Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect

Abstract

Due to its high strength and low temperature ductility, tungsten fibers (Wf) have been widely used as reinforcement elements in metallic, ceramic and glass matrix composites to improve the strength, toughness and creep resistance. Materials designed for future fusion reactors also utilize the option of Wf reinforcement, i.a. with a copper (Wf/Cu) or tungsten (Wf/W) matrix. Wf/W composites are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to embrittlement induced by neutrons resulting from fusion reaction. Operation of Wf/W in high temperatures (up to 1300 °C and even higher) fusion environment implies a risk of recrystallization and grain growth, which dimishes the attractive properties of tungsten fibers. In this work, we assess this modification of micro-mechanical and microstructural properties of tungsten fibers by means of nanoindentation, scanning electron microscopy, electron back-scattering diffraction analysis and corelate it with the ultimate tensile strength and fracture modes observed in the tensile tests. Both pure W and pottasium doped wires in the as-fabricated and annealed states are investigated and the results are compared with bulk tungsten, also exposed to several annealing temperatures. The results highlight the postive impact of potassium doping which shifts the threshold temperature for the grain growth by about 600 °C compared to pure tungsten wire. The results of the nanoindentation revealed systematic linear correlation with the ultimate tensile strength, which therefore offers a complimenatary way of micro-mechanical testing linking it with macro-scale properties of the wires.