Abstract
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 α → β and β → γ 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/β reveals a smooth power law decay with increasing heating rate (β). For the α → β 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 γ → β phase change on the other hand, is best described by the two-parameter Koistinen–Marburger empirical relation for the martensitic transformation.
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