A macro-scale ruck and tuck mechanism for deformation in ion-irradiated polycrystalline graphite

A vein structure, which becomes more pronounced with increasing ion dose, was found on the surface of polycrystalline HOPG (highly oriented pyrolytic graphite) implanted by ex situ C+ (up to 1.8 x 10(17) ions/cm(2)), and in situ Ar+ in a transmission electron microscope (TEM). These veins are found to be independent of the crystallographic orientations and are associated with the formation of pores. Underneath the veins, a triangular-shaped core was formed with the graphite platelet inside the core displaced up towards the surface. A macro-scale 'ruck&tuck' geometry was thus generated at these triangle structure boundaries. Progressive movement of dislocations along basal planes during irradiation was observed, and a mechanistic model was proposed on this basis to explain the vein formation. A small increase of cspacing was observed with irradiation but it is believed that macro-scale vein formation plays a more vital role in the dimensional and property changes in polycrystalline graphite, especially when a stress gradient is present. The model proposed also explains the change of thermal expansion in HOPG with irradiation. Together with Heggie's 'ruck&tuck' and Barsoum's 'ripplocations' models, the present model is considered to have provided an additional experimentally proven mechanism responsible for irradiation behaviour in graphite materials. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study found a vein structure on the surface of highly oriented pyrolytic graphite (HOPG) that became more pronounced with increasing ion dose, independent of crystallographic orientations and associated with the formation of pores. A triangular-shaped core was formed underneath the veins, generating a macro-scale 'ruck&tuck' geometry. The progressive movement of dislocations along basal planes during irradiation was observed, and a mechanistic model was proposed to explain vein formation and dimensional changes in graphite materials.