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 dynami c deformation (up to epsilon(t) = -0.8) or (c) pre-shocking (at epsilon(eff ) = 0.22) and then deforming it. Although at ambient temperature the deform ation of tantalum is macroscopically uniform to high strains (epsilon t con gruent to -0.8), at 77 K shear localization under the same loading conditio n was developed at a critical strain of -0.2 to -0.3. This higher propensit y to shear localization at low temperatures is a direct consequence of the combination of low er heat capacity and higher rate of thermal softening. A t the three temperatures investigated (77, 190 and 298 K), localization occ urs at strains significantly higher than the instability strains (the maxim a of the adiabatic stress-strain curves for these three temperatures). The thicknesses of the forced localization regions and sheer bands were found t o 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 deformati on of tantalum enhances its predisposition to subsequent shear localization , and this was demonstrated by subjecting shocked and unshocked specimens t o high strain, high strain rate deformation through the collapse of a thick -walled cylinder assembly. (C) 1999 Elsevier Science S.A. All rights reserv ed.