The selective laser melted (SLM) 316L stainless steel (316L SS) has shown superior tensile ductility and doubled yield strength compared to its wrought counterpart. The significantly improved yield strength has been attributed to the unique cellular substructures that are featured by Cr/Mo-segregation and trapped dislocations. However, the excellent tensile ductility was still vaguely ascribed to pronounced deformation twinning without clear reasons. The effect of cellular substructure on the deformation mechanism also remains unrevealed. In the present study, we investigated the deformation mechanism of the SLM 316L SS using comprehensive transmission electron microscope (TEM) analysis. Besides active deformation twinning, deformation faulting and dislocation cell refinement were also featured during the whole tensile deformation. Deformation faulting and dislocation cell refinement synergistically dominate the deformation behavior at low strain levels, and deformation twinning and faulting play a crucial role at medium and high strain levels. Our results showed that the cellular substructure is mainly responsible for the marked deformation faulting and twinning. The pre-existed SFs in cellular substructure promoted the deformation faulting by providing faulting nuclei. The overlapped wide SFs, together with the cellular boundaries, resulted in pronounced deformation twinning through the Fujita-Mori twining mechanism. The multiple deformation mechanisms jointly led to a steady strain hardening rate during tension and thus a superior tensile ductility of SLM 316L SS.
