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One of the main problems found in the nuclear applications of graphite is its dimensional instability under irradiation, involving both swelling and shape changes. In order to understand better the mechanisms that give rise to these changes, highly oriented pyrolytic graphite was irradiated with 300 keV electrons at temperatures between 25 and 657 °C in a transmission electron microscope (TEM). Microscopic dimensional changes and structural disordering were studied in directions parallel and perpendicular to the graphite basal plane. Changes in the specimen length were investigated by measuring the distance between two markers on the specimen surface in TEM images. Changes in the lattice parameter and the crystalline structure were studied by a TEM diffraction technique. In agreement with reported results, large increases in the specimen length and the lattice parameter were observed along the c-axis direction, whereas a relatively small decrease was observed along the a-axis. In irradiation studies conducted at room temperature, it was found that the dimensional change saturates at high dose, at an elongation along the c-axis direction of about 300%. High resolution microscopy revealed that the microstructure had become nanocrystalline. Electron energy loss spectroscopy results showed that the volume change was recovered at this stage. These observations are discussed in terms of point defect evolution and its effects on the microstructure of irradiated graphite.

A buildup of radiation-induced lattice defects is proposed as the cause for lattice instability that can give rise to a crystalline-to-amorphous transition. An analysis of published experiments on intermetallic compounds suggests that, when amorphization takes place, no microstructural evolution based on the aggregation of like-point defects occurs. This observation leads us to suggest that buildup of a different type of defect, which will destabilize the crystal, should occur. We thus propose that an interstitial and a vacancy may form a complex, giving rise to a relaxed configuration exhibiting a sort of short-range order. Two mechanisms of complex formation are analyzed, one diffusionless (limited by the point defect production rate) and the other temperature dependent. The amorphization kinetics as a function of temperature, dose, and point defect sink strength are studied. Theoretical predictions on the amorphization dose as a function of temperature are made for the equiatomic TiNi alloy and compared with available experimental results.