Large-Scale Tungsten Fibre-Reinforced Tungsten and Its Mechanical Properties

Abstract

Tungsten-fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidates for the first wall and divertor armour material in a future fusion power plant. As the development of Wf/W continues, the fracture toughness of the composite is one of the main design drivers. In this contribution, the efforts on size upscaling of Wf/W based on Chemical Vapour Deposition (CVD) are shown together with fracture mechanical tests of two different size samples of Wf/W produced by CVD. Three-point bending tests according to American Society for Testing and Materials (ASTM) Norm E399 for brittle materials were used to obtain a first estimation of the toughness. A provisional fracture toughness value of up to 346MPam1/2 was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied. A maximum value of the J-integral of 41kJ/m2 (134.8MPam1/2) was determined for the largest samples. Post mortem investigations were employed to detail the active mechanisms and crack propagation.

Bulk Tungsten Fiber-Reinforced Tungsten (WfW) Composites Using Yarn-Based Textile Preforms

“…To conduct a statistically significant analysis, several specimens from different areas should be cut and tested. Nonetheless, the measured average densities are especially for type 1 significantly higher than the maximum relative density of 92 % realized for the single-fiber based fabrics presented in [17].…”

“…The results show, that the usage of yarn-based fabrics for CVD-stacking are not just increasing the fiber volume fractions and densities of Wf/W composites, but also increasing the reproducibility compared to the results presented in [17] for 150 µm single fiber-based fabrics. In Table 1, the material properties of each composite material produced is shown in an overview.…”

“…The weakest point in the component determines the fracture toughness and therefore the material's lifetime is difficult to predict [8][9][10][11]. To mitigate this problem and to improve the toughness and reproducibility of tungsten-based components, tungsten fiber-reinforced tungsten (Wf/W) metal matrix composites are being developed [10,[12][13][14][15][16][17][18][19][20][21][22]. As shown in Figure 1, extrinsic toughening mechanisms like fiber pull-out, ductile deformation and/or crack bridging are applied to promote a pseudo-ductile behavior that significantly increases the application range of Wf/W composites compared to brittle tungsten.…”

“…Up to now, state of the art has been the usage of preforms, which are depicted in Figure 3. These preforms are based on single filaments of potassium doped tungsten wires with diameters of 150 µm warp and 50 µm weft filaments [17,35]. In Figure 4, 25 layers of these base fabrics have been stacked with the CVD-process to reach the geometrical range of the potential application field.…”

Material and component developments for the DEMO divertor using fibre reinforcement and additive manufacturing

“…Currently the main emphasis of the development is put on the upscaling of the sample production in order to arrive at material samples suited for large mechanical test specimen and mono-block-like mock-ups. First tests with sample sizes up 84 × 24 × 10 mm 3 and a fibre fraction of about 10% confirm the pseudo-ductile behaviour originating from the fibre properties and yield a fracture toughness of K Rmax = 346 MPam 1/2 [30]. CVD samples produced with yarn based preforms allowed a higher fibre fractions (14%-17%) and densities (97%) and increased reproducibility [31] compared to the 150 μm single fibre-based fabrics described above [30].…”