Effects of precipitation morphology on toughness of reduced activation ferritic/martensitic steels

Reduced activation ferritic/martensitic steels (RAFs) are leading candidates for structural materials of D-T fusion reactors. It is reported that 9Cr-2W-V, Ta steel (JLF-1), one of the RAFs, has superior phase stability, swelling resistance and mechanical properties against high-fluence neutron irradiation. Recently 9Cr-xW-V, Ta steels (x = 2.5, 3.0 and 3.5, JLS-series hereafter) were developed for use at higher temperatures. In this work, JLF-1 and JLS-series were thermal-aged at 823 and 923 K. Charpy impact tests were performed before and after thermal-aging. Microstructural features were observed using transmission electron microscope with energy dispersive X-ray spectrometer. From the results of Charpy impact tests, the ductile to brittle transition temperature was found to increase both by thermal-aging and by increasing tungsten content. This behavior was consistent with microstructural evolution of intergranular precipitates such as M₂₃C₆ and Laves phase coarsening.