Atomistic simulation of the stretching of nanoscale metal wires

Simulations have been carried out of nanoscale-width platinum and silver wires with the purpose of identifying the mechanism of failure under strain and the way in which this differs from failure in bulk materials. The wires studied were approximately cylindrical in crosssection and periodic boundary conditions were applied along their length; the atoms were arranged initially in a face-centred cubic structure with the [0 0 1] direction parallel to the long axis of the wire. Interactions between atoms were modelled by Sutton–Chen potentials, which are known to provide a good description of the relevant properties of the bulk metals. The strain along the length of the wire was increased linearly with time by an appropriate scaling of the interatomic distances and the resulting tensile force was computed as a function of strain. At low temperatures the tensile force varies with strain in a stepwise fashion. There is a period in which the wire stretches elastically, followed by an abrupt change in which the atoms rearrange; the pattern is repeated until eventually the wire breaks. At higher temperatures, steps are seen at low strain, but thereafter the wire becomes highly disordered. The nature of the structural rearrangements at low temperatures has been investigated in detail. In each case two domains of the wire are found to slip relative to each other over a {1 1 1} plane and in a ⟨1 0 1⟩ direction to yield an ordered structure of the same symmetry but with an extra layer of atoms inserted. Examination of the atomic trajectories shows that atoms in the planes adjacent to the slip move in curved paths in order to avoid each other. The main difference with respect to the bulk metals is that the slip observed in the wires is homogeneous whereas in the bulk it occurs via dislocations; in the nanoscale systems the small area of contact favours uniform slip rather than the initiation and propagation of dislocations.