Abstract: Ferritic-martensitic steels such as P92, which are potential candidates for Generation IV reactors, tend to precipitate Laves phases at elevated temperatures. In this study, the oxidation behavior of both non-aged and pre-aged P92 steels at 550 °C in a lead-bismuth eutectic (LBE) alloy was examined using scanning electron microscopy, transmission electron microscopy, and atom probe tomography. Results showed that the thickness of the outer oxide layer (OOL) as well as the inner oxide layer (IOL) decreased with longer pre-aging times. The OOL was primarily composed of Fe₃O₄, while the IOL displayed a network-like structure consisting of Cr-rich and Fe-rich oxides. A preferential oxidation zone developed along the grain and lath boundaries, serving as the oxidation front. Moreover, Laves phases exhibited a much-delayed oxidation rate in the LBE alloy and acted as inert markers of the original surface during the oxidation test. Between the Laves phase and the IOL, an interlayer approximately 10 nm thick was identified, containing a Cr-rich oxide, a Si-rich layer, and a W-rich oxide. Density functional theory calculations further revealed that Laves phases strongly adsorb oxygen and effectively hinder its diffusion compared with the steel matrix. Finally, the role of Laves phases in the oxidation mechanism and their implications for long-term oxidation kinetics of P92 steel in LBE were discussed.

Key words: Atom probe tomography, Density functional theory, Material characterization, Precipitation strengthening, Lead-bismuth eutetic