New perspectives on structural parameters of graphite materials revealed by micromechanics based on microstructural analysis

Graphite is used as a moderator and reflector in many nuclear reactors, where it is exposed to high temperatures, stress, radiation, and other harsh conditions. Under these conditions, graphite undergoes microstructural changes, the consequences of which can lead to deformation, cracking, and ultimately fracture. This research highlighted aims to investigate the microstructural based micromechanics of graphite materials and focus is on understanding the micromechanics based on microstructural degradation and changes in crystalline grain structure, their orientations, and boundaries in polycrystalline graphite for predicting structural damages in the graphite material. This study utilises analytical transmission electron microscopy to examine the nature of grain boundaries and extract important microstructural parameters. The crystallites and contained boundaries, including their orientations in a polycrystalline structure, all have the potential to influence the properties of the material. Under the harsh conditions of temperatures, radiation, and stress that graphite experiences in the reactor core, it is crucial to understand its microstructural behaviour and its influence on its properties. This research brings insight into the potential mechanisms of structural damage based on the nature of grain boundaries and crystalline orientations. The key investigations are the bending of crystallites along the interfaces, causing the division of coherently scattered domains into smaller crystallites. On analysing the traces across the crystallites, it was also observed that each crystallite was rotated in a similar direction. These crystallites bending is referred to as a micro-bending feature in graphite. This micro-bending deformation mode is less commonly observed in graphite, and the identification of these micro-mechanisms and their correlation with crystallite bending and rotation within polycrystalline structure provides insights into the deformation behaviour and potential degradation mechanisms in the graphite material.