Abstract: While nanoscale κ-carbide precipitation is a well-established strengthening mechanism in low-density steels, the unexpected enhancement of austenite strength observed with Mo addition, despite its known inhibitory effect on κ-carbide formation, has remained unclear. In this study, we identify a novel and controllable short-range ordering (SRO) structure mediated by the Mo element in Fe-28Mn-9Al-1.2C-5Mo (wt.%) steel, accompanied by the surrounding C atoms occupying at octahedral interstices within the face-centered cubic lattice. Theoretical calculations indicate that the localized ordering of C atoms originates from the strong Mo-C interaction at first-nearest-neighbor sites. Meanwhile, this chemical SRO behavior markedly increases the nanohardness and intrinsic strength of the austenite matrix. These findings demonstrate that Mo not only alters the precipitation behavior of κ-carbide but also induces SRO structure formation, providing a new pathway to tune the mechanical properties of low-density steels through atomic-scale structural design.

Key words: Low-density steels, Short-range ordering, Mo addition, Precipitation strengthening