An energetic and kinetic investigation of the role of different atomic grain boundaries in healing radiation damage in nickel

In this paper, we investigate the role that the atomic structure of grain boundaries (GBs) in Ni has in the interaction with point defects produced during the collision cascades. Using molecular dynamics and statics, we study the influence of different GB structures on the production and evolution of defects, through calculation of defect formation energy, vacancy diffusion barrier, interstitial-vacancy annihilation barrier, and the corresponding energetic and kinetic influence ranges near pristine and damaged GBs. We observe that, as previously reported, GBs preferentially absorb interstitials over vacancies, which result in accumulation of vacancies in the bulk grain and interstitials to highly localize at the GBs. Also, we find that, in several layers near the GBs, the formation energy of defects is substantially reduced, indicating that the GBs serve as a sink for the radiation-produced defects. Further investigation also reveals that the sink strength of the GBs is significantly stronger for interstitials than vacancies. Also in terms of kinetic behavior, we find that the energy barrier of vacancy diffusion reduces as the defect migrates to the close vicinity of the GBs, implying that in the presence of thermally assisted events, vacancies diffuse toward the GBs via low-barrier processes, which result in healing of the damaged boundaries at high temperatures. Moreover, we observe that in the neighboring region of the defect-loaded GBs, the annihilation of a close vacancy–interstitial pair is spontaneous. Finally, via continuous ion bombardment simulation, we study the stability of the GBs in severe irradiation condition. We find that the GBs lose their stability as the number of trapped interstitials increases at the GB plane. Besides, we conclude that the GBs have high efficiency in defect removal on picosecond time scale and act as efficient defect sinks, despite the instability and saturation observed in repeated ion bombardments.