Metal additive manufacturing is a promising route for producing complex, highly customized embeddedstructures for nuclear fusion environments, such as breeding blankets and divertors. These applicationsrequire steels with high thermomechanical stability and resistance to irradiation, yet AM processing oftenleads to undesired microstructural heterogeneities, including the formation of metastable phases. In thiswork, we investigate the formation and spatial distribution of retained austenite in Laser Powder BedFusion (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 Fluorescencemapping, combined with operando XRD during PBF-LB/M, reveal the presence of retained 𝛾-phase in periodicmesostructures at concentrations up to 0.5 wt%, depending on scanning strategy. We demonstrated thatthis result, gained from volumetric measurements based on μXRD scanning imaging, cannot be gatheredby any surface-sensitive technique (e.g. EBSD) due to depth limitations and phase transformation artifactsduring 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 geometriesexhibit rapid cooling (∼105 to 106 K/s) and complete martensitic transformation, whereas bulk samples coolmore slowly (∼104 K/s), allowing up to 0.5 wt.% of 𝛾-phase to be retained. These results demonstrate the stronginfluence 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.
Garrivier et al. (Thu,) studied this question.