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Ultra-fine grained molybdenum alloys with fine TiC particles at grain boundaries that were recently developed by the authors are very promising for the application to divertor components in advanced fusion devices. In order to examine the impact toughness and microstructural change of the alloys which were exposed to high temperature heating, miniaturized flat specimens, 1 mm by 1 mm by 20 mm, of both the alloys with TiC additions to 1.0 wt% and TZM alloy were subjected to vacuum heating up to 2273 K for 3.6 ks. The impact toughness was evaluated by three-point bending tests. It is shown that the developed alloys exhibit much improved toughness (lower ductile-to-brittle transition temperature and higher yield and maximum strengths) and the toughness increases both by increasing TiC content and by using mechanical alloying (MA) treatment for fabrication : the ductile-to-brittle transition temperature of the MA treated alloy with 1 wt% TiC is 75 K lower than that of TZM. In addition, the alloy with 1 wt% TiC shows no significant grain growth, though TEM observation indicate the occurrence of recrystallization.

The d-t fusion reactions produce neutron with energies up to 14.1MeV. These neutrons, in addition to producing atomic displacement damage, will induce nuclear transmutation reactions. In particular, helium will be generated at high rate in materials. Since helium is a potent agent in nucleating cavities, the mechanical properties of materials may be affected significantly. It is important, therefore, to determine experimentally the helium accumulation effect on radiation damage of materials under conditions similar to those in a fusion reactor. We indicated that proton beam experiments are a useful tool to study radiation damage by fast neutron in fusion reactor. Productions of both atomic displacement damage and helium in materials have been simulated by computer code. Energy deposition in target materials is also estimated to consider the heating problem. The transmutation calculations indicated that the proton beam experiments imitate satisfactorily the ratios of transmutation rates to displacement rates calculated for the first wall of fusion reactor. Constant concentration of accumulated helium can be achieved across the target thickness. Cooling of target of 1mm thickness could be handled without experimental difficulty in samples in the case of irradiation of 0.1mA/cm^2 proton beam.

Plasma-facing materials and components in a fusion reactor are the interface between the plasma and the material part. The operational conditions in this environment are probably the most challenging parameters for any material: high power loads and large particle and neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak or stellarator reactor, given the proven geometries and technological solutions, requires an improvement of the thermo-mechanical capabilities of currently available materials. In its first part this article describes the requirements and needs for new, advanced materials for the plasma-facing components. Starting points are capabilities and limitations of tungsten-based alloys and structurally stabilized materials. Furthermore, material requirements from the fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, defining directions for the material development. Finally, safety requirements for a fusion reactor with its specific accident scenarios and their potential environmental impact lead to the definition of inherently passive materials, avoiding release of radioactive material through intrinsic material properties. The second part of this article demonstrates current material development lines answering the fusion-specific requirements for high heat flux materials. New composite materials, in particular fiber-reinforced and laminated structures, as well as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical operation space towards regions of extreme steady-state and transient loads. Self-passivating