Self-organization of shear bands in titanium and Ti–6Al–4V alloy

The evolution of multiple adiabatic shear bands was investigated in commercially pure titanium and Ti–6Al–4V alloy through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (∼104 s−1). The shear-band initiation, propagation, as well as spatial distribution were examined under different global strains. The shear bands nucleate at the internal boundary of the specimens and construct a periodical distribution at an early stage. The shear bands are the preferred sites for nucleation, growth, and coalescence of voids and are, as such, precursors to failure. The evolution of shear-band pattern during the deformation process reveals a self-organization character. The differences of mechanical response between the two alloys are responsible for significant differences in the evolution of the shear band patterns. The number of shear bands initiated in Ti (spacing of 0.18 mm) is considerably larger than in Ti–6Al–4V (spacing of 0.53 mm); on the other hand, the propagation velocity of the bands in Ti–6Al–4V (∼556 m/s) is approximately three times higher than in Ti (∼153 m/s). The experimental shear-band spacings are compared with theoretical predictions that use the perturbation analysis and momentum diffusion; the shortcomings of the latter are discussed. A new model is proposed for the initiation and propagation that incorporates some of the earlier ideas and expands them to a two-dimensional configuration. The initiation is treated as a probabilistic process with a Weibull dependence on strain; superimposed on this, a shielding factor is introduced to deal with the deactivation of embryos. A discontinuous growth mode for shear localization under periodic perturbation is proposed. The propagating shear bands compete and periodically create a new spatial distribution.