Synergistic enhancement of strength, ductility, and toughness in a low carbon micro-alloy steel with an ultrafine-grained heterogeneous lamellar structure

Structural materials with heterostructure design have emerged as an efficient strategy to address the dilemma of balancing strength and ductility/toughness, resulting in increased interest from the scientific community. In this study, a novel ultrafine-grained heterogeneous lamellar (HL) structure has been architected in a common lowcarbon micro-alloyed steel. The heterostructured steel exhibits a lamellar microstructure comprising alternating ultrafine/nano-grained lamellae and micrometer-grained lamellae, with a texture gradient along the thickness. Results show that the HL structure has a tensile strength comparable to quenched and tempered martensite (QTM) steel while retaining similar ductility as coarse-grained dual-phase (CG) steel. Furthermore, the HL specimen exhibits enhanced fracture toughness (KQ = 232.8 MPa m1/2), which is 11.3% and 28.8% higher than that in the CG and QTM specimens, respectively. Extrinsically, the macroscopical heterogeneous lamellar structure and microscopical elongated grain shape in ultrafine/nano-grained lamellae could provide substantial lamellar grain boundaries to hinder the connection of voids and make the crack propagation path more tortuous, which effectively increases the energy release rate to contribute to the exceptional fracture toughness. Intrinsically, not only micrometer-sized CG lamellae with high plastic deformation tolerance capacity can develop coarse (deep) dimples for energy consumption, but also the elongated ultrafine martensite grains can undergo significant plastic deformation together with the coordinated deformation of coarse-grained ferrite. We hope the strategy in this study may provide insights for microstructure optimization and synergistic strength-plasticity/ toughness enhancement of conventional low-carbon micro-alloyed steels for structural applications.