Fracture behavior of tungsten materials and the impact on the divertor design in nuclear fusion power plants

M. Rieth, KIT, Institute for Materials Research I, Karlsruhe, Germany

A. Hoffmann, PLANSEE Metall GmbH, Development Refractory Alloys, Reutte, Austria

J. Reiser, KIT, IMF I, Karlsruhe, Germany

D.E.J. Armstrong, Department of Material Science, University of Oxford, Oxford, United Kingdom

Present US and European helium cooled DEMO divertor design studies make use of the high temperature strength and good heat conductivity of tungsten. In such outlines, refractory materials are used for structural parts. The most critical issue of tungsten materials in connection with structural applications is the ductile-to-brittle transition (DBT) which is already at rather high temperatures for tungsten materials. Depending on irradiation and dynamic load conditions, the DBT level could be even higher. A systematic study of microstructure and impact bending properties of standard tungsten materials was performed on different tungsten rod, plate, and round blank materials. The results of the tungsten materials look quite different compared to those of typical bcc metals. Only specimens of pure tungsten show a clear upper shelf area, starting at about 800 įC. All other rod materials donít show pure ductile fracture within the whole test temperature range. On the other hand, all tested materials tend to brittle fracture at temperatures below 500 įC. But above that temperature, the specimens show delamination fractures which propagate along the rod axis, that is, parallel to the specimenís long side and perpendicular to the notch. Compared to the rods, all plate materials show even worse properties: (1) the energies are lower by more than 50 %, and (2) the fracture is dominated by delamination within the whole temperature range. In summary, there are three types of fracture behaviour (brittle, ductile, and delamination) which is closer correlated to the materials microstructure than to the materials composition. This paper reviews the results and other relevant properties of tungsten materials with respect to application for structural divertor parts. Drawbacks and possible alternatives are discussed.