Kinetic analysis of the oxidation of Nb–Si eutectic alloy doped with boron

详细

The mechanism and the kinetics of the oxidation in an air flow of powdery Nb–Si eutectic alloy containing (wt %) 93.0 Nb, 6.7 Si, and 0.27 B are studied by X-ray diffraction (XRD), thermogravimetric (TG), and differential thermal analysis (DTA). The oxidation of alloy proceeds through three stages. At the first stage (600–923 K), the oxidation of a Nb_ss solid solution (with the formation of Nb₂O₅, NbO_0.76, NbO, and NbO₂ oxides) and boron (to B₂O₃) released during the conversion of the Nb₅Si_3–xB_x phase (T₂ phase) into Nb₅SiB_y (D8₈) occurs. At the second stage (923–993 K), the accumulation of the product layer and the formation of borosilicate occur, which prevents the oxidation. At the third stage, Nb₃Si and Nb₅SiB_y (D8₈) silicides and Nb₃B₂ niobium boride are oxidized. Under heating above 1023 K, the interaction of boron oxide with niobium oxide occurs with the formation of Nb₃BO₉. The possible oxidation mechanisms are considered. It is shown that are well described by the model of three successive stages, each one limited by the kinetic regime.