Theoretical Description of the Growth and Stability of Helium Platelets in Nickel

Abstract

A static computer simulation is performed in order to study the nucleation and the two-dimensional growth mode of helium-filled aggregates in nickel. The results indicate that a helium-filled aggregate expels neighbouring atoms at its outer edges. These atoms are displaced in the direction of a nearest octahedral position and they are bound to the cluster.The helium-filled aggregate grows in a planar way and an interstitial loop is formed by the emitted atoms. The two dimensional growth occurs parallel with the close-packed planes. To explain the collapse of the platelets into clusters of smaller aggregates, a theoretical model is developed to describe the total energy of a platelet. It is based partly on the results of the simulations. The variation of the total energy gives information regarding the platelet stability and is calculated in both a numerical and an analytical way. It is shown that small platelets are stable and that, once a critical radius is attained, the platelet becomes unstable and a transformation into a group of smaller aggregates takes place. The theoretical model can also explain the fact that two-dimensional helium aggregates are observed to be more stable in bcc than in fcc structures.