Mechanical properties and microstructure of HFR-irradiated ferritic/martensitic low-activation alloys

Within the framework of the European Fusion Technology Programme, a series of ferritic/martensitic developmental alloys have been investigated after irradiation in the HFR (Petten, Netherlands). These new 7-10%CrWVTa alloys have been optimized towards long-term activation and the properties are compared with the conventional 9-12% CrMoVNb steels. The comparison contains two alloys from FZK (OPTIFER - type), the commonly investigated IEA alloy F82H and an ORNL - 9Cr2WVTa material. The irradiation was carried out in the temperature range of 250 - 450 degrees C up to an accumulated dose of 0.8 dpa. The mechanical properties of the unirradiated and irradiated specimens were determined by tensile and impact tests. Radiation hardening, shift in DBTT to higher temperatures and partially a reduction of upper shelf energy were observed. Selected samples have been investigated by means of metallographic, SEM and TEM methods. The microstructures developed during neutron irradiation showed irradiation-induced precipitates, dislocation loops and He bubbles or small voids. These microstructures contribute to the observed irradiation-induced hardening and the changes of the fracture mode from transgranular to intergranular fractures.