In-situ study of 6 phase evolution in 25Cr-7Ni duplex stainless steel (DSS): Unrevealing critical insight

초록

The current understanding of the formation mechanism and kinetics of the sigma phase in DSS remains ambiguous. This study investigates the in-situ evolution of the sigma phase in DSS through a multi-faceted approach. Distinct sigma phase formation mechanisms were identified at 850 degrees C and 950 degrees C, with short-range diffusion of eutectoid decomposition at 850 degrees C and long-range diffusion of nucleation and growth at 950 degrees C. In-situ X-ray diffraction (XRD) revealed that the lattice parameter of the gamma phase peaks at similar to 600 degrees C. The thermal expansion coefficient (CTE) of the gamma phase is 1.8 times higher than that of the alpha phase at low temperatures. Interestingly, the convergence of the CTEs of the alpha and gamma phases above 600 degrees C triggers sigma nucleation. The strain field diagram shows maximum impinging strain at the lattice parameter maxima just before the sigma formation range. The adverse effects of the sigma phase on DSS heat transport properties were also explored, showing an initial increase in thermal conductivity with an increasing gamma volume fraction, followed by an 18 % decrease upon sigma evolution. First-principle calculations highlight the correlation between Ni enrichment and lattice parameter maxima, supporting these findings.

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