Abnormal Phase Transition Mechanism in the CoCrAlY Bond Coat Revealed by In Situ and Ex Situ Experiments

ABSTRACT

In Co‐based alloys, the γ phase is normally stable at high temperatures and the ε phase at low temperatures, and there is a martensitic transition on cooling to ∼417°C. In the γ/β dual‐phase CoCrAlY bond coat of turbine blade thermal barrier coating systems, we observed the opposite: γ remained stable at room temperature, yet it underwent a γ → ε martensitic transformation during high‐temperature service in gas turbine, which accelerated cracking and mechanical degradation. To elucidate the mechanism, we combined ex situ and in situ experiments at elevated temperatures with density functional theory and CALPHAD analyses. Room‐temperature γ stability arises from Al doping, outward Ni diffusion from the substrate, and interfacial energy effects associated with twins, stacking faults, and Y‐rich precipitates. During service, Al depletion by internal oxidation/segregation and grain growth diminishes these stabilizing factors, promoting γ instability and the γ → ε transformation. Subsequent annealing at 900°C drives the inverse ε → γ transformation, restoring γ stability at room temperature. Restabilization correlates with precipitation of Cr‐rich σ, which depletes Cr in matrix (weakening ε stabilization) and amplifies interfacial energy effects. σ precipitates exhibit temperature‐dependent morphologies and maintain a coherent relationship with the γ matrix. Their formation also disrupts the γ/β equilibrium, fostering phase coalescence. These results reveal a complete stability–destabilization–restabilization loop for γ‐Co in service and provide guidance for Co‐based bond coat design to suppress ε formation.