Microstructural stability of reduced activation ferritic/martensitic steels under high temperature and stress cycling

Reduced activation ferritic/martensitic steels are leading candidates for blanket/first-wall structures of the D-T fusion reactors. In fusion application, structural materials will suffer cyclic stresses caused by repeated changes of temperature and electromagnetic forces according to reactor operation scenarios. Therefore, creep-fatigue behaviors are extremely important to qualify reduced activation steels as fusion structural materials. In this work, microstructural stability of reduced activation ferritic/martensitic steels under various external stresses, such as constant stress cyclic stress, was studied. The materials used are JLF-1 steel (9Cr-2W-V,Ta) and JLS-2 steel (9Cr-3W-V,Ta). The microstructure inspection by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) was performed following creep rupture tests, fatigue and creep-fatigue tests at elevated temperatures. In order to examine precipitation morphology in detail, the improved extracted residue and extracted replica methods were applied. From the microstructural observation of creep rupture-tested specimen, intergranular precipitates such as M₂₃C₆ and Laves phase coarsened by applying the static stress.