Evidence of Embryo Formation as a Precursor to the Delta to Alpha-Prime Transformation in a Pu-Ga Alloy
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0893-JJ04-06.1
Evidence of Embryo Formation as a Precursor to the Delta to Alpha-Prime Transformation in a Pu-Ga Alloy K.J.M. Blobaum1, C.R. Krenn2, M.A. Wall1, T.B. Massalski3, and A.J. Schwartz4 1 Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550 U.S.A. 2 Lawrence Livermore National Laboratory, Livermore, CA 94550 U.S.A. 3 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 U.S.A. 4 Physics and Advanced Technologies Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550 U.S.A.
ABSTRACT It was experimentally observed that a single Pu-2.0 at% Ga sample can be thermally cycled many times, with nearly the same amount of transformation and reversion during each cycle, provided that the sample is annealed at 375°C for 8 hours and then conditioned at 25°C for at least 6 hours. The ambient temperature conditioning period is crucial for obtaining the same amount of transformation to alpha-prime in each thermal cycle. Here, we present results from a series of experiments that investigate the effects of conditioning time and temperature on alphaprime formation. When the sample is conditioned at 25°C for times between 0 and 6 hours, the amount of alpha-prime formed is a function of conditioning time. For conditioning treatments between 6 and 70 hours, however, the amount of alpha-prime formed is nearly constant. Conditioning treatments at –50°C, 150°C, and 370°C all resulted in less alpha-prime formation than the 25°C treatments. We hypothesize that embryos of the thermodynamically stable alpha phase form during the conditioning treatments, and these embryos form alpha-prime upon subsequent cooling. This result is an indirect confirmation of the Russian Pu-Ga equilibrium phase diagram. INTRODUCTION In a Pu-2.0 at% Ga alloy, the fcc δ phase is metastable at room temperature. Upon cooling, a partial transformation to the metastable monoclinic α’ phase occurs at approximately –120°C, and the α’ begins to revert to the δ phase upon heating to approximately 32°C. The transformation and reversion have been studied extensively under conditions of continuous cooling/heating in a differential scanning calorimeter (DSC). The transformation and reversion can occur reproducibly many times (>100) in a single sample, provided that an appropriate annealing scheme is implemented after each thermal cycle. Specifically, the sample is annealed at 375°C for 8 hours and then conditioned at 25°C for at least 6 hours. The ambient temperature conditioning step is vital for maximizing the amount of α’ formed upon subsequent cooling. The δ to α’ phase transformation is reported to be an isothermal martensite [1], although it can also occur on continuous cooling, like all isothermal martensitic transformations above their lower temperature limit. Both the transformation and the reversion occur via a burst martensite mode [2]. The Pu-rich side of the Pu-Ga equilibrium phase diagram is shown in Figure 1 [3, 4]. The martensite burst (Mb
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