STATIC STRENGTHS OF TA AND U UNDER ULTRAHIGH PRESSURES

We have performed high-pressure strength experiments on tantalum and u ranium using a diamond-anvil cell. These experiments determined the: f low stresses of tantalum and uranium at room temperature and in the lo w strain rate limit (epsilon'<10(-6) sec(-1)) by using x-ray diffracti on to measure the pressure gradients in the samples. We find that the flow stresses increase dramatically with increasing pressure and strai n, with the flow stress of Ta reaching 10.3 GPa at a pressure of 85.8 GPa and an estimated strain of approximate to 90%, and the flow stress of U reaching 19.8 GPa at a pressure of 109.0 GPa and an estimated st rain of approximate to 70%. With further increases in pressure and str ain, the flow stresses decrease. This apparent strain-softening effect has also been observed in static high-pressure how stress experiments on other materials, and has been suggested to be due to either materi al damage or preferred orientation of grains induced by large strains. [S0163-1829(98)05141-8].