Restoration in drawn tungsten wires of tungsten fiber-reinforced tungsten composites

Abstract

Fusion energy holds great promise as a sustainable solution to meet global energy demands, offering a quasi-inexhaustible, secure and environmentally friendly energy resource. Materials facing the burning plasma in fusion reactors must withstand extreme conditions. Tungsten, the current choice for plasma-facing materials, is at risk of embrittlement if exposed to high temperatures due to restoration processes increasing its ductile-to-brittle transition temperature. This embrittlement limits operation of tungsten under fusion-relevant conditions. Tungsten fiber-reinforced tungsten (W${}_f$/W) composites are developed to mitigate brittleness and achieve pseudo-ductile behavior, utilizing drawn, potassium-doped tungsten fibers embedded in a pure tungsten matrix to enhance toughness compared to pure tungsten significantly. In view of the high heat fluxes and the expected high steady-state operation temperatures in a fusion reactor, thermal stability of the plasma-facing material becomes crucial. Model W${}_f$/W systems containing a single tungsten fiber in a dense tungsten matrix, with or without an yttria interlayer, are annealed at 1450 °C for up to 2 days to evaluate their thermal stability. As the tungsten fibers are primarily responsible for the pseudo-ductile behavior, this investigation focuses on analyzing the thermal stability of the fibers and their immediate vicinity in the surrounding matrix. Changes within the tungsten fibers, including alterations of boundary spacing and crystallographic texture, are analyzed using electron backscatter diffraction (EBSD) and further post-processing of the orientation data. Quantification shows a substantial increase in boundary spacing in the fibers after annealing. This indicates recrystallization, where many boundaries with disorientation angles up to 50°are removed. The crystallographic texture in the fibers changes only slightly during annealing, if at all.