Resonance with Surface Waves Induces Forbidden Velocity Bands in Dislocation Glide

We use molecular dynamics simulations to demonstrate a pronounced influence of free surfaces on the mobility of dislocations at near-sonic velocities. By correlating instantaneous velocities v and resolved stresses tau, we show that the mobility relations of both edge and screw characters are composed of multiple ranges of stable uniform motion separated by bands of forbidden velocity. The lower limiting velocity for uniform motion of both edge and screw dislocations is the Rayleigh wave speed, cR. When forced to propagate at an average velocity greater than cR, dislocations exhibit cyclic, intermittent jumps in instantaneous velocity between cR and higher velocity branches of the mobility relation. Bands of forbidden velocity may be calculated directly from the velocity dependence of dislocation drag functions, which contain peaks near cR and-in the edge case-one other characteristic surface wave speed, which we associate with shear-horizontal (SH) waves. We propose that peaks in the drag function are due to resonance of free surface vibrations forced by nearby moving dislocations.