Development of V-Cr-Ti type alloys with small additives for advanced fusion applications

Advanced approach to develop a practically tough material was demonstrated in recent research activity of low activation vanadium alloys for fusion applications. It was shown that modification by small addition of yttrium to the V-4Cr-4Ti alloy improved integrity of their material performance. Vanadium alloy has been considered as a blanket structure material for fusion reactor applications because of their promising properties such as low induced radioactivity, high-heat-loading capability and compatibility with coolant materials like lithium. The critical issues to use vanadium alloys as blanket structure material in lithium self-cooled and breeding system include development of insulator coating for reducing magneto-hydrodynamic (MHD) pressure drop and filling up material performance database of the vanadium alloys. Significant progress has been made in development of the MHD coating and material database of vanadium alloys. In this paper, successful development of the vanadium alloys by means of advanced melting process and their material performance database will be described. Gaseous impurities such as oxygen in vanadium alloy affect hardening very much especially when radiation induced defects exist. Initial impurity levels were successfully reduced by modification of melting process. Purified V-4Cr-4Ti alloy have demonstrated improvement in the material properties including workability, high temperature strength, ductility and weldability. It has been shown that the impurity levels of the V-Cr-Ti alloys can be controlled by small additives such as yttrium by means of their scavenging effect. Tensile properties after neutron irradiation of the modified V-Cr-Ti alloys showed remarkable performance. Yttrium addition will reduce oxygen level by oxide slug formation on the melting ingot surface. Two possible mechanisms of the reduction were proposed, that is, suppression of oxygen penetration into molten metal at the surface and removal of oxygen from inside by forming Y2O3. A high-purity 15 kg-ingot of V-4Cr-4Ti-0.15Y alloy was made by a levitation melting method. It was demonstrated that the melting process successfully completed. Concentration of interstitial impurities in the ingot reached adequately low levels. (author)