Data in the literature on mechanical properties of neutron-irradiated tungsten are very limited.234’238’239 However, in combination with experimental results obtained for other refractory metals, it has been shown that in metals with a bcc lattice structure’ irradiation hardening causes a steep increase in yield stress and a decrease in ductility.110 Conse­quently, the material fails by brittle cleavage frac­ture as soon as the yield stress exceeds the cleavage strength. Therefore, the increase of the DBTT depends on the neutron fluence, the neutron spec­trum (will be addressed by the International Fusion Materials Irradiation Facility, IFMIF), and the irradiation temperature. The radiation hardening in bcc alloys at low temperatures (<0.3 Tm) occurs even for doses as low as ~0.15—0.6 dpa (irradiation of plasma facing materials for ITER and DEMO, PARIDE campaigns217), which corresponds to the expected ITER conditions. Therefore, operation of tungsten at temperatures >1000 °C would be preferred as full or at least partial recovery of defect-induced material degradation is achieved by annealing at 1200 °C.234 This implies that the near­surface part of a W component will retain its ductility, which has a beneficial effect on the crack resistance at the plasma loaded surface. However, such temperatures are not feasible at the interface to the heat sink where tungsten will be in contact with copper (ITER) or steel (DEMO), which are limited to significantly lower operational tempera­tures. Hence, better understanding of the irradiation effects on tungsten at temperatures between 700 and 1000 °C is needed, particularly related to reactor

application in DEMO.109,110,240

In addition to the influencing factors on the DBTT mentioned above, that is, neutron fluence, neutron spectrum, and irradiation temperature, the material’s composition also plays an important role. While the addition of Re has a beneficial effect on the material’s ductility in the nonirradiated state, under neutron irradiation it results in more rapid and severe embrit­tlement than it is observed for pure W2 9 Similarly, less mechanical strength and an increased loss of ductility compared to pure W is found for particle — strengthened W alloys (e. g., W—1% La2O3) when tested up to 700 °C. The only exception among all explored tungsten alloys might be mechanically alloyed W-TiC (see Section 4.17.3.3) that showed no irradiation hardening as measured by Vickers hard­ness at 600 °C.87

Finally, the mechanical properties are influenced by neutron-induced He-generation and the transmu­tation of tungsten. While He generation in W is, com­pared to CFC and Be, very small (~0.7 appm He per dpa) and its influence on the mechanical properties of W negligible,73,83,224 the transmutation of W into Re and subsequently Os significantly alters the mate­rial structure and its properties. The generation of significant amounts of ternary a and subsequently а-phases results in extreme material embrittlement and will cause shrinkage. In combination with ther­mally induced strains, this might produce high tensile stresses causing the extremely brittle material to extensively crack and perhaps even crumble to powder.36