Phase stability and phase transformations in plutonium and plutonium-gallium alloys
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6/30/04
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Phase Stability and Phase Transformations in Plutonium and Plutonium-Gallium Alloys JEREMY N. MITCHELL, MARIUS STAN, DANIEL S. SCHWARTZ, and CARL J. BOEHLERT The complexity of phase stability and transformations in plutonium alloys is reflected in the plutoniumgallium (Pu-Ga) phase diagram, which is perhaps the most complex of all binary systems. Although many investigations have explored phase equilibria, transformation systematics, and structure/property relations in the Pu-Ga system, many outstanding problems remain and new issues regularly appear. In this article, we describe recent dilatometry and calorimetry measurements on pure plutonium and plutonium-gallium alloys. We also present recent phase diagram modeling that attempts to unravel differences between the U.S. and Russian Pu-Ga phase diagrams. The ultimate goal of this work is to produce the first internally consistent database of thermophysical properties of this system so that a true equilibrium phase diagram can be produced and so that stability can be predicted over a range of conditions.
I. INTRODUCTION
UNDERSTANDING the stability of plutonium (Pu) and its alloys has been a scientific challenge since the early days of the Manhattan Project. This challenge is best represented by the six allotropes that pure Pu forms from room temperature to melting. These phases all have distinctly different crystal structures with significantly different cell volumes that are easily distinguished using a variety of in-situ analytical techniques, such as dilatometry and differential scanning calorimetry (Figure 1). Perhaps the most astonishing aspect of these allotropes is that the room-temperature phase, , has the low symmetry monoclinic structure yet is the highest density phase. Alpha Pu is hard and brittle, making it difficult to machine. Addition of 1 at. pct Ga will stabilize the softer face-centered cubic phase, which has proven to be technologically the most useful allotrope. Stabilization of phase presents additional complexities to plutonium, such as the appearance at low temperatures of the martensitic phase, the effects of aging on stability, and the sensitivity of physical properties to solute addition. Recent focus on the Russian Pu-Ga phase diagram and its differences with the more widely accepted U.S. diagram is an important example of the elusive nature of our knowledge of phase stability in plutonium alloys (Figure 2).[1] In this article, we will highlight recent dilatometry and calorimetry experiments on stability and transformations in pure Pu and Ga-stabilized Pu. In particular, we are studying the effects of composition, microstructure, and age on the stability of these materials. The intent of these experiments is JEREMY N. MITCHELL, and DANIEL S. SCHWARTZ, Technical Staff Members, Nuclear Materials Technology Division, and MARIUS STAN, Technical Staff Member, Materials Science and Technology Division, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] CA
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