Overcoming the strength-ductility trade-off in 300 M steel via spatially-controlled heterostructure design

A paradigm for spatially-controlled heterostructure design is proposed to overcome the strength-ductility trade-off in ultra-high-strength 300M steel fabricated via laser powder bed fusion (LPBF). This mechanism-guided approach deliberately modulates the spatial distribution of laser energy using a hybrid linear scanning (LS) and oscillating scanning (OS) strategy to construct lamellar heterostructured (HS) 300M steel with tailored microstructures. The innovation significantly enhances the mechanical property mismatch between the hard (LS-derived) and soft (OS-derived) regions, which is critical for amplifying back-stress strengthening, manifested as hetero-deformation-induced (HDI) stress. The HS materials fabricated via this optimized spatial control achieved a superior mechanical synergy: an ultimate tensile strength of 1761±4.4 MPa and an exceptional elongation of 17.5±0.3%. This performance represents improvements of 104 MPa in strength and 4.7% points in ductility compared to the non-HS samples. Furthermore, the study systematically investigates the influence of hard-phase fraction, lamellar thickness, and soft-hard phase contrast on HDI stress and mechanical performance. These findings establish a robust framework for designing high-performance additive-manufactured materials through coordinated control of lamellar geometry, phase contrast, and hard-phase fraction.