Microstructures of first-stage aluminized coatings on nickel-based superalloys

Microstructures of first-stage aluminized coatings on René 80, MAR-M 247 and Inconel 713LC nickel-based superalloys are studied. The σ-Ni 2Al 3 phase constitutes the major part of the coatings but a Ni 3Al(γ′ phase) layer and a β phase layer exist at the substrate-coating interface. There is no preferred orientation relationship between the γ′ and β phases. The grain size decreases abruptly from 20–30 μm to less than 0.3 μm across the substrate-coating interface. Very fine and dense precipitates elongated in shape are present in the coating near the coating-substrate interface whereas precipitates more distant from the interface are larger and more spherical but less dense. An M 23C 6-type precipitate is commonly found in the coatings and its crystallographic orientation relationship to the δ phase is determined.