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- The effects of tantalum addition on the microtexture and mechanical behaviour of tungsten for ITER applicationsPublication . Tejado, Elena; Carvalho, Patricia Almeida; Munoz, A.; Dias, Marta; Correia, J.B.; Mardolcar, U. V.; Pastor, Jose YgnacioTungsten (W) and its alloys are very promising materials for producing plasma-facing components (PFCs) in the fusion power reactors of the near future, even as a structural part in them. However, whereas the properties of pure tungsten are suitable for a PFC, its structural applications are still limited due to its low toughness, ductile to brittle transition temperature and recrystallization behaviour. Therefore, many efforts have been made to improve its performance by alloying tungsten with other elements. Hence, in this investigation, the thermo-mechanical performance of two new tungsten-tantalum materials has been evaluated. Materials with W–5wt.%Ta and W–15wt.%Ta were processed by mechanical alloying (MA) and later consolidation by hot isostatic pressing (HIP), with distinct settings for each composition. Thus, it was possible to determine the relationship between the microstructure and the addition of Ta with the macroscopic mechanical properties. These were measured by means of hardness, flexural strength and fracture toughness, in the temperature range of 300–1473 K. The microstructure and the fracture surfaces features of the tested materials were analysed by Field Emission Scanning Electron Microscopy (FESEM).
- New WC-Cu thermal barriers for fusion applications: high temperature mechanical behaviourPublication . Tejado, Elena; Dias, Marta; Correia, J.B.; Palacios, T.; Carvalho, Patricia Almeida; Alves, E.; Pastor, Jose YgnacioABSTRACT: The combination of tungsten carbide and copper as a thermal barrier could effectively reduce the thermal mismatch between tungsten and copper alloy, which are proposed as base armour and heat sink, respectively, in the divertor of future fusion reactors. Furthermore, since the optimum operating temperature windows for these divertor materials do not overlap, a compatible thermal barrier interlayer between them is required to guarantee a smooth thermal transition, which in addition may mitigate radiation damage. The aim of this work is to study the thermo-mechanical properties of WC-Cu cermets fabricated by hot pressing. Focus is placed on the temperature effect and composition dependence, as the volume fraction of copper varies from 25 to 50 and 75 vol%. To explore this behaviour, fracture experiments are performed within a temperature range from room temperature to 800 degrees C under vacuum. In addition, elastic modulus and thermal expansion coefficient are estimated from these tests. Results reveal a strong dependence of the performance on temperature and on the volume fraction of copper and, surprisingly, a slight percent of Cu (25 vol%) can effectively reduce the large difference in thermal expansion between tungsten and copper alloy, which is a critical point for in service applications. The thermal performance of these materials, together with their mechanical properties could indeed reduce the heat transfer from the PFM to the underlying element while supporting the high thermal stresses of the joint. Thus, the presence of these cermets could allow the reactor to operate above the ductile to brittle transition temperature of tungsten, without compromising the underlying materials.
- WC-Cu thermal barriers for fusion applicationsPublication . Dias, Marta; Guerreiro, F.; Tejado, Elena; Correia, J.B.; Mardolcar, U. V.; Coelho, M.; Palacios, T.; Pastor, Jose Ygnacio; Carvalho, Patricia Almeida; Alves, E.ABSTRACT: WC-Cu cermets have been devised for thermal barriers between the plasma facing tungsten tiles and the copperbased heat sink in the first wall of nuclear fusion reactors. Composite materials with 50 and 75 v/v% WC have been prepared by hot pressing at 1333 and 1423 K with pressures of 37 and 47 MPa, respectively. Microstructural changes have been investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. The materials consolidated have also been evaluated in terms of Archimedes' density, thermal diffusivity, Vickers hardness and elastic modulus. Implantation was carried out at room temperature with Ar+ at 100 keV ion beam with a fluence of 4 · 1020 at/m2. The materials consisted of homogeneous dispersions of WC particles in a Cu matrix and presented densifications of about 90%. Incipient swelling in copper-rich regions have been observed on the implanted surfaces, however no significant changes have been detected by X-ray diffraction. Higher WC content in the cermet materials increased hardness and the elastic modulus. The cermets' thermal diffusivity was significantly lower than that of pure copper or tungsten, as desirable for a thermal barrier.