Precipitate formations with self-adaptive elemental diffusion and segregation in T92 steel

•Compositional characteristics of precipitates at different stage of heat treatment were determined by STEM-EDS.•Precipitates having stable crystal structures but changing compositions during service are demonstrated.•Phenomenon of forming precipitates with elemental diffusion and segregation in T92 steel was found.•The subtle scenarios of compositional evolution of precipitate in the T92 steel were revealed.

A detailed characterization of the composition and evolution of precipitates during thermal exposure (0-10000 h at 650 °C) of T92 steel was carried out using a combination of transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, energy-dispersive X-ray spectrometry (EDS) analysis, scanning electron microscopy (SEM) and first-principles calculations. It is shown that MX in T92 steel can be expressed as V/Nb(C,N) rather than complete solid-solution (V,Nb)(C,N) in terms of composition. The segregation of P and S elements in the MX precipitates was found after normalization. Meanwhile the slow segregation of Si from the matrix into the MX took place during a long-term thermal exposure. The nucleation and growth of M23C6 carbides adjacent to the MX micrograin boundary carbonitrides always accompany by the diffusion of C and S from the neighboring MX, and the segregation of W and Mo from the matrix, respectively. Similarly this elemental diffusion behavior of W and Mo, as well as the segregation of Si, P and S were also found for the nucleation and growth of Laves phase in contact with M23C6. Our results indicate that these precipitates formation in T92 steel is a special process with self-adaptive elemental diffusion and segregation.

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