Yield Response of Tantalum for Quasi-Isentropic Loading and Unloading

Magnetic loading was used to study initial yield strength of pure annealed and cold-worked polycrystalline and single crystal Ta samples for ramp loading at strain rates of $\sim $10$^{6}$/s and the flow strength after peak compression to $\sim $17 GPa at about 5x10$^{4}$/s. For sample thicknesses of 0.5--6 mm, it was found that for annealed pure polycrystalline Ta, the quasi-isentrope elastic limit (IEL) was essentially constant at $\sim $3.2 GPa with propagation distance. There was no apparent effect of sample purity on precursor amplitude to within experimental uncertainty, although cold-working resulted in loss of the elastic peak and a reduced IEL of $\sim $1.7 GPa. The flow strength at peak loading stress was estimated from the quasi-elastic unloading and found to increase with peak stress. These results will be discussed in terms of constitutive models for Ta. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.