Thermodynamic description and solidified microstructure of the Co-Ge system

Abstract

The Co–Ge binary system was reassessed by CALPHAD method in view of new phase diagram data and drawbacks of the previous modeling. A three-sublattice model (Co,Va)1(Co)1(Ge)1 was used to describe the B82-type βCo5Ge3-phase based on its crystal structure. In order to describe the transformation between ordered L12-type phase (Co3Ge) and the disordered fcc_A1 phase (αCo), one single Gibbs energy function was used for both ordered and disordered phases. In almost all the previous thermodynamic calculations, the ordered phase with a negligible homogeneity range, such as Co3Ge, is treated as a stoichiometric compound. Such a treatment is not physically sound. For (αCo) and (εCo), the magnetic contribution to Gibbs energy is taken into account. Both substitutional solution model and associated model were applied to describe the liquid phase, and thus two sets of self-consistent thermodynamic parameters for this system were obtained. It was found that the associated model can account for the experimental data more satisfactorily than the substitutional solution model, especially for the partial enthalpy of mixing data for the liquid phase. Five representative as-cast Co-Ge alloys were prepared to compare the solidified microstructure with that predicted according to thermodynamic calculations. According to the calculated Scheil solidification curves of two key alloys, the solidified microstructure for the alloys was analyzed. Also, the calculated amounts of the solidified phases in five as-cast alloys are compared with the experimental data resulting from automatic image analysis of the BSE images, showing a good agreement between the calculation and experiment, in particular for the case in which the associated model is used to describe the properties of the liquid phase. It is demonstrated that the combined use of the thermodynamic calculation and decisive experiment is an efficient strategy to obtain the desired microstructure.