Phase Transformations in Uranium at High Pressures

The $\alpha -\beta$, $\beta -\gamma$ and the direct $\alpha -\gamma$ phase transformations in uranium have been studied up to 45 kbar at cooling and heating rates of 3-5°C/sec. The overheating and undercooling intervals for the $\alpha -\beta$ and $\beta -\gamma$ transformations at high pressures are about the same as the hystereses expected at zero pressure and do not vary appreciably with increasing contamination. Progressive contamination and resultant lowering of the transformation temperatures of the uranium samples was evident for all containers used, but was least for tantalum containers. The equilibrium $\alpha -\beta -\gamma$ triple point occurs at less than 31.5 kbar and greater than 803°C. The $\gamma$ phase is definitely more compressible than the $\beta$ phase and the $\beta$ phase is probably more compressible than $\alpha$. There is only a small hysteresis in the $\alpha -\gamma$ transformation.