Dynamic strength of uranium at high temperatures

Unalloyed uranium (PU) and uranium-0.78 wt% Ti alloy (UT) were studied in VISAR-monitored planar impact experiments with initial sample temperatures ranged from 27$^{\circ}$C to 860$^{\circ}$C. The recorded waveforms was used for obtaining the stress-strain $\sigma (\varepsilon )$ and deviator stress-strain $s(\varepsilon)$ diagrams, the conventional elastic limit $Y_{0.2}$, and the spall strength of the alloys at different testing temperatures. The strengths $Y_{0.2}$ of both the materials stay almost constant up to the temperature of $\alpha -\beta$ transformation, increase strongly in the $\beta$-phase domain, and abruptly drop above the temperature of $\beta -\gamma $ transformation. The temperature dependences of the spall strength of alloys differ from those of the compressive strengths indicating the prevailing role of the void nucleation (over the void growth) in the spallation process. The most striking finding of the work is the existence of $\rm beta$-uranium at pressures some 3 GPa higher than that permitted thermodynamically. The life time and the borders of existence of this non-equilibrium phase are unknown and should be determined in future