Modeling of structural and compositional homogenization of plutonium-1 weight percent gallium alloys
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SINCE the 1941 discovery of plutonium (Pu) by Glenn Seaborg and colleagues at the University of California, this enigmatic metal has been the subject of intense scientific investigation. Despite these efforts, there is still much to be learned about the unusual physical and mechanical properties of Pu and its alloys. The properties of Pu are a function of the six allotropes it forms during cooling from the melt (640 ⬚C) to room temperature (Figures 1 and 2). A seventh phase, ␣⬘, forms below room temperature in low-Al and low-Ga alloys, though the temperature and kinetics of this reaction are not well characterized.[1,2] Most of these phase transformations result in large volume changes and produce crystal structures that have distinctly different physical properties (Figure 1). These unusual characteristics have made the metallurgy of Pu and its alloys particularly challenging. For example, the highest density allotrope, ␣ (monoclinic), is extremely brittle and oxidizes readily. The addition of 1 wt pct Ga stabilizes the face-centered cubic ␦ structure, a phase with superior mechanical properties when compared to ␣ Pu. Other elements can act as ␦ stabilizers, including Al, Si, and In, but the Pu-Ga system has been studied in much more detail than these other systems. One of the key issues in understanding the phase stability of Pu-1 wt pct Ga is the solid-state microsegregation that occurs during cooling from the melt. This process results in a microstructure consisting of ␦-phase grains with Ga-rich cores and Ga-poor edges. The Ga-poor edges are metastable and can transform to ␣ phase during cold working, metallographic sample preparation, or storage.[1,2,3] Thermally driven homogenization is required to evenly redistribute Ga and to achieve complete ␦-phase stabilization. Homogenization is a function of the interdiffusion coefficient for Ga in ˜ Ga ␦ Pu (D ␦Pu), and tracking the extent of homogenization is JEREMY N. MITCHELL, THOMAS G. ZOCCO, and RAMIRO A. PEREYRA, Technical Staff Members, are with the Nuclear Materials Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. FRANK E. GIBBS, formerly Technical Staff Member with the Nuclear Materials Technology Division, Los Alamos Laboratory, is with the Rocky Flats Environmental Technology Site-Kaiser Hill, Golden, CO. Manuscript submitted January 24, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
important in predicting the behavior of the alloy. Although ˜ Ga D ␦Pu has been measured in several careful diffusion-couple experiments, there has been minimal application of these data toward predicting the homogenization of Pu-Ga alloys. In this article, we discuss the results of analytical models that have been used to determine the homogenization characteristics of Pu-Ga alloys. The calculated times required to achieve compositional homogenization and ␦-phase stability are compared with experimental data to develop a more detailed understanding of Ga diffusion and the homogenization kinetics of cored Pu-Ga alloys. II. BACKGROUND A. Micros
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