Effects of detrimental diffusion zone and heat treatment on creep strength of aluminized nickel-based single crystal superalloys

The detrimental effect of the diffusion zones between coating and the substrate had been investigated by performing creep tests on aluminized Ni-based single crystal superalloys. In present study, creep test-pieces that had two kinds of sub-surface crystallographic orientations, 〈100〉 and 〈110〉, were aluminized by the pack-cementation process at 1000°C for 5 h. To exclude the influence of the coating heat treatment on the creep strength of the bulk material, a set of specimens without coating was heat treated in vacuum at 1000°C for 5 h, which was equivalent to the condition during aluminizing process, for comparisons the creep rupture test was carried out at 900°C and 392 MPa. The cross-sectional microstructures of the as-aluminized and creep rupture specimens were observed by SEM using the backscattered electron image mode (BSE) and TEM. Analysis indicated that creep strength of the aluminized samples was decreased by the formation of the inter-diffusion zone (IDZ) and the substrate diffusion zone (SDZ) ; both of them reduced the load bearing thickness, thus resulting an increase in the effective stress. Furthermore, it was found that the disappearance of the secondary precipitate during coating treatment also decreased the creep strength. The IDZ, which formed at the coating/substrate interface grew more rapidly on the {110} plane than that on the {100} plane because of the preferential growth of the Topological-Close Paced (TCP) phase. Ru addition on the substrate could enhance the creep strength and the SDZ formation between the IDZ/substrate interface.