On the spinodal decomposition phenomena in a Ni-Al alloy system aided by thermodynamic calculation

Formation and evolution of order-disorder microstructure of γ/γʹ phases plays an important role in high temperature strength of cast nickel-based superalloys. Accordingly, the characterization of formation mechanism, as well as morphology and interface of γ and γʹ phases, are very important to control the mechanical properties of nickel-based superalloys. Formation of γ' precipitates inside the γ phase matrix often occurs by classical nucleation and growth mechanism. Recently, some researches have reported the formation of γ' precipitates by spinodal decomposition mechanism in some Ni-based superalloys. In the present study, the spinodal decomposition phenomena in a Ni-Al binary system (as a simple model alloy for Ni-based superalloys) has been studied. For this purpose, the Ni-rich region of Ni-Al alloy system was assessed thermodynamically using CALPHAD method. The Gibbs free energy of γ and γʹ phases for this alloy system was calculated employing two sublattice model. Then, the possibility of spinodal decomposition was investigated from the thermodynamic point of view by the study of the Gibbs energy curves in different ranges of temperature. Due to the thermodynamic modeling results, a proper heat treatment procedure was designed for Ni-18Al (at.%) alloy and characterized by means of 3D atom probe tomography (APT) analysis. According to the modeling and experimental results, it was found that γ' precipitates were formed by spinodal decomposition under higher supersaturation conditions.