Deformation faulting and dislocation-cell refinement in a selective laser melted 316L stainless steel

Abstract

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 featured by Cr/Mo-segregation and trapped dislocations. The excellent ductility of SLMed 316L SS has been mainly understood from the pronounced deformation twinning, which, however, is just one of the dominant plastic deformation mechanisms in 316L SS. Here instead, we report two other fundamental deformation micro-mechanisms, i.e., deformation faulting and dislocation cell refinement, in the SLM 316L SS. Our results showed that deformation faulting and dislocation cell refinement characterize the whole tensile deformation significantly except for deformation twinning. These newly found mechanisms synergistically dominated the deformation behavior at low strain levels while deformation faulting and twinning play a crucial role at medium and high strain levels. The pre-existing stacking faults (SFs) in cellular substructure is mainly responsible for the significant deformation faulting. At the early deformation stage, these pre-existing SFs provide faulting nuclei for deformation faulting, leading to wide SFs. These wide SFs in different cellular interiors penetrate the cellular boundaries and overlap as the strain increases, resulting in long stacking fault ribbons (SFRs). We attributed the formation of DCs to the measured medium stacking fault energy (SFE) of ∼18 mJ/m2. Upon straining, equiaxed dislocation cells (DCs) were generated by new dislocation cell walls (DCWs) inside the cellular substructure, and refined by either the dissociation of fully developed DCWs or the continuously formed new DCWs. Together with deformation twinning, these two fundamental deformation mechanisms jointly led to a steady strain hardening rate during tension and thus a superior tensile ductility of the SLM 316L SS with high yield strength. These findings provide new insights into the excellent strength-ductility combination of SLMed 316L SS and the experimental basis for crystal plasticity modeling and simulation, as well.