A molecular dynamics study of plastic deformation mechanisms in tungsten: The role of grain size and vacancies

Tungsten is a candidate for plasma-facing components in fusion reactors, where it must withstand extreme heat and neutron irradiation. Nanocrystalline tungsten (NC-W) shows improved radiation tolerance due to grain boundaries acting as defect sinks, but its plasticity degrades after irradiation. Although the mechanical response depends on the interplay between grain size and defect microstructure, the underlying mechanisms remain unclear. Using molecular dynamics, we investigate the combined effects of grain size and vacancy defects on the plastic deformation of NC-W. In defect-free samples, decreasing grain size shifts deformation from dislocation glide to twinning, leading to strengthening followed by softening, with a Hall–Petch to inverse Hall–Petch transition at approximately 25.0 nm. Vacancy defects alter this behavior: randomly distributed single vacancies cause only minor softening, whereas vacancy clusters suppress twinning and promote dislocation-mediated and grain-boundary-mediated deformation. Consequently, vacancy clustering weakens the grain-size dependence of flow stress and promotes vacancy-assisted softening, accompanied by pronounced strain localization. These results indicate that grain refinement alone cannot ensure mechanical stability in irradiated W. Instead, optimal performance requires coordinated control of grain size and vacancy defects, particularly through microstructures that suppress vacancy-cluster accumulation.