Generalized stability mediated two-step tailoring of B2 precipitates: Overcoming strength-ductility trade-off in low-density steel

: Fe–Mn–Al–C–Ni low-density steels, strengthened primarily by Ni–Al–type B2 intermetallic, are promising for advanced energy structures yet suffer from poorly controlled precipitate size and distribution, with coarse grain-boundary phases severely degrading ductility. Here, rolling deformation and a two-step heat treatment were employed to construct a multi-scale B2 precipitate morphology originating from Ni- and Al-rich regions, while simultaneously introducing nanoscale intragranular κ-carbides. Quantitative thermodynamic and kinetic analyses demonstrate that a 20 min short-term aging treatment effectively suppresses further coarsening of the deleterious banded structure, while maximizing the microstructural evolution into a relatively stable kinetic plateau regime. The high strength originates from dynamic interactions of multi-scale B2 precipitates and κ-carbides with dislocation configurations within the FCC matrix, whereas the uniform elongation results from the ability of dense planar slip bands therein to accommodate plastic strain and postpone necking. This strategy reconciles the inherent conflict between B2 strengthening and ductility loss, achieving an ultrahigh ultimate tensile strength exceeding ∼ 1.4 GPa together with a notable elongation of ∼ 26 %, the yield strength surpassing that of a one-step heat-treated sample at a comparable ductility level by approximately 20.7 %. By integrating thermodynamic calculations and kinetic simulations, this work elucidates the precipitation pathway during two-step heat treatment, offering a transferable design rationale for ultrahigh-strength, precipitate-engineered low-density steels.