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

Reinforcement of tungsten by tungsten fibers (W-f) 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 (W-f/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 degrees C) were assessed, and the role of potassium (K) doping on the modification of the mechanical properties of as-annealed wires was studied. While K-doping was found to delay the brittleness induced by heat exposure at least up to 1600 degrees 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 degrees C resulted in the full re-crystallization of the elongated grains, otherwise formed due to the extrusion fabrication process. Even at 1900 degrees 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 degrees C. Few scans done for 2300 degrees C annealed wire revealed that the microstructure contains one or several grains with a dimension of 70-150 mu m. The obtained results are discussed and analyzed in the frame of mechanistic model connecting microstructure with the mechanical response.