Thermodynamic assessment of the Co–Fe–Ni system and diffusion study of its fcc phase

Abstract

A specially designed diffusion triple and three diffusion couples were assembled to respectively determine isothermal sections of the Co–Fe–Ni alloy system at 1073 K and 1173 K. The binary thermodynamic parameters of the fcc and bcc phases in the Fe–Ni system were modified and further used to reassess the Co–Fe–Ni system above 873 K, neglecting the low-temperature ordered phases. Based on the present thermodynamic data, the atomic mobilities of Fe–Ni fcc phase were assessed to reproduce the literature data as well as the present extracted interdiffusion coefficients from two Fe–Ni diffusion couples. Furthermore, nine ternary Co–Fe–Ni diffusion couples annealed at 1173 K were scanned by using electron probe microanalysis and used to deduce the interdiffusion coefficients. Based on these coefficients and experimental data from 1373 K to 1588 K in the literature, the atomic mobilities of Co, Fe, and Ni coupled with the present thermodynamic description were assessed by means of DICTRA software, and verified by comparing with experimental composition profiles and diffusion paths. Although interdiffusion coefficients are treated as solvent-dependent so three sets of interdiffusion coefficients can be deduced from the same sets of ternary experimental data, the assessed atomic mobilities are unique and applicable in the whole composition range. This methodology is especially vital when studying diffusion behaviors for multi-principal element alloys (also known as complex concentrated alloys, or high entropy alloys).