Thermodynamics of Pd-Mn phases and extension to the Fe-Mn-Pd system

A thermodynamic description of the Pd-Mn system and the extension to the Fe-Pd-Mn system is developed within the framework of the CALPHAD approach. Thermodynamic data of the Pd-Mn system are critically reviewed and parameters of thermodynamic phase descriptions are optimized by selective combination of experimental data from the literature as well as published and new first-principles results. Phase diagram data are obtained from published microscopic and x-ray analysis. A previous calorimetric study and density functional theory results deliver enthalpies of formation of intermetallic Pd-Mn phases, and assessed Mn-activities are compared with experimental data derived from electromotive force and isopiestic experiments. A large difference between calorimetric high-temperature CALPHAD-assessed and first-principles enthalpies at 0 K of the PdMn-B2 phase is revealed and discussed. For the extension to the Fe-Mn-Pd system, thermodynamic descriptions of Pd-Fe intermetallics are revised using recent experimental and theoretic thermodynamic standard data and measured temperatures of magnetic transformations from the literature. A comprehensive thermodynamic standard data set of the intermetallic phases PdMn-B2, PdMn-L10, Pd5Mn3, Pd2Mn, Pd3Mn-D023, Pd3Mn-L12, Pd3Fe-L12, and PdFe-L10 is proposed. A set of experimental data of Fe-solubility in PdMn-L10, the phase compositions of the L10-phase in equilibrium with austenite, and the observed austenite+liquid equilibrium is used for extensions of model descriptions to ternary Fe-Mn-Pd. Heat-treated Fe-rich Fe-Mn-Pd alloys containing 5-20 wt% Mn and 1-10 wt% Pd reveal considerable stabilization of the tetragonal PdMn-L10 by dissolved Fe leading to an increase of the solvus temperature by more than 650 K relative to pure PdMn-L10. Assessed isothermal sections of the ternary Fe-Mn-Pd system are presented.