Microstructural evolution in modified 9Cr–1Mo ferritic/martensitic steel irradiated with mixed high-energy proton and neutron spectra at low temperatures

Modified 9Cr–1Mo ferritic/martensitic steel was exposed at 32–57 °C to a mixed proton/neutron particle flux and spectrum at the Los Alamos Neutron Science Center. The microstructure of unirradiated 9Cr–1Mo consists of laths, dislocations and carbides. Examination of electron diffraction patterns obtained from extraction replicas of unirradiated 9Cr–1Mo revealed that the precipitate microstructure was primarily dominated by M23C6 carbides. The post-irradiation microstructure contained black-spot damage in addition to precipitates and dislocations. Examination of electron diffraction patterns revealed diffuse rings from M23C6 carbides, indicating amorphization and/or nanocrystallinity. Crystalline MC carbides were also found. No cavity formation was found although a significant amount of helium and hydrogen generation had been generated. TEM–EDS examination of extraction replicas for carbides from unirradiated and irradiated samples did not show any detectable changes in composition of either M23C6 or MC carbides. There was also no evident change in carbide size. Lattice images of M23C6 carbides revealed an amorphous microstructure following irradiation, but MC carbides were still crystalline.