Effect of thermal history on microstructural changes in aluminized nickel-based single-crystal superalloy

The effect of thermal history on microstructural changes in the aluminized and Pt-aluminized Ni-based single-crystal superalloy was investigated. The substrate superalloys were firstly electro-polished in order to get rid of the surface residual strain. Then the electrodeposition of Pt followed by vacuum annealing was conducted for some of the substrates, and the aluminized and Pt-aluminized specimens were prepared by the conventional aluminizing process. It was found that in the case of the aluminized specimens, voids were formed in the vicinity of substrate/coating interfaces, by thermal cyclic heating, whereas secondary diffusion zones (SDZ) were formed by isothermal heating. These differences in microstructural changes of the aluminized specimens can be explained by the diffusion kinetics between the coating layer and the substrate during heating/cooling processes. In the case of the Pt-aluminized specimen, on the other hand, secondary reaction zone (SRZ) formation was observed by both the thermal cyclic and isothermal treatments. These results can be explained by the poly-crystallization of the substrate surface by the annealing process that could promote the interdiffusion, resulting in the formation of SRZ.