Mechanical properties and structural analysis of martensitic low-activation alloys after neutron irradiation

In spite of the good irradiation behavior of the martensitic 8-10%CrWVTa alloys further developments and optimizations were carried out to improve the ductility at lower irradiation temperatures. A critical effect of low-temperature irradiation-induced hardening and embrittlement occurs in the fusion-relevant temperature range of 250 - 400 °C. This behavior was found at moderate neutron doses of only few dpa up to 15 dpa. A remarkable shift of DBTT towards higher temperatures can be observed, together with an increase in the ultimate strength. However, it was also shown that the effect can be reduced by chemical modifications, for example, by operating with Cr contents in the range of 9 wt.% and strongly reduced B contents. Within the framework of the European Fusion Technology Program, a series of ferritic/martensitic developmental alloys have been investigated after irradiation in the HFR (Petten, Netherlands) up to an accumulated dose of 2.4 dpa. Six alloys from Forschungszentrum Karlsruhe (OPTIFER-type) were compared with the internationally investigated IEA alloy F82H mod. an 8%Cr2WVTa material. These new 8-10%CrWVTa alloys are optimized towards long-term activation. Mechanical properties were determined by tensile and impact tests. The highest strength was observed in the range of 250 and 350 °C as radiation hardening, and the shift of DBTT to higher temperatures. Neutron irradiation induced precipitates, dislocation loops and He bubbles or small voids have been revealed by transmission electron microscopic analysis.