Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics. polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti-6Al-4V, and stainless steel), granular and prefractured ceramics (Al(2)O(3) and SiQ, a polymer (teflon) and a metallic glass (Co(58)Ni(10)Fe(5)Si(11)B(16)). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti. Cu, Al-Li, and Ta. and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are ggeater than a critical size a(e). When they are smaller than a(e), particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady-Kipp (momentum diffusion), Wright-Ockendon, and Molinari (perturbation) models. (C) 2001 Elsevier Science B.V. All rights reserved.