Calculating the elemental diffusivities during homogenization of as-cast Inconel 718 superalloy

Since the diffusion of alloying elements in nickel has been studied based on the diffusion couples and the presence of other alloying elements can alter the diffusion of a given element, there is need for a method for direct calculation of the diffusivities form the superalloys. In the present work, the as-cast microstructure of Inconel 718 superalloy was studied, where this microstructure was composed of austenitic dendrites that rejected alloying elements such as Nb, Mo, and Ti to the interdendritic regions. Moreover, the Laves phase/austenite eutectic structure and NbC and TiC phases were characterized in the interdendritic regions. Subsequently, the microsegregation of Nb, Mo, and Ti in the as-cast ingot and its amendment during homogenization heat treatment was studied. Based on the concept of residual segregation index, during homogenization treatment, the diffusivities and the corresponding activation energies for diffusion of these elements in the austenitic matrix were obtained. The activation energy for the interdiffusion of Nb, Mo, and Ti was determined as 232, 286, and 256 kJ/mol, respectively. The outcome of these calculations is important for the precipitation reactions, high-temperature creep, and thermomechanical processing of superalloys.