Modeling of Structural and Compositional Homogenization


of Plutonium-1 Weight Percent Gallium Alloys


The microstructure of as-cast Pu-1 wt pct Ga alloys is characterized by extensive Ga microsegregation often referred to as “coring.” This process results in grains that consist of Ga-rich cores (,1.6 wt pct) with Ga-poor (,0.1 wt pct) edges. Cored grains can be homogenized at moderately high (i.e., >400 °C) temperatures, though the time required to achieve chemical homogeneity is not well constrained. In this article, we apply several analytical diffusion modeling techniques to characterize the kinetics of alloy homogenization as a function of time and temperature. We also review the experimental investigations that have used analytical tools such as X-ray diffraction, density, dilatome­try, and electron microprobe analysis to characterize Pu-Ga alloy homogenization. Data from these studies are used as a basis of comparison with modeling results. In particular, Ga coring-profile modeling appears to be a powerful tool for predicting alloy homogenization.


I. INTRODUCTION


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, a8, 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, a (monoclinic), is extremely brittle and oxidizes readily. The addition of 1 wt pct Ga stabilizes the face-centered cubic S structure, a phase with superior mechanical properties when compared to a Pu. Other elements can act as S 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 6--phase grains with Ga-rich cores and Ga-poor edges. The Ga-poor edges are metastable and can transform to a phase during cold working, metallo­


graphic sample preparation, or storage.[1,2,3] Thermally


driven homogenization is required to evenly redistribute Ga and to achieve complete 6--phase stabilization. Homogeniza­tion is a function of the interdiffusion coefficient for Ga in S Pu (,6GPau), 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.


important in predicting the behavior of the alloy. Although I)GaPu 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 6--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. Microsegregation in the Pu-Ga System


Phase relations within the Pu-Ga system have been studied in detail[4,5] and provide an initial basis for the understanding of the metallurgical properties of the system. Recent reinves­tigation of the Pu-Ga phase diagram produced by Russian scientists during the 1970s indicates that Ga-stabilized 6­


­


phase Pu may be metastable at room temperature.[6,7] How­ever, the kinetics of transformation to the proposed equilib­rium assemblage a Pu + Pu3Ga are apparently extremely sluggish.[8] The area of focus in this article is the Pu-rich portion of the phase diagram (Figure 2) and, in particular, the two-phase e + S region. The compositional path of a grain of Pu-1 wt pct Ga is depicted in Figure 2. Starting