Abstract: Breaking through the strength-toughness (ductility) tradeoff of ultrastrong steels has been an enduring pursuit in the scientific community. In the present work, superior synergistic combinations of ultrahigh strength (ultimate tensile strength, ∼2026 MPa), desirable ductility (total elongation, ∼13 %), and exceptional fracture toughness (K_JIC, ∼119 MPa m^-1/2) were achieved in a low-carbon alloy transformed-induced plasticity (TRIP) steel by microstructural architecture, specifically, by utilizing multi-delamination crack toughening evolved from microvoids induced toughening and deformation-induced martensitic transformation (DIMT) toughening strategy. Firstly, nanoscale lamellar ferrite (α) and metastable austenite (γ) strengthened by high-density Cu-rich precipitations were obtained by warm rolling (WR). Fabricated numerous lamellar interfaces and surrounding ductile α and γ phases in the WR620 steel trigger a microvoids-induced toughening mechanism that is characterized by high-density non-aggregated microvoids (∼2.5 × 10⁴ mm^-2), resulting in superior crack-initiation and crack-growth toughness. Secondly, maintaining these microstructure characteristics, further strengthening of constituent phases was achieved by cold rolling deformation at cryogenic temperature, which implants preferentially high-density dislocations in γ and ensures ultrahigh mechanical stability regarding DIMT in WR620-N2CR steel. Combined with ultrastrong α/α', optimized TRIP-assisted lamellar microstructure conquers the dilemma of strength-ductility tradeoff at ultrahigh yield strengths, meanwhile, without significantly deteriorating fracture toughness. These fabricated lamellar interfaces serve as preferential sites for the initiation of delamination microcracks, resulting in excellent crack-initiation fracture toughness. Nanoscale lamellar constituent phases with ultrahigh strength and deformed γ with ultrahigh critical martensitic transformation stress further retard catastrophic propagation and coalescence of delamination microcracks, which demonstrates a multi-delamination crack toughening mechanism from the perspective of observed fractographic features and derived micro-mechanically ductile fracture model. Additionally, the in-situ DIMT toughening enhances fracture toughness by simultaneously absorbing energy, which determined contribution accounts for 28.0 % and 21.3 % of the total fracture toughness in WR620 and WR620-N2CR steels, respectively. To summarize, our findings provide a microstructural architecture strategy and reveal synergistic strengthening-toughening mechanisms to design steel with ultrahigh strength-ductility and superior damage-resistance synergy.

Key words: Nanoscale lamellar microstructure, Ultrastrong ductility-toughness mechanisms, Fracture toughness, In-situ synchrotron X-ray diffraction, Microvoids and delamination microcracks induced toughening mechanisms