Irradiation embrittlement in pure chromium and chromium-tungsten alloy in a view of their potential application for fusion plasma facing components

Abstract

Design of plasma-facing components (PFC) for DEMO divertor unravels new challenges to be met by the in-vessel materials. Embrittlement induced by 14 MeV neutrons in the baseline first wall material - tungsten (W) endangers structural integrity of PFCs. Chromium (Cr) and/or Cr-W alloy has been considered as a candidate material in the design of mid heat flux PFCs as structural body of the monoblock. In our preceding work, the potential of the vacuum arc melting for the fabrication of pure Cr and Cr-W solid solution (with 10 at.%) has been demonstrated. The pure Cr exhibits ductile deformation above 50°C, while the ductile to brittle transition in Cr-10%W is at ~300°C. The particular advantage of the alloying was seen in the strong increase of the work hardening capacity as well as much higher fracture strength compared to pure Cr. Moreover, the solid solution formation as a result of alloying might improve the resistance of the structural materials against irradiation, as was shown for the case of iron-chromium alloys/steels. In this work, the mechanical properties of two pure Cr grades and Cr-10at.%W alloy are assessed after neutron irradiation performed in the conditions relevant for the application of PFCs. The mechanical tests are performed over a wide temperature range to investigate the strength at the irradiation temperature as well as to clarify the role of limited defect annealing. The neutron irradiation resulted in remarkable degradation of mechanical properties making all the studied materials brittle at the applied irradiation temperature. In the case of Cr-10at.%W, the irradiation caused such severe modifications that the fracture strength has become lower than in non-irradiated state. The fracture surface analysis is made to demonstrate the nature of the brittle fracture and to discuss possible reasons for the observed brittleness.