High strain rate constitutive response of thin tantalum foils and tungsten heavy alloys

Dynamic plastic behavior of tantalum is known to be strongly dependent on deformation induced textures. This investigation was conducted to examine these effects at very high strain rates as well as to see whether or not tantalum exhibits the pronounced increase in rate sensitivity that is observed in f.c.c. metals. Results of pressure-shear plate impact experiments on thin rolled tantalum foils are presented. Experiments were conducted on 10 mu m and 25 mum thick, high purity, rolled foils in which the shearing direction was either coincident with the rolling direction, perpendicular to the rolling direction or at 45° relative to the rolling direction. In these experiments the nominal shear strain rates were of the order of 106 s-1 . Similar experiments were performed on 127 mum thick, high purity, rolled foils which had been annealed subsequent to the rolling. Again the relative orientations between rolling and shear directions were either 0°, 45° or 90°, and the nominal shear strain rates were of the order of 2 x 105 s -1. The texture of the foils was determined using electron back-scatter patterning techniques on a scanning electron microscope. The experiments show a strong dependence of the initial flow stress to the foil orientation at high strain rates; however there does not appear to be a pronounced change in the rate controlling mechanism up to strain rates of 5 x 10 5 s-1. The experiments do indicate a strong rate sensitivity of the flow stress of rolled tantalum at strain rates of 106 s-1 and higher. Finite element simulations of the experiments have been performed using a crystal plasticity constitutive formulation in which the measured textures provide the initial textures of the model. In these simulations, the shearing rates on the slip systems are taken to be governed by the thermally activated motion of dislocations over Peierls barriers, which are known to be significant in the rate dependent plastic flow of b.c.c. metals.