The interaction of the alloy niobium-25Titanium with air, oxygen and nitrogen I. The unusual oxidation behavior of Nb-25Ti at 1000 ° C

Abstract

The oxidation behavior of Nb-25 wt.% Ti (39 at.% Ti) in air and oxygen at 1000 °C has been studied. The oxidation kinetics are linear in air with a pressure dependence of approximately 0.4. In oxygen the kinetics are in agreement with the parabolic rate law after about 8 h and are less dependent on the gas pressure. Oxidation in either air or oxygen results in the development of a unique substructure in the alloy. The external oxide layer at the alloy-oxide interface consists of TiO 2 and Ti 2Nb 10O 29. At the oxide-gas interface TiNb 2O 7 is observed formed by the solid state reaction of TiO 2 and Ti 2Nb 10O 29. Immediately below the external oxide layer, a gas-hardened zone consisting of two phases is observed. One of the phases is niobium-rich, the other is titanium-rich. The hardened zone is consistently thicker in specimens oxidized in oxygen than in air for equal exposure times. In either atmosphere, the growth of the hardened zone is diffusion controlled. Below the hardened zone, a small, but finite transition zone is observed followed by an extensive region containing a titanium oxide internal precipitate. The growth of this zone is also diffusion controlled, but the kinetics are complex since a portion of the zone is continuously being consumed by the inward growth of the gas-hardened zone. The oxidation behavior of the alloy, which is similar in some respects to that described by other investigators for Nb-Cr alloys, can be divided into two interrelated processes, dissolution of oxygen in the alloy and growth of an external scale. The poorer oxidation resistance of the alloy in air is assumed to be due to the interaction of nitrogen during the conversion of metal to external oxide. The substructure developed during exposure in air or oxygen is the same and is due to oxygen dissolution.