The thermodynamic assessment of the ternary Co–Ni–Sb system

Abstract

The Co–Ni–Sb ternary system is critically assessed using the CALPHAD technique on the basis of three optimized binaries, the Co–Ni, the Co–Sb and the Ni–Sb systems. The NiAs-type structure phase γ-(Co,Ni)Sb with a wide non-stoichiometric range is described by three sublattices, (Sb) 1/3(Co,Ni,V a) 1/3(Co,Ni,V a) 1/3, following the extension of the sublattices of the phases γ(CoSb) and γ(NiSb) in the related binary systems. The sublattice model is also used to describe the terminal ternary solid solution phases: three sublattices (Sb) 1/4(Co,Ni,V a) 1/2(Ni,V a) 1/4 for β(Co,Ni) 3Sb and two sublattices (Co,Ni) 3/4(Ni,Sb) 1/4 and (Co,Ni) 5/7(Ni,Sb) 2/7 for δ(Co,Ni) 3Sb and θ(Co,Ni) 5Sb 2 respectively. The intermetallic compounds ζ(Co,Ni)Sb 2 and η(Co,Ni)Sb 3 are treated as linear stoichiometric compounds. The liquid, the α Fcc_A1 and the ε Hcp_A3 phases are assumed to be substitutional solutions with Redlich–Kister polynomials for the expressions of the excess Gibbs energies. The Sb Rhombo_A7 phase is considered to be the pure Sb due to the very small and negligible solubilities of Co and Ni in it. A set of self-consistent thermodynamic parameters of the Co–Ni–Sb system is obtained according to eleven experimental isothermal sections and one isoplethal section. Most of the experimental data can be satisfactorily reproduced by the present thermodynamic modeling. The liquidus projection and the reaction scheme are also presented.