Conversion electron Mossbauer spectroscopy (CEMS) and X-ray diffraction studies of 40 keV He-irradiated SUS304 austenitic stainless steel

In order to elucidate the martensitic transformation process induced by He gas accumulated in SUS304 austenitic stainless steel, the relationship between the hyperfine parameters for the austenitic phase and the induced martensitic phase and He-ion fluence for He-irradiated steel samples was investigated using Conversion Electron Mossbauer Spectroscopy. The changes of lattice constant for austenitic and induced martensitic phases were also investigated using X-ray diffraction. The samples were irradiated by 40 keV He ions at fluences of 3.6×10 ¹⁷, 7.2×10¹⁷ and 10.8×10¹⁷ ions/cm² at a temperature of 70°C. We have found that the hyperfine field and lattice constant of the induced martensitic phase are both slightly larger than those for the martensitic phase formed by rolling. An austenitic phase modified by He irradiation has also been formed; the lattice constant of the modified austenitic phase is by about 0.5% larger than that of unirradiated austenitic phase, which may suggest the formation of a precursor preceding the martensitic transformation induced by He bubbles