Microstructural characterization of inhomogeneity in 9Cr ODS EUROFER steel

Abstract Ferritic-martensitic ODS steels are one of the candidate materials for Gen-IV nuclear fission and fusion reactors. Residual ferrite was often found in the microstructure of 9Cr ODS steels. This constituent was reported to be responsible for the superior creep and high-temperature strength. Using optical microscopy of an air-cooled batch of ODS EUROFER, inhomogeneous regions in the microstructure have been found with similar appearance to previously reported residual ferrite. In order to avoid a potential misinterpretation of inhomogeneous regions as residual ferrite, detailed microstructural investigations have been carried out on the inhomogeneous regions using site-specific nanoindentation, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. It is demonstrated that the inhomogeneous regions are free of oxide nanoparticles, which possibly form due to imperfect mechanical alloying. These regions also exhibit lower hardness which is attributed to the absence of nanoparticles and a lower dislocation density. It is concluded that optical microscopy alone is insufficient to distinguish beneficial residual ferrite from undesired particle-free regions. Our findings are underpinned by the consistency between the calculated theoretical yield strength, the yield strength converted from the indentation hardness and the yield strength obtained from tensile testing.