Orientation dependent interface morphology and oxide stability in a commercial niobium alloy: Explaining experimental results with density functional theory

The metal-oxide interface of a commercial niobium (Nb) alloy showed clear presence of 'hills' and 'plateaus' on the metallic side. This interface morphology was orientation-dependent and appeared to determine the presence of the surrounding oxide phase(s). In particular, the hills were associated with non-(111) Nb crystallographic orientations. The corresponding oxide phase was mostly tetragonal Nb2O5, which also contained significant porosities. Oxide phases around the plateaus, on the other hand, were primarily orthorhombic Nb2O5. In addition, the oxygen (O) concentrations also differed on the metal-lic side of the hills and plateaus, the latter showing a sharper compositional gradient. Density functional theory (DFT) calculations were performed to explain the experimentally observed orientation dependence and oxide stability. The calculations showed that (111)-Nb grains not only had the highest surface energy (ESurface) and O adsorption energy (EAds), but also had the highest activation energy barrier for O diffusion. On the other hand, the tetragonal Nb2O5 was energetically stable at higher O partial pressure. An analyt-ical model, based on O adsorption and short-range diffusion at the metal-oxide interface, was proposed. This study not only related the experimental results with DFT simulations, but also provided an atomistic framework to interface-controlled oxidation in a commercial Nb alloy.(C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.