[en] Highlights: • Tungsten fibre (W_f) based CuCrZr and W compounds are proposed for high heat flux components. • W Fibre preforms have been manufactured with industrially viable textile techniques. • W_f/Cu tubes have been successfully produced in cooperation industrial partner. • Strongly increased fracture toughness of W_f/W confirmed in cyclic load tests. - Abstract: The European Fusion Roadmap foresees water cooled plasma facing components in a first DEMO design in order to provide enough margin for the cooling capacity and to only moderately extrapolate the technology which was developed and tested for ITER. In order to make best use of the water cooling concept copper (Cu) and copper-chromium-zirconium alloy (CuCrZr) are envisaged as heat sink whereas as armour tungsten (W) based materials will be used. Combining both materials in a high heat flux component asks for an increase of their operational range towards higher temperature in case of Cu/CuCrZr and lower temperatures for W. A remedy for both issues- brittleness of W and degrading strength of CuCrZr- could be the use of W fibres (W_f) in W and Cu based composites. Fibre preforms could be manufactured with industrially viable textile techniques. Flat textiles with a combination of 150/70 µm W wires have been chosen for layered deposition of tungsten-fibre reinforced tungsten (W_f/W) samples and tubular multi-layered braidings with W wire thickness of 50 µm were produced as a preform for tungsten-fibre reinforced copper (W_f /Cu) tubes. Cu melt infiltration was performed together with an industrial partner resulting in sample tubes without any blowholes. Property estimation by mean field homogenisation predicts strongly enhanced strength of the W_f/CuCrZr composite compared to its pure CuCrZr counterpart. W_f /W composites show very high toughness and damage tolerance even at room temperature. Cyclic load tests reveal that the extrinsic toughening mechanisms counteracting the crack growth are active and stable. FEM simulations of the W_f/W composite suggest that the influence of fibre debonding, which is an integral part of the toughening mechanisms, and reduced thermal conductivity of the fibre due to the necessary interlayers do not strongly influence the thermal properties of future components.

[en] The EU has funded the Fusion Energy Materials Science project Coordination Action (FEMaSCA) with the intension to utilize the know-how in the materials community to help overcome the material science problems with fusion related materials. In this framework three different material concepts, tungsten-copper-composite (W/Cu), vacuum plasma sprayed tungsten (VPSW), and tungsten-fiber/tungsten-matrix-composite (W_f/W_m) were investigated by means of insitu tomography during mechanical testing. The measuring campaign was conducted at the high energy beamtine ID ISA at the European Synchrotron Radiation Facility (ESRF) in Grenoble. A tensile testing machine was used to perform displacement controlled tension tests. At the end of each well defined displacement step a tomogram was taken. Tomographic reconstructions were successfully produced of samples with high tungsten content and sample diameters up to 1 mm. Force-displacement curves were measured during loading to complete fracture. Crack propagation could he observed in the tomographic reconstructions. This paper describes the first results with special focus on the experimental work and the role of FEMaS-CA. (Author)

[en] 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. (paper)

[en] Due to the unique combination of excellent thermal properties, low sputter yield, hydrogen retention and activation, tungsten is the main candidate for the first wall material in future fusion devices. However, its intrinsic brittleness and its susceptibility to operational embrittlement is a major concern. To overcome this drawback, tungsten fiber reinforced tungsten composites featuring pseudo ductility have been developed. Bulk material can be successfully produced utilizing chemical vapor deposition of tungsten fabrics. However, a fully dense composite with a high fiber volume fraction is still a huge challenge. Therefore, a model is currently developed in Comsol including the complex coupling of transport phenomena and chemical reaction kinetics. To validate the model with experimental data, fibers were deposited in heated tubes under controlled parameter variation. The temperature and tungsten growth rate were measured along the fibers and inner tube surfaces for different heater temperatures, partial pressures and gas flows. With the experimental results the prediction of the model has been improved. As next step the model will be applied to design infiltration experiments to fabricate fully dense Wf/W composites with a high fiber volume fraction.

[en] In this study, the interaction of energetic oxygen ions with the beryllium-tungsten alloy Be₂W is investigated by depth-resolved x-ray photo-electron spectroscopy. A 4 nm tungsten layer is deposited on polycrystalline beryllium. By alloying at 900 K for 60 min, Be₂W is formed. In successive steps, oxygen ions are implanted with 500 and 1000 V acceleration voltage with a fluence of 5×10¹⁴ cm^-2 in each step. Between the single implantation steps the sample is heated for 30 min at 600 K. Depth-resolved photoelectron spectroscopy is performed after each experimental step. The W 4f and Be 1s spectra reveal the formation of beryllium-tungstate BeWO₄ already at room temperature (RT) in significant amounts due to energy deposition of implanted oxygen ions. No indication of oxygen diffusion at RT is observed. In previous experiments in thermal equilibrium, the ternary compound has proven to be stable at temperatures up to 1073 K. Contrary to these results, annealing at 600 K leads to decomposition of the BeWO₄ formed by energetic oxygen ions. After decomposition at 600 K, oxygen stays in the sample bound as BeO.

[en] The operating conditions in DEMO and a fusion power plant will add a number of new aspects to the list of requirements for plasma-facing materials. Other than for current fusion experiments and even ITER, the high neutron dose changes material properties with respect to mechanical properties (embrittlement) and composition (transmutation). Efficient energy production requires high coolant temperatures, calling for material and component solutions with sufficient high-temperature strength. The operation with D-T and the breeding of tritium require solutions against continuous tritium losses into structural materials and cooling media. Finally, a fusion power device must demonstrate passive safety properties in case of off-normal operational events, e.g. loss of coolant. Currently available materials for the first wall and divertor components of fusion devices fail in one or more of these criteria. In addition, essentially no data for first wall materials is available which allows predictions of erosion and hydrogen isotope retention due to plasma exposure after extended neutron exposure. In this presentation several material solutions for the issues mentioned above will be discussed. In recent years, a number of advanced material concepts have been developed in order to address DEMO and power plant requirements. In particular, composite materials, smart tungsten alloys and hydrogen isotope permeation barriers are investigated. The status of these material developments will be summarized and future research needs, in particular also with respect to plasma-material interaction experiments with neutron-irradiated materials, are identified. (author)

[en] Highlights: • Preparation of purpose-built samples of tungsten fibre reinforced tungsten. • Laterally resolved measurements of surface deuterium retention in W fibre reinforced W. • Quantitative comparison of deuterium retention of W fibre reinforced and power metallurgical W. • Identification of D retention mechanisms. In future fusion reactors, plasma-facing materials (PFMs) have to withstand unique conditions such as high temperatures, ion and neutron irradiation. Tungsten (W) has been established as main candidate material due to its favorable properties regarding the fusion environment but brings one major challenge: Its brittleness at moderate temperatures can lead to failure of tungsten components. Tungsten fiber-reinforced tungsten (W_f/W), a tungsten matrix containing drawn tungsten fibers, was developed to mitigate this problem by using extrinsic toughening mechanisms to achieve pseudo-ductility. The deuterium (D) retention in W_f/W manufactured by chemical vapor deposition (CVD) has been investigated using W_f/W single layered model systems consisting of a single plane of unidirectional tungsten fibers embedded in a tungsten matrix produced by CVD. Various parameters with potential influence on the D retention, such as the choice of an erbium oxide interface and potassium doping, have been included in the investigation. The samples have been ground to varying distances between surface and fiber plane - exposing distinct details of the W_f/W microstructures at the surface. The samples were exposed to a low temperature D plasma at 370 K for 72 h resulting in a total fluence of 10²⁵ D/m². The D retention of all samples was measured by nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS). The D retention in W_f/W composites is higher than in reference samples made from hot-rolled W by factors between 2 and 5. In addition, a comparison of NRA and TDS data indicates that D penetrates faster into the depth of W_f/W material than into hot-rolled tungsten.

[en] Full text: Heat, particle and neutron loads are a significant challenge to first wall material lifetime when extrapolating from present devices to DEMO. For DEMO only, early design studies exist and detailed operational requirements are currently being developed. For a future fusion power plant, multiple issues with respect to materials and components need to be evaluated. Tungsten is currently the main candidate material for the first wall of a fusion reactor as it is resilient against erosion, has the highest melting point of any metal and shows rather benign activation behaviour under neutron irradiation, as well as low tritium retention, which is beneficial to sustain the tritium fuel cycle. Despite its beneficial properties and its proposed use in ITER and DEMO as divertor material, W reveals various intrinsic problems when extrapolating to reactor conditions. Among those are the power handling capabilities and lifetime of the first wall and divertor plasma-facing components (PFCs). Crack formation by thermal shock and thermal fatigue, material modification due to neutrons as well as oxidation during accidental exposures are driving issues when deciding for new materials. Material solutions coping with neutron-induced effects such as transmutation, embrittlement and afterheat from are essential. The development of advanced materials is essential for fusion, especially as for the next step devices, requirements on power exhaust, availability and lifetime are far more stringent, e.g., due to neutron irradiation. In particular, the PFCs can benefit from new approaches such as composites or new alloys. Here Wf/W composites as well as strengthened CuCrZr materials together with oxidation resilient W-alloys are promising solutions towards the realization of fusion reactor compatible materials. As part of the European strategy towards a first DEMO reactor, the development of advanced materials is addressed in several work packages, dealing with the divertor, the plasma-wall interface and the material development itself. When using materials in a fusion reactor environment a highly integrated approach is required. This contribution highlights the progress for theses advanced materials and composites. Focus is put on the materials roadmap of the EUROfusion consortium as a development line to enable construction and operation of a fusion reactor. (author)

[en] 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. (paper)

[en] Full text: The development of advanced materials is essential for sophisticated energy systems like a future fusion reactor, where multiple issues with respect to materials and components need to be evaluated. Brittle behaviour is the limiting factors when operating any W based plasma facing components (PFCs) in a tokamak. This is particularly crucial when considering material degradation from neutron-induced transmutation and embrittlement. Here tungsten fibre-reinforced tungsten composites (Wf/W) can mitigate these issues by utilizing extrinsic toughening mechanisms and therefore overcoming both intrinsic and neutron embrittlement. Extrinsic toughening for Wf/W is achieved similar to other composites by incorporating an interface between fibre and matrix allowing for additional energy dissipation without relying on intrinsic material properties such as ductility. The use of Wf/W could broaden the temperature window of W significantly and mitigate problems of cracking occurring typically in cyclic high heat flux loading. Wf/W as a material has been successfully produced and tested during the last years and the focus is now put on the technological realization for the use in PFCs as well as the further enhancement of production methods. Here we present a way to utilize Wf/W composites for divertor applications by a fabrication route based on the both chemical vapour deposition or infiltration (CVD, CVI) of W and powder metallurgy (PM). Mock-ups based on the ITER typical design can be realized by the implementation of Wf/W-flat tiles or mono-block like approaches. In both cases, varying geometries for the introduction of fibres can be envisioned. For the CVD route, the furthest developed method is a concept based on a layered deposition approach allowing the production of flat tiles in the required geometry. One fibre layer after the other is positioned and ingrown into the W-matrix until the final sample size is reached. For the PM route development is progressing towards the use of pressure assisted sintering methods like field assisted sintering (FAST) and hot isostatic pressing (HIP). For multifibre PM-Wf/W special care needs to be taken developing a method for incorporating fibre and powder, while for the CVD Wf/W already large multifibre composites can be achieved. PM-Wf/W has only recently been shown to be able to achieve multifibre Wf/W. (author)