Stratified plastic flow in metals

Abstract

Experimentally determined possibility to control the localization of plastic flow (shear bands) and the mechanical properties of metals (viscosity) during a plastic deformation process that is relevantly changed, validates questioning the nature of the shear bands, the mechanisms behind their existence and their morphological form.In this paper, indications for a hypothesis, that there are possibilities and certain conditions in which a stratified plastic flow similar to a rigid (dislocation less) shear strain in metallic crystals can appear, are presented. The analysis conducted for the paper was based on experiments indicating the possible dominant role of point defects in structural processes (diffusion) and their influence on the mechanical properties of crystals (viscosity). The dependency between the diffusion coefficient and the point defects concentration and their migration enthalpy as well as the physical dependence of the viscosity coefficient on the diffusion coefficient might explain the low viscosity of the metal with high (in relation to thermodynamic equilibrium) concentration of self-interstitial atoms. It was proved that a high supersaturation of metal with such defects can be obtained either through a mechanically activated reverse dislocation movement or through changing the dominant slip system. The defects, generated as a result of collapse of dislocation dipoles, and their spatial distribution, change the properties of the plane in which they were generated.In other words, a planar zone along the active slip plane supersaturated by self-interstitial atoms with very low migration energy levels (which means their bonds within the lattice are weak) is treated as a layer (stratum), where the inter-planar bond in a crystal is weakened. A high saturation of vacancies in a given layer, which lowers the number of bonds within the first coordination sphere, gives similar results. High local saturation of metal with point defects results in the lattice's lower resistance to shear strain, and therefore, lower viscosity.