Oxidation processes of alloys of the Ni-Ta system. II. Oxidation of the intermettalic compound NiTa

The oxidation kinetics of the intermetallic compound NiTa was studied by th e continuous thermogravimetry in air at temperatures ranging from 600 to 10 00 degrees C . The scale formed was subjected to X-ray and metallographic s ectioning phase analysis. Oxidation of NiTa was shown to occur because of t he preferential diffusion of oxygen toward the scale-alloy interface. The k inetics is described as a parabolic function of time. The isotherms indicat e that the parabolic oxidation rate constant K-p periodically decreases for t less than or equal to 800 degrees C but increases and periodically decre ases for t > 800 degrees C. The temperature dependence of K-p is exponentia l At t similar to 850 degrees C the oxidation rate decreases, indicating a change in the oxidation mechanism. The scale formed on NiTa was found to co ntain the oxides NiO, NiO . Ta2O5(NiTa2O6), and Ta2O5, as well as Ni. The s olid solutions Ni(Ta) and Ni3Ta were detected in the sublayer of scale adja cent to the alloy. At high temperatures those phases are distributed among the layers: NiO + NiTa2O6 + Ta2O5 in the first, NiTa2O6 + Ta2O5 in the seco nd, Ni + Ta2O5 in the third and Ta2O5 + Ni(Ta) + Ni3Ta in the fourth. By an alogy with the oxidation of unalloyed tantalum the explanation for the expe rimental results is that at a p --> n phase transition accompanied by the f ormation of oxygen vacancies and tantalum interstitials, occurs in the latt ice att similar to 850 degrees C.