Toughening an ultra-strong medium Mn steel by interface delamination

Overcoming the inherent trade-off relationship between strength and crack resistance has always been the pursuit goal in the development of engineering materials. This study assessed the fracture toughness of a steel with directionally distributed fibrous ferrites in martensite. The fracture toughness reaches 145.3 ± 4.9 MPa m^1/2, with the yield and tensile strengths of 1310 ± 40 and 1516 ± 55 MPa, respectively. Interface strength between fibrous ferrite and martensite is 840.5 ± 40.3 MPa, which is predicted by experimental data on void nucleation. The "high mechanical stress" and "relatively weak interface" activate the interface delamination, altering the crack propagation path and creating hierarchical bridging structure. It is astounding that the divergence between the predicted and observed paths of the principal crack reaches as much as ∼85°. Based on the fracture energy principle, the energy release rate is reduced to ∼29 % of the initial value, which needs to be compensated by substantially improving the external load. The individual contributions of different toughening mechanisms have been quantified. Specifically, the intrinsic toughness is 74.7 ± 3.5 MPa m^1/2, and the extrinsic toughness is mainly attributed to the deflection (43.2 ± 9.9 MPa m^1/2) and bridge (27.4 ± 1.5 MPa m^1/2) of the main crack. The research methodology is expected to be applicable to various hetero-structured materials.