Investigation into the Phase Stability, Microstructure and Oxidation Behavior of Cryomilled MCrAlY Bond Coat

Improved thermal cycling lifetime has been observed in thermal barrier coatings (TBCs) with cryomilled MCrAlY (M = Ni and/or Co) bond coat. To understand this improved behavior, the phase stability, microstructure and oxidation behavior of the MCrAlY bond coat, with a focus on the influence of the cryomilling process, were investigated through a robust experimental study coupled with mechanistic explanations. It is found that cryomilling results in two significant changes in the NiCrAlY bond coat: unintentional Fe additions and creation of a homogeneous distribution of ultrafine oxide/nitride dispersoids. Through extensive microstructural analysis combined with computational simulation using Thermo-CalcRTM software, it is determined that the presence of Fe stabilizes the high temperature gamma and beta phases in the NiCrAlY bond coat, corresponding to a decrease in the transformation temperature. Computational investigation into the phase equilibria in the NiCoCrAl system suggests that the Co additions decrease the stability of the single gamma' phase region within the temperature range 700-1100°C while they lead to the appearance of a four phase (gamma + gamma' + sigma + beta) co-existing region by changing the stability of the sigma phase. Characterization of the thermally grown oxide (TGO) in TBCs after isothermal oxidation with rigorous statistical evaluation indicates that the TGOs in the TBCs with the cryomilled bond coats are more uniform in thickness and slower growing. Theoretical analysis based on diffusion theory and quantum mechanics indicates that the improved TGO growth behavior can be attributed to the more homogeneous distribution of oxide dispersoids, which are a direct result of the cryomilling, yet remain stable after extensive thermal exposure.