Tungsten Fiber Research Articles

Insight into the Microstructure and Tensile Behavior of the W–Cu Composite Reinforced with Tungsten Fibers and Particulates

Herein, the microstructure and tensile property of the tungsten fiber (Wf)‐reinforced W–Cu composite are investigated by both experimental and simulation methods. An electron backscattering diffraction (EBSD) analysis reveals the elongated <110> textured grains in the Wf compared with the matrix part, ensuring the small quantities of the grain boundaries along the direction perpendicular to the tensile direction. Tensile tests further confirm its improved strength by ≈8.0% (526.8 ± 5.3 MPa) compared with the composite without inserted Wf, with a satisfactory tensile strain of 5.9 ± 0.3%. Based on the fractography observation and finite‐element analysis, typically stable energy dissipation behavior of the composite is considered to be ascribed to the interactive competition between complex fracture modes, including frictional fiber pullout, elastic bridging, interface debonding, and crack deflection.

EBSD characterization of pure and K-doped tungsten fibers annealed at different temperatures

Electron Backscatter Diffraction was used to investigate the grain boundary character and triple junction distributions as well as the microtexture on drawn pure and potassium doped (60–75 ppm) tungsten wires. With an approximate diameter of 150 μm, pure W wires were annealed at 1300, 1600 and 1900 °C, whereas K-doped material was annealed at 1300, 1600 and 2100 °C. The annealing was performed under hydrogen atmosphere for 30 min. Both longitudinal and transversal sections were analyzed to assess anisotropic features. Up to 1600 °C, all conditions presented a strong <110> fiber texture parallel to the drawing axis. With increasing annealing temperature, the pure W material developed a more heterogeneous fiber texture while for the K-doped material, it remained homogeneous. Orientation correlation function (OCF) analysis suggested sub-grain coarsening as the recrystallization mechanism while grain boundary density and grain boundary character distribution exhibited anisotropic behavior, as well as the triple junction distribution network. On the other hand, the coincidence site lattices (CSL) distribution did not present any anisotropy and followed the empirical law of the inverse cubic root of Σ-value. For all conditions, the most abundant CSL boundaries were Σ3, Σ9, Σ11, Σ17b, Σ19a, Σ27a and Σ33a. Based on the statistics of the triple junction types and their resistance to intergranular cracking, it was revealed that increasing the annealing temperature might play a role in crack deflection since the resistance to intergranular crack growth is increased in the transversal section and reduced in the longitudinal section. This anisotropic behavior is preserved up to a higher annealing temperature in the K-doped material.

The effect of various factors on the strength and ignition of the used Ni-Al system

The aim of this work was to obtain structural reactive materials from high-energy mixtures. The effect of various additives on the parameters of the burning rate and the ignition temperature was studied. The object of the study was a powder mixture based on nickel and aluminum in a stoichiometric ratio with a particle size of less than 50 microns. Boron and tungsten fibers were used as reinforcing and energy elements. Mixtures of powders were compacted to a relative density of 0.7 and 0.8. Specimens were made in the form of a parallelepiped. The strength of the samples was studied using three-point bending. It was possible to increase the initial strength of the samples by 9 times when reinforced with tungsten fibers.

The use of tungsten yarns in the production for Wf/W

Material issues pose a significant challenge for the design of future fusion reactors. Recently progress has been made towards fully dense multi short-fibre powder metallurgical production of tungsten-fibre reinforced tungsten (Wf/W) as well as optimizing the process understanding for the routes using chemical vapour deposition (CVD). For CVD-Wf/W weaves and textile preforms are being used to facilitate large scale production. Classically 150 μm tungsten fibres supplied by OSRAM GmbH have been used. In order to facilitate the better use of textile processes less stiff 16 μm filaments are being evaluated. The strength of the 16 μm filament is at 4500 MPa and thus significantly higher than the strength of the 150 μm fibre (∼2500 MPa) (in the as-fabricated state). Better weavability allows a more flexible use of fibre preforms Two main yarn production routes have been investigated: covered yarns where a set of tungsten filaments is held together by a PVA (Polyvinyl alcohol) cover and braided yarns. In order to allow a comparison to the previously used single fibres, yarns with ∼140 μm effective diameter were produced. Braided yarns with tensile strength of 2500 MPa and 6% strain at fracture and twisted yarns with tensile strength of 4500 MPa and 3% strain at fracture. For both yarns single fibre CVD samples have been produced to investigate the infiltration properties of the yarns and thus their applicability for the CVD route. A dense infiltration is observed for all yarns under investigation.

Micro-structuring of tungsten for mitigation of ELM-like fatigue

Fusions reactors have to handle numerous specifications before being able to show viable commercial operation, one of which is to find a proper Plasma Facing Material (PFM) which can withstand the high heat loads of several tens of megawatts per square meters combined with the pulse operation of a tokamak and many other problematics (Brezinsek et al 2017 Nucl. Fusion 57 116041). Nowadays, only tungsten is considered as a PFM for high heat flux areas of a tokamak divertor. Tungsten has been selected due to its favorable physical properties, but tungsten has a major drawback: it is brittle under temperatures typically used for water-cooled plasma-facing components (PFC). Under these temperatures the damage threshold due to thermal fatigue induced by ELM is very low, which will dramatically reduce the life-time of the tungsten PFC. The ANSYS simulations and experiments with a millisecond pulsed laser demonstrate a strongly improved ability to withstand thermal fatigue by micro-structuring of the tungsten surface with the help of 150–240 μm diameter tungsten fibres.

Infiltrated tungsten-copper composite reinforced with short tungsten fibers

To achieve an enhancement of mechanical properties, the WCu composite reinforced with short tungsten fibers was advocated. Characteristics including phase constituents, microstructure and mechanical properties (tensile, compressive and frictional behaviors) of the Wf reinforced WCu composite have been systematically investigated. The magnetron sputtered Ti layer on the surface of Wf effectively improved the sinterability of the composite, leading to excellent electrical conductivity of 56.55% IACS and hardness of 182 HB. Cross sectional EBSD results clearly reveals the sharp <110> fiber texture according to the {110} pole figure of the Wf part, indicating a reserved texture structure although after high-temperature sintering and infiltration, which ensures the fiber strengthening effect. Further tests confirmed the enhancement of mechanical properties including high tensile strength of 549.0 MPa and compressive strength of 1047.8 MPa with large plasticity, as well as low friction coefficient of 0.494. A three dimensional model via numerical simulation further revealed that the smaller the angle between the axial direction of the inserted fibers and the loading direction, the easier it was to bear a higher stress, and the middle part of the fiber was more vulnerable to the accumulated damage.

Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Ceramic matrix composites usually utilize carbon or ceramic fibers as reinforcements. However, such fibers often expose a low ductility during failure. In this work, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibers to reach pseudo ductile behavior during failure. Tungsten and molybdenum fibers were chosen as reinforcement in SiCN ceramic matrix composites manufactured by polymer infiltration and pyrolysis process. The composites were investigated with respect to microstructure, flexural- and tensile strength. The single fiber strengths for both tungsten and molybdenum were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. The W fiber became brittle across the entire cross section, while the Mo fiber showed a superficial, brittle reaction zone but kept ductile on the inside.

Performance of tungsten fibers for Wf/W composites under cyclic tensile load

Toughness improved tungsten-based composites are one of the currently considered material option for future fusion reactors capable to withstand both high heat flux and irradiation induced embrittlement. Today, fiber-reinforced composites (Wf/W) are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to neutron irradiation embrittlement especially below 800 °C. Operation of a material as an element of a plasma facing component (i.e. divertor monoblock or first wall armour) implies not only high heat flux exposure but also thermal cyclic fatigue caused by repetitive oscillations of the heat loads due to the nature of the plasma and the limitations on the capacity of its confinement. In this work, we assessed the performance of potassium doped tungsten fibers under cyclic loading applied in tensile mode. Stress-controlled fatigue tests were performed at room temperature, 300 °C and 500 °C increasing the load from 50% of the yield strength up to the ultimate tensile strength of the studied fibers. It is revealed that significant cyclic hardening emerges as the fatigue stress limit exceeds the yield strength already within a few cycles. Despite the noticed cyclic hardening, the wire can sustain few hundreds of cycles without any detectable damage unless the cycle stress is increased to reach the value above the mean ultimate tensile strength. Given this observation, we have studied the impact of the cyclic stress (σC) on the rupture strength and total elongation of the wires exposed to twenty loading cycles varying test temperature in the range 23–500 °C. At room temperature, the rupture stress after cyclic deformation progressively increases with σC and saturates at 2.7 GPa with a moderate reduction of the total elongation, while the nominal ultimate tensile strength of the wire is 2.5 GPa. Thus, the strength of the wire is increased by 200 MPa, on average. At elevated temperature, the rupture stress after the cyclic deformation increases by more than 300 MPa.

Memristive Behavior in Core-Shell Nanowire Networks for Neuromorphic Architectures

Continuous enhancement of performance and processing powers of computing devices will soon reach a technological and physical limit and to overcome this, systems emulating the human brain are being developed. Presently, mathematical models known as artificial neural networks (ANNs) are designed to simulate the computational abilities of the brain. Due to large simulation times, the current digital ANNs cannot be efficiently scaled. By contrast, analog (as opposed to digital) neuromorphic devices with dedicated and adaptable synapses are expected to rival the scale and efficiency of the brain. Memristors are two-terminal electrical component where the resistance is a function of the amount and direction of current and therefore they can uniquely emulate biological synapse behavior. Here, we discuss memristive behavior of Ti/HFO2/Pt core shell nanowire systems and networks for neuromorphic architectures and discuss their potential application as ANNs. In prior studies, memristive architectures for neuromorphic computing are connected through a crossbar array of neurons. With a limit on the number of neurons and regular connectivity, the networks lack sparsity and randomness. As a result, they have high wiring cost and poor functional connectivity. To address such limitations, we have fabricated a random array of core-shell memristive wires to form the connectivity matrix for neuromorphic hardware. In the systems listed above, the conductive core (Pt or Ti) serves as the bottom electrode with a memristive shell (HfO2) that can be electroformed with a set of top electrodes (Ti or Pt) deposited on the surface. Prior to miniaturization, as proof of concept, the core-shell wire networks were fabricated using 20 μm tungsten fibers as conductive cores with tungsten oxide as memristive shells. After imprinting of top silver electrodes and electroforming, IV measurements revealed memristive behavior with switching between resistive states (i.e., LRS-HRS) at ±1 V. Subsequently, nanowire networks based on the Ti/HFO2/Pt system were fabricated using electron beam lithography (EBL). For example, in one embodiment, over 90 overlapping core (40 nm)-shell (5 nm) nanowires were written on a 200 μm × 200 μm write field. To complete the connectivity matrix, 64 nodes and individual vias connecting the resulting network to electrode pads were written and deposited. Network quantifying simulations revealed a small-world coefficient of 2.89, shortest path length of 3.61 and clustering coefficient of 0.057. After electroforming, the core-shell structures exhibited switching between LRS and HRS at ±7 V. In this presentation, the fabrication process and the memristive characteristics (endurance, resistance retention, pulse measurements, etc.) of the fabricated nanowire network structures will be discussed in detail.

Evolution of microstructure, texture and grain boundary character distribution of potassium doped tungsten fibers annealed at variable temperatures

The effect of the annealing temperature on the microstructure and grain boundary character distribution of potassium doped tungsten fibers made of drawn wire was investigated by Electron Backscatter Diffraction. Samples, with a diameter of 148.7 μm, in the as-received condition and annealed at 1300, 1600, 1900, 2100 and 2300 °C were analyzed at the center of the transversal sections. Up to 1900 °C, a uniform microstructural coarsening and primary recrystallization followed by normal grain growth was observed. Between 1900 and 2100 °C abnormal grain growth took place. The strong texture (<110> parallel to the drawing axis) remained present in all conditions. With increasing the annealing temperature, the low angle grain boundary fraction increased at the expense of high angle grain boundaries while the amount of coincidence site lattice boundaries reached its maximum at 1600 °C. At this temperature, the most resistant configuration of triple junctions against intergranular crack propagation was obtained.

Modeling and validation of chemical vapor deposition of tungsten for tungsten fiber reinforced tungsten composites

Tungsten is the most promising first wall material for nuclear fusion reactors. One disadvantage, however, is its intrinsic brittleness. Therefore, tungsten fiber reinforced tungsten (Wf/W) is developed for extrinsic toughening. Wf/W can be produced by chemical vapor deposition (CVD), e.g. by reducing WF6 with H2 using heated W-fibers as substrate. However, it still needs to be optimized regarding relative density and fiber volume fraction. The decisive factor is the tungsten deposition rate, which depends on the temperature and the partial pressures. For this dependence, however, there are controversial results in the literature. In this article, a new rate equation is presented, in which different literature equations are partially adapted and combined. It adjusts the WF6 reaction order between one and zero, depending on the temperature and the H2 and WF6 partial pressure. For validation, a simplified experimental setup with a single fiber was designed, which provides very well defined boundary conditions while varying the CVD process parameters heating temperature, pressure, gas flow rate and gas inlet composition. The experimental runs were simulated with COMSOL Multiphysics. The model was successfully validated by measurements of the WF6 consumption rates (< 2 to 100 %), deposited tungsten masses and spatially high-resolved tungsten deposition rates.

Fracture behavior of random distributed short tungsten fiber-reinforced tungsten composites

In future fusion reactors, tungsten is considered as the main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is of great concern during operation. To overcome this drawback, tungsten fiber-reinforced tungsten composites (Wf/W) are being developed relying on extrinsic toughening principles. Tungsten (W) fibers with extremely high tensile strength and ductility are used to reinforce a tungsten matrix.In this work, field assisted sintering technology (FAST) is used to produce Wf/W material. Mechanical characterizations including Charpy impact and 3-point bending tests are performed. Based on the 3-point bending test results, the Wf/W materials can facilitate a promising pseudo-ductile behavior even at room temperature, similar to fiber reinforced ceramic composites. Fracture energy density and fracture toughness together with the crack-resistance curves (R-curves) are measured. Compared to conventional pure tungsten, Wf/W shows significant improvement in fracture toughness.

Synthesis and property of short tungsten fibre/Zr-based metallic glass composite

ABSTRACTThe short tungsten fibre reinforced Zr41.2Ti13.8Ni10.0Cu12.5Be22.5 bulk metallic glass composites with macroscopic isotropic mechanical properties were prepared by infiltration and rapid solidification. The diameters of the tungsten fibres are 300, 500 and 700 µm with a length of 1000 µm and the fibre volume fraction in three kinds of composites is between 60 and 65%. The room temperature compressive deformation behaviours of these composites were investigated systematically. The results show that the strength and plasticity of the composites increase with the decrease in the tungsten fibre diameter. The maximum compressive strength and plastic strain of the composite with 300 µm fibre, respectively, reach 3079 MPa and 37%. The fracture modes of all the composites are shear fracture. The superior compressive property of the composites with short tungsten fibre is due to the competition among different fracture modes and the inhibition effect of the interface on the shear band extension.

Design, development and recent experiments of the CIMPLE-PSI device

The CIMPLE-PSI laboratory in India’s northeast corner has been engaged over the last decade in the development of advanced experimental systems for controlled plasma fusion-relevant plasma surface interaction (PSI) studies and material testing. The CIMPLE-PSI experimental device was commissioned recently, where a magnetized collimated helium plasma beam is produced under steady-state conditions inside a linear vacuum chamber, which can be made to interact with remotely placed material targets, under controlled experimental conditions. This paper reports on the design, development of the major sub-systems of this device and the performance of the integrated system. The peak helium ion flux and heat flux were measured as 1024 m−2 s−1 and 5.1 MW m−2, respectively; this confirms that extreme ITER divertor-like parameters are successfully reproduced by this simulation device. Steady-state operation of the electromagnet allowed prolonged operation of the plasma leading to a very high fluence of 0.3 × 1028 m−2, which may be enhanced further in the future. Tungsten samples were exposed in this device under helium plasma that had led to the formation of nanometer-sized tungsten fibre foam structures on the target. The technique of grazing incidence small angle x-ray scattering was successfully utilized to measure the average size of the helium bubbles remaining buried in the exposed sample and its variation with depth from the top of the surface.

On the nature of carbon embrittlement of tungsten fibers during powder metallurgical processes

As a candidate material for plasma facing material in future fusion reactor, tungsten (W) fiber reinforced tungsten (Wf/W) composite has been recently developed. The crack resistance of Wf/W is proven to be significantly higher compared to normal tungsten. However, the W-fibers used always become embrittlement during the powder metallurgy (PM) processes. In order to understand this significant issue, in this work, a series of Wf/W composites have been prepared. Microstructural and mechanical studies revealed that microstructural and mechanical studies revealed that the nanosized carbides in the grains and the carbide-layer on the grain boundaries are formed during PM processes. Especially, the carbide-layer on the grain boundaries can cause the brittle fracture of those W-fibers affected. Meanwhile, W-foil protection of the green body during the sintering process can reduce the carbon contamination effect and allows to preserve the ductility of the tungsten fibers used.

Self-sharpening ability enhanced by torque gradient in twisted tungsten-fiber-reinforced Cu-Zn matrix composite

The self-sharpening ability is s of great importance for penetrators when penetrating the targets. In this work, we propose a novel and simple structural design of penetrator material based on tungsten-fiber-reinforced Cu-Zn matrix composite. The clusters of tungsten fibers within the matrix are twisted with different amount in the central and edge parts to create a torque gradient. The dynamic compressive behavior of the central and edge parts of the composite is studied under a high strain rate of 2000 s−1. Their yield strength reaches 2180 MPa and 1820 MPa, with a maximum true strain at fracture being 0.2 and 0.05 respectively. The tungsten fibers from the central part fracture by splitting along the direction of the fibers, while the tungsten fibers in the edge part are more easily fractured by shearing. The penetrator made of the tungsten-fiber-reinforced Cu-Zn matrix composite shows a high penetration capability and good “self-sharpening” capacity, which can be attributed to the torque gradient which induces different mechanical responses between the central and edge parts in the composite.

Evolution of interfacial microstructures between tungsten wires/flakes or graphite flakes and Al2O3/ZrO2 eutectic ceramics via a novel combustion synthesis centrifugal casting

Abstract The Al2O3/ZrO2 eutectic ceramics with adding tungsten wires/flakes and graphite flakes were successfully prepared via a novel Al/Zr(NO3)4 combustion synthesis centrifugal casting under low-pressure, and the interfacial bonding between them and the Al2O3/ZrO2 eutectic was studied via using the characterization methods of Energy Dispersive Spectrometry (EDS), X-ray diffraction (XRD), Hardness and Raman. The results show that there was a thin chemical reaction layer at the interface between the tungsten wires or flakes and the Al2O3/ZrO2 eutectic system, which increased slightly with the increase of the reaction temperature from 3000 K to 3600 K, indicating that the interface was well combined. When the adiabatic temperature was 3600 K, the reaction layer thickness can reach ∼9.2 μm with the composition of WO3/Al2Zr. However, when graphite flakes were added to the combustion system, they can be oxidized by Al2O3 or ZrO2 to form CO or CO2, and the gas generation leaded to poor interface bonding between them and Al2O3/ZrO2 eutectic system. These studies provided some theoretical guidance for the future addition of tungsten fibers and carbon fibers to Al2O3/ZrO2 eutectic ceramics to improve the fracture toughness and strength of the materials. Moreover, it has guiding significance for preparing various fibers or woven fibers reinforced composite materials with large size, high density and excellent mechanical properties under ultra-high temperature.

Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect

Due to its high strength and low temperature ductility, tungsten fibers (Wf) have been widely used as reinforcement elements in metallic, ceramic and glass matrix composites to improve the strength, toughness and creep resistance. Materials designed for future fusion reactors also utilize the option of Wf reinforcement, i.a. with a copper (Wf/Cu) or tungsten (Wf/W) matrix. Wf/W composites are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to embrittlement induced by neutrons resulting from fusion reaction. Operation of Wf/W in high temperatures (up to 1300 °C and even higher) fusion environment implies a risk of recrystallization and grain growth, which dimishes the attractive properties of tungsten fibers. In this work, we assess this modification of micro-mechanical and microstructural properties of tungsten fibers by means of nanoindentation, scanning electron microscopy, electron back-scattering diffraction analysis and corelate it with the ultimate tensile strength and fracture modes observed in the tensile tests. Both pure W and pottasium doped wires in the as-fabricated and annealed states are investigated and the results are compared with bulk tungsten, also exposed to several annealing temperatures. The results highlight the postive impact of potassium doping which shifts the threshold temperature for the grain growth by about 600 °C compared to pure tungsten wire. The results of the nanoindentation revealed systematic linear correlation with the ultimate tensile strength, which therefore offers a complimenatary way of micro-mechanical testing linking it with macro-scale properties of the wires.

Μicro-structured tungsten: an advanced plasma-facing material

Abstract A micro-structuring of the tungsten plasma-facing surface can strongly reduce near surface thermal stresses induced by ELM heat fluxes. This approach has been confirmed by numerical simulations with the help of ANSYS software. For experimental tests, two 10 × 10 mm2 samples of micro-structured tungsten were manufactured. These consisted of 2000 and 5000 vertically packed tungsten fibres with dimensions of O240 µm × 2.4 mm and O150 µm × 2.4 mm, respectively. The 1.2 mm bottom parts of the fibres are embedded in a copper matrix. The top parts of the fibres have gaps about of 10 µm so they are not touching each others. The top of all tungsten fibres was electro-polished. A Nd:YAG laser with a pulse duration 1 ms and a pulse repetition frequency of 25 Hz was used to simulate up to 105 ELM-like heat pulses. No damage on either of the micro-structured tungsten samples were observed. Neon plasma erosion rate and fuel retention of the micro-structured tungsten samples were almost identical to bulk tungsten samples.

Fracture surfaces of tungsten wires used in fiber-reinforced plasma facing components: Effect of potassium doping and high temperature annealing

We have studied the microstructure of tungsten fibers, which are considered as reinforcement elements for the advanced tungsten composites (Wf/W) known to exhibit pseudo ductile behaviour at room temperature. The potentially negative impact of the high temperature annealing, expected under operation in fusion environment, remains to be explored and mitigated. Doping by potassium is considered as a main option to delay recrystallization and grain growth inside the drawn tungsten wires to a higher temperature. Here, we have performed a systematic analysis of the fracture surface of pure and K-doped tungsten wires which were annealed prior to a uniaxial tensile test. The results demonstrate that the fracture mechanism depends strongly on the annealing temperature and presence of potassium doping. Four fracture mechanisms were clearly distinguished and classified by converting into deformation maps as a function of annealing and test temperature. By summarizing all previously available results and current ones, one can demonstrate that potassium doping delays massive grain growth (and subsequent loss of strength) by ∼600 °C as compared to the pure W wire and this positive effect holds for the deformation not only at room temperature but at least up to 500 °C.