Molecular dynamics study of vacancy diffusion in a forced Lennard-Jones system

Using constant-temperature molecular dynamics simulations, we have investigated the effect of an alternating uniaxial external stress on vacancy migration in a fcc argon crystal in which the atoms interact with each other through a 12-6 Lennard-Jones potential. The crystal is confined between two smooth walls which interact with the atoms through a similar Lennard-Jones potential, and an alternating stress field is exerted by moving both the walls inwards (compressive) and outwards (expansive) sinusoidally with a frequency of 93 MHz. In the other two directions periodic boundary conditions are used. The amplitude of the sinusoidal wall movement along the [100] axis corresponds to a strain of 0.0138 and a stress of about 15 MPa. At this small strain amplitude, the vacancy jump frequency, when averaged over a full stress cycle, has a value similar to that in the bulk unstressed crystal. However, the vacancy jump frequency is higher with a larger proportion of jumps being in planes normal to the stress axis, during the expansive half-cycle than during the compressive half cycle. Thus, the jump behaviour is anisotropic under stress, and this anisotropy is opposite for the two half-cycles