Phase transformations in uranium, plutonium, and neptunium

Abstract

The actinide metals uranium and neptunium are characterized by three allotropes, and plutonium has six solid phases with two of the allotropes having complex, low symmetry, crystal structures. Further, many phase changes in these metals are accompanied by a large specific volume change and a large enthalpy change. Phase transformations in uranium and plutonium have been studied extensively but very little experimental work has been conducted to understand the phase transformations in neptunium. The low temperature transformations occur isothermally and exhibit simple TTT curves similar to many alloys. The kinetics in the metals are controlled more by purity and thermal history than by any inherent feature of the metals. Mechanisms in the pure metals are inadequately understood, and the crystal-lographic relations are complex. The β → α transformations in uranium and plutonium are diffusional near the α β equilibrium temperature, and they are both reminiscent of shear transformations at low temperatures. Skip transformations have been studied in these metals and the role of grain size on transformation behavior has been investigated more than in other metals. Accordingly, the roles of metal purity, thermal history, microstructure, mechanical properties, and thermodynamic variables are discussed, particularly as they might be related to phase transformations in other systems. Generalities have been difficult to establish for plutonium and uranium alloys. Among well-known metals the zirconium and titanium transformations form the closest analogs to the uranium transformations, but there is no obvious analog to the plutonium transformations.