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

Abstract

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