Spontaneous and forced shear localization in high-strain-rate deformation of tantalum

High-strain-rate shear localization was induced in tantalum by (a) lowering the deformation temperature or (b) subjecting it to high strains by dynamic deformation (up to εt=−0.8) or (c) pre-shocking (at εeff=0.22) and then deforming it. Although at ambient temperature the deformation of tantalum is macroscopically uniform to high strains (εt≅−0.8), at 77 K shear localization under the same loading condition was developed at a critical strain of −0.2 to −0.3. This higher propensity to shear localization at low temperatures is a direct consequence of the combination of lower heat capacity and higher rate of thermal softening. At the three temperatures investigated (77, 190 and 298 K), localization occurs at strains significantly higher than the instability strains (the maxima of the adiabatic stress–strain curves for these three temperatures). The thicknesses of the forced localization regions and shear bands were found to be a function of temperature, and decreased with decreasing temperature (at the same strain) in accord with the equation proposed by Y. Bai et al. (Y. Bai, C. Cheng, S. Yu, Acta Mechanica Sinica 2 (1986) 1). Shock deformation of tantalum enhances its predisposition to subsequent shear localization, and this was demonstrated by subjecting shocked and unshocked specimens to high strain, high strain rate deformation through the collapse of a thick-walled cylinder assembly.