Microstructural characterization of inhomogeneity in 9Cr ODS EUROFER steel

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. 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 formed due to imperfect mechanical alloying leading to the absence of oxide nanoparticles and consequently lower hardness. It is concluded that optical microscopy is insufficient to distinguish beneficial residual ferrite from undesired particle-free regions. The weakness of the inhomogeneous regions is attributed to the absence of nanoparticles and a lower dislocation density. 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.