Fundamental Studies of Plutonium Aging
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Fundamental Studies of Plutonium Aging Brian D. Wirth, Adam J. Schwartz, Michael J. Fluss, Maria J. Caturla, Mark A. Wall, and Wilhelm G. Wolfer Introduction Plutonium metallurgy lies at the heart of science-based stockpile stewardship.1–3 One aspect is concerned with developing predictive capabilities to describe the properties of stockpile materials, including an assessment of microstructural changes with age. Yet, the complex behavior of plutonium, which results from the competition of its 5f electrons between a localized (atomic-like or bound) state and an itinerant (delocalized bonding) state,4,5 has been challenging materials scientists and physicists for the better part of five decades.6 Although far from quantitatively absolute, electronic-structure theory provides a description of plutonium that helps explain the unusual properties of plutonium, as recently reviewed by Hecker.5 (See also the article by Hecker in this issue.) The electronic structure of plutonium includes five 5f electrons with a very narrow energy width of the 5f conduction band, which results in a delicate balance between itinerant electrons (in the conduction band) or localized electrons and multiple lowenergy electronic configurations with nearly equivalent energies.4,5 These complex electronic characteristics give rise to unique macroscopic properties of plutonium that include six allotropes (at ambient pressure) with very close free energies but large (25%) density differences, a lowsymmetry monoclinic ground state rather than a high-symmetry close-packed cubic phase, compression upon melting (like water), low melting temperature, anomalous temperature-dependence of electrical resistance, and radioactive decay.5 Additionally, plutonium readily oxidizes and is toxic; therefore, the handling and fundamental research of this element is very challenging due to environmental, safety, and health concerns. Unalloyed Pu has a dense, monoclinic structure ( phase) at room temperature that is extremely brittle and highly reactive.
MRS BULLETIN/SEPTEMBER 2001
The remarkably less dense but more ductile fcc phase is stable between temperatures of 360C and 463C.7 Not surprisingly, the phase is preferred by Pu metallurgists and can be stabilized down to room temperatures by the addition of Group IIIb elements like aluminum and gallium.8,9 The binary phase diagrams have been extensively studied during the period of 1950–2000.7–14 Thus, it is well established that additions of Al, Ga, and Am promote the stability of the phase, while additions of U and Np reduce the stability of the phase, although the underlying mechanisms responsible for this behavior are not well understood.5 While the binary phase diagrams have been studied extensively, the kinetics responsible for transitions to thermodynamic equilibrium are often sluggish at best, especially near room temperature, making equilibrium difficult to determine. Indeed, Timofeeva and co-workers8,14 have recently shown that the U.S. phase diagram for the Pu-Ga system may not be correct and
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