Correlation of microstructural and mechanical properties of K-doped tungsten fibers used as reinforcement of tungsten matrix for high temperature applications

Abstract

Reinforcement of tungsten by tungsten fibers (Wf) is considered an attractive option to mitigate the intrinsic brittleness of this material and to possibly extend the operational temperature window to ensure safe operation of the plasma facing component. By now, it has been demonstrated that tungsten fiber-reinforced tungsten composites (Wf/W)acquire pseudo ductility even at room temperature, and crack propagation is determined by the interaction of the fibers with the propagating crack. In view of strong temperature oscillations, expected during operation in the fusion plasma, the mechanical properties of tungsten fibers annealed at different temperatures (up to 2300°C) were assessed, and the role of potassium (K) doping on the modification of the mechanical properties of asannealed wires was studied. While K-doping was found to delay the brittleness induced by heat exposure at least up to 1600°C, still a strong reduction of the fiber strength was observed in tests performed at elevated temperatures. In this work, we investigate the reasons for this effect by performing scanning electron microscopy coupled with electron backscatter diffraction measurements. The longitudinal and transversal cross-sections of W fibers were analyzed to deduce the morphology and size distribution of the grains. Consistent with the mechanical data, we found that annealing at 2100°C resulted in the full recrystallization of the elongated grains, otherwise formed due to the extrusion fabrication process. Even at 1900°C, the longitudinal cross-section still exhibits elongated grains. The transversal shape of the grains undergoes a change from needle-like fine structure to equiaxed grain shape upon annealing above 1600°C. Few scans done for 2300°C annealed wire revealed that the microstructure contains one or several grains with a dimension of 70-150 µm. The obtained results are discussed and analyzed in the frame of mechanistic model connecting microstructure with the mechanical response.

Tungsten–potassium: a promising plasma-facing material

“…The microstructural and mechanical properties of K-doped W wires were investigated to suppress the degradation of the mechanical properties for the fiber-reinforced composite at high temperatures. K-doping was confirmed to retard the brittleness induced by thermal at specific conditions by Terentyev et al [35]. After the thermal treatment at 2573 K and examination of those wires between 295 and 873 K, it was demonstrated that the local strain of the necking region could reach up to 50%.…”

“…In Europe, WVWM (a K-doped W material by vacuum metallizing) bulks whose diameter was 15 mm and the deformation degree was 1.7 [31] were fabricated by PLANSEE AG. OSRAM GmbH, Schwabmuenchen [35] manufactured K-doped (6 × 10 −3 -7.5 × 10 −3 wt%) W wires by drawing. Allied Material Corp. [24] produced pure W (PW) and WK rods via using similar powder metallurgy and swaging processes.…”

“…7. The inset displays the zoom of the yield point area at which the wires annealed from 2173 to 2373 K [34,35,63]. Their works interpreted the role of K-doping on the improvement of the mechanical properties of W wires and gave an indication for the service temperature range of W f /W composites [35].…”

“…Ti-and Y-doping had a positive effect to some extent on the thermal resistance of WK materials. Transient thermal shock tests were conducted under the absorbed power density of 0.37 GW m −2 thermal pulse (5 ms) for WK-based multicomponent alloy [35,65,66]. Cracks were found in all the samples apart from W-K-2 wt% Mo-Ti-Y, which might be attributed to the increase of ductility [39,64].…”

“…The accompanied stress concentration makes a significant reduction in toughness. Tungsten-potassium or potassium-doped tungsten (WK), which brings in nano-sized K bubbles to pin GBs and dislocations, and simultaneously enhances the toughness and strength, is of great importance in PFMs [12,13,16,[27][28][29][30][31][32][33][34][35]. The main mechanical properties and preparation of WK are reviewed in this paper.…”