Effects of cellular segregation for high strength and ductility of additively manufactured 304L stainless steel

This study characterized the microstructural features of additively manufactured 304L stainless steel (AM304L) which exhibited high strength and high ductility. To investigate the relationship between the cellular segregation and deformation-induced martensitic transformation (DIMT) in AM304L, microstructural changes during the deformation were examined through the interrupted tensile tests. The microstructure of undeformed AM304L showed element segregation in cellular structures formed through dendritic solidification during the AM process. From the tensile test, AM304L exhibited higher yield strength, higher tensile strength, and even higher elongation compared to wrought 304L (SS304L). After tensile deformation, deformation-induced martensite (α'-martensite) was observed in both deformed samples (AM304L and SS304L). Coarse martensite was found in the deformed SS304L while fine and uniform martensite was found in the deformed AM 304L. Notably, in deformed AM304L, deformation-induced martensitic transformation (DIMT) to a cellular structure with martensite surrounded by austenite was observed. The region of martensite phase coincided with nickel-depleted regions and the region of austenite phase coincided with nickel-enriched regions, resulting from element segregation in the cellular structures. This indicates that element segregation influenced the DIMT by changing the local chemical composition and the austenite phase stability. This conclusion was supported by simultaneous EBSD and EDS analysis. The high strength and ductility of the AM304L is considered to result from homogeneous martensitic transformation in cellular structures of the proper scale, induced by element segregation.