Material issues pose a significant challenge for the design of future fusion reactors. Recently progress has been made towards fully dense multi short-fibre powder metallurgical production of tungten-fibre reinforced tungsten (W f /W) as well as optimising the process understanding for the routes using chemical vapor deposition (CVD). For CVD-W f /W weaves and textile preforms are being used to facilitate large scale production. Classically 150 µm tungsten fibres supplied by OSRAM GmbH have been used. In order to facilitate the better use of textile processes less stiff 16 µm filaments are being evaluated. The strength of the 16 µm flament is at 4500 M P a and thus significantly higher than the strength of the 150 µm fibre (∼2500 MPa) (in the as-fabricated state). Better weavability allows a more flexible use of fibre preforms. Two main yarn production routes have been investigated: covered yarns where a set of tungsten filaments is held together by a PVA (Polyvinyl alcohol) cover and braided yarns. In oder to allow a comparison to the previously used single fibres, yarns with ∼ 140 µm effective diameter were produced. Braided yarns with tensile strength of 2500 MPa and 6% strain at fracture and twisted yarns with tensile strength of 4500 MPa and 3% strain at fracture. For both yarns single fibre CVD samples have been produced to investigate the infiltration properties of the yarns and thus their applicability for the CVD route. A dense infiltration is observed for all yarns under investigation.
Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall and armor material for future fusion devices. However, its intrinsic brittleness and its embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (Wf/W) composites exhibiting extrinsic toughening are being developed. A possible Wf/W production route is chemical vapor deposition (CVD) by reducing WF6 with H2 on heated W fabrics. The challenge here is that the growing CVD-W can seal gaseous domains leading to strength reducing pores. In previous work, CVD models for Wf/W synthesis were developed with COMSOL Multiphysics and validated experimentally. In the present article, these models were applied to conduct a parameter study to optimize the coating uniformity, the relative density, the WF6 demand, and the process time. A low temperature and a low total pressure increase the process time, but in return lead to very uniform W layers at the micro and macro scales and thus to an optimized relative density of the Wf/W composite. High H2 and low WF6 gas flow rates lead to a slightly shorter process time and an improved coating uniformity as long as WF6 is not depleted, which can be avoided by applying the presented reactor model.
In future fusion reactors, tungsten is a main candidate material for the plasma facing material. To overcome the brittleness of tungsten, tungsten fiber-reinforced tungsten (Wf/W) composites have been developed using a powder metallurgy processes. In this study, a novel type of Wf/W with porous matrix has been developed using field assisted sintering technology (FAST). Compared to conventional Wf/W, the avoiding of fiber/matrix interface simplified the production process. Initial mechanical testing showed Wf/W with porous matrix can establish a promising pseudo ductile behavior with an increased fracture toughness compared to pure W.
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.
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