Object kinetic Monte Carlo simulations on the difference between fission neutron and heavy ion irradiation induced void evolution in Fe-Cr alloys

Abstract: Experimental results show significant difference between neutrons and ions irradiation of alloys, while the underlying reasons remain unclear. Herein, we performed object kinetic Monte Carlo (OKMC) simulations on void evolution in Fe-Cr alloys under neutron and ion irradiations, focusing on the effects of dose rate, irradiation particle type and temperature. Binary Collision Approximation and Molecular Dynamics are applied to obtain the cascade morphology of ion irradiation in order to study the effect of spatial correlation of cascades along the ion track, which is considered as a significant difference between the neutron and ion irradiations. Systematic OKMC simulations were performed at a wide range of dose rate from $10^{-7}$ to $10^{-3}$ dpa/s and temperature from 300 to 500$^\circ C$. Simulation results show that both a higher dose rate and a lower temperature can lead to a higher density and a smaller average size of voids. High dose rate greatly promotes the interaction frequency between small defects and inhibit the absorption of vacancies by one vacancy cluster, thus enhancing the nucleation of vacancy clusters. This dose rate effect explains the major difference of microstructure between fission neutron and heavy ion irradiation. High temperature enhances the migration of small defects and the absorption of vacancies by vacancy clusters, and thus enlarge the vacancy clusters. The impact of irradiation particle types that has influence on the primary knock-on atom spectrum and cascade morphology is less important to void evolution compared with dose rate and irradiation temperature. This work provides fundamental insights into the difference in void evolution between fission neutron and heavy ion irradiations that are important to the application of ion irradiations in study irradiation effects of nuclear materials.