Molecular Dynamics Study On The Influence Of Grain Boundaries On The Growth And Rupture Of Near-Surface Helium Bubbles

With the development of society,the demand of energy is increasing.Developing new energy resources has become an urgent task.Fusion energy is considered as one of the best future energy because it is safe and produces no pollution along with abundant fuels.The performance of plasma facing materials is one of the key issues that impact the realization of fusion energy.Tungsten is the best candidate material for the divertor due to its good mechanical and thermal properties.However,existing experimental studies have found that under fusion-relevant conditions,high-flux low-energy helium irradiation can lead to the formation of a large number of helium bubbles near surface and may induce the formation of "fuzz" nano-tendril structures on the surface.Such microstructures can reduce the thermal conductivity of tungsten by several orders of magnitude and may threaten the steady operation of the plasma.Ultrafine grained tungsten and nanocrystalline tungsten can greatly improve irradiation resistance due to their large number density of grain boundaries that acts as defect sinks.Such materials have attracted more and more attentions in recent years.Besides,a large number of grain boundaries and helium bubbles have been observed inside fuzz nanotendrils.The study of the interactions between grain boundaries and helium bubbles can help to understand the mechanism of helium resistance of ultrafine grained tungsten and nanocrystalline tungsten and the mechanism of fuzz growth under low-energy helium irradiation.However,there are limited studies on the effects of grain boundaries on the growth and rupture of helium bubbles near surfaces.This work presents Molecular Dynamics(MD)simulations that study the interactions between near-surface helium bubbles and grain boundaries in tungsten,and the temperature effects on the diffusion of tungsten interstitial atoms in grain boundaries.The simulations of the growth and rupture of helium bubbles in nanotendrils in the vicinity of various grain boundaries indicate that certain grain boundaries can lead to the stacking of interstitial atoms on the surface to form significant adatom islands.Due to limited space inside the nanotendrils,integrated prismatic dislocations can be rarely formed during the growth of helium bubbles.Instead,a more complex hybrid dislocations are formed and the type of the hybrid dislocation is closely related to the structure of grain boundaries.By studying the movement of these two kinds of hybrid dislocations under stress,the origin of differences in the shape of ad-atom islands are explained.Based on the results of this part of work,we propose that the growth of helium bubble may be one of the reasons for the intricate and complex branched structures of nanotendrils.By combining molecular statics,dynamics,quasi-equilibrium harmonic approximation and other methods,the temperature effect of tungsten interstitial atom diffusion in the Σ5<100>{310} grain boundary is studied in this work.An abnormal diffusion behavior is discovered,which is that the diffusivity of interstitial atoms at high temperature is lower than that at low temperature.By the analysis of the diffusion path,it is found that the diffusion patterns can be categorized into two patterns,namely,the parallel diffusion with a low diffusion barrier and the vertical diffusion with a high diffusion barrier.The calculation of formation energy at stable sites and the attempting frequency of diffusion indicates that the main diffusion pattern has switched at different temperatures due to thermal expansion and atomic vibration.The main diffusion pattern at high temperature is vertical diffusion,which is slow.On the contrary,the quick parallel diffusion pattern dominates at low temperature.