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Abstract: The present research deals with work hardening analysis of two types (4Mn & 8Mn) high strength-high plasticity metastable duplex stainless steel (DSS). A three-stage (I, II, III) strain hardening rate (SHR) was recognized in both alloys, and the involved mechanism(s) at each stage were investigated using extensive TEM and EBSD analysis. The SHR slopes and extremum points differed for each stage due to variations in austenite instability. Microstructural observations revealed that the ε-martensite formation affects the slope of stage I in the 8Mn alloy. In contrast, the lower stacking fault energy (SFE) of 4Mn alloy resulted in faster kinetics of ε-martensite formation masking its effect on stage I. The minimum of SHR in both alloys corresponded to the α´-martensite nucleation and its initial relaxation effect at the end of stage I. The sharp SHR increase in 4Mn alloy was ascribed to the deformation-induced α´-martensite formation and its hardening effect during straining. In 8Mn alloy, however, the complex supplementary mechanisms comprising mechanical twins, α´-martensite, and stacking faults (SFs) formation were responsible for SHR increment during stage II. It was shown that the appearance of sustained SF bands was influential in enhancing work hardening and the superior mechanical properties of 8Mn steel. The deformation-induced α´-martensite formation was simulated through the Olson-Cohen model in both alloys, and the origin of austenite instability was thermodynamically discussed. The results revealed that aside from SFE, the chemical driving force of α´-martensite to austenite (-ΔG^γ→α´) needs to be considered for predicting the austenite instability and work hardening of both alloys.