Abstract: The microstructural evolution of an aluminide coating, prepared by chemical vapor deposition on the inner surface of a superalloy blade, was comprehensively investigated during service in a real gas turbine. The as-deposited coating exhibited a dual-layer structure: the outer layer predominantly composed of β and γ' phases, and the inner layer consisting mainly of β, γ', and σ phases. A secondary reaction zone (SRZ), enriched with needle-like σ phases, developed inside the superalloy substrate beneath the coating. Despite the blade not being enriched with refractory elements, three types of topologically close-packed (TCP) phases—σ, μ, and P—precipitated. During the aluminizing process, the needle-like σ phase formed first, followed by the transformation of σ to P and μ phases, with P and μ nucleating within the σ phase. The transformation coordinate was proposed and simulated through density functional theory (DFT) calculations. Coherent relations were observed between different TCP phases and between TCP phases and superalloy substrate. The aluminide coating exhibited varying oxidation behaviors due to its microstructural diversity. Interfacial oxidation dominated the oxidation process, attributed to the enrichment of Ti in the inner coating layer and the co-oxidation of Ti and Al. The γ → γ' transformation within the SRZ led to the shrinkage of the original γ phase into line-like structures. Additionally, the aluminizing and homogenization processes were simulated using the DICTRA code, with results that align well with experimental observations.

Key words: Aluminide coating, Phase transformation, Topologically close-packed phase, High-temperature oxidation, Elemental interdiffusion