Interface-associated creep degradation mechanisms of coating/superalloy after ultra-high-temperature thermal cycles

During aero-engine service, turbine blades are subjected to severe thermal cycling characterized by oxidation, elemental interdiffusion, and the generation of thermal stresses. The resulting interfacial instability between the protective coating and the superalloy substrate profoundly influences the mechanical properties of the coating/superalloy. At present, the creep degradation mechanisms of coated superalloys under cyclic thermal conditions are not fully understood, specifically concerning the synergistic interplay among interfacial diffusion, phase transformations, and thermal mismatch stresses. Using multiscale characterization and computational simulations, this study investigates the creep degradation mechanisms (at 850 °C/500 MPa) of an Al gradient NiCrAlYSi coating/superalloy system following thermal cycling between 25 and 1200 °C. The results reveal that while the as-prepared coating exerts a negligible influence on the creep properties of the superalloy, subsequent thermal cycling systematically curtails the creep life. This degradation is primarily driven by the pronounced structural evolution of the interface. Comprehensive post-cycling failure analysis highlighted phase structure evolution, thermal stress distributions, dislocation pile-ups, localized recrystallization, and crack dynamics. Specifically, thermal cycling degrades the interfacial γ/γ′ structure into a complex β + topologically close-packed (TCP) + γ/γ′ multiphase structure. Under creep loading, the sharp tips of the TCP phases act as severe stress concentrators, promoting localized dislocation nucleation. Moreover, progressive thermal cycling drives the accumulation of severe thermal mismatch stresses around these TCP phases, triggering subgrain formation within the interdiffusion and secondary reaction zones during subsequent creep. This localized structural degradation accelerates interfacial crack initiation and propagation, ultimately leading to premature macroscopic fracture. Based on these findings, we propose a novel design paradigm to mitigate interfacial thermal cycling damage. This work establishes a theoretical foundation for understanding interface-dominated creep failure and provides critical insights for tailoring coating compositions.