Microstructure of high-strain, high-strain-rate deformed tantalum

Hat-shaped specimens of polycrystalline tantalum are subjected to high plastic shear strains (gamma = 170-910%) at strain rates exceeding 5 x 10(4)/s in a compression split Hopkinson bar. The dynamic shear tests are performed at room and 600 K initial temperatures, under adiabatic and quasi-isothermal conditions, using UCSD's recovery Hopkinson technique [1]. The microstructure of the post-test specimens is examined with transmission electron microscopy (TEM). The plastic deformation is highly concentrated, producing a narrow shear-localization region of approximately 200 mu m in width. Slip of perfect screw dislocations, on the {110} primary planes along the <111> directions, is found to be the dominant deformation mechanism. Dynamic recovery takes place in the shear-localization regions of all adiabatically tested specimens, and evidence of dynamic recrystallization is observed in the specimen deformed to a shear strain of 910% at a 600 K initial temperature. The substructures of the adiabatically tested specimens include well-defined dislocation arrays, grouped dislocations, elongated dislocation cells, subgrains, and recrystallized micron-sized grains. The microstructure of isothermally tested specimens, on the other hand, features high dislocation density and inhomogeneous dislocation distribution. In light of the TEM observations, the relation between the microstructure and shear stress, the causes of strain inhomogeneity, the estimated adiabatic temperature within the shear-localization zone, the rapid quenching of the shearband al the end of the dynamic testing, the slip characteristics of dislocations in tantalum, and the formation mechanisms of dislocation loops, are discussed.