Interstitial loops in graphite, their motion and their effect on elastic modulus

Abstract

Electron microscope observations of interstitial loops in graphite have provided many examples of their interactions with basal plane dislocation lines. The two most important effects discussed in this paper are the motion of the loops due to the force of an approaching dislocation, and the pinning of these dislocation lines by a high density of small loops. Both effects are analysed in the light of a recent theoretical treatment of such interactions in anisotropic materials. Conditions under which loop movement occurs are examined and compared with theory and the process is shown to be unfavourable for loops of radius greater than 150 Å. Calculated criteria are obtained for the pinning of the dislocation lines by the introduction of a saturation pinning stress which is independent of irradiation temperature, and when combined with the theory of nucleation and growth of loops in graphite it is possible to calculate the irradiation dose for which saturation of elastic modulus is obtained as a function of irradiation flux and temperature. The application of these results to polycrystalline graphite is considered and a comparison made with experimental observations. Suggestions are made for further analysis and experiment.