Investigating the formation mechanism of void lattice in tungsten under neutron irradiation: from collision cascades to ordered nanovoids

The "void lattice" formed by periodic arrangements of voids, usually replicating the symmetry and crystallographic orientation of the host matrix, is an interesting phenomenon in materials under neutron irradiation. In this work, taking tungsten (W) as an example, we explore the formation mechanism of the void lattice using an object kinetic Monte Carlo (OKMC) model together with the collision cascades simulated by the molecular dynamics method. The entire formation processes from the chaotic neutron irradiation defects to the observable void lattice are reproduced via OKMC simulation, which could be divided into three stages: nucleation, incubation and growth. It is found that both the one-dimensional (1D) migration of SIAs and the fraction of clustered vacancies in cascades play a critical role in the formation of the void lattice. On the one hand, the 1D migration of SIAs leads to the mutual protection of voids aligned in <111> directions. The self-shielded voids may therefore grow faster than the unaligned ones. On the other hand, a moderate fraction of clustered vacancies in cascades guarantees the stable nucleation and growth rates of voids. Once the density of the <111> aligned voids reaches a critical value, the shrinkage rate of the unaligned voids will overwhelm their growth rate, leading to the formation of void-free channels and thus the void lattice. Our results reveal the synergistic effects of the fraction of clustered vacancies in cascades and 1D migration of SIAs for the formation of the void lattice in W under neutron irradiation, which improves the fundamental understanding of the self-assembled microstructures in irradiated materials.