No abstract

Recent efforts dedicated to the mitigation of tungsten (W) brittleness have demonstrated that tungsten fiber-reinforced composites acquire extrinsic toughening even at room temperature, which is due to the outstanding strength of W wires. However, high temperature operation/fabrication of the fiber-reinforced composite might result in the degradation of the mechanical properties of W wires. To address this, we investigate mechanical and microstructural properties of potassium-doped tungsten wires, being heat treated at 2300°C and tested in temperature range 22-600°C. Based on the microscopic analysis, the engineering deformation curves are converted into actual stress-strain dataset, accounting for the local necking. The analysis demonstrates that local strain in the necking region can reach up to 50% and the total elongation monotonically increases with temperature, while the ultimate tensile strength goes down. Preliminary transmission electron microscopy analysis using FIB-cut lamella from the necking region revealed the presence of curved dislocation lines in the sample tested at 300C, proving that plastic deformation occurred by dislocation glide.

Recent efforts dedicated to the assessment of mechanical properties of tungsten wires, as means for fiber-reinforced composites, have shown that potassium (K) doping in the as-drawn state does not modify the mechanical properties of the wire. High temperature annealing (Ta up to 2300°C) leads to the severe embrittlement of the wire associated with the loss of fracture strength. In this work, we assess the transition behavior of pure and K-doped W wires exposed to the annealing in the temperature range of 1000-2300°C to identify and recommend temperatures suitable for operation and fabrication of the fiber-reinforced composites. The results of mechanical tests performed in the temperature range of RT-500°C are reported and substantiated by the electron microscopy analysis. Room temperature tests demonstrate that pure W wires become fully brittle after annealing above 1300°C, whereas K-doped wires loses ductility above 2100°C. With raising the test temperature to 300-500°C, it is found that the strength of pure W wire reduces by a factor of two at Ta=1000°C (as compared to non-annealed wire), and goes down to 100 MPa at Ta=1900°C. The K-doping suppresses the reduction of the fracture strength at least up to Ta=1900°C, thus offering a temperature gap of ~600°C for the use as reinforcement.

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.

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.