Fig. 1. Microstructure of the current samples before tensile tests: (a) Typical EBSD pole-figure maps of metastable austenite (γ) grains with heterogeneous size distributions in ferrite-based low-Mn steel samples with compositions of Fe-2.8Mn5.7Al-0.1C (wt.%; left) and its 0.3 wt.% C analogue (right) [28], processed by hot-rolling, cold-rolling, intercritical annealing (ICA) at 850°C, and isothermal annealing. These samples were denoted as 0.1C-850 and 0.3C-850, respectively. Heterogeneously layered metastable γ grains are embedded in δ-ferrite matrix, where transverse direction is normal to the plane view of the EBSD maps. RD: rolling direction, ND: normal direction. (b) APT-reconstructed maps of the C-partitioned small (～0.5 μm in diameter, left) and large γ grains (3.0 μm in diameter, right) for low-temperature partitioning (LTP)-treated steel (0.3C-850-LTP). (c) APT-measured values of C and Mn concentrations, obtained from small and large γ grains in the undeformed steel samples of 0.3C-850-LTP, 0.3C-850, and 0.1C-850. (d) Representative APT mass-spectrum obtained from the LTP steel, showing all peaks are fully revolved.

Fig. 2. Room-temperature tensile properties of the current ferritic base duplex lightweight steels. Tensile properties for the LTP steel (0.3C-850-LTP) and non-LTP counterpart (0.3C-850) with the same alloy composition are provided by red and blue curves [28]. Curves of other non-LTP steels with 0.1 wt.% C subject to the ICA at 850°C (0.1C-850; cyan) and 950°C (0.1C-950; black) are also included. Bottom inset: Steel alloy compositions and heat-treatment processing strategies applied in this study. Loading direction was parallel to the RD.

Fig. 3. Tensile strength-ductility map including the values of alloy strength-ductility balance. Besides our LTP-treated steel sample with a composition of Fe-2.8Mn5.7Al0.3C (wt%), the tensile properties of several lightweight steels with bulk Mn compositions of <5 wt% (low-Mn grade; gold clouds), 5-12 wt% (medium-Mn grade; cyan clouds), and >12 wt% (high-Mn grade; silver clouds) are included. Our LTP steel, which can be realized with existing industrial production lines, exhibits a superior strength-ductility balance (see gray dotted lines) to that of bcc-based low-Mn lightweight alloys, even those of deformed and partitioned medium-Mn (10 wt%) steel [25], low-C medium-Mn TRIP steel [24], typical quenching and partitioning steel [51], ultrafine-grained steel [48], Ni(Fe,Al)-maraging steel [3], nanoprecipitate-hardened medium-Mn (12 wt%) steel [2], while a comparable strength-ductility balance to that of high-Mn base nanotwinned steel [54], high-Mn TWIP steels [53], and high-Co bearing high-entropy alloys (HEAs) [55].

Fig. 4. (a) Synchrotron XRD profiles of the LTP steel at different plastic strain levels, i.e., ε = 0 (undeformed), ～13.5%, 25.2%, and 47.1% (fractured). Distortion Fourier coefficients of (b) FCC-austenite and (c) BCC-based structures, including ferrite matrix together with mechanically transformed martensite, as a function of Fourier length under applied strains, determined from synchrotron XRD of tensile tested LTP steel. Each point was obtained by the MWA method from synchrotron XRD profiles, and the corresponding curves were fitted by the Wilkens model-based Berkum function. (d) Increased density of dislocations at different strains in BCC and FCC-structures. Details of dislocation density determination are given in the Supplementary materials.

Fig. 5. The volume fraction of austenite at different plastic strain levels measured by X-ray diffraction (4 peaks method)

Fig. 6. Bright-field TEM image of the dislocations in the deformed ferrite matrix at ε = 47 %. The yellow arrows indicate the Burgers vectors of the observed grain.

Fig. 7. Conventional TEM bright-field images of the deformed, untransformed small austenite (γ) grains in an LTP steel at different strains: (a) ε = ～13.5%, which shows both the easy passage of planar dislocations across boundaries and the planar configuration of dislocations (the enlarged view; inset); (b) ε = ～47% (sample fractured state), which reveals the constriction in the initial cross-slipping state. The corresponding SAED pattern (right), captured from the constriction region, includes the streaks of diffraction scattering due to the faulted ribbons. (c) Conventional TEM image and corresponding SAED pattern from the deformed, untransformed large austenite grains in an LTP steel at ε = ～13.5%, showing presence of a common K-S relationship between the mechanically transformed α'-martensite and its parent austenite. (d) Scanning (S)TEM annular bright field (ABF; left) and annual dark field (ADF; right) images of the large austenite grains in a fractured LTP steel upon ε = ～47%, which displays the wavy configuration of dislocations.

Fig. 8. (a) Typical SAED pattern along a low-index ${{\left[ 1\bar{1}1 \right]}_{\text{BCC}}}$ zone, obtained from the deformation-induced α'-martensite from small austenite grains in the tensile-tested LTP steel (ε = ～47%) to reach three series of two-beam conditions, i.e., three diffraction vectors, g1 = (101); red arrow, g2 = (110); green arrow, and g3 = $\left( 01\bar{1} \right)$; blue arrow, respectively. (b) Corresponding TEM dark-field images, where most dislocations in the transformed α'-martensite are regarded as screw-components. Three Burgers vectors are indicated by the yellow arrows. (c) Typical SAED pattern along a low-index ${{\left[ 1\bar{1}1 \right]}_{\text{BCC}}}$ zone (left), obtained from deformation-induced α'-martensite from large austenite grains in the tension-fractured LTP steel (ε = ～47%). (d) Corresponding TEM bright-field image and additional images (bottom panel), revealing the formation of fine dislocation cells (red arrows) and dense dislocation wall (green arrows)

Fig. 9. APT results of small austenite (γ) grain zone in the fractured LTP steel under ε = ～47%: (a) APT-reconstructed C (red sphere) map with iso-concentration envelopes of 4.5 at% Mn (cyan) and 8.0 at% C (gold). The iso-concentration wire-frame from the outlined red box shows the wolf-shaped interaction between dislocations and C atoms (right inset); (b) 1D concentration profiles of Mn and C across the BCC/FCC interface in (a), revealing the strong concentration spike of C at the interface region. By the same iso-concentration analysis, the distinct C-dislocation interaction did not visible in the APT results of the LTP-processed, undeformed austenite grains.

Fig. 10. (a) APT map of C atoms, taken from the large austenite zone, showing a high density of C decoration at dislocations inside the large austenite grain zone for the fractured LTP steel under ε = ～47%. The (b) 1D concentration profiles of C and Mn across the austenite/BCC interface in (a). The iso-concentration wire-frame from the outlined red box shows the cell formation by the C-decorated dislocations.