Recently, a research team led by Associate Prof. LI Xiangyan from the Institute of Solid State Physics of the Hefei Institutes of Physical Science of the Chinese Academy of Sciences uncovered how nano-voids evolve under irradiation and proposed a strategy to mitigate helium-induced damage in fusion reactor materials.

The findings were published in Tungsten and Nuclear Fusion.

Fusion reactor structural materials are exposed to intense neutron irradiation, which can generate defects such as nano-voids, leading to swelling, hardening, and creep. Neutron reactions also produce helium in structural materials, where it can accumulate and form bubbles. Grain boundaries can trap helium, but the trapped helium can accumulate over time and damage the material.

In this study, the researchers first developed a computational framework based on the differential evolution algorithm to explore stable nano-void structures. The simulations showed that small nano-voids tend to form compact polyhedral structures, while larger ones develop faceted shapes bounded by low-energy surfaces such as (110) and (100).

The team also found that vacancies around nano-voids have different energy levels depending on their local atomic environments. Based on these findings, the researchers linked the energy levels of nano-voids to their atomic structures, helping to explain differences in their stability and migration behaviors.

The researchers then examined whether certain grain boundaries could help helium move out of the material. Through high-throughput calculations and object kinetic Monte Carlo simulations, they found that some grain boundaries with suitable atomic structures can serve as rapid helium diffusion channels. When connected to free surfaces, these boundaries can help release helium from the material. Similar behavior was observed in both iron and tungsten.

Based on these results, the researchers proposed a "capture-transport-expulsion" strategy, providing a theoretical basis for using grain-boundary engineering to shift from passive helium capture to active helium removal and thereby improve the resistance of fusion reactor materials to helium-induced damage.

"By understanding how these defects form and evolve, we hope to provide useful guidance for developing fusion materials with better radiation resistance," LI said.

Schematic of the competing mechanisms of helium transport and expulsion in nanocrystalline metals, including the grain-boundary-assisted transport-expulsion and trapping-retention regimes. (Image by LI Xiaolin)