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 (W f) 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 W f reinforcement, i.a. with a copper (W f /Cu) or tungsten (W f /W) matrix. W f /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 W f /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 were 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.

EBSD characterization of pure and K-doped tungsten fibers annealed at different temperatures

“…All the maps from both As can be seen in Figure 1, the "as-received" condition of both materials exhibits elongated grains oriented along the drawing axis (vertical direction) and there are no clear differences in the average length and width, measured by the linear intercept method, that are about 6-9 µm and 0.3-0.6 μm, respectively. As previously reported [26], with increasing annealing temperature, the grains tend to become longer and thicker. At 1600°C, these values change to 10-12 and 2-4 μm for the pure W and to 7-9 and 0.8-0.9 μm for the K-doped material.…”

“…Pure W exhibits clearer and faster microstructural modifications when compared to the doped wire. As previously reported, recrystallization and grain growth occur already at 1300°C and the grains keep growing up to 1900°C, but no secondary recrystallization was observed [26]. On the other hand, the K-doped fiber shows minor and homogeneous changes in the microstructure up to 1600°C, which might be attributed to extended recovery as suggested by Engler and Randle [40] and/or recrystallization as suggested by Zhao et al [33].…”

“…Drawn pure (99.9%) and potassium doped (60-75 ppm) W wires, similar to the ones used in [26,27] were provided by OSRAM GmbH, Schwabmünchen, with a diameter of approximately 150 μm [21]. Pure W wires were annealed at 1300, 1600 and 1900°C, whereas K-doped material was annealed at 1300, 1600 and 2100°C.…”

“…Potassium doping has already proved its efficiency on suppressing secondary recrystallization and controlling grain growth up to 1900°C in tungsten thin wires [26][27][28]. Although these materials have been characterized in terms of microstructure, texture and mechanical behavior [26][27][28][29][30][31][32][33][34], a dedicated study on the role of both triple junction and grain boundary character distributions is still missing. The main goal of this paper is to assess potential anisotropy in the material and to compare the evolution of both distributions in pure and K-doped W fibers annealed at different temperatures.…”

“…The findings could help to improve the performance of the fibers under cyclic tensile load [35]. For a better understanding of the link between microstructure and mechanical behavior, the reader is referred to the literature [26][27][28]31].…”