In-situ study of σ phase evolution in 25Cr-7Ni duplex stainless steel (DSS): Unrevealing critical insight
Raj Narayan Hajra, Byoung Jun Han, Jin Woong Park, Woo Hyeok Kim, Joonho Lee, Won‐Seok Ko, Jeoung Han Kim
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.