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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