Thermodynamic Properties and Phase Equilibria of Nd–Ni Alloys

Isoperibolic calorimetry has been used for the first time to determine the mixing enthalpies of binary Nd–Ni liquid alloys in the range 0 < x Ni < 0.5 at 1733 K and range 0.55< x Ni < 1 at 1773 K. The binary Nd–Ni melts are characterized by significant negative mixing enthalpies with a minimum of –33.9 ± 0.8 kJ/mole at x Ni = 0.63 and 1750 K. In the temperature range studied (1733–1773 K), the mixing enthalpies of the melts are described by polynomial ΔH = x Ni (1 – x Ni )× × (–75.32 – 103.41 x Ni + 273.45 x Ni 2 – 817.02 x Ni 3 +548.58 x Ni 4 ). The ideal associated solution (IAS) model was used to calculate the activities of components, molar fractions of associates, Gibbs energies, and mixing entropies using our ΔH and Δ H ¯ i and literature data on the formation enthalpy of nickelides and the Nd–Ni phase diagram. All intermediate phases were considered stoichiometric, with zero excess heat capacity. Five associates were selected for the calculation: Nd 2 Ni, NdNi, NdNi 2 , NdNi 3 , and NdNi 5 . Most of them agree in composition with the intermetallides in this system. Only one of them does not exist in solid state. This is associate Nd 2 Ni, whose composition is close to that of the Nd 7 Ni 3 intermetallide. The activities of the Nd–Ni melt components show high negative deviations from ideal solutions. Associates of simplest composition, NdNi and NdNi 2 , are predominant in the melts. The mixing entropies of Nd–Ni liquid alloys are negative (minimum value is close to –6.4 J/mole · K). All our thermodynamic properties indicate that there is strong energy of interaction between Ni and Nd. The liquidus curves of the Nd–Ni system were calculated using the IAS model parameters.