  We reformulate the theory of polycrystalline plasticity, in externally
driven, nonequilibrium situations, by writing equations of motion for the flow
of energy and entropy associated with dislocations. Within this general
framework, and using a minimal model of thermally assisted depinning with
essentially only one adjustable parameter, we find that our theory fits the
strain-hardening data for Cu over a wide range of temperatures and six decades
of strain rate. We predict the transition between stage II and stage III
hardening, including the observation that this transition occurs at smaller
strains for higher temperatures. We also explain why strain-rate hardening is
very weak up to large rates; and, with just one additional number, we
accurately predict the crossover to power-law rate hardening in the
strong-shock regime. Our analysis differs in several important respects from
conventional dislocation-mediated continuum theories. We provide some
historical background and discuss our rationale for these differences.
