3 - Nanostructural and Microstructural Aspects of Shear Localization at High-Strain Rates for Materials

Abstract

We describe the principal characteristics that define the structure of adiabatic shear bands in metals at the nanostructural and microstructural scales. We comment on deformed and transformed bands, a classification that was created based on optical microscopy observations related to the appearance of the shear band. Modern analytical techniques, described in Section 3.2, have added a great deal to our capability to look into the nanostructure of shear bands. Thus, a band in the optical microscope which appears to be transformed consists actually of nanocrystalline grains that resulted from the breakup of the original microstructure and exhibit radically different etching characteristics. The formation of shear bands is dictated by the thermoplastic instability induced under high strain and high-strain rate, and the resulting structure is governed by three principal factors: (a) the temperature rise in conjunction with the kinetics and thermodynamics of phase transitions; (b) the shear strains and stresses that govern the generation of defects (primarily dislocations and twins, but also displacive phase transitions); and (c) the temperature decrease and kinetics and thermodynamics of phase transitions. The rotational recrystallization mechanism, proposed to be responsible for the creation of ultrafine-grained and nanocrystalline structures, is presented.