Thermodynamic description of the Co–Ni–Ta system

Abstract The temperatures of two invariant reactions λ3 → fcc(Co) + Co3Ta and λ3 → Co3Ta + λ2 in the Co–Ta system were identified to be 1320 and 1303 K, respectively, by Differential thermal analysis (DTA). The Co–Ta, Ni–Ta and Co–Ni–Ta systems were optimized using the CALculation of PHAse Diagram (CALPHAD) method based on the present experimental results and literature data. Three Laves phases λ1, λ2 and λ3 were described using a two-sublattice model (Co,Ni,Ta)0.6667(Co,Ni,Ta)0.3333, and compound (Co,Ni)Ta was modeled as (Co,Ni,Ta)1Ta4(Co,Ni,Ta)2(Co,Ni,Ta)6 by a four-sublattice model. A set of reliable and self-consistent thermodynamic parameters was obtained, which can be used for a variety of thermodynamic calculations and database establishment of the Co–Ni-based superalloys.