Spatially propagating martensitic transformation enables strength–ductility synergy in gradient-structured L-PBF 316L stainless steel

Structural integrity, particularly porosity and tensile residual stresses, remains a major challenge limiting the application of additively manufactured alloys. Here, ultrasonic impact treatment (UIT) was employed on laser powder bed fused (L-PBF) 316L stainless steel to engineer a gradient hierarchical nano-architecture and residual stress state. The high-frequency impacts induce intensive dislocation and twinning activities, generating deformation-induced martensitic transformation (DIMT) and depth-dependent hierarchical architecture, including intermixed martensitic–austenitic nanocrystalline composite structures on the top surface (<6 μm), dense nanotwins and stacking faults (6∼23 μm), and refined cellular substructures with pronounced dislocation tangles (23∼150 μm). Compared with untreated specimens, the yield strength of the UIT-treated material increases significantly from 504 ± 2.0 MPa to 592 ± 0.5 MPa, while preserving ductility and enhancing work-hardening capability. Quasi-in-situ tracking reveals that, during tensile deformation, the gradient distribution of crystallographic defects and phase stability activates a spatially propagating martensitic transformation, which continuously renews hetero-interfaces and sustains geometrically necessary dislocation accumulation, thereby preventing saturation of back stress and enabling persistent hardening. The exceptional mechanical performance stems from the dynamic synergy between gradient-induced mechanical incompatibility, sustained hetero-deformation-induced stress, compressive residual stress, reduced porosity, and spatially propagating DIMT, collectively enabling persistent strain-hardening capability and enhanced strength–ductility synergy. HighlightsUltrasonic impact treatment (UIT) enhances strength-ductility synergy of L-PBF 316L.High-frequency impacts induce intensive dislocation and twinning activities.UIT creates depth-dependent intermixed martensitic–austenitic hierarchical structures.Spatially propagating martensitic transformation renews interfaces and delays back-stress saturation.Residual stress, lower porosity, and sustained HDI stress boost strength–ductility synergy.