Revisiting the Diffusion of Niobium in an As-Cast Nickel-Based Superalloy During Annealing at Elevated Temperatures

Abstract

While understanding the interdiffusion of Nb in the austenite matrix of Ni-based superalloys is important for studying the precipitation reactions (Laves phase, γ″-precipitates and δ-phase) and homogenization of the as-cast structure, there is no report on the activation energy (Q) and pre-exponential factor (D0). Moreover, for the Ni–Nb diffusion couple, the reported Q values of 202.6 and 257 kJ/mol are not consistent. In the present work, based on the concept of residual segregation index during homogenization treatment and elemental analyses from dendrite cores and interdendritic segregated regions, Q and D0 were respectively determined as ~ 236 kJ/mol and 0.0053 cm2/s for the Ni-based superalloys. Improved analysis of the Ni–Nb system resulted in Q and D0 values of ~ 234 kJ/mol and 0.1618 cm2/s, respectively. Therefore, it was concluded that the activation energy for interdiffusion of Nb in superalloys and Ni–Nb diffusion couple is the same. However, due to the presence of other alloying elements in the superalloys, D0 for Ni–Nb diffusion couple is ~ 30 times that of the superalloys.