Engineering heterostructures via layer-wise in-situ alloying in additive manufacturing: Compositional and architectural design in heterogeneous 316 L stainless steel for strength-ductility synergy

Achieving a favorable balance between strength and ductility remains a critical challenge in additive manufacturing (AM) of structural metals. This study proposes a universally applicable design strategy based on layer-wise in-situ alloying, which enables flexible and coordinated control over composition and architecture within a single-alloy system. Using 316 L stainless steel as a model material, titanium (Ti) was selectively introduced into alternating layers during laser-directed energy deposition (L-DED) to construct laminated structures comprising Ti-alloyed (hard) and pure (soft) domains. The resulting heterostructures exhibit superior combinations of strength and ductility as compared to the homogenous samples. The high strength arises from the rule of mixture and hetero-deformation-induced (HDI) strengthening, while HDI strain hardening and twinning-induced plasticity (TWIP) delay necking and contribute to sustained ductility. Systematic investigations revealed that Ti content and laminate spacing critically influence microstructural heterogeneity and mechanical incompatibility due to the dilution phenomenon. Optimal mechanical performance was achieved with 1.5 wt.% Ti and a bilayer alloying pattern, yield a tensile strength of 726.1 MPa and uniform elongation of 41 %. These findings establish layer-wise in-situ alloying as a general design route for tailoring mesoscale heterogeneity and unlocking new performance regimes in AM-fabricated structural components.