Theoretical and experimental study of the γ and γ′ equilibrium in Ni-based superalloys

Ni-based superalloys with aluminium belong to systems with order–disorder transition for the γ and γ′ structures. Because of the complexity of these alloys, quaternary systems can be taken as suitable and sufficiently representative model systems for a theoretical study of their phase equilibria. In recent years several assessments including various model descriptions of ordered phases and ordering based on the CALPHAD formalism have been made with the aim to describe phase diagrams of relevant binary, ternary and some quaternary systems. For our study, the Ni–Al–Cr–Co system was chosen and phase boundaries of γ and γ′ phase equilibrium were calculated based on the data for lower-order systems. The calculations were performed by means of the software ‘Thermo-Calc’. Values obtained were compared with experimentally determined phase information for five alloys with constant Ni content (70 at.%) after long-term annealing at 900°C. The experiments were realized by means of energy-dispercive microanalysis in scanning transmission electron microscopy. For better agreement of theory with experiment, an thermodynamic optimization was performed.