Tests were conducted to determine the compatiblity of vanadium alloys with reactor - grade helium and to define the helium gas chemistry requirements for fusion reactors. Miniature tensile specimens of V-5Cr-5Ti, V-10Cr-5Ti. and V-12.5Cr-5 Ti were exposed in a once-through system to helium with 70 vppm-H2 (measured oxygen partial pressures of 10-12 atm) and bottle helium (measured oxygen partial pressures of 10-4 atm) between 500 and 700°C for up to 1008 h. The weight changes in the specimens were recorded. The helium-exposed specimens were tensile tested, and the effects of exposure on mechanical properties were assessed. Exposure between 500 and 700°C for 1008 h in He+70 vppm-H2 resulted in complete embrittlement of all the alloys in room temperature tensile tests. The fracture mode was primarily cleavage, probably caused by a hydrogen-induced shift in the ductile to brittle transition temperature (DBTT). Weight gains increased with temperature and were largest for the V-5Cr-5Ti alloy. Specimens exposed for 531 h between 500 and 700oC in bottle He exhibited two distinct fracture morphologies on the fracture surfaces. Brittle cleavage around the edges of the specimens gave way to ductile dimpling in the center of the specimens. The brittle region around the periphery of the specimen is most likely the highest vanadium oxide, V2O5.
