Experimental investigation and thermodynamic description of the Co–Si system

The Co–Si system is investigated via experiment and modeling. Based on the critical evaluation of the phase diagram data available in the literature, one key Co/Si diffusion couple and eight decisive alloys are prepared. The diffusion couple, which is annealed at 1323.15 K for 8 days, is first examined by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), and then by electron probe microanalysis (EPMA) to determine homogeneity ranges of the phases. The alloys, which are annealed at 1373.15 K for 3 days and then water-quenched, are analyzed using X-ray diffraction (XRD), optical microscopy, differential thermal analysis (DTA), and EPMA. The thermodynamic optimization for the Co–Si system is then conducted by using the assessed literature data and the present experimental data. For (αCo) and (εCo), the magnetic contribution to the Gibbs energy is taken into account. The sublattice model is employed to describe βCo2Si (the high-temperature form of the Co2Si phase), αCo2Si (the low-temperature form of the Co2Si phase) and CoSi. A set of self-consistent thermodynamic parameters is finally obtained. Comprehensive comparisons show that the calculated phase diagram and thermodynamic properties agree well with the experimental ones. Significant improvements have been made, compared with the previous assessments.