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

Using constant-temperature molecular dynamics simulations, we have investig ated the effect of an alternating uniaxial external stress on vacancy migra tion in a fcc argon crystal in which the atoms interact with each other thr ough a 12-6 Lennard-Jones potential. The crystal is confined between two sm ooth walls which interact with the atoms through a similar Lennard-Jones po tential, and an alternating stress field is exerted by moving both the wall s inwards (compressive) and outwards (expansive) sinusoidally with a freque ncy of 93 MHz. In the other two directions periodic boundary conditions are used. The amplitude of the sinusoidal wall movement along the [100] axis c orresponds to a strain of 0.0138 and a stress of about 15 MPa. At this smal l strain amplitude, the vacancy jump frequency, when averaged over a full s tress cycle, has a value similar to that in the bulk unstressed crystal. Ho wever, the vacancy jump frequency is higher with a larger proportion of jum ps being in planes normal to the stress axis, during the expansive half-cyc le than during the compressive half cycle. Thus, the jump behaviour is anis otropic under stress, and this anisotropy is opposite for the two half-cycl es.