Effect of austenite stability on the fracture micromechanisms and ductile-to-brittle transition in a medium-Mn, ultra-fine-grained steel for automotive applications

This work reports on the tensile and Charpy impact behaviour of a high-strength, ultra-fine-grained, medium-manganese steel with a ferrite + austenite microstructure. The mechanical stability of austenite was varied by varying the test temperature, while keeping all other microstructural features constant. By lowering the tensile test temperature, austenite decomposition into martensite occurred earlier, inducing strong work hardening, decreasing tensile elongation and triggering ferrite–martensite interfacial decohesion. Under impact testing, the material exhibited a broad transition in absorbed energy and in thickness reduction at fracture initiation. A continuous transition in fracture mechanism, from fully ductile to fully interfacial fracture, occurred with lowering the test temperature. Besides chemically induced weakening of ferrite–austenite interfaces, the temperature-dependent mechanical stability of retained austenite, as well as the strength of the ferrite phase, is thought to control local stresses at interfaces between ferrite and freshly formed martensite. It thus drives the extent of interfacial microcrack initiation and propagation before blunting and development into voids.