Microstructure evolution of an EB-PVD NiAl coating and its underlying single crystal superalloy substrate

A NiAl coating was deposited onto a Ni-based single crystal superalloy with (001) crystal orientation by electron beam physical vapor deposition (EB-PVD). The as-deposited NiAl coating showed a columnar microstructure with (110) preferred orientation. The microstructure evolution behavior near interface between the NiAl coating and superalloy substrate at 1100 °C was investigated. Kirkendall voids were formed in the NiAl coating, indicating the different elements diffusion coefficients in the coating and substrate. Interdiffusion zone (IDZ) with rod-like and granular topological close-packed (TCP) phases and substrate diffusion zone (SDZ) with needle-like TCP phases were formed during diffusion annealing at elevated temperature. The equi-axed β-NiAl grains were developed in the IDZ after diffusion annealing at 1100 °C for 10 h, which showed different orientations from the coating and substrate. However, after 50 h diffusion annealing, the equi-axed β-NiAl phases in the IDZ were transformed into γ′-Ni3Al phases which had the same orientation as the substrate. Furthermore, oriented rafting of the substrate occurred during diffusion annealing and the rafts were parallel to the coating/substrate interface.