The steam oxidation behavior of three heterogeneous HR3C alloys was investigated at 650 °C comparatively. After a long-term oxidation process for 1000 h, the results demonstrated that the commercial HR3C alloy already exhibited a high oxidation resistance. However, the spallation resistance of the oxide scale was low during the initial oxidation period. The addition of a moderate amount of Nb into the alloy (1#HR3C) increased the oxidation resistance of the alloy. In addition, the improvement of the microstructural stability was substantially influenced by solid solution strengthening and fine grain strengthening. However, the addition of excessive Nb could significantly affect the growth model of the oxide scale and negatively affect the oxidation performance and microstructural evolution of the alloy (2#HR3C).

A new framework for a nanocrystalline coating system is established and prepared to study the oxidation behavior with a significant difference in elemental composition. K38 superalloy is selected as a substrate alloy and the composition of the 2nd-generation single-crystal superalloy Rene N5 is used as the sputtered nanocrystalline coating. The oxidation behavior of the newly designed nanocrystalline coating is comparatively studied with the original K38 coating and its substrate alloy at 1050 °C for 500 h. Moreover, microstructure evolution on the interface is used for studying the influence of element interdiffusion behavior on the substrate alloy. Results show that the nanocrystalline coatings increase the oxidation performance of alloys at 1050 °C for 100 h. The sputtered SN-N5 nanocrystalline coating exhibits the best oxidation resistance among the three groups of specimens for 500 h. Interdiffusion occurred and is observed on the SN-N5 coating after long-term oxidation. However, no topologically close-packed phases participated in the substrate alloy.

Nanocrystalline coatings with the same chemical composition as an N5 superalloy were prepared on K38 and N5 superalloys by magnetron sputtering. The effect of different superalloys on the high temperature oxidation behavior of nanocrystalline coatings was investigated through oxidation kinetics, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and wavelength dispersive spectrometry (WDS). The results indicated that K38-N5 had better oxidation resistance than N5-N5 due to the diffusion of Zr from the K38 superalloy into the oxide scale. In addition, no interdiffusion occurred in the K38 superalloy. The formation of Ta-rich phases in the Al₂O₃ scale leads to decrease the oxidation resistance of nanocrystalline coatings. However, the presence of Zr inhibits the formation of Ta-rich phases.