Superior fracture toughness in a high-strength austenitic steel with heterogeneous lamellar microstructure

The development of steels with a combination of high strength, ductility, and toughness has been highly desirable for structural applications. Here we achieve high fracture toughness (129 MPa m½) in a high-strength (∼1.5 GPa yield strength) and ductile (∼37% uniform elongation) austenitic stainless steel. Through cold rolling, flash annealing, and tempering processes, the steel displays a heterogeneous lamellar microstructure that is comprised of reversed austenite (RA) lamellae with high-density dislocations and partially recrystallized austenite (PRA) lamellae with sub-micron grains. The lamellar interfaces are prior-austenite grain boundaries (PAGBs) with thin layer precipitates. Intrinsically, a large number of grain boundaries hinder the propagation of cracks, enhancing fracture resistance in PRA lamellae. The dislocation cells in the RA lamellae act as soft barriers to crack propagation, blunting the crack tips and reducing the adverse effect of high-density dislocations on fracture toughness. Extrinsically, the toughening mechanisms include the deep expansion of dimples in the PRA lamellae, the interfacial delamination of lamellar interfaces, and transformation-induced plasticity (TRIP) effect. Our study may promote the development of high-strength and high-toughness austenitic steels and alloys for structural applications.