Effects of Trace Aluminum and Titanium on High Temperature Oxidation Behavior of Inconel 690 Alloy

Inconel 690 alloy is a nickel-based alloy with a high chromium content that provides excellent oxidation and corrosion resistances. The high oxidation resistances of the alloy is attributed to the protective Cr2O3 that forms during oxidation, which prevents the outward diffusion of alloy elements and the inward diffusion of oxygen. However, when the temperature exceeds 1000 degrees C, the volatilization and spallation of the Cr2O3 oxide scale severely reduce the oxidation resistance of the Inconel 690 alloy. The addition of trace active elements is an effective way to improve the oxidation resistance of superalloys; however, the effects of these elements on the oxidation behavior and mechanism of the Inconel 690 alloy remain unclear. In this study, the oxidation behavior of Inconel 690 alloys with varying contents of Al and Ti elements was systematically studied through oxidation kinetics, morphology observation, and element analysis. In addition, the effects of Al, Ti, and their coadditions on the oxidation behavior and mechanism of the Inconel 690 alloy were investigated. The results indicate that the addition of Al reduces the oxidation mass gain and improves the oxidation resistance of the Inconel 690 alloy. Moreover, the addition of Al and Ti accelerates the oxidation rate of the alloy at 850 degrees C, but retards it at 1000 and 1200 degrees C. The positive influence of Al addition can be attributed to the fact that Al2O3 particles precipitated at the grain boundary hinder the diffusion of Cr3+ along the grain boundary. The slow diffusion of Cr3+ inhibits the growth of the Cr2O3 oxide scale and reduces the number of holes in the alloy. As a result, the oxidation resistance of the alloy increases owing to the decrease in the oxidation rate and the increase in adhesion between the oxide scale and the substrate. When Al and Ti are added, Ti-4+(,) which acts as a high-valence ion, is doped into the Cr2O3 scale, promoting the outward diffusion of Cr3+ and accelerating the oxidation rate of the alloy at lower temperatures (850 degrees C). However, during oxidation, Ti tends to converge toward the surface of the Cr2O3 scale and form a nonvolatile Ti-rich oxide layer. The formation of this layer inhibits the volatilization and peeling of Cr2O3, thereby increasing the oxidation resistance of the Inconel 690 alloy at 1000 and 1200 degrees C.