An atom probe tomography study of internal oxidation processes in Alloy 600

Internal oxidation in Alloy 600 (Ni–16Cr–9Fe) involves the inward diffusion of O, which preferentially reacts with solute elements (e.g. Cr, Fe) forming internal oxide precipitates. In this work, the internal oxidation process and its resultant metal-oxide heterostructures are systematically analyzed in 3D at the finest length scales, using atom probe tomography (APT). Internal oxidation is induced by exposure of Alloy 600 to 480 °C hydrogenated steam, with an oxygen partial pressure below the dissociation pressure of the solvent metal oxide, NiO. Following exposure, nodules of metallic Ni are observed on the sample surface, originating from material that is expelled to relieve compressive stresses generated by internal oxidation. Probing the material immediately below the surface reveals these nodules to be directly connected to their parent grains via Ni-rich metal channels. The matrix metal is intertwined with a continuous network of oxides, which are primarily FeCr2O4. The continuous interface between oxides and matrix provides the short-circuit diffusion pathways necessary for Ni expulsion. Deeper below the surface, oxides are observed as particles, rather than a continuous network, and are often aligned on matrix planes. The small oxide particles are Cr2O3, which is also found to be the composition at the centres of many larger FeCr2O4 oxides. This indicates that oxides first precipitate as Cr2O3, then later grow as FeCr2O4 when local depletion of Cr changes which phase is most energetically-favourable. Initial comparisons indicate that some fundamental findings in this work are applicable to known internal oxidation systems at lower and higher temperatures.