Abstract
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, NbO0.76, NbO, and NbO2 oxides) and boron (to B2O3) released during the conversion of the Nb5Si3–x B 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.
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