Yield strength of tantalum for shockless compression to 18 GPa

A magnetic loading technique was used to study the strength of pure, annealed, and cold-rolled polycrystalline tantalum under planar ramp loading at strain rates of ∼ 10⁶/s. Both the initial yield strength and the flow strength after compression to peak loading stresses of 18 GPa were determined. For sample thicknesses ranging from 0.5-6.0 mm, it was found that the elastic limit of ∼3.2 GPa, corresponding to a yield strength of 1.6 GPa, for annealed Ta was sharply defined and essentially independent of sample thickness. After elastic yielding, relaxation of the longitudinal stress occurred for sample thicknesses greater than ∼0.5 mm, approaching an asymptotic value of ∼1.6 GPa. Two different purities of annealed Ta showed no difference in initial yield strength. Cold-rolling annealed Ta to 26% plastic strain resulted in a more dispersed elastic precursor with an amplitude of about 1.6 GPa and with no stress relaxation after yielding. Analysis of unloading wave profiles from the peak loading states allowed determination of the flow stress, which increased to about 0.9 GPa for annealed Ta and 1.3 GPa for cold-rolled Ta at peak stresses of 17-18 GPa.