Static strengths of Ta and U under ultrahigh pressures

We have performed high-pressure strength experiments on tantalum and uranium using a diamond-anvil cell. These experiments determined the flow stresses of tantalum and uranium at room temperature and in the low strain rate limit $({\varepsilon }^{\prime }<{10}^{-6}\hspace{0.5em}{\sec }^{\mathrm{-}1})$ by using x-ray diffraction to measure the pressure gradients in the samples. We find that the flow stresses increase dramatically with increasing pressure and strain, with the flow stress of Ta reaching 10.3 GPa at a pressure of 85.8 GPa and an estimated strain of ≈90%, and the flow stress of U reaching 19.8 GPa at a pressure of 109.0 GPa and an estimated strain of ≈70%. With further increases in pressure and strain, the flow stresses decrease. This apparent strain-softening effect has also been observed in static high-pressure flow stress experiments on other materials, and has been suggested to be due to either material damage or preferred orientation of grains induced by large strains.