A detailed 3D stoichiometric map of spinels and oxides in structurally critical passivation layers on Inconel 718 superalloy after heating up to 650 °C

This research presents a detailed structure and time evolution for forming the passivation layer during oxidation of Selective Laser Melted (SLM-IN718) and commercially cast (Comm-IN718) Inconel 718 at 650 °C. It reveals that this layer has a complex 3D structure, with a top layer of Fe-Ni Chromite spinels (NiFe_xCr_(2−x)O₄) and a sublayer of Cr₂O₃. Raman maps and statistical analysis estimate x in NiFexCr_(2−x)O₄ for different heating times. The heating process initially forms iron-rich NiFe_xCr_(2−x)O₄ with x≈2, decreasing to ≈1 after 72 h of heating for both Comm- and SLM-IN718. Interestingly, a slightly different result was observed for the heat-treated SLM sample (HTSLM-IN718), where x increased again to 1.2 and 1.4 after 72 h. Secondary Ion Mass Spectrometry (SIMS) results reveal the depth hierarchy of the passivation layer profile, with the chromite layer covering a thicker Cr₂O₃ layer forming above the niobium-rich intermetallic layer above the alloy. Passivation layers are critical to the endurance of superalloys during extreme thermal cycling conditions. Therefore, by precisely determining the complex stoichiometry and hierarchy of the layer, this study can guide the development of durable superalloys with improved resistance to extreme thermal cycling conditions.