Interpretable machine learning integrated with physicochemical feature for developing additively manufactured ultra-high strength and ductility steel

The multi-component complexity of ultra-high strength and ductility steels (UHSDS) imposes significant challenges for physical metallurgy-guided machine learning (ML) and CALPHAD-combined ML optimization design. Currently, the reported additively manufactured UHSDS necessitate multi-step heat treatments and expensive alloying elements while exhibiting poor corrosion resistance. In this study, a physicochemical feature-integrated, interpretable ML strategy was employed to design multi-component UHSDS fabricated via laser-directed energy deposition (LDED). SHapley additive exPlanation (SHAP) analysis was performed to reveal the critical intrinsic factors and their effects on the tensile mechanical performance, including ultimate tensile strength (UTS), yield strength (YS), and elongation (EL). Accordingly, the candidate alloying elements were identified, and a simple single-step tempering heat treatment was adopted, enabling the synchronous enhancement of strength and ductility. As a consequence, we designed a novel cost-effective and short-process LDEDed Fe-15Cr-3.2Ni-0.8Mn-0.6Cu-0.56Si-0.4Al-0.16C UHSDS, followed by tempering at 480 °C for 6 h. The as-tempered specimen mainly consisted of lath martensite, together with some austenite, carbides and nano-precipitates (AlN, NiAl, and ɛ-Cu). Both striking mechanical properties (i.e., UTS: (1 713 ± 17) MPa, YS: (1 502 ± 33) MPa, EL: (15.5 ± 0.7)%) and excellent corrosion resistance, with a corrosion rate of 0.105 mm·a^-1, have been achieved, superior to the reported additively manufactured UHSDS. The ultra-high strength and ductility are attributed to solid-solution strengthening, precipitate strengthening and transformation-induced plasticity (TRIP) effects. This work will be of great significance to provide new insights into the development of low-cost and process-simplified UHSDS, especially for the laser fabrication of high-value-added steel components with excellent comprehensive performance.