Q: Dislocations structures — how far from equilibrium? A: Very close indeed

Abstract

The combination of Newton's third law, action equals reaction, and of the second law of thermodynamics, leads to the LES (Low-Energy Structures) hypothesis, saying that among all potentially accessible structures, plastic deformation will generate the one with the lowest free energy. For the particular case of dislocation-mediated plastic deformation, this means that, limited only by dislocation mobility, availability of slip systems and insignificant entropy, dislocation structures always approach the lowest possible mechanical energy of the present dislocation population. This insight is a most valuable aid in understanding the plastic properties of technological metals. Among the many aspects of plastic behavior that have been successfully treated by this means are the evolution of the major dislocation structure types (Taylor lattices and cell structures), characterizing ‘planar-glide’ and ‘wavy-glide’ metals, respectively, and the resulting differences in workhardening behavior. Further explained are the shape and temperature dependence of the wavy-glide workhardening curve, the strain rate dependence of flow stress, thermal recovery of wavy- and planar-glide materials, the two observed types of worksoftening, and the shape of the hysteresis curve in constant amplitude fatigue. Lastly, deformation banding does not depend on dislocation behavior and is governed by the LES hypothesis in metals as well as in polymers, and presumably also on a geological scale.