| Summary: | The selection of the material for the blanket first wall (FW) is one of the crucial aspects to
overcome in the framework of the EUROfusion program towards DEMO. In this reactor, a loss-ofcoolant accident (LOCA) with simultaneous air ingress into the vacuum vessel would lead to
temperatures of the in-vessel components above 1000 °C up to almost 1200 °C due to the decay
heat. Under such a scenario, the use of pure tungsten, which is currently the main candidate,
represents a potential safety risk because of its poor oxidation resistance that may result in a full
oxidation of the armor layer. Such tungsten oxides, which are volatile at the involved temperature
and radioactive after exposure of the FW to the fusion plasma, would be partially released to the
atmosphere.
A possible way to mitigate this important safety issue is the addition of oxide-forming alloying
elements to pure tungsten, since in case of LOCA with simultaneous air ingress, the alloying
elements will diffuse to the surface forming an adherent and stable protective scale, preventing
further tungsten oxidation. Under normal operation conditions, these alloying elements would be
eroded preferentially by sputtering from the fusion plasma, resulting in a pure tungsten layer
exposed to the plasma. Initially, thin films alloys of the systems W-Cr-Si, W-Cr-Ti and W-Cr-Y were developed at the
Max-Planck-Institute for Plasma Physics (IPP) Garching, Germany, by means of magnetron
sputtering. These W-alloys exhibited a reduction of the oxidation rate of several orders of
magnitude compared to pure tungsten due to the formation of a stable Cr2O3 protective scale when
exposed to air at 1000 °C. Although these thin films are not applicable because thicknesses of
several mm are required for the blanket FW of DEMO, they served as model systems for the
manufacturing of bulk materials by powder metallurgy. Previous work have shown the good
oxidation behavior of the W-10Cr-0.5Y alloy manufactured by mechanical alloying (MA) and
subsequent hot isostatic pressing (HIP). Besides, several authors demonstrated the improvement
of the mechanical properties of pure tungsten by the addition of Zr or ZrC.
In the present dissertation, W-Cr-Y(-Zr) alloys manufactured by MA and HIP have been studied
and tested under relevant DEMO operating conditions. After HIPing, the alloys exhibited a twophase microstructure since the W-Cr system has a miscibility gap below 1680 °C. The application
of a heat treatment at a temperature above the miscibility gap enabled to obtain a dense material
formed by a single metastable phase, which remained stable for long periods of time under the
maximum expected operating temperature. The addition of Zr to the W-Cr-Y system resulted in
an improvement of thermal shock resistance while maintaining similar oxidation resistance and
thermo-mechanical properties. The heat-treated W-10Cr-0.5Y alloy has been exposed to 0.19 to
0.26 dpa neutron irradiation at 600, 800 and 1000 °C, after which an increase of fracture strength
was recorded under all irradiation conditions. This increase was especially relevant after
irradiation at 1000°C, where a value of 1.7 GPa was achieved, being a factor of about 3 higher than
the one of the non-irradiated material. This significant increase in strength after 1000 °C irradiation
is thought to be mainly associated to the solid-solution decomposition of the initial metastable
single phase, resulting in an ultrafine-grained, vermicular shaped, two-phase microstructure. The
as-HIPed W-10Cr-0.5Y alloy has been also exposed to W ion irradiation for producing damage and
subsequently deuterium implanted at temperatures up to 250 °C. The amount of retained
deuterium after implantation at 250 °C was 1/3 lower than that of pure tungsten, being the presence
of Cr responsible for this reduction. Regarding technological aspects, joints between W-10Cr-0.5Yalloy and P91 steel have been produced using diffusion bonding by HIP, resulting in a high shear
strength of 354 MPa, comparable to the one obtained by brazing this alloy to Eurofer. These
strength values are among the highest found in the literature for joining pure tungsten to steel.
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