Structural and chemical length scales at near-stationary austenite-ferrite interfaces in a model duplex stainless steel

The structural and chemical properties of austenite/ferrite (γ/α) interfaces were investigated in a model duplex stainless steel (Fe–Cr–Ni–Mo–N) using a correlative approach combining scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), and atom probe tomography (APT). Two interface types were selected based on their orientation relationship (OR): a near-Kurdjumov-Sachs (K-S) interface and a non-K-S interface. The near K-S interface presents a planar terrace-ledge morphology with atomically sharp structural transitions at the terraces, whereas the non-K-S interface is rough with protrusions of 3–5 nm amplitude. In both cases, the chemical transition width measured by EELS and APT exceeds the crystallographic interface width determined from STEM imaging, indicating that chemical potential perturbations extend beyond the structural discontinuity. The OR is found to have a clear effect on the solute segregation behavior: Mo and N segregate exclusively to the non-K-S interface, while Cr segregates to both interface types. Ni shows no detectable interfacial excess at either interface. The 2D APT composition maps reveal a spatially heterogeneous distribution of Cr at the near K-S interface, with a strong spatial correlation between Cr and N enrichment attributed to their attractive thermodynamic interaction. Gibbsian interfacial excess values and segregation energies are reported for all solutes, distinguishing between effective and intrinsic contributions. Thermodynamic self-consistency of the derived segregation energies is only achieved for an interface width of 1–2 nm, consistent with the chemical width from EELS. This provides an experimentally grounded estimate of the interface width directly applicable to solute drag models of γ/α interface migration.