New insights into annealing induced hardening and deformation mechanisms in a selective laser melting austenitic stainless steel 316L

Abstract

Annealing softening is commonly observed in traditional coarse–grained materials. Herein, an annealing–induced hardening mechanism in selective laser melted 316L stainless steel (SLM–ed 316L SS) was investigated. The SLM–ed 316L SS, without prior cold–working history, displayed evident hardening behaviour as the annealing temperature increased from 400 °C to 500 °C. Several dedicated scanning transmission electron microscope and quasi–in–situ/electron backscatter diffraction techniques were employed to investigate the intrinsic characteristics evolution of the samples, including cellular/wall dislocation structure, nano–particles/segregation, dislocation density, crystallographic orientations, and low–angle grain boundaries (LAGBs).This phenomenon primarily arises from unique guardrail–like dislocation walls decorated with nano–particles (O, Cr, Mo, and Si) and a high proportion of LAGBs, hindering movement of dislocations and leading to their accumulation. Furthermore, this structure and the stable configuration of columnar crystals can synergistically affect the 500 °C annealed sample, resulting in a high yield stress of 628 MPa. On the other hand, complex deformation substructures, such as stacking faults, Lomer–Cottrell locks, and forest dislocations, also proliferated during deformation. These substructures enabled multiscale plastic strain partitioning, intensifying strain hardening and realizing a strength–ductility combination of a comparable yield/ultimate tensile strength of 628 MPa/789 MPa and tensile ductility of 32%. Dislocation motion was the dominant deformation mechanism based on the strengthening mechanism model in this study.