Effects of Rare Earth Elements on the Isothermal Oxidation of the Alumina-Scale-Forming NbSiTiAlHf Alloys

The microstructures and oxidation behavior of the NbSiTiAlHf alloys doped with rare earth elements at 1300 °C were investigated. The nominal compositions of the selected alloys are Nb-13.5Si-23Ti-37Al-5Hf-0.5X (at.%), where X = Y, Dy, and La, respectively. It was shown that the whole scales were mainly composed of the major phases of Al2O3 and the minor phases of TiO2, where the TiO2 formed on the surface or in the upper layer of scales, for the undoped, Y, and Dy-doped alloy. But, for the 0.5 at.% La-doped alloys, the whole scales were constituted with the major phases of both Al2O3 and TiO2, and contained plenty of large voids. The 0.5 at.% Dy-doped alloys exhibited the lowest scale growth rate with the value of 1.87 × 10−11 cm2/s, and the benefits of Y on the oxidation rates were short-term, while 0.5 at.% La-doped alloys had the highest scale growth rate of 4.55 × 10−10 cm2/s compared with those of all the selected alloys. Then, the effects of Y, Dy, and La on the oxidation behavior of the alumina-scale-forming NbSiTiAlHf alloys were discussed.