Modeling of the radiation-induced microstructural evolution in ionic crystals

Results of experimental and theoretical investigations are presented on heavily irradiated natural and synthetic NaCl crystals in the temperature range where anion defects are mobile. They give a strong evidence for the formation of vacancy voids, which cannot be explained by the Jain-Lidiard model used up to date for description of metal colloids and dislocation loops formed in ionic crystals during earlier stages of irradiation. We consider an additional set of reactions between experimentally observed extended defects (metal colloids, gas bubbles and voids) and point defects. The latter include F and H centers that are the primary defects produced by irradiation, and cation vacancies (with a trapped hole) that are secondary defects, produced in the process of dislocation climb due to absorption of extra H centers. We show that highly overpressurized bubbles of fluid halogen are strongly biased for absorption of H centers, which makes them grow via punching out interstitial dislocation loops. The loops grow and produce cation vacancies that are subsequently trapped at the incoherent colloids together with extra F centers giving rise to the colloid-void transition. Elastic interaction between extended defects and point defects is shown to play a major role, since it determines the bias factors of extended defects, which is a major driving force of the microstructural evolution under irradiation. A quantitative comparison of the new model for radiation damage in NaCl with experimental data is presented. Mean sizes and volume fractions of all types of observed defects are calculated. It is shown that voids formed due to agglomeration of F centers and cation vacancies can grow to the dimensions exceeding the mean distance between colloids and bubbles, eventually absorbing them, hence, bringing the halogen gas and metal to a back reaction. Impurities play a major role in the void development with increasing irradiation dose, which strongly affects the radiation stability of NaCl. (C) 1999 Elsevier Science B.V. All rights reserved.