Effect of the accumulated vacancies and interstitials on the tungsten surface on the surface's role as defect sinks

Neutrons induce displacement damage to the tungsten-based plasma facing material by creating radiation-defects, e.g., vacancies (Vs) and self-interstitial atoms (SIAs). These defects will accumulate on the surface and further affect surface's role as defect sinks. In this work, by combining molecular statics (MS) and object kinetic Monte Carlo (OKMC) methods, we study the clustering of the V/SIA on the two typical low-index W surfaces (1 0 0) and (1 1 0) to uncover the effect of the agglomerated surface V/SIA on the segregation and annihilation of the V and SIA nearby. Results show that the Vn/SIAn could form on the two surfaces via the agglomeration of the V/SIA on the surfaces with the energy release and the reduced energy barrier for the V/SIA diffusion near the Vn/SIAn as respective energetic and kinetic driving force. Yet, the Vn/SIAn is more easily formed on surface (1 1 0) compared to surface (1 0 0) due to the much larger binding energy of a V with a Vn (or a SIA with a SIAn) and lower diffusion energy barrier of the V/SIA on surface (1 1 0) than that on surface (1 0 0). Long-term OKMC simulations of the V/SIA behavior near the surfaces at 1000 K show that surface (1 0 0) could develop to locally convex structures due to the SIA agglomeration, while surface (1 1 0) will evolve to locally concave-convex structures due to the V/SIA clustering. Meanwhile, the interaction mechanism of the V/SIA with the surface is changed from the fundamental segregation, annihilation and clustering of the V/SIA near a pure surface to the trapping for the V/SIA by the locally concave-convex structure and the junction.