The Effect of the Rate of Cooling on the Allotropic Transformation Temperatures of Uranium*

Allotropic transformation in uranium has been studied over a range of cooling rates from 5 to about 8000°C/sec. The transformation temperatures of both gamma-to-beta and beta-to-alpha were found to decrease continuously with increasing rates of cooling. The extent of the beta-range increased with increasing cooling rates. For rates of cooling up to 1000°C/sec, recalescence was observed in both transformations. For higher cooling rates, there was usually no recalescence. In most of the recorded cooling curves, a small but definite thermal arrest was observed, between the two main arrests which correspond to the two known phase transformations. This additional thermal arrest was also present in a heating curve, where it occurred at about 740°C, compared with 666 and 771 •c for the two known phase transformations. Possible explanations of the additional arrest are discussed.