Multimodal synchrotron characterization of the formation and spatial distribution of retained austenite in PBF-LB/M-manufactured ferritic–martensitic steel

Metal additive manufacturing is a promising route for producing complex, highly customized embedded structures for nuclear fusion environments, such as breeding blankets and divertors. These applications require steels with high thermomechanical stability and resistance to irradiation, yet AM processing often leads to undesired microstructural heterogeneities, including the formation of metastable phases. In this work, we investigate the formation and spatial distribution of retained austenite in Laser Powder Bed Fusion (PBF-LB/M) — processed ferritic–martensitic stainless steel (AISI 415) using multimodal synchrotron-based characterization. Micron-resolution 2D and 3D synchrotron X-ray Diffraction and X-ray Fluorescence mapping, combined with operando XRD during PBF-LB/M, reveal the presence of retained γ -phase in periodic mesostructures at concentrations up to 0.5 wt%, depending on scanning strategy. We demonstrated that this result, gained from volumetric measurements based on μ XRD scanning imaging, cannot be gathered by any surface-sensitive technique ( e.g. EBSD) due to depth limitations and phase transformation artifacts during sample preparation. No correlation between γ -phase formation and elemental segregation was observed. Operando XRD measurements show that cooling rates critically affect phase evolution: wall-like geometries exhibit rapid cooling ( ∼ 1 0 5 to 1 0 6 K/s) and complete martensitic transformation, whereas bulk samples cool more slowly ( ∼ 1 0 4 K/s), allowing up to 0.5 wt.% of γ -phase to be retained. These results demonstrate the strong influence of both scanning strategy and thermal history on phase stability in PBF-LB/M steels, supporting the qualification of AM-built components for nuclear applications. • Retained austenite is detected in PBF-LB/M 415 steel and mapped using synchrotron XRD. • Operando XRD shows retained austenite (RA) when the cooling rates are ≲ 1 0 4 K/s. • Cooling rates impact the formation of RA; scanning strategy its spatial distribution.