Research Progress on the Influence of Metastable Austenite on the Fracture Toughness of High-Strength Steels

Incorporating metastable austenite is the one of the key strategies for achieving synergistic improvement in the strength and ductility of high-strength steels. Through in situ deformation-induced martensitic transformation during tensile loading, metastable austenite can delay necking while enhancing work-hardening capacity. Concurrently, ultrahigh-strength steel components are facing increasing demands in terms of lightweightness and service in complex environments; hence, they will be required to have a higher fracture toughness without compromising strength. Research has focused on incorporating the tougher austenite phase in high-strength steels to improve their fracture toughness and preserve ductility. Metastable austenite contributes to enhanced fracture toughness through transformation toughening and its interactions with cracks, which can deflect or blunt cracks. However, freshly formed martensite, a product of martensitic transformation, can reduce the toughening effect or even deteriorate fracture tough ness due to its inherent brittleness and effect on the local stress state. This paper reviews recent re- search progress on the relationship between metastable austenite and fracture toughness of high- strength steels, examining the toughening and embrittlement mechanisms of the phase. In addition, it out- lines future design principles for metastable austenite incorporation in high-strength steels to achieve syn- ergistic improvements in strength and toughness.