Intermetallic phase precipitation and oxidation behavior of Fe–20Cr–0.5Nb–2Mo (at%) high-Cr ferritic alloy at high temperatures

The precipitation behavior of intermetallic compounds, such as C14 Laves phase and Chi (χ) phase, and oxidation behavior of an Fe–20Cr–0.5Nb–2Mo (at%) high-Cr ferritic alloy were investigated at high temperatures between 1023 K and 1123 K. During aging/oxidation heat treatment at 1073 K, the protective Cr ₂O ₃ is rapidly formed on the surface, consecutively, a high density of fine Laves precipitates, (Fe,Cr) ₂(Mo,Nb), are formed throughout the α-Fe matrix and preferably at the α/Cr ₂O ₃ interfaces as well as α-Fe grain boundaries. The Laves precipitation causes local changes of solute concentrations in the surrounding α-matrix, i.e., decrease in Nb and increase in Mo. These local changes of phase equilibria in the vicinity of the α-grain boundaries and α/Cr ₂O ₃ interfaces result in the formation of the χ-phase which is not supposed to be precipitated. Moreover, χ precipitation at the α/Cr ₂O ₃ interfaces is most favored in case the alloy is oxidized at 1073 K. In the short-term oxidation at 1073 K, the Cr-rich χ interfacial precipitates, Fe ₃₆Cr ₁₂(Mo ₈,Nb ₂), suppress the growth of Cr ₂O ₃. However, as the interfacial χ precipitates coarsen in the long-term oxidation at 1073 K, localized stress concentrations are generated at intersections between the Cr ₂O ₃ layer and the α/χ interfaces. Consequently, these facilitate cracking of the adjacent surface oxide, then oxide ‘rehealing’, and formation of the abnormal-Cr ₂O ₃ process. Eventually, the precipitation of interfacial χ phase indirectly causes an acceleration growth of the surface Cr ₂O ₃ on the alloy.