Effect of free surfaces on transonic edge, screw, and twinning dislocation mobility in copper

The mobility of dislocations in transonic/supersonic regime is not yet fully understood. We use molecular dynamics to investigate the effect of free surfaces on the mobility of edge, screw, and twinning dislocations in single crystal copper. We find that the upper limit of dislocation velocity depends on its character. While twinning-dislocation can reach supersonic speeds, edge-dislocation velocity remains below the longitudinal sound speed, and screw-dislocation velocity below the higher transverse sound speed. By correlating instantaneous velocities v and resolved stresses tau, we show that the mobility law of each dislocation type can be decomposed into piece-wise continuous ranges of uniform motion separated by bands of forbidden velocity. The lower limiting velocity for uniform motion of all dislocations is the Rayleigh wave speed, c^R. When forced to propagate at an average velocity greater than c^R, dislocations exhibit periodic, intermittent, discrete jumps in instantaneous velocity between c^R and higher velocity branches of the mobility law. Bands of forbidden velocity may be calculated directly from the velocity dependence of dislocation drag coefficients. These findings provide atomistic insights of plastic deformation near free surface under high strain-rate loading.