The phase equilibria and solidification microstructure of La–Co–Cu alloys were investigated experimentally. The results confirmed the existence of the ternary compound LaCu12Co with the NaZn13-type structure and Fm \(\overline{3 }\) c space group at different temperatures of 873 K, 1073 K, and 1273 K. The homogeneity range of this ternary compound was measured to be 1.5–2.9 at. pct Co at 873 K and 1.8–3.4 at. pct Co at 1273 K, while the composition of La in this ternary compound was found to be approximately 7.7 at. pct. It was observed that a continuous solid solution La(Co, Cu)5 phase was formed from LaCo5 and LaCu5. The solubility of Co in β-LaCu6 and LaCu2 at 873 K was determined to be 3.1 and 4.1 at. pct, respectively, while that of Cu in LaCo13 was measured to be 1.8 at. pct at 873 K and 5.4 at. pct at 1273 K. Furthermore, based on the phase equilibria results obtained in this study and the reported experimental information, a thermodynamic calculation of the La–Co–Cu system was conducted using the CALPHAD method, incorporating the previous calculations of the La–Co, La–Cu, and Co–Cu systems. The solidification microstructure of as-cast La–Co–Cu alloys was analyzed through experimental investigations and thermodynamic calculations using the Scheil-Gulliver non-equilibrium model. The simulation results showed the good agreement with the experimental findings, indicating that self-consistent and reasonable thermodynamic parameters for the La–Co–Cu system were obtained. These parameters provide a solid foundation for the establishment of a thermodynamic database for Sm–Co–Cu-based permanent magnets.
