Experimental determination of phase equilibria in the Co–Si binary system

Accurate phase-equilibria data for the Co–Si binary system are crucial for thermodynamic assessment and CALPHAD-based design of Co-based alloys. However, existing experimental data show notable discrepancies. Therefore, in this study, the phase equilibria of the Co–Si binary system were experimentally investigated using differential scanning calorimetry (DSC) and a field-emission electron probe microanalyzer equipped with wavelength-dispersive-X-ray spectroscopy (FE-EPMA/WDS). The phase diagram was determined over the entire composition range, and critical revisions were made to invariant reaction temperatures, phase boundaries and solubility ranges of intermetallic compound phases. DSC measurements revealed that the solidus line of γ(Co) is higher than values reported in earlier studies. The invariant reaction temperatures associated with the Co 3 Si phase were accurately determined using slow-heating (2 °C/min) DSC measurements, confirming that Co 3 Si exists as a stable phase in the temperature range of 1197–1206 °C. FE-EPMA/WDS measurements demonstrated that the Co 3 Si phase exhibits an extremely narrow solubility range, whereas the CoSi 2 phase has a finite solubility range extending toward the Si-rich side. For the α- and β-Co 2 Si phases, the single-phase region of β-Co 2 Si phase was found to be larger than that calculated in the previous assessment. The revised Co–Si phase diagram presented herein provides valuable insights for the design and optimization of Co-based alloys for thermoelectric and biomedical applications. • The phase equilibria in the Co–Si binary system were experimentally determined over the entire composition range. • The Co₃Si phase is stable in a narrow temperature range of 1197–1206 °C. • The CoSi₂ phase exhibits a solubility range extending toward the Si-rich side. • The magnetically influenced bending of the ε(Co) and γ(Co) phase boundaries was experimentally identified between 700 and 900 °C.