COMPUTER-SIMULATION OF THRESHOLD DISPLACEMENT EVENTS IN ALLOYS

Point defect production in alloys by recoil of atoms at the threshold for displacement has been investigated by computer simulation. Two alloy types have been considered, namely the ordered L1(2) structure and a dilute, fcc solid solution: we have taken Ni3Al and a Au solute in copper as examples of these systems. Many-body interatomic potentials for these alloys have been used, but modified at small separations to provide suitable descriptions of collisions at near-threshold energies. The properties of point defects associated with a single Au solute atom in a copper matrix are presented, and it is then shown that the threshold energy, E(d), for the Au recoil in this alloy is smaller and has much less orientation-dependence than is found for a Cu recoil in the pure metal. This effect is related to the number of atoms temporarily displaced from their sites during the creation of a stable Frenkel pair. E(d) for Al in Ni3Al is much higher than in pure aluminium and similar in size to that of a Ni atom. The stable defect arrangements produced at threshold are in general much more complex than for pure metals, and defects on the Ni sublattices are usually the majority species. A striking effect occurs for [110] recoils along mixed Ni-Al-Ni-Al- ... rows, for chains of antisite defects created by replacement sequences break to form two Frenkel pairs for each primary recoil atom.