Mechanical behaviour of W particulate-reinforced Cu composites: Fracture toughness and R-curves

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One of the most critical technology challenges regarding the European demonstrational fusion reactor (DEMO) project is to develop a heat-exhaust system (divertor) that fulfils the required structural design criteria at the heat flux loads and irradiation dose level foreseen for the DEMO divertor, which is predicted to be an order of magnitude higher in the DEMO divertor compared to the ITER divertor [1]. To address the material issues posed by this substantial gap in nuclear loads, a dedicated technology R&D work to develop advanced composite materials for the high-heat-flux and heat-exhaust systems is being carried out in the framework of Eurofusion work package "Divertor".These advanced materials are mostly tungsten-copper composites aiming to achieve high-temperature strength and toughness and reduce thermal stresses [2]. However, the limitations of the testing environment and the actual grade of development of these materials make it necessary the evaluation of their fracture properties out of the standard. With this aim, a general analytical model to obtain the crack resistance curve (R-curve) for non-standard samples and with any span-to-depth ratio has been developed.The case study addresses the three-point-bending fracture of three W-Cu (15, 30 and 40 wt.%Cu) metal matrix composites for heat sink applications in future fusion devices. The preliminary experiments show predominant rising R-curves, though their shape and fracture energy slopes depend strongly on the Cu-content and temperature. Thus, it is evident that the energy of the high Cu-content samples is higher than that of the low Cu-content ones, and this difference can be quantified in terms of initial fracture energies and microstructure of the fracture surfaces. Though intense degradation is observed at the highest temperature tested, the mechanical properties at operational temperatures, i.e. below approximately 350 °C, remain relatively high and even better than W/Cu composites reported previously.