Multimodal Synchrotron Characterization of the Formation and Distribution of Retained Austenite in LPBF-Manufactured Ferritic-Martensitic Steel

Recent advancements in metallic additive manufacturing (AM) for nuclear fusion reactor components demands extensive control over microstructural evolution during laser-based processes. In this work, we show that without operando and multimodal characterization, critical transformations in ferritic-martensitic (FM) steels during Laser Powder Bed Fusion (LPBF) remain hidden — specifically, the formation of localized metastable austenite driven by carbon segregation. Using time-resolved operando synchrotron X-ray diffraction (XRD), combined with 2D and 3D synchrotron XRD, and X-ray fluorescence (XRF) microscopy, we map the emergence and spatial distribution of these unwanted phases. Our results demonstrate that rapid thermal cycling and scanning strategy intricately influence austenite formation and microstructural heterogeneity, meaning direct consequences for material performance and structural integrity in-service. This study highlights that post-mortem analyses alone are insufficient to qualify AM processes for nuclear applications. Only by combining real-time tracking of phase dynamics with high-resolution chemical and crystallographic mapping can critical insights be captured across multiple length and time scales. The methods and results presented in this work establish a blueprint for future qualification frameworks for AM components in extreme environments, emphasizing that predictive control of process-structure-property relationships is unreachable without a fully integrated, operando, multimodal approach.