Effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys

Abstract

The effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys at 1000 °C and 1100 °C has been investigated. Interdiffusion occurs between the NiCrAlYSi coating and the substrate, leading to the formation of inter-diffusion zone (IDZ) and secondary reaction zone (SRZ). After exposure at 1000 °C for 100 h, the initial γ/γ′ transforms to γ′ phases in the IDZ, and SRZ with σ and μ topologically close-packed (TCP) phases is formed primarily due to the inward diffusion of Al. Different behavior of SRZ formation in the alloys with various Ta content can be observed after exposure at 1000 °C, which highlights the potency of Ta addition in promoting SRZ formation. This may be ascribed to the fact that Ta promotes the precipitation of TCP phases. After exposure at 1100 °C for 100 h, the initial γ/γ′ transforms to γ layer and coarse γ′ phases in the IDZ, and no SRZ can be found. The formation of IDZ is significantly suppressed by Ta addition, which may be related to the retardation of interdiffusion between the coating and the substrate due to the increased γ′ volume fraction. Furthermore, γ′ rafting parallel to and/or perpendicular to the coating/substrate interface in the substrate beneath IDZ are found in the alloys after exposure at 1000 °C and 1100 °C. Microstructural observation shows that Ta has a potent influence on the development of perpendicular γ′ rafting. The mechanism of parallel γ′ rafting and perpendicular γ′ rafting has been discussed, respectively.