Helium segregation and transport behavior near ⟨100⟩ and ⟨110⟩ symmetric tilt grain boundaries in tungsten

We report the results of a systematic atomistic modeling study of small helium cluster behavior near tungsten symmetric tilt grain boundaries. This behavior was studied qualitatively by molecular dynamics simulations and quantitatively by molecular statics simulations combined with elastic inclusion theory. The sink strength is used to describe the magnitude of the clusters' attraction to the grain boundary. We find that small helium clusters show impeded transport behavior relative to the bulk around all types of grain boundaries, including low-angle, high-angle, low-Sigma-value, and high-Sigma-value grain boundaries. Helium clusters tend to become trapped near, but typically not directly on, the grain boundary plane. Both the distance between the helium cluster and the grain boundary when the cluster first becomes immobilized and the sink strength are correlated with helium cluster size, grain boundary formation energy, grain boundary tilt angle, excess volume, and other aspects of grain boundary structure. We expect similar impeded transport behavior for other types of grain boundaries and in other metals, because helium is effectively insoluble in most materials and has a similar interstitial-based diffusion mechanism in most metals.