On the constitution and thermodynamics of Ni-La alloys

The thermodynamic properties of Ni-La alloys were determined between 923 and 1023 K by EMF measurements using CaF2 single crystals as solid electrolytes. The results yield a complete set of thermodynamic functions for the intermetallic phases (Ni), Ni,La, Ni17La4, Ni7La2, Ni16La7, Ni3La2, NiLa, Ni3La7, and NiLa3 as well as information on the phase relations. The system is characterized by pronounced negative deviations from ideality. The relative partial excess Gibbs energy of La at infinite dilution, Delta(G) over bar (E)(La)(x(La) --> 0), was determined to be - (83+/-3) kJ/mol. This value is interpreted by taking electronic and elastic effects into account. The liquidus curve was fixed over the entire range of composition by differential thermal analysis. The present results were used as basis for a thermodynamic optimization of the system using a computer program based on the least squares method. Optimized and experimental data turned out to be in good agreement. (C) 2000 Elsevier Science B.V. All rights reserved.