Evolution of the oxide layers of an aluminum-forming austenitic heat-resistant steel with high-Mn substitution for Ni: Experiment and first-principle calculations

High Mn (8 wt%) was used to save Ni resources to replace 10 wt% Ni in Aluminum-forming austenitic (AFA) heat-resistant steel; the changes in oxidation resistance and morphology evolution of multiple oxide scales for the 8Mn-AFA steel were then studied. The 8Mn-AFA steel formed a three-layer scale on the surface oxidized at 750 °C for 100 h: inner dense Al2O3 sub-layer closed to the austenite matrix, intermediate Cr2O3 sub-layer with partial Cr substituted by Mn, and outer loose Mn2O3 + Mn3O4 sub-layer with FeMnO3 surface nodules. Mn from the matrix to the α-Al2O3 diffusion is spontaneous. However, Mn moving from an α-Al2O3 to α-Cr2O3 leads to partial Cr in the Cr2O3 sub-layer was substituted by Mn. When the 8Mn-AFA steel oxidated at 750 °C for 360 h, the Mn oxides, including Mn2O3, Mn3O4 and FeMnO3, increased significantly with the prolongation of the oxidation time. First-principle calculations based on density functional theory (DFT) also confirmed the diffusion of Mn did not destroy the dense Al2O3 scale. The oxidation weight gain of 8Mn-AFA steel oxidized at 750 °C in the air for 360 h follows a parabolic law, and the oxidation rate constant is 2.65 × 10−4.