A STUDY OF AMORPHIZATION AND MICROSTRUCTURAL EVOLUTION OF GRAPHITE UNDER ELECTRON OR ION IRRADIATION

In order to clarify the mechanism of irradiation-induced phenomena, such as amorphization, swelling and microstructural evolution in highly oriented pyrolytic graphite, in situ observation through high-voltage and high-resolution electron microscopy, electron energy loss spectroscopy and convergent-beam electron diffraction has been performed under irradiation with 16-160 fJ electrons or 4.8 fJ He+, Ar+ or Xe+ ions in the temperature range from 110 to 470 K. The induced amorphization with electron irradiation accompanies swelling along the c-axis due to the accumulation of di-carbon molecules and their clusters between the basal planes, and is fully completed with excess vacancies introduced by atom displacements of around 1 dpa. Heavy-ion irradiations were also performed to understand the effects of the impact and/or the deposited energy density in cascades and the projected range of ions on the irradiation-induced phenomena. Twinning, amorphization and microcrystal evolution in the amorphized region emerge, depending on the heterogeneous damage rate with respect to the depth from the incident surface of ions. The microstructural evolution under ion irradiation is successfully analyzed in terms of the damage profile and the deposited energy density, which are directly related to the mass and the energy of projectiles.