Enabling the Double-C Curve in Pu-Ga Alloy Time-Temperature-Transformation Diagrams
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1104-NN01-07
Enabling the Doulbe-C Curve in Pu-Ga Alloy Time-Temperature-Transformation Diagrams Jason R. Jeffries, Kerri J. M. Blobaum, Mark A. Wall, and Adam J. Schwartz Lawrence Livermore National Laboratory, Livermore, CA, 94550 ABSTRACT Under ambient conditions, a Pu-2.0 at.% Ga alloy is retained in the metastable δ phase. Upon cooling to approximately -120 °C, the face-centered-cubic δ phase partially transforms to the metastable monoclinic α’ phase via a martensitic transformation. The kinetics of the δ → α’ transformation are reported to have double-C curve kinetics in a time-temperature-transformation (TTT) diagram, but the mechanisms responsible for this unusual behavior are not understood. Our work focuses on determining the underlying cause of the two noses. Optical microscopy has been used to investigate the role of “conditioning”—an isothermal hold at sub-anneal temperatures—on the δ → α’ transformation and to illuminate any disparities in transformation products. Conditioning was found to affect substantially the amount of transformation that occurs at particular points corresponding to both the upper- and lower-C of the TTT diagram. INTRODUCTION The metastable face-centered-cubic (fcc) δ phase of Pu can be retained at room temperature by adding a few atomic percent of any of several dopants including gallium, aluminum, and americium. In the case of Ga, the addition of 2.0% Ga to Pu yields a δ phase that is metastable, albeit with exceedingly long kinetics [1, 2], with respect to eutectoid decomposition (α + Pu3Ga) as well as a martensitic phase transformation at sub-ambient temperatures near -120 °C yielding the metastable α’ phase, which crystallizes in the same structure as that of α-Pu. The formation of the α’ martensite is sufficiently fast to prohibit diffusion of Ga, which is relatively insoluble in the α phase of Pu, leading to the expanded α’ lattice. This latter martensitic instability has been found to proceed isothermally with anomalous double-C kinetics on a time-temperature-transformation (TTT) diagram, wherein there are two temperatures that define the maximal rate of transformation [3]. It has been postulated that the δ → α’ transformations in the upper- and lower-C proceed through different mechanisms, such as the presence of intermediary phases [2, 4, 5] or a massive transformation [3, 5]. Previous differential scanning calorimetry experiments suggest multiple transformation products, mechanisms, or a combination thereof [6, 7]. In addition, these calorimetry experiments have intimated a significant connection between a “conditioning” process—an isothermal hold at subanneal temperatures—and the amount of δ → α’ transformation that occurs upon cooling. Metallography can be used to illuminate several aspects of a phase transformation including the amount of transformation as well as details of the resultant morphology. In this article, we present the results of optical metallography experiments designed to reveal the microstructure resulting from the δ → α’ transformation and elucidat
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