Abstract: Lightweight steels display microstructural configuration- and unit size-dependent mechanical properties, yet how can these factors be synergistically adjusted to achieve superior performance? In this study, two kinds of microstructural configurations are fabricated in the lightweight steel by thermo-mechanical processes: (i) heterogeneous structures with gradually optimized hetero-unit sizes; (ii) homogeneous structures, alongside partially/ non-recrystallized variants for comparison. The heterogeneous structure with optimized hetero-unit size achieves an exceptional mechanical synergy, delivering an excellent ductility of 36.1 % at an ultrahigh tensile strength of 1441 MPa. This synergy arises from a dual-improvement effect synergized by hetero-structural design and hetero-unit size optimization, in which the physical mechanism is ascribed to grain boundary strengthening and coupling-enhanced hetero-deformation induced (HDI) strengthening and hardening. A physical-based mechanical model is proposed to elucidate the coupling effect, jointly governed by volume fractions of hetero-zone boundary affected region and disparities of geometrically necessary dislocation density between hetero-boundaries and grain cores, theoretically confirming the ideal hetero-unit size. A mechanical comparison across massive configurations reveals that heterostructure designs reshape strength-ductility synergy, in which the size-optimized (approaching the ideal size) heterostructure is the optimal structural design. This work advances high-performance lightweight steel designs and enriches theories of HDI effects.

Key words: Heterostructure, Hetero-unit size optimization, Strength-ductility synergy, HDI effects, Lightweight steel