Duplex-Phase Fe-Mn-Al-C Low-Density Steels: A Review on Their Alloy Design, Processing, Mechanical and Application Performances

Duplex-phase low-density steels are attracting interest for lightweight structural applications, as reducing vehicle mass is an effective route to lower fuel consumption and emissions. This review summarizes recent progress in alloy design, processing, microstructure control, and performance of duplex-phase low-density steels. The roles of major alloying elements are discussed in terms of phase stability and precipitation tendency, followed by an overview of typical processing routes from melting to hot and cold rolling and subsequent heat treatments used to tailor phase fractions and defect structures. Strengthening mechanisms are reviewed with emphasis on precipitation control, including the beneficial contribution of fine intragranular κ′ precipitates and the ductility penalty associated with coarse intergranular κ* films, as well as the use of B2-based particles for high specific strength. Deformation behavior is then discussed in terms of transformation-/twinning-induced plasticity (TRIP/TWIP), planar versus wavy slip, and strain partitioning between ferrite and austenite. Finally, key challenges are outlined, including quantitative interface-based mechanism description, gaps in service property data, stable industrial production and compositional uniformity, and the development of forming and welding windows for engineering implementation.