Understanding the Formation Mechanism of Void Lattice Under Irradiation: From Collision Cascades to Self-Assembled Nanovoids

Abstract

The formation of periodic arrangements of voids is an interesting phenomenon occurring in materials under neutron irradiation, usually replicating the symmetry and crystallographic orientation of the host matrix, hence called “void lattice”. Here, taking tungsten as an example, we explore the formation mechanism of the void lattice using an object kinetic Monte Carlo (OKMC) model through the collision cascades simulated using molecular dynamics method. Specifically, the detailed processes from the chaotic neutron irradiation defects to the observable void lattice are obtained via OKMC simulation, which is a prerequisite for understanding its formation mechanism. The formation of the void lattice could be divided into three stages: nucleation, incubation and growth. Both the one-dimensional (1D) migration of SIAs and the fraction of clustered vacancies in cascades play a critical role in the emergence 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 compared to the unaligned ones. On the other hand, a moderate fraction of clustered vacancies in cascades guarantees a stable rate for both nucleation and growth of voids. Once the density of the <111> aligned voids reaches a critical value, the dissolution rate of the unaligned voids will overwhelm their nucleation rate, leading to the formation of void-free channels and thus the void lattice. Our results reveal the interrelated mechanism of the 1D migration of SIAs and intra-cascade vacancy clustering for the formation of the void lattice under neutron irradiation, which improves our fundamental understanding of the self-assembled microstructures in irradiated materials.