Molecular dynamics study of primary damage in the near-surface region in nickel

Abstract

We carried out a large-scale molecular dynamics (MD) simulation to study displacement cascades in the near-surface region in pure nickel. These MD simulations were performed both in the bulk and near-surface regions with primary knock-on atom (PKA) energies of EPKA=1,5, or 10keV and at temperatures T=300,425, or 525K. In our study, the primary knock-on atom was taken to be created by a neutron knocking off a lattice atom. Accordingly, cascades were initiated by giving a lattice atom the appropriate velocity corresponding to the PKA energy. Assuming isotropic neutron fluence, the PKA was initiated in random directions (including toward and parallel to the free surface) in both bulk and near-surface simulations. This contrasts with previous studies in which the cascades were initiated by the projectiles directed toward the free surface to mimic ion irradiation. Therefore, for every (T,EPKA), near-surface cascade simulations were performed also as a function of PKA's initiation depth. Present results provide quantitative information about defect production and clustering in the primary damage state for near-surface and bulk displacement cascades. The effect of EPKA and T on the defect production (averaged over all PKA directions) is similar for both the near-surface and bulk cascades. In both cases, the defect production increases with EPKA, but the temperature has minimal effect. However, the production and clustering of vacancies are higher for near-surface cascades. In contrast, the production and clustering of self-interstitial atoms are lower and increase monotonically with depth. The decrease in the average number of vacancies and average vacancy cluster size with increasing depth is not monotonic.