The dual role of TRIP effect on ductility and toughness of a medium Mn steel

In this study, the tensile and fracture behaviors of a medium Mn steel fabricated by two thermomechanical processes, namely intercritical annealing (IA) and room-temperature quenching and partitioning (RT Q&P), were investigated. The IA steel consists of ultrafine-grained, fully recrystallized ferrite and austenite, while the RT Q&P steel is comprised of martensite matrix and retained austenite. The austenite in the IA steel is less stable due to its lower carbon content. High-resolution micro-digital image correlation (micro-DIC) results reveal moderate strain localization at boundaries in the RT Q&P steel. On the contrary, strain is highly localized in the austenite of the IA steel, making austenite transform into fresh martensite quickly and promoting the work hardening rate, i. e., transformation-induced plasticity (TRIP) effect. However, this intensive TRIP leads to premature decohesion at boundaries and sudden fracture without post-elongation. The J-integral-based resistance curves (J-R curve) were also measured for both steels. The crack-initiation toughness of the IA steel is 22% less due to its frequent martensite cracking and intergranular decohesion. In contrast, the hierarchical microstructure in the RT Q&P steel suppresses brittle fracture and enables significant crack tip blunting, resulting in a high fracture toughness. The present work illustrates the dual role of TRIP on ductility and fracture toughness, namely, intensive TRIP is useful for tensile strength and uniform elongation, but is harmful to fracture toughness, which contradicts the common belief that TRIP is beneficial for both ductility and toughness.