Oxidation behavior in the system Nb-Ti-Al

Alloys of the system Nb-Ti-Al are candidate materials for a variety of high temperature applications in oxidizing environments. The role of the α, γ, and β phases during oxidation at 1,200 C and 1,400 C in air was investigated. Oxidation kinetics were monitored continuously by thermogravimetry. Oxidized test samples were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, and electron microprobe analysis. At 1,200 C, an increase in the volume fraction of the γ phase in two phase σ + γ microstructures decreased the rate of oxidation. Further, when the γ phase was the continuous matrix phase, near parabolic oxidation kinetics were observed. However, the rates of oxidation were too high to be considered protective. At 1,400 C, a protective continuous α-Al2O3 layer was formed when the aluminum content was increased from 40 atom percent to 50 atom percent. The increase in aluminum content corresponded to the formation of the γ phase in the microstructure. A second high-temperature phase, either β or α, was also present. The γ phase was depleted underneath the α-Al2O3 layer, which suggests that the γ phase was the aluminum source for the protective α-Al2O3 layer. The alloy containing 40 atom percent aluminum was single phase β at 1,400 C. Although this alloy formed a continuous α-Al2O3 layer, the oxide scale was not protective because of excessive transient oxidation and occasional breakdown of the α-Al2O3 layer