In situ observation of a transient intermediate state during metastable solidification of a Fe–Cr–Ni alloy

Liquid-to-solid phase transformations mediated by intermediary transitional metastable states have garnered significant interest due to their potential to uncover materials with unique morphologies and functionalities absent under equilibrium conditions. In this study, the metastable-to-stable transformation from a body-centered cubic (bcc) to a face-centered cubic (fcc) structure is investigated during the solidification of an undercooled Fe–16Cr–12Ni alloy using containerless electromagnetic levitation combined with in situ synchrotron X-ray diffraction (XRD). Time-resolved diffraction patterns reveal unexpected peak shifts and splitting during the early stage of the transformation, indicating pronounced lattice distortions in the emerging fcc phase. Detailed analysis of multiple symmetry-sensitive reflections suggests that the transformation proceeds through a transient intermediate state characterized by a short-lived, symmetry-lowering lattice distortion with an fct-like character. This distortion relaxes rapidly as the system evolves toward the thermodynamically stable fcc structure. These observations point to a non-equilibrium transformation pathway in which localized strain and defect-mediated processes play an important role during rapid solidification. The findings provide new insight into multistep, non-equilibrium phase transformation dynamics relevant to alloy design, additive manufacturing, and rapid solidification processing.