Phase equilibria in Ti-rich portion and thermodynamic re-optimization of Co–Ti system

The experimental and thermodynamic studies of the Co–Ti system are an important part of the project to build a thermodynamic database for multicomponent Co–Ti-based alloys. Several key alloys were prepared and then examined for microstructural, compositional and thermal analyses to determine the Ti-rich phase equilibria. According to the available experimental information, the Co–Ti system was thermodynamically re-optimized on the basis of CALPHAD method. Four disordered solutions, liquid, fcc-A1 (α-Co), bcc-A2 (β-Ti) and hcp-A3 (ε-Co and α-Ti) were modeled as substitutional ones. CoTi₂ with limited solubility was treated as a stoichiometric compound, while γ-Co₂Ti and β-Co₂Ti with certain solubility were described in the form of (Co,Ti)₂(Co,Ti)₁ using a two-sublattice model. A single Gibbs energy function was employed to model two order–disorder transformations from fcc-A1 to fcc-L1₂ (Co₃Ti) and from bcc-A2 to bcc-B2 (CoTi). A group of self-consistent thermodynamic parameters of the Co–Ti system were obtained. With these thermodynamic parameters, the experimental data can be described more reasonably and satisfactorily.