Temporal Ordering of Oxide Sublayers in Single-Crystal Superalloys

Nickel-based single-crystal superalloys operate in components exposed simultaneously to mechanical loading, high temperatures, and oxidation. Performance depends on the γ/γ′ microstructure: a Ni-rich γ matrix containing several solid-solution elements surrounds γ′ precipitates based principally on Ni₃(Al,Ta) and this two-phase arrangement contributes to the alloy’s high-temperature mechanical behaviour, but it also creates a chemically heterogeneous surface when the material is exposed to an oxidizing atmosphere. Local differences in Al, Ni, Co, Cr, Ta, and other alloying elements can influence which oxides form first, where oxygen penetrates, and how the surface reaction zone develops with time.

Oxidation becomes especially important when a protective coating cracks, peels away, or is otherwise damaged, leaving the underlying alloy directly exposed. The oxide scale that forms under such conditions is not a passive and uniform reaction product. It develops through coupled transport of oxygen inward and alloying elements outward, while the phases within the scale may themselves change as oxidation proceeds. After prolonged exposure near 1100 °C, the oxide layer of Ni-based single-crystal superalloys is often described in terms of an outer Ni–Co-rich oxide, intermediate complex oxides, and an inner Al₂O₃-rich region. Such descriptions establish the final layered morphology, but they do not reveal how the individual oxide layers developed over time. The mature scale records the outcome of oxidation, not the sequence of structural and chemical changes that produced it. Early oxidation involves surface disordering, selective oxidation of alloying elements, diffusion through chemically distinct γ and γ′ regions, and reactions among oxides that may already be present. Aluminium has a strong tendency to oxidize, but its behaviour must be considered together with the availability and mobility of Ni, Co, Cr, and Ta, as well as the local oxygen concentration. The γ and γ′ phases add further complexity because γ′ contains more Al and may respond differently to oxygen exposure, whereas the γ matrix can provide pathways for oxygen to penetrate beneath the immediate surface.

Earlier studies have proposed different sequences for the earliest stages of oxidation. Some suggest that Al₂O₃ forms first, followed by development of the Ni–Co-rich surface oxide, whereas others indicate that Ni and Co may react directly with oxygen as exposure begins. The respective roles of the γ and γ′ regions during this early period have also remained unclear. Clarifying the process requires observations that track the evolving microstructure over short oxidation times instead of inferring the sequence solely from the final oxide scale.

In a recently published research paper in Corrosion Science, Dr. Zhiqiang Zhou, Professor Pan Xie, Professor Cuilan Wu, and Professor Jianghua Chen from Hunan University and Hainan University examined how the oxide scale forms on a nickel-based single-crystal superalloy. Their analysis separates the early solid–gas oxidation of alloying elements from later solid–solid reactions among the oxides already present. It also identifies γ channels as preferred routes for inward oxygen diffusion and explains why secondary Al₂O₃ forms before the intermediate NiAl₂O₄, CoCr₂O₄, and CrTaO₄ layers.

The researchers performed alloy oxidation in air at 1100 °C for periods ranging from 10 s to 1 h. The longest exposure established the stable oxide sequence from the external surface inward: (Ni₀.₉Co₀.₁)O, NiAl₂O₄, CoCr₂O₄, CrTaO₄, and α-Al₂O₃. Atomic-resolution imaging and compositional mapping identified the crystal structures and dominant elemental distributions within each layer.   They found after only 10 s, both γ and γ′ surface regions were covered by a thin, continuous amorphous-like oxidation product.   By 15 s, Ni-rich and O-rich nanoparticles had appeared at the outer surface, while an internal oxidation front beneath them showed Al and O enrichment together with Ni depletion. That compositional separation already indicated two linked processes: outward formation of a Ni-rich oxide and inward penetration of oxygen into the near-surface alloy.

The authors also found at 60 s, the developing scale had acquired a bilayer character. An outer Ni–Co oxide layer lay above an ultrafine-grained primary Al₂O₃ layer. Both γ and γ′ regions participated, although the Al-rich γ′ regions oxidized more rapidly.  Higher Al availability in γ′ favoured faster formation of alumina, whereas the γ channels carried oxygen more deeply into the substrate. The γ/γ′ microstructure therefore influenced not only local oxide composition but also the route by which oxygen reached regions below the initial surface reaction zone. The team performed elemental line scans to understand the sequence within the two phases and noticed in γ regions, Ni and Co first migrated toward the surface and formed the outer Ni–Co oxide. The region below became enriched in Al and O before converting more slowly toward alumina. In γ′ regions, the same outer oxide appeared first, followed by more rapid transformation into Al₂O₃. Oxygen concentration was higher within γ channels than in adjacent γ′ regions, supporting the interpretation that γ channels acted as preferred inward diffusion paths.

This diffusion route became especially consequential after 120 s. A second Al₂O₃ layer appeared at depth, separated from the primary Al₂O₃ layer by a remaining slice of unoxidized alloy. Its position showed that oxygen had travelled through γ channels and reacted with Al in deeper γ′ blocks before the intermediate oxide layers had fully developed. The secondary alumina layer contained chiefly Al and O, whereas the earlier primary alumina layer retained a more mixed chemical character, including contributions from Ta, W, Co, Cr, and other alloying elements.

The primary Al₂O₃ layer then evolved into three intermediate sub-layers. At this stage, a thin NiAl₂O₄ layer appeared near the boundary with the outer Ni–Co oxide. Within the primary alumina, Ta-rich oxide particles were identified as Ta₂O₅, while α-Al₂O₃ remained the dominant phase. With continued oxidation to 30 min, reactions among the simple oxides produced stable NiAl₂O₄, CoCr₂O₄, and CrTaO₄ layers. The final five-layer architecture therefore developed through overlapping oxidation and solid-state transformation processes, not through a single outward-to-inward sequence.

The authors’ findings provide a more specific basis for evaluating oxidation resistance in nickel-based single-crystal superalloys under high-temperature exposure. The mature oxide scale should be considered in relation to the local γ/γ′ arrangement, the availability of Al-bearing regions, and the paths available for oxygen transport. The observation that γ channels promote inward oxygen diffusion is especially relevant when interpreting subsurface oxidation.

According to the authors, this behaviour matters for alloy microstructure design and for post-exposure assessment of service components. A surface oxide scale may appear continuous at low magnification while the substrate beneath it has already undergone internal chemical redistribution. The formation of a γ′-free region below the secondary alumina layer shows that oxidation can alter the near-surface microstructure beyond the visible external scale. For components exposed after coating cracking or local coating loss, inspection strategies may therefore need to consider both the oxide layer and the modified alloy region immediately below it. The distinction between primary and secondary Al₂O₃ formation also has value for interpreting oxidation kinetics.   These two alumina regions arise through different local conditions even though both are Al-rich oxides. Assessments based only on the presence or thickness of alumina may not fully capture how oxygen has moved through the underlying microstructure and where Al has been consumed.

The later emergence of NiAl₂O₄, CoCr₂O₄, and CrTaO₄ further indicates that intermediate oxide layers should be understood as products of reactions among earlier simple oxides.   This provides a basis for interpreting oxide scales formed after different exposure durations, because the absence of a mature complex oxide layer does not necessarily indicate limited oxidation. For materials development, the refined scenario offers a route for linking alloy chemistry and γ/γ′ morphology to oxidation-layer evolution.  Elemental diffusion rates, local oxygen concentration, and phase-specific composition act together in determining which oxide products appear, where they form, and when they transform.