Effect of heating and cooling rate on the kinetics of allotropic phase changes in uranium: A differential scanning calorimetry study

摘要

The kinetic aspects of allotropic phase changes in uranium are studied as a function of heating/cooling rate in the range 10(0)-10(2) K min(-1) by isochronal differential scanning calorimetry. The transformation arrest temperatures revealed a remarkable degree of sensitivity to variations of heating and cooling rate, and this is especially more so for the transformation finish (T-f) temperatures. The results obtained for the alpha -> beta and beta -> gamma transformations during heating confirm to the standard Kolmogorov-Johnson-Mehl-Avrami (KJMA) model for a nucleation and growth mediated process. The apparent activation energy Q(eff) for the overall transformation showed a mild increase with increasing heating rate. In fact, the heating rate normalised Arrhenius rate constant, k/beta reveals a smooth power law decay with increasing heating rate (beta). For the alpha -> beta phase change, the observed DSC peak profile for slower heating rates contained a distinct shoulder like feature, which however is absent in the corresponding profiles found for higher heating rates. The kinetics of gamma - beta phase change on the other hand, is best described by the two-parameter Koistinen-Marburger empirical relation for the martensitic transformation. (C) 2008 Elsevier B.V. All rights reserved.