Abstract: Austenite stability governs micromechanical stress/strain evolution and macroscopic strength-ductility synergy in martensite-austenite duplex steels. While its role in stress/strain partitioning and uniform plastic deformation has been established, its effect on damage initiation and post-uniform elongation (PUE) remains rarely addressed. Herein, three 9Ni steel specimens with different austenite stability, namely T5h, T10h, and T30h, were obtained by varying tempering durations at 590 °C. Interestingly, these specimens exhibit different damage behavior but similar PUE. Two types of voids, viz. voids away from the martensite-austenite islands (MA, consisting of deformation-induced martensite and retained austenite) and voids near the MA, are observed in specimens T5h and T10h, while only voids near the MA are detected in specimen T30h. Crystal plasticity finite element method (CPFEM) simulations are used to reveal the underlying mechanism. As for the specimens T5h and T10h, the existence of voids away from the MA is mainly induced by strain localization on the tempered martensite (TM), while the voids near the MA are attributed to both stress-localization-induced and strain-localization-induced nucleation on the MA. However, reduced austenite stability for the T30h specimen strongly suppresses strain localization on the MA and thus deteriorates strain compatibility between the MA and TM. This results in void nucleation preferentially at the TM/MA interface (voids near the MA). The decreased austenite stability also produces excessive stress concentration on the MA, which results in a higher probability for stress-localization-induced voids near the MA. The distinct stress/strain distributions explain the different void types for these three specimens. Additionally, martensitic transformation impedes stress triaxiality localization on the MA, which yields homogeneous and low-magnitude stress triaxiality across all specimens. Consequently, the void propagation is inhibited, and thus similar PUE values are obtained for the three specimens. This work highlights the importance of austenite stability in stress/strain distribution, damage mechanisms, and PUE of martensite-austenite dual-phase steels.

Key words: Austenite stability, Stress/strain distribution, Damage mechanism, Crystal plasticity, Martensite-austenite dual phase steels