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

Advanced tungsten fibre-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key component of the composites, thus their mechanical properties and thermal stability define the allowed operation / fabrication temperature of the composite material itself. In this work, a comprehensive characterization of the pure and potassium doped tungsten wires was performed, focusing on the influence of various heat treatments on different microstructural features (nature of grain boundaries, grain shape and size, texture analyses) and mechanical properties. Annealing in the temperature range from 900-1600°C enables the investigation of the microstructural stability of the two materials and arising annealing phenomenarecovery, recrystallization and grain growth. The results demonstrate that the pure tungsten recrystallizes fully in the temperature range 1300-1500°C accompanied with tremendous coarsening and a complete loss of the initial fibrous, elongated grain structure. In contrast to this, potassium doped wire shows superior high temperature properties, where performed heat treatments cause only milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures well above the investigated ones. Furthermore, hardness measurements and observed softening complement the discussion of the grain morphology evolution.

Advanced tungsten fibre-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key component of the composites, thus their mechanical properties and thermal stability define the allowed operation / fabrication temperature of the composite material itself. In this work, the room temperature fracture behaviour of the pure and potassium doped tungsten wires was investigated, focusing on the evolution of the fracture micromechanisms in respect to annealing. Single-edge-notched specimens were used, with the crack growth direction perpendicular to the drawing axis of the wire. The occurrence of either a brittle or a ductile response in the as-received state of both materials is a strong indication that the ductile-to-brittle transition temperature is about room temperature. Pure, annealed tungsten wire experiences a tremendous deterioration of the fracture toughness with a very prominent transition of the failure mode. The observed embrittlement by annealing can be related to the loss of the fibrous, elongated microstructure. In contrast to this, the results of the annealed, doped wire demonstrate that the microstructural stability and preservation of the initial, beneficial grain structure is directly reflected in the crack resistance of the material. Predominately ductile behaviour, with characteristic knife-edge necking, is seen even after annealing at 1600 °C.