Metastable δ-ferrite and twinning-induced plasticity on the strain hardening behavior of directed energy deposition-processed 304L austenitic stainless steel

Abstract

Rapid melting and solidification cycles during laser-based additive manufacturing create non-equilibrium microstructures in stainless steels (SSs) including atomic segregation-mediated ultrafine δ-ferrite, in contrast to coarse δ-ferrite in typical casting-produced SSs. The formation of metastable ultrafine δ-ferrite in additively manufactured SSs generates a new coherent interface in austenitic matrix. However, currently a consensus on how ultrafine δ-ferrite interacts with dislocations is lacking, particularly in directed energy deposition-processed 304L SS. Herein, the role of ultrafine δ-ferrite on the mechanical properties of directed energy deposition-processed 304L SS was investigated by modifying the laser scan speeds of 850 and 1150 mm/min and by performing electron microscopy-based characterization. We find that the ultrafine δ-ferrite maintains coherency with a γ-austenite matrix in the undeformed state and interacts with dislocations during plastic deformation. Additionally, the twin volume fraction depends on the initial grain size of 304L SSs, which results in a 23 MPa (for strength) and 5% (for ductility) mechanical property difference between the 850 (SLOW) and 1150 (FAST) mm/min scan speed conditions. Through the synergetic effects of ultrafine δ-ferrite, deformation-induced twins and twin intersections, the present additively manufactured 304L SS achieves an outstanding ductility that is larger than that of the previous directed energy deposition-processed SSs. This result proves that the ultrafine metastable phase contributes to the prolonged plasticity of the additively manufactured metallic alloys if the metastable phase maintains coherency with the matrix during plastic deformation.