V-alloy embrittlement by irradiation in a cooling gas environment

Sub-sized tensile and Charpy specimens of the vanadium-alloy V–4Cr–4Ti have been irradiated in a piggy-back experiment with ferritic–martensitic material in the HFR at Petten at 250–450°C to about 2.5 dpa, and subsequently tested. A complex set of post-irradiation mechanical properties was found. Impact measurements showed severe shifts in ductile–brittle transition temperatures (DBTT) regardless of irradiation temperature and no clear trends of recovery at higher irradiation temperatures. The fracture surfaces always indicated transcrystalline crack propagation. Tensile tests generally revealed irradiation hardening with a maximum around 300°C, accompanied by total elongations which increased with increasing temperature. Extremely small uniform elongations at the lower temperatures suggest high localized plastic deformation under small strain rates. Deviations from expected irradiation effects led to a scrutiny of the atmospheric conditions during irradiation which revealed that the capsule had several times been flushed with He and Ne containing traces of interstitial gases or N2 for temperature adjustment. A skin was found on the specimens irradiated at the highest temperatures. X-ray diffraction results showed an uptake of nitrogen, whereas microprobe analysis suggested the presence of oxygen in the bulk. The limited data allow only a qualitative discrimination between chemical and irradiation effects.