Effects of temperature and rare earth content on oxidation resistance of Ni-based superalloy

The oxidation behaviors of the Ni-22Cr-14W-2Mo superalloys with various La contents were investigated at 1 000 °C and 1 100 °C up to 500 h. The experimental results show that the oxidation resistance is improved by La addition. The mechanism of the reactive-element effect (REE) exhibits some difference at various oxidation temperatures. The REE is dependent on the solutionized La content when the alloy is oxidized at 1 000 °C. The dispersed LaCrO3 segregates on the grain boundaries of Cr-oxides, and hence the diffusion of metallic ions, e.g., Mn2+ and Cr3+, are prevented. The oxidation resistance decreases with the increase of La content. However, the REE ascribes not only to the solutionized La, but also to the La-Ni intermetallic phases, when the alloy is oxidized at 1 100 °C. The oxidation resistance is influenced by two effects: firstly, the formation of the dispersed La2O3 from the solutionized La on the grain boundary of Cr-oxides reduces the oxidation rate, and secondly, the aggregation of the La-O oxides from the large amount of La-Ni phases accelerates the oxidation process. The alloy with 0.026% La (mass fraction) exhibited excellent oxidation resistance at 1 100 °C.