Search

In future fusion reactors, tungsten is a main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is an issue under the extreme fusion environment. To overcome this drawback, tungsten fiber-reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughening principle. In this study Wf/W composites are produced by a Field-Assisted Sintering Technology (FAST) process with different fiber–matrix interfaces. The fracture behavior was studied by 3-point bending tests on notched samples. 4-point bending tests and tensile tests are performed to measure the flexural strength and tensile strength, respectively. Wf/W with a weak interface shows a typical pseudo-ductile fracture behavior, similar to ceramic matrix composites. A strong interface is beneficial to achieve higher flexural strength and tensile strength, but in turn, weakens the pseudo-ductile behavior.

Plastic deformation of tungsten wire is an effective source of toughening tungsten fibre-reinforced tungsten composites (Wf/W) and other tungsten fibre-reinforced composites. To provide a reference for optimization of those composites, unconstrained pure tungsten wire is studied after various heat treatments in terms of microstructure, mechanical behaviour and fracture mode. Recrystallization is already observed at a relatively low temperature of 1273 K due to the large driving force caused by a high dislocation density. Annealing for 30 min at 1900 K also leads to recrystallization, but causes a rather different microstructure. As-fabricated wire and wire recrystallized at 1273 K for 3 h show fine grains with a high aspect ratio and a substantial plastic deformability: a clearly defined tensile strength, high plastic work, similar necking shape, and the characteristic knife-edge-necking of individual grains on the fracture surface. While the wire recrystallized at 1900 K displays large, almost equiaxed grains with low aspect ratios as well as distinct brittle properties. Therefore, it is suggested that a high aspect ratio of the grains is important for the ductile behaviour of tungsten wire and that embrittlement is caused by the loss of the preferable elongated grain structure rather than by recrystallization. In addition, a detailed evaluation of the plastic deformation behaviour during tensile test gives guidance to the design and optimization of tungsten fibre-reinforced composites.

Material issues pose a significant challenge for future fusion reactors like DEMO. When using materials in a fusion environment a highly integrated approach is required. Damage resilience, power exhaust, as well as oxidation resistance during accidental air ingress are driving issues when deciding for new materials. Neutron induced effects, e.g. transmutation adding to embrittlement is crucial to material performance. Here advanced materials, e.g. W f /W or W/Cu, W f /Cu composites allow the step towards a fusion reactor. Recent developments in the area of multi-fibre powder-metallurgical W f /W mark a possible path towards a component based on standard tungsten production technologies. Field assisted sintering technology is used as production route to achieve 94% dense materials. Initial mechanical tests and micro-structural analyses show potential for pseudo-ductile behavior of materials with a reasonable (30%) fibre fraction. In the as-fabricated condition samples showed step-wise cracking while the material is still able to bear rising load, the typical pseudo-ductile behavior of a composite. Yttria is used as the interface material in order to allow the energy dissipation mechanisms to become active. W f /W as plasma facing material contributes here to advanced material strength and crack resilience even with a brittle matrix embrittlement, while W/Cu, W f /Cu composites at the coolant level allow for higher strength at elevated cooling temperatures. In addition to the use of pure tungsten it is demonstrated that tungsten-based self-passivating alloys can also be used in the composite approach.

Advanced engineering materials (2020). doi:10.1002/adem.201901242, Published by Wiley-VCH Verl., Weinheim

Damage-tolerant ceramic matrix composites (CMC) are prone to high temperature applications under severe environmental conditions and usually utilize carbon or ceramic fibres (e.g. SiC) as reinforcements of ceramic matrices with inherent low elongation to break compared to common metals. However, CMC reveal an elongation to break and stiffness similar to the ceramic matrices, and thus need a fibre coating in order to improve the elongation to break length and thus to achieve damage tolerance of the composite. In addition, such fibers often expose a low ductility during failure. As a consequence, design criteria for components of such CMC materials are limited by the low strain of failure. In order to overcome this problem, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibres to reach pseudo ductile behaviour during failure. Tungsten (W) and molybdenum (Mo) fibers were chosen as reinforcement in SiCN CMC manufactured by polymer infiltration and pyrolysis process. These fibres are commercially available since they are widespread used in light bulbs, etc. , and possess an intrinsic higher elongation to break, compared to ceramic fibres, as well as high stiffness even at high temperatures. W/SiCN and Mo/SiCN composites were manufactured via filament winding and resin transfer moulding of commercially available polysilazanes, pyrolysed and re-densified by multiple reinfiltration and pyrolysis steps. These composites were investigated with respect to microstructure, flexural and tensile strength. Single fibre strengths for W and Mo 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. W fibre became brittle across the entire cross section, while the Mo fibre showed a superficial, brittle reaction zone but kept ductile on the inside.

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.

Tungsten is traditionally sintered at very high temperatures. The master sintering curve (MSC) for densification is a functional model that describes sintering under an arbitrary time–temperature regime of a particular material during heating. The MSC for potassium-doped tungsten (W-K) has been determined by fitting experimental relative density data results versus work of sintering data with a modified sigmoid function. Five independent parameters of the fitting function are identified by minimizing error in terms of mean residual square. For measurement of relative density during sintering, we developed a non-contact high temperature dilatometer experiment using an optical method. Densification was continuously recorded at constant heating rates of 5, 10, 20, 30 and 40 °C/min. The work presented here was carried out to predict and control densification evolution of W-K during free sintering. The results demonstrate, that the MSC model of W-K describes densification independently of selected temperature regime.

This paper presents a density dependent modified Drucker–Prager Cap (DPC) model for the compaction behavior of potassium doped tungsten powder. The experimental calibration of the model is done using an instrumented cylindrical die in powder compaction tests. The die is designed for direct determination of loading and unloading forces of powder compact inside the die in compaction and transverse directions. The cap surface parameters are characterized by fitting stress and strain curves acquired during loading to different green density values. The material parameters for the DPC failure surface are obtained by subsequent diametrical and axial compression tests. Additionally the contact pressure dependent wall friction coefficient is determined. The material model is implemented in the ABAQUS finite element code by user subroutines. The calculated compaction forces for a double-action compaction process are in excellent agreement with experimental data. The results show, that the proposed material model for doped tungsten can be used in finite element simulations of the compaction process.

For the next step fusion reactor the use of tungsten is inevitable to suppress erosion and allow operation at elevated temperature and high heat loads. Tungsten fibre-reinforced composites overcome the intrinsic brittleness of tungsten and its susceptibility to operation embrittlement and thus allow its use as a structural as well as an armour material. That this concept works in principle has been shown in recent years. In this contribution we present a development approach towards its use in a future fusion reactor. A multilayer approach is needed addressing all composite constituents and manufacturing steps. A huge potential lies in the optimization of the tungsten wire used as fibre. We discuss this aspect and present studies on potassium doped tungsten wire in detail. This wire, utilized in the illumination industry, could be a replacement for the so far used pure tungsten wire due to its superior high temperature properties. In tensile tests the wire showed high strength and ductility up to an annealing temperature of 2200 K. The results show that the use of doped tungsten wire could increase the allowed fabrication temperature and the overall working temperature of the composite itself.

Under certain conditions alkali vapours form dipole monolayers on metallic electrodes that can lower the work function of the bulk material. In this case, the power balance of the electrode, the electrode fall voltage and the electrode loss power can change considerably. To verify this effect a pyrometric technique was adapted and optimized for the diagnostics of tungsten electrodes in high pressure sodium discharges. Using an already verified model of thermally emitting cathodes the effect was observed in a Na DC discharge and the range of existence was investigated. An interpretation of the results is given using a Langmuir description of forming the Na monolayers and first-principles electronic structure calculations using a pseudopotential plane wave method to solve the Kohn-Sham equations of density-functional theory.

In tungsten fibre-reinforced tungsten composites (Wf/W) the brittleness problem of tungsten is solved by utilizing extrinsic toughening mechanisms. The properties of the composite are very much related to the properties of the drawn tungsten wire used as fibre reinforcements. Its high strength and capability of ductile deformation are ideal properties facilitating toughening of Wf/W. Tensile tests have been used for determining mechanical properties and study the deformation and the fracture behaviour of the wire. Tests of as-fabricated and straightened drawn wires with a diameter between 16 and 150 μm as well as wire electrochemically thinned to a diameter of 5 μm have been performed. Engineering stress–strain curves and a microscopic analysis are presented with the focus on the ultimate strength. All fibres show a comparable stress–strain behaviour comprising necking followed by a ductile fracture. A reduction of the diameter by drawing leads to an increase of strength up to 4500 MPa as a consequence of a grain boundary hardening mechanism. Heat treatment during straightening decreases the strength whereas electrochemical thinning has no significant impact on the mechanical behaviour.

In future fusion reactors, tungsten is the prime candidate material for the plasma facing components. Nevertheless, tungsten is prone to develop cracks due to its intrinsic brittleness—a major concern under the extreme conditions of fusion environment. To overcome this drawback, tungsten fiber reinforced tungsten (Wf/W) composites are being developed. These composite materials rely on an extrinsic toughing principle, similar to those in ceramic matrix composite, using internal energy dissipation mechanisms, such as crack bridging and fiber pull-out, during crack propagation. This can help Wf/W to facilitate a pseudo-ductile behavior and allows an elevated damage resilience compared to pure W. For pseudo-ductility mechanisms to occur, the interface between the fiber and matrix is crucial. Recent developments in the area of powder-metallurgical Wf/W are presented. Two consolidation methods are compared. Field assisted sintering technology and hot isostatic pressing are chosen to manufacture the Wf/W composites. Initial mechanical tests and microstructural analyses are performed on the Wf/W composites with a 30% fiber volume fraction. The samples produced by both processes can give pseudo-ductile behavior at room temperature.

A novel method applying a micro-channel plate as collimator in front of a position sensitive detector (CCD-camera) allows the imaging of large areas (≈1 cm 2 ) of polycrystalline materials using the diffracted radiation. Using this set-up the spatially resolved intensity of selected reflections can be measured with exposure times in the order of seconds. This method has successfully been applied to study lattice spacing and orientation as function of position in the specimen in order to reveal the influence of the surrounding and the impact of processing parameters.

The BaO/W interaction is responsible for the emission properties of barium–oxide coated tungsten electrode coils in fluorescent lamps. The electronic structure of the BaO/W(001) interface is investigated by first-principles calculations within the local-density approximation of the density functional theory using the full-potential linearized augmented plane wave method. Results are presented for the total density of states (DOS), the atom- and orbital-resolved partial DOS and charge density distributions. Partial covalent character in the W–O and W–Ba bonding is shown. The main contribution to chemical bonding is caused by the tungsten d states and the adsorbate valence states, which interestingly also involve barium d states. This leads to a stabilization of the adsorbed configuration, with respect to cathode operation temperatures. The calculated work function is in agreement with experimental data.

A numerical model is developed to study the optical properties of phosphor powder coatings. The method derived uses a combination of ray tracing and Monte Carlo modeling which allows us to calculate the absorption, reflection, and transmission for the exciting ultraviolet radiation as well as for the emitted visible light. Size, shape, morphology, density of packing of the phosphor grains, and the roughness of the coating can be studied in detail with our model. Calculations for Y1.91Eu0.09O3 red phosphor powder layers used in commercial fluorescent lamps are presented. Good agreement with experiment was found.A numerical model is developed to study the optical properties of phosphor powder coatings. The method derived uses a combination of ray tracing and Monte Carlo modeling which allows us to calculate the absorption, reflection, and transmission for the exciting ultraviolet radiation as well as for the emitted visible light. Size, shape, morphology, density of packing of the phosphor grains, and the roughness of the coating can be studied in detail with our model. Calculations for Y1.91Eu0.09O3 red phosphor powder layers used in commercial fluorescent lamps are presented. Good agreement with experiment was found.

This paper describes a three-dimensional lattice model for the study of the recrystallization and grain growth of tungsten wires in comparison to additional experiments. Microstructures similar to experimental ones were obtained. The experimentally obtained grain size data was used to determine the grain growth exponent and the activation energy for grain boundary migration at temperatures ranging from 1500 to 1900 °C. The evolution with time and temperature of the average grain size has been calculated by the Monte-Carlo method. For this, a relation between length and time scale in simulation and experiment, based on intrinsic material parameters, was developed. In general, the results agree very well with the experimental observations.

High strength and creep resistance also at high temperature, combined with a high thermal conductivity and high melting point make tungsten (W) an ideal material for highly loaded areas in future fusion reactors. However, as a typical bcc metal tungsten features an intrinsic brittleness up to very high temperature and is prone to operational embrittlement. Tungsten fibre-reinforced tungsten composite (Wf/W) utilizes extrinsic toughening mechanisms similar to ceramic fibre-reinforced ceramics and therefore overcomes the brittleness problem. The properties of the composite are to a large extend determined by the properties of the drawn tungsten wire used as reinforcement fibres. W wire exhibits a superior strength and shows ductile behaviour with exceptional local plasticity. Beside the typical mechanisms observed for ceramic composites the ductile deformation of the fibres is therefore an additional very effective toughening mechanism. Tension tests were used to investigate this phenomenon in more detail. Results show that there is a region of enhanced localized plastic deformation. The specific energy consumption in this region was estimated and used to suggest optimisation options for Wf/W composites.