High fracture toughness of an ultra-strong medium Mn steel with fibrous ferrite in a martensitic matrix

Abstract

Developing ultrahigh strength steels that are fracture resistant and economical would be appealing for structural applications. In this work, an ultra strong-and-tough steel was fabricated by introducing fibrous ferrite into martensite matrix. This steel exhibited an ultrahigh yield strength and tensile strength, 1560 and 2103 MPa respectively, along with a 7.3 % uniform elongation. The fracture toughness and smoothly rising R-curve were measured through the elastic-plastic unloading compliance method using miniaturized side-grooved compact tension (CT) specimens. A high fracture toughness (KIC≈105 MPa m1/2) that is superior to many steels at the same strength level was achieved. The heterogeneous microstructure of fibrous ferrite embedded in a martensitic matrix allows for high strain hardening rate and adequate micro-void formation, consequently leading to higher fracture strain and the occurrence of localized necking. The toughening mechanisms of CT samples are discussed according to the energy criterion for fracture. The expanded plastic deformation area (intrinsic toughening mechanism), deflecting and bridging the primary crack propagation as well as interface delamination between fibrous ferrite and martensite (extrinsic toughening mechanisms) significantly improve the overall fracture resistance. The current findings promote the development of high-strength and high-toughness medium-Mn steels for structural applications.