Linear collision sequences in fcc and L1 2 metals: A computer simulation study

Abstract

The mechanisms of energy loss in 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 head on collision sequences are studied in fcc copper and gold, as well as in Ll 2Cu 3Au by molecular dynamics (MD). An interaction potential based on the second moment tight binding approximation is employed. Its repulsive branch is splined to a Molière potential for close encounters, in order to allow comparisons between full molecular dynamics and its binary collision approximation (BCA). In 〈1 0 0〉 and 〈1 1 0〉 directions of fcc lattices, the energy loss is found to result from the overlap in time between the interaction of an atom with its next neighbour in a sequence and the intermediate ring of atoms surrounding the row axis. In 〈1 1 1〉 directions, interactions with rings of atoms and next neighbours in the sequences are well separated in time and the energy loss is shown to result from the interaction with the rings. No energy is lost in the interaction with the next neighbour in the row. In Ll 2 lattices, non-symmetric rings may have a defocusing effect, which may compete with focusing at energies below the focusing threshold for an isolated row. Half of 〈1 1 0〉 rows are of mixed alternate composition. Because of the large mass difference between copper and gold, copper is found to shuttle between its two gold neighbours with the consequence of storing energy locally in the row. If one excepts this latter case and the case of non-symmetrical rings, the energy lost per unit length in a row is found close to constant and to depend only on the orientation of the row. This is shown to allow for correcting the underestimate of the energy loss predicted in the binary collision approximation by adding an artificial “binding” energy of the atoms in a sequence to their lattice sites. This correction is direction dependent and its values are tabulated.