High dose 30 MeV 58Ni5+ ion irradiation causes microstructure evolution in nuclear graphite at 400 °C

A 30 MeV 58Ni5+ ion beam was used to irradiate fine-grained graphite grade IG110 and ultrafine-grained graphite grade G1 at 400 °C to study their microstructure evolution under irradiation. Taking advantage of the depth dependence of the damage rate and cumulative damage of ion irradiation, the microstructure change exploration of graphite with multiple damage levels within a single fluence specimen was achieved by characterizing the sample cross-section with HR-TEM and micro-Raman. The Raman 2D maps of the cross-sections of the graphite samples irradiated with various fluences and displacement damages up to ∼18 displacements per atom were analyzed. Evidenced by the saturation of the intensity ratio of graphite Raman D and G band (ID/IG), the irradiation damage and annealing equilibrium was observed. Moreover, ID/IG in combined with the full width at half maximum of the G band (FWHM (G)), shows an inverse evolution compared with the graphitization process, which is also supported by the HR-TEM observation. Demonstrated by the increasing rate of ID/IG with FWHM (G) and the saturation ID/IG, the microstructure changes of fine-grained graphite IG110 and ultrafine-grained graphite G1 was distinguished.