Description:
<jats:sec><jats:label /><jats:p>In the current study, the 3D nature of the melt pool boundaries (MPBs) in a 316 L austenitic steel additively manufactured by laser‐based powder bed fusion (L‐PBF) is investigated. The change of the cell growth direction and its relationship to the MPBs is investigated by transmission electron microscopy. A hitherto unreported modulated substructure with a periodicity of 21 nm is further discovered within the cell cores of the cellular substructure, which results from a partial transformation of the austenite, which is induced by a Ga<jats:sup>+</jats:sup> focused ion beam. While the cell cores show the modulated substructure, cell boundaries do not. The diffraction pattern of the modulated substructure is exploited to show a thickness ≥200 nm for the MPB. At MPBs, the cell walls are suppressed, leading to continuously connecting cell cores across the MPB. This continuous MPB is described either as overlapping regions of cells of different growing directions when a new melt pool solidifies or as a narrow planar growth preceding the new melt pool.</jats:p></jats:sec>