Abstract
The Ni3Al–Ni3Nb alloys are oxidized as a result of the diffusion of oxygen ions toward the interface between the alloy and the oxidation product. This diffusion produces a relatively thick inner layer of complex composition; in addition, diffusion of nickel ions toward the interface between the oxidation product and the gas results in the formation of a thin outer layer of NiO. At any temperature, NiO in the inner oxide is reduced to Ni by niobium atoms. During the initial stages of the oxidation, the reduction occurs at the oxide-alloy inter face; during the later stages, it occurs at the interface between the oxide and the suboxide layer. Protective double oxides of NiO · Nb2O5 (t = 700–725 °) and NiO · Al2O3 (t = 800–850 °) form in the oxidation product. An α → β-Nb2O5 conversion occurs at 825–900 ° and considerably reduces the oxidizability of the alloys. The β-Nb2O5 lattice probably contains fewer oxygen vacancies than the α-Nb2O5 lattice and thus has better protective properties.
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