# Development of a CALPHAD Thermodynamic Database for Pu-U-Fe-Ga Alloys
## 3 Literature Review
### The Plutonium-Iron Phase Diagram  
The Pu-Fe system has been experimentally investigated by [PERSON] [43], [PERSON] [44], [PERSON] [45] and [PERSON] [46] using thermal analysis, dilatometry, metallography, x-ray diffraction (XRD), microhardness and viscosity measurements. The full phase diagram was studied by [43] and [45] whereas the authors in [44] focused on the PuFe\({}_{2}\)-Fe region and [46] investigated the liquidus range on the Pu-rich side up to 13 at. % Fe. Formation enthalpies of the intermetallic PuFe\({}_{2}\) and Pu\({}_{6}\)Fe phases were measured/estimated by [47; 48; 49] and [49] respectively. Elemental plutonium exists in 6 allotropic phases, including: simple-monoclinic (\(\alpha\)-Pu), body-centered monoclinic (\(\beta\)-Pu), face-centered orthorhombic (\(\gamma\)-Pu), face-centered cubic (fcc or \(\delta\)-Pu), body-centered tetragonal (\(\delta^{\prime}\)-Pu) and the body-centered cubic (bcc or \(\varepsilon\)-Pu). The lower-temperature Pu-allotropes (\(\alpha\), \(\beta\) and \(\gamma\)) exhibit limited (negligible) solubility of iron [50], whereas the higher-temperature phases (except \(\delta^{\prime}\)-Pu) were found to have between 0.6-1.5 at. % Fe and 2-2.5 at. % Fe for \(\delta\)-Pu and \(\varepsilon\)-Pu respectively, with [PERSON] [45] predicting higher solubilities than [PERSON] [51]. An interesting attribute of the Pu-rich side of the phase diagram includes a catatectic reaction (430 \({}^{\circ}\)C) between \(\varepsilon\)-Pu and \(\delta\)-Pu; upon cooling of \(\varepsilon\)-Pu, a reappearance of the liquid phase (partial melting) occurs and is in equilibrium with the \(\delta\)-Pu phase. The eutectic reaction: [Liquid \(\leftrightarrow\) Pu\({}_{6}\)Fe + \(\delta\)-Pu] was found at 90 at. % Pu by [45], whereas other authors located the eutectic around 89-89.5 and 91.5 at. % Pu by [44] and [46] respectively. Pu\({}_{6}\)Fedecomposes peritectically into PuFe\({}_{2}\) and liquid phases at 430 \({}^{\circ}\)C according to [43] or 428 \({}^{\circ}\)C according to [45]. Thermal analysis measurements of Pu\({}_{6}\)Fe by [52] indicate an onset melting temperature at 411.5 \({}^{\circ}\)C with a melting enthalpy of 6.77 kJ/mol. The PuFe\({}_{2}\) intermetallic compound consists of 3 distinct polymorphs, with transition temperatures of 771 \({}^{\circ}\)C and 1020 \({}^{\circ}\)C for \(\alpha\)-PuFe\({}_{2}\)\(\rightarrow\)\(\beta\)-PuFe\({}_{2}\) and \(\beta\)-PuFe\({}_{2}\)\(\rightarrow\)\(\gamma\)-PuFe\({}_{2}\) respectively; the compound melts congruently at 1240 \({}^{\circ}\)C. A eutectic reaction [Liquid \(\leftrightarrow\) PuFe\({}_{2}\) + \(\gamma\)-Fe] was identified by [45] at 18 at % Pu and 1165 \({}^{\circ}\)C, while other studies [43, 44] placed the same reaction at 19 at. % Pu and 1180 \({}^{\circ}\)C [43]. Elemental iron transforms from bcc (\(\alpha\)-Fe) to fcc (\(\gamma\)-Fe) at 912 \({}^{\circ}\)C and back to bcc (8-Fe) at 1394 \({}^{\circ}\)C and melts at 1538 \({}^{\circ}\)C. Very low solubility of Pu is reported in \(\alpha\)-Fe, at less than 0.02 at. % Pu [53] and a eutectoid reaction [\(\gamma\)-Fe \(\leftrightarrow\)\(\alpha\)-Fe + Fe\({}_{2}\)Pu] at 907 \({}^{\circ}\)C is observed by [44]. Higher solubility of Pu in the fcc (\(\gamma\)-Fe) phase was reported by [44] with an average of 0.7 at. % Pu. The peritectic transformation [L + \(\delta\)-Fe \(\leftrightarrow\)\(\gamma\)-Fe] is suggested by [54] due to the presumed lower solubility of Pu in the high temperature bcc (\(\delta\)-Fe) phase. Based on these experimental investigations, a complete thermodynamic assessment of the Pu-Fe phase diagram has been reported by [2, 3]. In these assessments, the Pu-rich side did not include the solubility of Fe in the \(\delta\)-Pu and \(\varepsilon\)-Pu, which are pertinent to this study. Additionally, the intermetallic compounds were modelled using the liquid phase as their reference state, which is not consistent with the most recent and generally accepted CALPHAD-formulation for the standard reference state.