Enhanced strength–ductility synergy and transformation-induced plasticity of the selective laser melting fabricated 304L stainless steel

The microstructure, mechanical properties and deformation mechanisms of the 304L stainless steel (SS) additively manufactured by selective laser melting (SLM) were systematically investigated. The SLM fabricated 304L SS contains two phases (face-centered-cubic γ-austenite and body-centered-cubic δ-ferrite) and exhibits a hierarchical microstructure with length scales spanning several orders of magnitude. The hierarchical microstructure includes the melt pools and slightly elongated columnar grains at the micron scale, cellular structures decorated with a high density of dislocations at the sub-micron scale and oxides at the nanoscale. Stacking faults formed due to the residual stress in addition to the low stacking fault energy of the 304L SS (19.2 mJ/m²) while massive annealing twins were generated arising from the combined effects of residual stress and intrinsic heat treatment. The as built 304L SS exhibits a significantly enhanced strength–ductility synergy compared to that of wrought and annealed counterparts. The enhanced yield strength stems from the hierarchically heterogeneous microstructure, while the outstanding tensile elongation is ascribed to the activation of multiple deformation mechanisms, involving the dislocation activities, the formation of stacking faults and mechanical twins, and the transformation-induced plasticity.