Thermodynamic Properties of Melts in the Ni–Tb System

The isoperibolic calorimetry method was used to determine the mixing enthalpy of liquid alloys in the Ni–Tb system in the composition range 0 < x_Ni < 0.6 at 1660 ± 1 K. The minimum mixing enthalpy of melts in this system was –41.8 ± 0.9 kJ/mol at x_Ni = 0.6. The activities of components and the mole fractions of associates in these melts were calculated according to the ideal associated solution (IAS) model with our and literature values of formation enthalpies for compounds in the Ni–Tb system and with phase diagram data. Two associates were selected for the calculations: TbNi and TbNi₅. The activities of the components showed large negative deviations from the ideal solution, with the simplest associate, TbNi, being predominant (x_max = 0.65). The second associate was present in a much smaller proportion (x_max = 0.22). These data correlate with the mixing enthalpies of the melts, formed with significant exothermic effects. To assess the reliability of the formation enthalpies of compounds and melts in the Ni–Tb system, they were compared with those of LnNi₅ compounds and liquid alloys in the Ni–Ln system. All were determined with different options of the calorimetry method. Hence, to be compared, they were plotted as a function of the Ln atomic number. Most of the data points aligned with two trend lines, except for the data for compounds in binary Ni–Gd(Dy, Er) systems and melts in binary Ni–Ce (Eu, Yb) systems. Regarding these ΔH_min values, which are more exothermic (Ni–Ce system) and less exothermic (Ni–Eu(Yb) systems) than all others, they may be attributed to the electronic structures of atoms in the components of the melts. The Eu and Yb atoms are known to have half-filled and completely filled 4f orbitals, while the Ce atom contains one electron in the 4f orbital. Therefore, Eu and Yb are divalent and Ce is tetravalent in the nickel alloys. Since nickel is a strong electron acceptor, the energy of its interaction with Ce is greater and that with Eu and Yb is lower compared to other neighboring lanthanides.