Application of the cluster/site approximation to fcc phases in Ni–Al–Cr system Original Research Article

The cluster/site approximation (CSA) has been used to model the face-centered cubic (fcc) phases (disordered γ with A1 structure and ordered γ′ with L12 structure) in the Ni–Al–Cr ternary system. The CSA takes into account short-range order (SRO), which is essential to satisfactorily describe the thermodynamics of order/disorder transitions such as occur between the fcc phases in the Ni–Al–Cr system. It possesses computational advantages over the cluster variation method (CVM) while offering comparable accuracy in the calculation of multi-component phase diagrams. This makes the CSA a practical method for carrying out calculations on real alloy systems. These points are illustrated in the application of the CSA to fcc phases in the Ni–Al–Cr system. The CSA-calculated phase diagrams, which use fewer model parameters than used in previous descriptions, based on the point approximation, are in good accord with the experimental data. In addition, the fcc metastable phase diagrams calculated by using the CSA show reasonable ordering/disordering behavior and phase relationships.