Propagative modes of plastic deformation

A major objective of the theory of defects is to relate the mechanical behaviour of macroscopic materials to the spatio-temporal evolution of the microstructure. The present paper deals with the dislocation-dynamical foundations of plastic instabilities and the propagation of slip in coherent plastic deformation modes (solitary plastic waves). Solitary waves arise from a proper balance between nonlinear localization effects and dispersion. By the dislocation dynamical approach, both nonlinear interactions and spatietemporal couplings are accessible in a quantitative way. Particularly, intrinsic length scales may be identified, in order to address the problem of propagation velocity selection. This is illustrated by means of various models of propagative plastic instabilities observed in tensile tests. The model assumptions are as follows: 1.) The microscopic cause of the repeated non-uniform yielding of the Portevin-Le Chdtelier (PLC) effect i s dynamic strain-ageing, while the macroscopic propagation of PLC bands is controlled by intergranular incompatibility stresses. 2.) Llders bands in polycrystals result from a dislocation dynamics which is diffusion-like (owing to the random grain orientation) and bistable (owing to the stabilizing effect of the grain boundaries). 3.) This is to be compared with Llders bands in single crystals where the dislocation-poor initial state is unstable and band propagation is traced back to an interplay of cross-slip and non-axial stresses. 4.) Thermomechanical fronts arise from the interplay between heat generation during plastic deformation, heat conduction, and strain hardening.