Overcoming strength-toughness trade-off by improving the strength of martensite in dual-phase heterolamellar steel

Ultra-high strength steels typically exhibit low impact toughness due to the conflict between strength and toughness. Here we show that a dual-phase heterogeneous lamellar (HL) microstructure can mitigate the strength-toughness trade-off dilemma, a simultaneous enhancement of strength and impact toughness was achieved by increasing the tensile strength of martensite. The impact toughness of HL steels at − 40 °C was elevated by a factor of 1.6, rising from 258 to 424 J, while tensile strength increases from 1 300 to 1 571 MPa. The underlying toughening mechanisms were clarified by analysing microcrack formation and the associated plastic deformation microstructures during crack initiation and propagation. The lamellar structure can impede crack propagation and lead to multiple crack nucleation. The superior impact toughness of HL steels is primarily attributed to the plastic dissipation energy consumed by multiple crack nucleation. The increase in tensile strength of HL steel results in higher plastic dissipation energy, thereby leading to a linear enhancement of impact toughness with tensile strength. This research provides an in-depth investigation into the toughness mechanisms of the dual-phase heterolamellar structure, offering valuable insights that could help promote the application of these materials in modern industrial applications.