characterized by a prompt component from relativistic particles and by late components predominantly connected to neutrons in the cascade. The sensitivity to this late component thus depends on the choice of the active medium for hadronic calorimeters. The time structure and the differences originating from different readout technologies in a calorimeter with tungsten absorbers are studied with two dedicated setups using scintillator tiles read out with SiPMs and glass RPCs. In both cases, a radial strip of 15 cells with a size of 3 × 3 cm² each is read out with fast digitizers with deep buffers, providing detailed information on the time structure of the recorded signals over a long sampling window. We will discuss the technical aspects of these systems, and present results on the time structure measurements, which demonstrate sizeable differences in the response of scintillator and gaseous detectors to the neutron components of the hadronic cascade.