• Duplex Fe–Mn–Si SMAs were designed by adjusting Si and Cr contents, yielding controlled austenite–ferrite phase fractions. • Duplex microstructures significantly increased yield strength while maintaining comparable recovery stresses to the single-phase alloy. • Higher Cr contents improved corrosion resistance in chloride-containing environments. • Results establish composition–microstructure guidelines for strengthening Fe–Mn–Si SMAs without degrading functional performance. In this study, no