Development of chromium and chromium-tungsten alloy for the plasma facing components: Application of vacuum arc melting techniques

Abstract

Designof 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 is currently considered as a candidate material in the design of mid heat flux PFCs as structural body of the monoblock. Cr has the superior mechanical properties in the low temperature range where the commercial tungsten products are brittle. However, the fabrication of Cr requires high level purity control and is therefore challenging for mass production.In this work, vacuum arc melting (VAM) equipment is employed for the fabrication of chromium (Cr) and Cr-10at.%W alloy targeted for the PFC applications. VAM techniques represents new promising alternative route with a high upscale potential. VAM fabrication improves Cr quality by avoiding the introduction of interstitial impurities, while the produced ingots can be further mechanically treated as well as solution alloyed by W to design a dedicated microstructure thus enhancing important mechanical properties, e.g. yield strength and fracture toughness.The produced heats of pure Cr and Cr–10 W are investigated by means of chemical and microstructural analysis as well as by mechanical testing. The obtained results are compared with those obtained for pure Cr and W products fabricated by Plansee (Austria). The VAM-produced pure Cr (without any thermo-mechanical optimization) shows the transition to ductility deformation mode just above the room temperature proving the principal advantage of this fabrication route. Solid solution with 10% of W significantly improves the proof stress while sustaining good ductility at elevated temperatures. The ductile to brittle transition in Cr–10%W is observed around 300 °C, which likely can be reduced further by thermo-mechanical treatment. The bending strength of the tested pure Cr grades is considerably lower than that of pure tungsten, when compared at the technologically relevant temperature i.e. 300-500 °C. Whereas the bending strength of Cr–10%W constituted about 80% of the strength of pure tungsten. Hence, the developed VAM Cr and Cr–W alloys require a next step assessment with respect to neutron irradiation testing and improvement by thermo-mechanical treatment.