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

Experimental results show significant differences between neutron and ion irradiation of alloys, while the underlying reasons remain unclear. Herein, we performed object kinetic Monte Carlo (OKMC) simulations to investigate the 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 (MD) were applied to obtain the cascade morphology of ion irradiation and to explicitly account for the spatial correlation of displacement cascades along ion tracks, which is considered a significant difference between neutron and ion irradiations. Systematic OKMC simulations were performed at a wide range of dose rates and temperatures. Simulation results show that the increase in dose rate and the decrease in temperature lead to a higher density and a smaller average size of voids. From a microscopic interaction perspective, a high dose rate significantly promotes the interaction frequency between small defects and inhibits the absorption of vacancy-type defects by vacancy clusters, thereby enhancing vacancy cluster nucleation. This dose rate effect accounts for the major differences in microstructural evolution observed between fission-neutron and heavy-ion irradiation. By explicitly simulating the PKA energy spectra and cascade spatial correlation under both neutron and heavy-ion irradiation conditions, we quantify the relative contributions of dose rate, temperature, PKA spectrum, and cascade spatial correlation to vacancy cluster evolution, and demonstrate that the influence of PKA spectrum and cascade spatial correlation between HFIR fission-neutron and 5 MeV Fe heavy-ion irradiation is less pronounced than that of irradiation dose rate and temperature within the displacement-cascade-dominated regime considered here. This work provides a quantitative ranking of the relative contributions of multiple irradiation parameters, including dose rate, temperature, PKA energy spectrum, and cascade spatial correlation, to void evolution, thereby complementing the established rate-theory framework and existing experimental ion–neutron correlation studies.