Nanocrystallization can increase the diffusion rate of alloying elements, such as Cr, Al, and then improve the high-temperature oxidation resistance of metallic alloys due to the presence of high-volume fraction of grain boundaries. Up to now, the studies on the effect of nano-structure on the oxidation behavior of alloys mainly focus on the oxidation kinetics and the structure of oxide scale at steady stage. The microstructure of oxide scale formed on nano-structured alloys during oxidation, especially at the initial stage, is lack of characterization. At the same time, nano-structured alloys are far away from thermodynamic equilibrium state, which inevitably result in the grain coarsening during oxidation process. Therefore, it is necessary to characterize the microstructure of oxide scale on nano-structured alloys to understand the relationship between grain coarsening of alloy and the formation of oxide scale on its surface. In this study, the oxidation behavior of coarse-grained (CG) and nanocrystalline (NC) Ni-12Cr alloys (mass fraction in nominal chemical composition) at 800 ℃ in air were studied. The NC alloy was prepared by severe plastic deformation, and the average grain size is around 42 nm. Focused ion beam (FIB) microscope and scanning transmission electron microscope (STEM) were used to characterize the microstructure and composition distribution of the scale. The results demonstrate that a two-layered scale, external NiO and internal Cr-rich layer, is formed on both of CG and NC alloys after oxidation for 25 s. The scale on CG alloy develops into a multilayered structure, which includes NiO/(NiO+NiCr2O4)/porous Cr2O3/internal oxidation zone after oxidation for 10 min, while the Cr-rich layer on NC alloy crystallizes and forms a protective Cr2O3 scale after oxidation for 2 min. The oxidation kinetics of NC alloy consists of two stages, both of which follow the parabolic law. The parabolic rate constants of the two stages are 1.76×10-13 cm2·s-1 (within 1 h) and 1.58×10-14 cm2·s-1 (1-109 h) respectively, both of which are about 3-4 orders of magnitude lower than that of CG alloy. The study on grain growth kinetics of alloy indicates that the protective chromia scale forms before the significant coarsening of nano-grain. According to Wagner theory with the effective diffusion coefficient, the critical grain size for Ni-12Cr alloy to change from internal oxidation to external oxidation is about 94 nm at 800 ℃.

Keywords： Nanocrystalline ; Ni-Cr alloy ; high temperature oxidation ; selective oxidation