Helium segregation and migration behavior near symmetric tilt grain boundaries in tungsten
Tungsten, one of the most important divertor materials in nuclear fusion devices, will be subject to high-flux helium plasma, which can lead to unexpected structural defects and irregular microstructure such as fuzz and blistering. Helium transport in tungsten is affected by grain boundary (GB) defects in the metal. To improve the lifetime and long-term performance of plasma-facing tungsten, it is important to have a better understanding of helium behavior near GBs. In the first part of this thesis, helium cluster transport near symmetric tilt GBs is studied by molecular dynamics (MD) simulation. We find that helium cluster diffusion is significantly impeded, compared to its bulk diffusion rate, near various symmetric tilt GBs with different tilt angles and [sigma] values. Secondly, various cluster-GB interactions and the resulting drift forces for helium cluster transport near GBs are quantified by molecular statics (MS) simulations and continuum elastic inclusion theory. We conclude that the GB sink strength for helium cluster segregation increases with cluster size and depends strongly on various GB properties. The third part of this thesis focuses on analyzing small helium cluster migration on symmetric tilt GBs. We find that small helium clusters show much higher migration energies when bound to the GB than in the bulk. Helium cluster migration is highly affected by the size of helium cluster and the structure of the GB. Vacancy migration energies are relatively low compared to the bulk, and are also much lower than helium cluster migration energies on the GB plane. This suggests that helium cluster migration on grain boundaries is actually governed by the rate of vacancy migration.
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