Thermodynamic assessment of the Co-Cr-Fe-Ni quaternary system and diffusion study of its fcc phase

As a core of the high entropy alloys, the Co-Cr-Fe-Ni system has been widely investigated. In the present work, the thermodynamics of the Co-Cr-Fe-Ni system and the atomic mobilities of its fcc phase have been evaluated by means of the CALPHAD approach. First-principles calculations were performed to obtain the total energies for the end-member compounds of the σ phase in the Co-Cr-Fe-Ni system. Combining with the experimental data and thermodynamic modeling of the sub-systems from the literature, a set of self-consistent thermodynamic parameters were derived and extrapolated to obtain a thermodynamic description of the Co-Cr-Fe-Ni quaternary system. In order to verify the accuracy of the model parameters, the phase equilibria of a series of the CoCr_xFeNi alloys with different Cr contents were determined using DSC, BSE and XRD analysis. Subsequently, based on the diffusion experimental data, the atomic mobilities of the fcc Cr-Fe-Ni alloys were reassessed using the DICTRA software. A mobility database for the fcc Co-Cr-Fe-Ni quaternary system was constructed by directly extrapolating the atomic mobilities of all sub-systems, and comprehensive comparisons prove the consistency between the present assessments and the experiments. In addition to the direct extrapolation approach, extra four-body interaction parameters concerning all four components were added and assessed. The results demonstrate that the extra interaction contributions are ignorable, so that the direct extrapolation from the sub-systems to the quaternary system is feasible in the fcc Co-Cr-Fe-Ni quaternary system.