RADIATION EFFECTS AND GAS CAVITIES IN PYROLYTIC-GRAPHITE IMPLANTED WITH HELIUM-IONS

Changes in the microstructure of edge oriented (EO) and basal oriented (BO) pyrolytic graphite (PG), implanted with 40 keV and 3.5 MeV heliu m ions, respectively, (in both cases at 300 and 770 K) have been inves tigated by transmission electron microscopy. In the as-received specim ens lenticular cavities and other imperfections were found. Ion irradi ation at 300 K significantly modifies the microstructure, mainly due t o submicroscopic defect accumulation, giving rise to radiation-induced swelling, curing of original cavities and strengthening of the matrix . With increasing dose pressurized helium bubbles of lenticular and el lipsoidal shape appear in EO specimens while disk-shaped helium filled cavities are observed in BO PG. In BO PG flaking occurs at high doses , preceded by a latent exfoliation which originates along the interfac e between the radiation-strengthened, helium-rich layer and the virgin matrix. At 770 K blistering occurs in BO PG while the microstructural modifications are small compared to the effects at 300 K. Finally, ex perimental evidence is presented which shows that ion irradiation even at room temperature up to doses of 10 dpa does not cause amorphizatio n in graphite.