The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load ...capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme’s main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments.
Plasma-facing materials and components in a fusion reactor are the interface between the plasma and the material part. The operational conditions in this environment are probably the most challenging ...parameters for any material: high power loads and large particle and neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak or stellarator reactor, given the proven geometries and technological solutions, requires an improvement of the thermo-mechanical capabilities of currently available materials. In its first part this article describes the requirements and needs for new, advanced materials for the plasma-facing components. Starting points are capabilities and limitations of tungsten-based alloys and structurally stabilized materials. Furthermore, material requirements from the fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, defining directions for the material development. Finally, safety requirements for a fusion reactor with its specific accident scenarios and their potential environmental impact lead to the definition of inherently passive materials, avoiding release of radioactive material through intrinsic material properties. The second part of this article demonstrates current material development lines answering the fusion-specific requirements for high heat flux materials. New composite materials, in particular fiber-reinforced and laminated structures, as well as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical operation space towards regions of extreme steady-state and transient loads. Self-passivating tungsten alloys, demonstrating favorable tungsten-like plasma-wall interaction behavior under normal operation conditions, are an intrinsic solution to otherwise catastrophic consequences of loss-of-coolant and air ingress events in a fusion reactor. Permeation barrier layers avoid the escape of tritium into structural and cooling materials, thereby minimizing the release of tritium under normal operation conditions. Finally, solutions for the unique bonding requirements of dissimilar material used in a fusion reactor are demonstrated by describing the current status and prospects of functionally graded materials.
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 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.
•EBSD characterization is performed on pure and K-doped W-fibers.•Strong //DA fiber texture is present regardless the annealing temperature.•The most abundant low-Σ CSL boundaries are identified.•Good agreement with the empirical law of the inverse cubic root of Σ-value was found.•Anisotropy of triple junctions distribution is observed.
Tungsten materials are candidates for plasma-facing components for the International Thermonuclear Experimental Reactor and the DEMOnstration power plant because of their superior thermophysical ...properties. Because these materials are not common structural materials like steels, knowledge and strategies to improve the properties are still under development. These strategies discussed here, include new alloying approaches and microstructural stabilization by oxide dispersion strengthened as well as TiC stabilized tungsten based materials. The fracture behavior is improved by using tungsten laminated and tungsten wire reinforced materials. Material development is accompanied by neutron irradiation campaigns. Self-passivation, which is essential in case of loss-of-coolant accidents for plasma facing materials, can be achieved by certain amounts of chromium and titanium. Furthermore, modeling and computer simulation on the influence of alloying elements and heat loading and helium bombardment will be presented.
Tungsten-fibre-reinforced tungsten composites (Wf/W) are supposed to enable enhanced toughness owing to extrinsic energy dissipation mechanisms such as interface debonding and plastic deformation of ...fibre. In particular, the latter is an effective source of toughening, since ductile tungsten fibres can absorb a considerable amount of plastic work. For a precise evaluation of the toughening capability, the energy dissipation mechanisms need to be analysed in detail. To this end, single-fibre tungsten composite specimens are fabricated and the stress–strain behaviour of the tungsten fibre bridging a matrix crack is measured by means of in situ high-energy synchrotron microtomography during a uniaxial tensile test. Despite the high X-ray attenuation in tungsten, a sufficiently high resolution is achieved and clear images of crack extension and deformation are obtained. The amount of absorbed energy due to plastic deformation of the tungsten fibre is determined and compared with values obtained conventionally from single-fibre tensile tests.
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.
Abstract
High heat fluxes in future fusion reactors pose big challenges on the materials of plasma-facing components due to restoration processes occurring at high temperatures. Tungsten is ...considered most suitable as plasma-facing material. To overcome its inherent brittleness at low temperatures, tungsten fiber-reinforced tungsten composites are developed which contain ductile, potassium-doped, drawn tungsten wires in an undeformed tungsten matrix. Such composites show pseudo-ductile behavior, an improved toughness and a more controlled fracture compared to undeformed tungsten. Model systems containing a single fiber either without any interlayer or with an yttria interlayer between fiber and matrix are annealed and characterized by electron backscatter diffraction (EBSD) in order to investigate their thermal stability. The restoration process in wire and matrix differ from each other: Recrystallization followed by grain growth occurs in the deformation structure of the wire. Grain growth is the sole mechanism affecting the undeformed matrix. An yttria interlayer between fiber and matrix is supposed to separate the differently restoring microstructures from each other and thereby preserve the improved mechanical properties of the composite. The investigation focuses on characterizing the as-processed condition and the microstructural changes after annealing at 1450 °C for either four days or two weeks. After two weeks of annealing, grains in the region or the vicinity of the wire have coarsened so much that former fiber and matrix cannot be distinguished any longer; not even in a model composite with a 1 μm thick yttria interlayer.
•Stable crack propagation of tungsten reinforced with tungsten fibers.•Fracture surface observations.•Using fiber reinforcement improves the fracture toughness of tungsten.•The fracture toughness of ...tungsten fibre-reinforced tungsten composites was investigated with different methods.•Discussion of the applicability of the used methods for tungsten fibre-reinforced tungsten composites.
Tungsten fibre-reinforced tungsten composites (Wf/W) have been developed to overcome the inherent brittleness of tungsten, which is a promising candidate for the plasma-facing material in a future fusion power plant. As the development of Wf/W evolves, the fracture toughness of the composite is in the focus of interest for further component design. In this contribution fracture mechanical tests on two different types of chemical vapour deposited (CVD) Wf/W are presented. Three-point bending tests according to ASTM E399 as a standard method for brittle materials were used to get a first estimation of the toughness. A provisional fracture toughness value of up to 241 MPa m1/2 was calculated for the as-fabricated and of up to 20.5 MPa m1/2 for a heat-treated and thus embrittled state. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied for this state. A maximum value of the J-integral of 7.5 kJ/m2 (57.6 MPa m1/2) was determined. A detailed post mortem investigations was used to obtain the active mechanisms.
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