THERMODYNAMIC PROPERTIES AND PHASE EQUILIBRIA OF Nd–Ni ALLOYS

Abstract

Isoperibolic calorimetry has been used for the first time to determine the mixing en-thalpies 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 х_Ni = 0.63 and 1750 K. In the studied temperature range (1733–1773 K), the mixing enthalpies of the melts are described by polynomial ΔH = x_Ni (1-x_Ni) • (-75,32 - 103,41x_Ni + 273,45x²_Ni  - 817,02x³_Ni  + 548x⁴_Ni)^,. 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 ∆Н and and literature data on the formation enthalpy of nickelides and the Nd–Ni phase diagram. All intermediate phases were considered stoichiometric, which zero excess heat. Five associates were selected for the calculation: Nd₂Ni, NdNi, NdNi₂, NdNi₃, and NdNi₅. Most of them agree in composition with the intermetallics in this system. Only one of them does not exist in solid state. This is associate Nd₂Ni, whose composition is close to that of Nd₇Ni₃ intermetallide. The activities of the Nd–Ni melt components show high negative deviations from ideal solutions. Associates with simplest composition, NdNi and NdNi₂, are predominant in the melts. The mixing entropies of Nd–Ni liquid alloys are negative (minimum value is close to –6.4 J/mole • К). 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.