Thermodynamic description of the Ni–Sc system

• The Ni–Sc system was not previously thermodynamically assessed.
• The Calphad method was used to optimize the thermodynamic parameters.
• For the solution phases, two models were compared for the temperature dependence of the excess parameters: linear and exponential.
• Both optimizations led to very similar calculated phase diagrams.
• A good agreement was obtained with the thermodynamic and experimental phase diagram data.

The Ni–Sc system was thermodynamically assessed by the CALPHAD approach based on the available experimental data including the thermodynamic properties and phase equilibria. The excess term of the Gibbs energy of the solution phases (liquid, b.c.c., f.c.c. and h.c.p.) was assessed with the recent exponential temperature dependence of the interaction energies by Kaptay (Calphad 28–2 (2004) 115–124; Calphad 32–2 (2008) 338–352; Mat. Sci. Eng. A 495 (2008) 19–26) and compared with Redlich and Kister (Ind. Eng. Chem. 40 (1948) 345–348) polynomial equation results. The intermetallic compound Ni2Sc in this binary system which has a homogeneity range, was treated by a two-sublattice model (Sundman et al., Calphad 9 (1985) 153–190; Hillert and Staffansson, Acta Chem. Scand 24 (1970) 3618). The others compounds were modeled as stoichiometric. A consistent set of thermodynamic parameters was optimized to give account of the available experimental and thermodynamic data.