Theory of Dislocation Mobility in Pure Slip

The mobility during glide of uniformly moving dislocations or dislocation segments supposed not to be obstructed by any Peierls' barrier is estimated. For a straight freely moving dislocation, the strong anharmonicities in the core region, the thermoelastic (edge dislocation) and the phonon viscosity effect give rise to a drag stress at ordinary temperatures T∼θ, θ being the Debye temperature, of the order σ∼110ε×V/c in insulators. In metals the thermoelastic effect is negligible, while the core anharmonicity effect and the phonon viscosity effect will be of the same order of magnitude as in insulators. In the above formula, ε=thermal energy density, V=dislocation velocity, and c=velocity of shear waves. The scattering of phonons by the dislocation also causes a drag stress at ordinary temperatures of the order of magnitude of the above formula. All of the above mentioned contributions to the drag stress go rapidly to zero with decreasing temperature. However, if the dislocation is constrained by the Peierls' barrier except at freely moving kinks, the kink mobility determines the dislocation mobility. It is shown that the scattering of phonons of a half-wavelength longer than the kink width causes a drag stress which may outweigh all other contributions up to ordinary temperatures, and which persists with decreasing temperature as T down to a temperature ∼θb/D, where b=the lattice spacing constant and D is the kink width.