Elevated temperature plastic flow of high purity uranium in the alpha, beta and gamma phases

Abstract

The high temperature mechanical properties of high purity α-, β- and γ-uranium were investigated by compression testing, from which steady state creep parameters were obtained. Stress exponents of 6 and 4.2 were obtained for α- and β-uranium respectively, while a varying stress exponent was found for γ-uranium which increased from 5 to about 7 with decreasing stress. For α- and γ-uranium, the activation energies for creep, after suitable correction for the temperature dependence of the elastic modulus (a very large correction of 20 kcal for α-uranium), were found to correspond closely to measured values of the volume self-diffusion activation energy. It is concluded, therefore, that creep of α- and γ-uranium is diffusion controlled. The creep of β-uranium could not be concluded to be diffussion controlled, since the creep activation energy was found to be significantly greater than the self-diffusion activation energy. When the creep data are plotted as diffusion compensated creep rate versus modulus compensated stress, the creep of α- and γ-uranium are seen to superimpose. This is significant since a six-fold ratio of elastic modulus exists between the two phases, and selfdiffusion in γ is anomalously rapid with a low activation energy. When compared at the same value of ε ̇ D , β-uranium is seen to be about 3 times stronger on a σ E basis than α- and γ-uranium.