Thermodynamic equilibrium in the low-solute regions of Pu-group MIA metal binary systems
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INTRODUCTION
T H E nature of the equilibrium phase boundaries in the low-solute regions of the Pu-A1 and Pu-Ga binary diagrams has been disputed by Pu metallurgists and thermodynamicists for a number of years. The first two Pu-AI diagrams, presented by Schonfeld flj in 1957 and followed by Bochvar e t a l . ~21 in 1958, are shown in Figures 1 and 2, respectively, and illustrate contended phase boundaries. Specifically, the diagram shown in Figure 1 indicates the 6 phase is stable to low temperatures, whereas Figure 2 shows a eutectoid decomposition at 448 K of 6 --~ fl + Pu3A1. Since supporting data on which these diagrams were based were not included in the publications, Ellinger et al. ~3~ presented a revised Pu-A1 diagram, shown in Figure 3, in which accurate determinations of 6 phase boundaries were made using dilatometric, X-ray diffraction, and thermal analyses techniques. Other than lowering the 6 / 6 + a boundary to about 2 at. pct AI at room temperature, indicating a greater compositional range of 6-phase stability, Ellinger e t a l . ' s diagram is essentially the same as that in Figure 1. A similar dispute surrounding the Pu-Ga phase diagram arose in 1975 when Chebotarev et a l . [4] presented X-ray, metaUographic, diffusion layer, and high-pressure evidence indicative of a 6---> a + Pu3Ga eutectoid reaction at 373 K for this system. Previous work by Ellinger et al. tS] in 1964 and Hocheid et a l . [61 in 1965 had shown an extended 6-stability range similar to that of the Pu-AI diagram in Figure 3. The Pu-Ga phase diagrams from the work of Ellinger et al. and Chebotarev et al. are presented in Figures 4 and 5, respectively. The most recent Pu-A1 and Pu-Ga phase diagram assessments were published in 1986 as part of the A S M / N B S binary alloy phase diagram assessment pro-
gram. t7'81 Both diagrams are essentially those given by Ellinger et al. p,51 Low-temperature phase boundary determinations involving in situ measurements of transformation temperatures or pressures naturally lead to the investigation of mechanisms responsible for the observed transformations. Transformations involving the high-temperature bcc(e)-Pu phase are fairly well established as being diffusional-nucleation and growth, [9"10] while the very low diffusivity of substitutional atoms in the 6 phase (D 10 -12 cm2s-1) [11'121 has raised the issue of diffusionless mechanisms, and hence, the degree of thermodynamic stability, for low-temperature transformations involving this structure. In particular, the 6 ---> a transformation exhibits many characteristics indicative of a martensitic mechanism. We here discuss fundamental kinetic characteristics of a martensitic transformation which result in a metastable configuration of the product phase and present a thermodynamic model which provides sufficient information for prediction of the low-solute regions of Pu-A1 and Pu-Ga phase diagrams via Gibbs free energy minimization techniques such as used by the thermochemical-based computer code F * A * C * T (Facility for the Analysis of C
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