A thermodynamic description of the Al–Co–Ni system and site occupancy in Co + AlNi3 composite binder phase

Abstract

Phase equilibria of the Al–Co–Ni system is extremely useful for understanding phase behavior of AlNi3-based L12 phases, regarding its attractive properties, such as high-temperature oxidation resistance and corrosion resistance, high hardness and strength. In the present work, all the experimental data about the phase equilibria and thermodynamic properties of the Al–Co–Ni system were critically evaluated, and a thermodynamic description for this system over the whole composition and temperature ranges was obtained. Most importantly, the ordered/disordered transition between fcc_A1 and L12 phases was described using a four-sublattice (4SL) model. Based on the crystallographic and experimental data, three stable ternary intermetallic phases were described using sublattice model. Two of them were treated as semi-stoichiometric compounds with homogeneity ranges for Co and Ni, and the other one was treated as a stoichiometric compound. The other phases were described by using substitutional solution model. The present work provides a more complete and accurate thermodynamic description compared with previous ones. A reasonable agreement was obtained between calculations and experimental data in the ternary system. The occupancy behavior of Co in AlNi3 is thermodynamically predicted, and the computed occupancy shows a much better fit to the experimental data than previous first-principles calculations. In addition, the composition range of the Co + AlNi3 two-phase region, concerning a new type of composite binder for cemented carbides, was found to shift to the Co-rich side with the decrease of the temperature.