Accurate Fe–He machine learning potential for studying He effects in BCC-Fe

Abstract

Nanostructured Ferritic Alloys (NFAs) containing oxide nanoparticles are prospective advanced structural materials in future fusion reactors. Lacking fusion neutron sources with an adequate flux, modeling is critical to understand radiation damage in the NFAs, including helium effects. Machine learning interatomic potentials (MLPs) have been demonstrated to be capable of describing atomic interactions in chemically complex systems with comparable accuracy to density functional theory (DFT) but with much less computational costs. Hence, MLPs are chosen to study He bubble accumulation in chemically complex NFAs. As a preliminary step, we develop a Fe–He potential, based on ∼10,000 atomic configurations. The developed MLP accurately predicts the bulk properties of BCC-Fe compared to DFT, including the lattice constant (2.832 Å), elastic constants (c11=271 GPa, c12=141, c44=93), and phonon frequencies (maximum error < 3.5%). The MLP predicts He binding energies in HenV bubbles and Hen clusters with a mean absolute error (MAE) of only ∼70 meV, which is ∼3 to ∼5 times smaller than the MAE of binding energies estimated using empirical potentials. The MLP also accurately predicts the migration barrier of interstitial He (64 meV). Furthermore, the MLP correctly predicts the maximum number of He atoms (critical size) at which a HenV bubble (Hen cluster) can kick-out a self-interstitial atom (SIA), forming a bigger HenV2 bubble (HenV bubble). Subsequently, we use the MLP to perform molecular dynamics simulations to investigate the effect of temperature on the critical size of HenV bubbles and Hen clusters. We find that the critical size decreases with increasing temperature — indicating that smaller bubbles and clusters can kick-out an SIA at higher temperatures, thereby escalating the He bubble growth. The current Fe–He MLP can be further developed to include all chemical interactions in NFAs.