Cyclic ring fiber textures in single tungsten fiber-reinforced tungsten composites

Abstract

Tungsten fiber-reinforced tungsten composites are considered as plasma-facing material in future fusion reactors. Such composites are obtained by chemical vapor deposition of tungsten on potassium-doped, drawn tungsten wires. In model composites containing single fibers, particular texture types develop due to the cylindrical geometry of the deposition process. The vapor-deposited tungsten layers form a cyclic <100> ring fiber texture with one of the <100> directions pointing radially along the growth direction. The procedure for revealing this non-standard texture type from orientation data obtained by electron backscatter diffraction is presented. Identification of cyclic textures requires carefully chosen acquisition regions as well as a suitable coordinate system for their analysis. It is demonstrated that quite erroneous conclusions about the texture of the layer would be drawn if this is not accounted for properly.

Recrystallization and grain growth in single tungsten fiber-reinforced tungsten composites

“…All three orientation maps clearly reveal the microstructure of the matrix with large columnar grains having grown radially outwards during CVD, confirming the impression from figure 2.2a. This is similar to what has been observed previously in single fiber composites with and without oxide interlayers [9,10]. Even the same peculiar spatial arrangement of orientations can be recognized from the colors: Horizontally elongated grains appear red when the crystallographic direction along the x-axis is shown, indicating a preference for having a ⟨100⟩ direction along their growth direction.…”

“…Such a cyclic fiber texture, different from a cyclic ring fiber texture, has not been observed before in single fiber model composites; they always showed a cyclic ⟨100⟩ ring fiber texture, even in the absence of any oxide interlayer [9,10]. Nevertheless, the reason for this specific texture occurring in the absence of an interlayer has to be linked to this absence establishing a direct interface between matrix and wire and the texture of the wire itself.…”

“…In order to establish the preferential growth direction unambiguously, an evaluation procedure has been developed [10] taking into account the more appropriate cylindrical coordinate system. As marked in blue in figure 2.2a, the radial direction r of a cylindrical coordinate system points in a cross section outwards from the origin and the azimuthal direction φ along the perimeter.…”

“…As marked in blue in figure 2.2a, the radial direction r of a cylindrical coordinate system points in a cross section outwards from the origin and the azimuthal direction φ along the perimeter. The crystallographic direction along this radial direction can be highlighted in orientation maps by adjusting the Euler angle φ 1 describing the orientation at each pixel individually according to its azimuthal angle φ in the map (note that in [10] erroneously φ 2 is mentioned). The resulting orientation map highlighting the crystallographic direction along the radial direction in figure 3.1 appears almost entirely red confirming the preferred alignment of ⟨100⟩ directions with the radial direction and hence the existence of a cyclic ⟨100⟩ ring fiber texture.…”

“…The 100 pole figure shows a high pole density along the radial direction and rotational symmetry around that; the 110 pole figure substantiates the latter. An ideal ⟨100⟩ fiber texture along the radial direction in the cylindrical coordinate system translates to a cyclic ⟨100⟩ ring fiber texture in Cartesian coordinates [10]. Quantification of the strength of the cyclic ⟨100⟩ ring fiber texture by analyzing the individual orientations reveals a volume fraction of 89 % when allowing a maximal inclination angle of 15°between the radial and the closest ⟨100⟩ direction.…”