The present work extends a recent model for plastic deformation of polycrystalline metals based on irreversible thermodynamics. A general dislocation evolution equation is derived for a wide range of strain rates. It is found that there is a transitional strain rate (∼10³ s^-1) over which the phonon drag effects play a dominant role in dislocation generation resulting in a significant raise in the dislocation density and flow stress. The model reduces to the classical Kocks-Mecking model at low strain rates.