Oxidation processes of alloys of the Ni—Ta system. II. Oxidation of the intermetallic compound NiTa

Abstract

The oxidation kinetics of the intermetallic compound NiTa was studied by the continuous thermogravimetry in air at temperatures ranging from 600 to 1000°C. The scale formed was subjected to X-ray and metallographic sectioning phase analysis. Oxidation of NiTa was shown to occur because of the preferential diffusion of oxygen toward the scale-alloy interface. The kinetics is described as a parabolic function of time. The isotherms indicate that the parabolic oxidation rate constant Kp periodically decreases for t≤800°C but increases and periodically decreases for t>800°C. The temperature dependence of Kp is exponential. At t∼850°C the oxidation rate decreases, indicating a change in the oxidation mechanism. The scale formed on NiTa was found to contain the oxides NiO, NiO·Ta2O5(NiTa2O6), and Ta2O5, as well as Ni. The solid solutions Ni(Ta) and Ni3Ta were detected in the sublayer of scale adjacent to the alloy. At high temperatures those phases are distributed among the layers: NiO+NiTa2O6+Ta2O5 in the first, NiTa2O6+Ta2O5 in the second, Ni+Ta2O5 in the third, and Ta2O5+Ni(Ta)+Ni3Ta in the fourth. By analogy with the oxidation of unalloyed tantalum the explanation for the experimental results is that at a p→n phase transition. accompanied by the formation of oxygen vacancies and tantalum interstitials, occurs in the lattice at t∼850°C.