| The high-strain, high-strain-rate deformation of commercially pure tantalum was investigated. Four experimental methods were used: split Hopkinson bar, planar shock impact, hat-shaped specimen and thick-walled cylinder (TWC) techniques. The constitutive behavior of this material was studied at temperatures 77-998 K and strain rates {dollar}10sp{lcub}-4{rcub}-7times 10sp3 ssp{lcub}-1{rcub}.{dollar} Parameters of Zerilli-Armstrong and modified Johnson-Cook constitutive models were established.; Planar shock impact at a pressure of 45 GPa and 1.8 {dollar}rmmu s{dollar} duration at 298 K produced significant mechanical twinning in tantalum. The shocked tantalum exhibits higher flow stress and work hardening rate than tantalum quasi-statically deformed to the same effective strain because of the contribution by twin boundaries. Twinning was also produced at 77 and 190 K at high strain rates; the morphology and density of these twins were quite different to those formed by shock loading.; The microstructural evolution at {dollar}4times 10sp4 ssp{lcub}-1{rcub}{dollar} was studied with hat-shaped specimen (up to {dollar}rmvarepsilonsb{lcub}eff{rcub}sim 3){dollar} and TWC (up to {dollar}rmvarepsilonsb{lcub}eff{rcub}sim 10){dollar} techniques. The thickness of the forced localization regions in hat-shaped specimens varies with strain, temperature, and stress and is consistent with the prediction by Bai et al. (213,214). The microstructures in this region were mainly elongated dislocation cells and subgrains. However, in TWC samples, elongated dislocation cells, subgrains, dynamically recrystallized micrograins, and post-deformation recrystallized grains were observed. A microstructural evolution sequence is proposed. At the plastic strains, {dollar}rmvarepsilonsb{lcub}eff{rcub} > 1,{dollar} grain scale localization produced by anisotropic plastic flow and localized recovery and recrystallization were observed. Shear bands were also observed in the cylindrical specimens. The critical strains for shear localization were obtained and found to decrease, from approximately -0.8 at room temperature to -0.23 at 77 K. These strains are significantly higher than the instability strains.; A constitutive model for the threshold stress for mechanical twinning is proposed, which incorporates the effects of grain size, temperature, and strain rate. The constitutive model is combined with the Swegle-Grady relationship for the strain rate at the shock front, enabling the prediction of critical shock pressures for twinning. The predictions of the model are in qualitative agreement with experimental results. |
