Abstract: Martensitic-based microstructures in low-density steels offer high strength and improved specific strength, combined with the lightweight effect of aluminum (Al). However, while Al effectively reduces density, it simultaneously promotes the formation of coarse ferrite and expands the two-phase (α + γ) intercritical temperature range. Thus, increasing the Al content for higher weight reduction inevitably leads to ferrite formation and impedes further strengthening. To achieve both high strength and ductility while incorporating ferrite, it is crucial to elucidate the effects of ferrite fraction, size, and distribution on mechanical properties and deformation behavior, particularly in relation to phase interactions. In this study, three model steels were developed through controlled annealing temperatures, producing distinct triplex microstructures comprising ferrite, martensite, and retained austenite (RA). The role of each phase in strain partitioning was investigated using ex-situ microscopic digital image correlation and electron back-scattered diffraction analysis. Key findings reveal that the martensitic matrix ensures an ultrahigh strength level (1758 MPa), while a moderate fraction (∼17 %) and homogeneous distribution of intercritical-ferrite (IC-ferrite) enable sustainable strain-hardening behavior by delaying the transformation-induced plasticity (TRIP) effect. Strain partitioning into IC-ferrite reduces local strains in the martensitic matrix, preventing early exhaustion of the TRIP effect and facilitating ductile fracture behavior. This strategy leverages the presence of ferrite, offering significant advantages for applications requiring both ultrahigh strength and ductility.

Key words: Martensitic low-density steel, Intercritical ferrite, Retained austenite, Strain partitioning, Transformation-induced plasticity