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 °C. All other rod
materials don’t 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 °C. But above that temperature, the specimens show
delamination fractures which propagate along the rod axis, that is, parallel to
the specimen’s 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.