W–Cu metal matrix composites for next generation fusion devices

One of the main challenges in the development of magnetic confinement fusion, as a reliable and predictable baseload source of energy, is the design of materials that can withstand the harsh operating conditions inside the reactor. Whilst the experimental reactor ITER – the machine that will demonstrate the technological and scientific feasibility of fusion energy – is under construction, the next step, the demostration fusion power plant (DEMO), will bring fusion energy research to the threshold of a prototype fusion reactor.

In this regard, the operating conditions for highly heat loaded plasma-facing components (PFCs) in DEMO will be unlike any encountered in current tokamak experiments. Heat fluxes of up to 20?MW/m2 are expected in the divertor region, in combination with substantial fusion neutron irradiation. So it is conceivable that the baseline strategy, with a conventional divertor as pursued in ITER, cannot be extrapolated to DEMO or commercial fusion power plants. Research is now focused on improved plasma-facing materials (PFMs) and components, and on new divertor configurations for enhancing both heat removal capability and mechanical integrity of the divertor PFC performance [1].

The preferred PFM in magnetic confinement fusion devices is tungsten (W) because of its high threshold energy for sputtering by hydrogen isotopes as well as its low retention of tritium within the material [2], while Copper–Chromium–Zirconium – or more accurately Cu–1.0%Cr–0.1%Zr – is considered the primary material for the water-cooling pipes for the divertor (in the monoblock design) due to its superior thermal conductivity, ductility, water-tightness and relatively low activation [3]. But the joint between two materials as different as W and Cu-based alloys will experience high thermal stress when exposed to high heat loads. To overcome these limitations, a metal matrix composite (MMC) between W and Cu could prove very beneficial concerning PFC applications. The interest in these MMC composites is twofold: the W matrix provides the necessary composite strength at high temperatures, while the Cu-based phase provides the required high thermal conductivity to effectively dissipate and transfer the heat to the cooling system. Furthermore, the development of a functionally graded material would mitigate thermal stresses in the joint, since thermo-mechanical properties could be tailored just by tailoring the content of each component.

However, despite its interesting performance, the production of perfectly dense W–Cu MMCs is quite challenging as the significant difference between the melting points of W (approximately 3400?°C) and Cu/CuCrZr (about 1083?°C) makes it difficult to produce a conventional alloy. Furthermore, radioprotection requirements limit the presence of possible wetting aids elements, such as cobalt. Among the preferred methods for producing fully dense W–Cu composites, laser [4] and plasma sintering [5] have gathered much attention in the last few decades; but both processes result complex and achieving an adequate electrical conductivity of the powders and a homogeneous temperature distribution is particularly challenging [6].

Against this background, melt infiltration of Cu in a porous W skeleton has indeed many advantages [7]. In this synthesis route, a W rigid skeleton with proper relative density is produced via powder metallurgy, consolidated by uniaxial cold pressing and then sintered at 1150?°C for 2?h in a high purity hydrogen atmosphere, to finally obtain skeletons with desirable density values. These latter were infiltrated by oxygen-free molten copper. This technique was applied by Louis Renner GMBH Company (Amtsgericht Stuttgart, Germany) to produce composites with 15, 30 and 40 wt% Cu. Their thermophysical and strength properties were investigated and measured to assess their feasibility as a component of next-generation fusion reactors [8], [9]. Thus, a graded structure could be developed with these composites, tailoring the properties mismatch between the W-PFM and the Cu-based cooling pipes.

This issues’ cover image shows the false-coloured electron micrograph of the fracture surface of an MMC composed of 70?wt.%W (orange) and 30?wt.%Cu (green) after tensile testing under quite extreme conditions (800?°C under very high vacuum) to recreate the operating conditions inside the future fusion reactor. The present image was captured using a Zeiss Auriga 40 Field Emission Scanning Electron Microscope fitted with GEMINI column located at the Department of Materials Science of the Civil Engineering School at the Universidad Politécnica de Madrid. Posmortem specimen was micrographed using secondary detector at an accelerating voltage of 10?kV, and at a working distance of 7.4?mm.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The author is thankful to the Materials for energy under extreme service conditions Research Group of the UPM for encouraging the research and providing the necessary facilities. Author would also like to thank Dr Müller and Prof You from the Max-Planck-Institut für Plasmaphysik (Garching, Germany) for providing the materials under study.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support from the Ministerio de Economía y Competitividad of Spain (MAT2015-70780-C4-4-P), the Comunidad de Madrid (S2018/NMT-4411 ADITIMAT-CM) the Universidad Politécnica de Madrid in the line of action for encouraging research from young doctors (project M190020074EMTG, COAT4FUSION) is also gratefully acknowledged.

W–Cu metal matrix composites for next generation fusion devices

Declaration of Competing Interest

Acknowledgments

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