Embrittlernent of irradiated F82H in the absence of irradiation hardening

Neutron irradiation of 7-12% Cr ferritic/martensitic steels below 425-450 °C produces microstructural defects and precipitation that cause an increase in yield stress. This irradiation hardening causes embrittlement, which is observed in a Charpy impact or fracture toughness test as an increase in the ductile-brittle transition temperature. Based on observations that show little change in strength in steels irradiated above 425-450 °C, the general conclusion has been that no embrittlement occurs above these temperatures. In a recent study of F82H steel, significant embrittlement was observed after irradiation at 500 °C. This embrittlement is apparently due to irradiation-accelerated Laves-phase precipitation. Furthermore, observations of the embrittlement in the absence of hardening has been examined and analyzed with thermal-aging studies and computational thermodynamics calculations to illuminate and understand the effect.