Collective micro shear processes and plastic instabilities in crystalline and amorphous structures

The mechanisms that may lead to plastic instabilities are reviewed briefly, and their intimate connection with localization of plastic shear is emphasized. The combination of macro and micro experiments, ie, analysis of stress fluctuations and serrations, and of slip and shear band evolution during deformation, for investigations of these mechanisms is demonstrated with examples of LÜDERS and PORTEVIN-LeCHATELIER band propagation in single glide oriented crystals, and of shear band formation in predeformed mono- and polycrystals as well as amorphous materials. The problems connected with collective effects of the micro units of deformation (ie, dislocations in crystalline, local shear transformations in amorphous materials) are indicated, and the necessity to study the mechanism of transfer of slip (shear) to neighbouring regions is stressed.