Irradiation Response and Mechanical Properties of 316L Stainless Steel Fabricated by Direct Energy Deposition

Abstract

Additive manufacturing (AM) techniques have been widely used to fabricate structural components with complex geometries. Understanding AM materials under extreme environments is crucial for their implementation in various engineering sectors. In this study, the deformation behavior and irradiation tolerance of 316L stainless steel (SS) fabricated by the direct energy deposition (DED) process were investigated. The fabrication-induced nanopores with an average diameter of 200 nm exhibited a core-shell structure. The strain mapping around the pores suggests that the core-shell structure showed a local tensile strain. In situ tensile testing in a scanning electron microscope (SEM) showed a high density of deformation twins forming in the DED fabricated specimen at room temperature. Precession electron diffraction (PED) revealed that the martensitic phase transformation preferentially occurs around the nanopores. Proton irradiation experiments were performed at 360℃ for both conventional and DED fabricated specimens. The DED fabricated 316L SS exhibits a stronger void-swelling resistance and lower dislocation loop density than its wrought counterpart. Fabrication-induced features, such as nanopores and sub-grain boundaries, could serve as defect sinks to absorb irradiation-induced defects. Single crystal micro-pillar compression revealed the critical resolved shear stress of AM 316L SS decreased from 106 MPa to 87 MPa after annealing and significantly increased to 246 MPa after proton irradiation. Twinning and martensitic phase transformation were developed at the later stage of deformation in non-irradiated AM 316L SS. Twinning became the primary deformation mechanism after irradiation. Irradiation hardening results were comparable with the prediction from Orowan dispersed barrier model. The model inferred dislocation loop was the primary factor to strengthen the AM 316L SS. Characterization showed radiation-induced voids could be compressed by twin or cut by dislocation while nanopores from fabrication can pin down dislocations. Although nanopores have shown the capability to strengthen the material and mitigate swelling, the residual stress and sub-grain boundary were considered the mechanical properties and swelling behavior dominator.