Thermal Stability of Tungsten Fiber-reinforced Tungsten Composites as Plasmafacing Components

Tungsten or a tungsten-based material will be used as armor material for plasma-facing components in future fusion reactors. The environment for plasmafacing materials is harsh as they will be bombarded with neutrons and exposed to high heat fluxes. The most extreme conditions in the divertor will also see impact of α particles (He²⁺). The high heat fluxes will impose a high operating temperature on the plasma-facing components which impose the risk of microstructural rearrangements through recovery, recrystallization and grain growth in metallic materials. 

Tungsten and tungsten alloys are sensitive to such microstructural rearrangements: Deformed tungsten behave in a ductile manner at room temperature, recrystallized tungsten behaves brittle at room temperature. The brittleness of recrystallized tungsten makes it susceptible to embrittlement when exposed to high temperatures. 

The development of tungsten fiberreinforced tungsten (W_f/W) composites seeks to remedy the inherent brittle behavior of tungsten. In W_f/W composites, tungsten wires are embedded as fibers in a tungsten matrix. During deformation of Wf/W composites extrinsic toughening mechanisms are activated achieving a pseudoductile behavior. Two main routes are currently investigated for the manufacturing of W_f/W composites: through chemical vapor deposition and powder metallurgy. This study only involves W_f/W composites manufactured by chemical vapor deposition. 

The thermal stability of W_f/W composites is of great importance for a potential use as plasma facing material. In this study the thermal stability of W_f/W model systems containing a single fiber without any interlayer or with a 1 μm or a 3 μm thick yttria interlayer are investigated using electron backscatter diffraction following annealing at 1400 °C and 1450 °C for various durations. 

The restoration processes differ significantly between the two constituents of the composite. In the fibers, recrystallization initially occurs, as the stored energy introduced during manufacturing — wire drawing — provides the driving force. In the matrix, abnormal grain growth dominates. Quantification of grain boundaries show that the fibers are recrystallized after 1 day at 1450 °C, with the fiber and matrix remaining distinct. However, after 1 week at 1450 °C, the microstructures of fiber and matrix are fully interconnected and indistinguishable from each other. The interlayers are completely disintegrated after this annealing treatment. 

Additionally, the mechanical behavior of multifiber W_f/W composites is quantified before and after annealing through 3-point bending tests. The tests show a significant reduction in fracture toughness and work done during the bending tests after 2 days at 1450 °C. Traces of the pseudoductility appears to be preserved after the annealing treatment.