Investigation into the effects of Fe additions on the equilibrium phase compositions, phase fractions and phase stabilities in the Ni–Cr–Al system

NiCrAl alloys are commonly used for the bond coat in thermal barrier coatings (TBCs). Previous work has investigated the effects of intentional chemical additions on phase stability, but there is also a need to consider the effects of unintentional additions. For instance, cryomilling of NiCrAlY powder, which has been demonstrated to increase TBC lifetime, results in unintentional Fe additions. In this study, the effects of Fe additions are evaluated both through experimentation and computational simulation. The combined results indicate that the presence of Fe stabilizes the high-temperature γ and β phases, corresponding to a decrease in the transformation temperature. These results are explained on the basis of Gibbs free energy for individual phases. Phase composition is also investigated, indicating that, even without Fe, the phases are more complex than typically assumed and that, at the low concentrations associated with cryomilling, the Fe goes into solution in the Ni-rich phases.