Atomistic Simulations For Behaviors Of Helium Atoms In Nickel And Palladium

Helium atoms could be produced in materials by several approaches, such as tritium decay in tritium-storage materials, the bombing to the first wall of fusion reactor fromα-particles escaping from the plasma regions, plutonium decay and so on. Due to the closed–shell electronic configuration, helium could not be solved in materials matrix, the precipitated helium atoms tend to form He-Void cluster or even helium bubbles. The bubbles could grow up easily at lattice defects, e.g. grain boundaries, dislocations, and they could cause depression of macro physical and mechanical properties to the materials. Hence, it is essential for nuclear and materials industry to understand the micro mechanisms of helium interaction with all defects in metals. Currently, stainless steels are used as the principal wall materials in fusion reactors, and nickel is one main ingredient of stainless steel, moreover, the research into helium behaviors in nickel could provide a direction for the research into plutonium because of the same FCC structure. Meanwhile, palladium has the same electronic configuration as helium and it could be applied for tritium-storage owing to its strongest capacity to retain helium in metals as far as we know. Thus, we choose nickel and palladium to perform our research.In the present paper, using molecular dynamics simulation, we studied the behaviors of helium atoms in nickel and palladium systematically with an analytical embedded atom method which was developed constantly by our research group these years. The interstitial diffusion of helium atoms were studied comparatively in these two perfect metals. The diffusion coefficients were calculated at different temperatures; consequently, according to the Arrhenius relation, the activation energy of interstitial diffusion was evaluated, which agreed well with the available experimental result. The interactions of helium atoms with vacancies investigated in these two metals showed that the formed He-Void clusters would grow up via emission of self interstitial metal atoms, and palladium could retain more helium atoms than nickel. Via the research of diffusion behaviors of helium atoms in several grain boundaries, we found helium atoms tend to cluster in grain boundaries. And the determination of diffusion coefficients of single helium atom in grain boundaries suggested that grain boundaries provided fast-diffusion paths. The study of behaviors of helium atoms in nickel dislocations indicated that helium atoms were either repelled from or trapped at edge dislocations in nickel, depending on the direction of approach. It also demonstrated that helium interstitial on slip face had a strong interaction with dislocation and may give a pinning force to the dislocation.