PNNL

Abstract

Energetic primary recoil atoms from ion implantation or fast neutron irradiation produce isolated point defects and clusters of both vacancies and interstitials. The migration mechanisms and mobility of these defects are crucial to the successful multiscale modeling of microstructural evolution during ion-implantation, thermal annealing, or under irradiation over long periods of time. In this paper, the dimer method is employed to search the possible transition states of interstitials and small interstitial clusters in SiC and alpha-Fe. The method uses only the first derivatives of the potential energy to find saddle points without knowledge of the final state of the transition. In SiC, the possible migration pathway for the C interstitial is found to consist of the first neighbor jump via a Si site or second neighbor jump, but the relative probability for the second neighbor jump is very low. In alpha-Fe, the possible transition states are studied as a function of interstitial cluster size, and the lowest energy barriers correspond to defect migration along directions, as seen in molecular dynamics simulations. However, this paper addresses whether migrating interstitial clusters can thermally change their direction. The activation energies for changing the direction of these clusters are determined, and the corresponding mechanisms are discussed in detail. Finally, the results are discussed in terms of modeling the long time scale dynamics of defects under irradiation or thermal annealing.