Additive manufacturing (AM) of metallic alloys has gained momentum in the past decade for industrial applications. The microstructures of AM metallic alloys are complex and hierarchical from the macroscopic to the nanometer scale. When using laser-based powder bed fusion (L-PBF) process, two main microstructural features emerge at the nanoscale: the melt pool boundaries (MPB) thickness and the solidification cellular substructure.

Here, details of the MPB are revealed to clearly show the three-dimensional nature of MPBs with changes of cell growth of direction and their relation to their surrounding cellular substructure, as investigated by transmission electron microscopy (TEM) for L-PBF 316L austenitic stainless steel. A hitherto unknown modulated substructure with a period of 21 nm is further discovered within cells as the result of a partial Ga⁺-focused ion beam-induced ferritic transformation of the austenite. Cell cores and cell boundaries differ notably with regard to the modulated substructure. The distinct diffraction effects of the modulated substructure are exploited to show that the MPB has a finite thickness of at least ≈ 200 nm. The MPB suppress cell walls within this region, continuously connecting cell cores of both of its sides. This continuous MPB wall can be described either 1) as overlapping regions of cells of different growing directions when a new melt pool solidifies, or 2) as a short stretch of planar growth preceding the new melt pool.