Research ArticleMATERIALS SCIENCE

Chemical boundary engineering: A new route toward lean, ultrastrong yet ductile steels

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Science Advances  27 Mar 2020:
Vol. 6, no. 13, eaay1430
DOI: 10.1126/sciadv.aay1430
  • Fig. 1 Schematic illustration of a PB, a GB, and a CB.

    (A) PB, a boundary between two grains of different lattice type. (B) GB, a boundary between two grains of the same lattice type but with different crystallographic orientations. (C) CB, defined by a sharp discontinuity of at least one elemental concentration inside a lattice-continuous region, e.g., a very sharp chemical gradient. Note that our CBs do not involve any change in crystal structure or lattice orientation. The different colors represent atoms of different element type.

  • Fig. 2 Microstructural evolution of the steel processed via the CBE strategy confirming the strong effect of CBs on martensitic transformation.

    (A) EBSD image quality map with superposed phase color map of the face-centered cubic (FCC) phase (red region) of the ART-processed steel, showing the equiaxed microstructure of austenite (γ) with ferrite (α), and (C) the ultrafine dual phase microstructure of γ and martensite (α′) of the CBE-processed steel. (B) Sketch of the microstructural evolution of the steels during ultrafast heating and quenching via the CBE strategy to illustrate the role of GBs, PBs, and CBs. (D and E) TEM images with EDS analysis showing the microstructure and Mn concentration profile between γ and α of the ART-processed steel, while (F and G) is that between γ and α′ of the CBE-processed steel. Near-atomic level Mn distribution is revealed by 3D-APT, (H and I) between γ and α, and (J and K) between γ and α′.

  • Fig. 3 Nano-Auger–EBSD and TEM analysis of the CBE-processed steel.

    (A) Inverse pole figure map of austenite combined with image quality map of martensite. (B) Prior austenite reconstruction of (A) showing that the martensite blocks in the center share the same parent austenite orientation with the surrounding austenite. Blue and green lines in the EBSD maps correspond to high- and low-angle GBs, respectively (misorientation of ≥15° and between 5° and 15°, respectively). (C) Mn concentration profile obtained by nano-Auger line scan along the white line in (A), further confirming that the CB hinders martensitic transformation. (D) TEM bright field micrograph showing nanocrystalline martensitic laths stop at the CB. Prior austenite grain boundary (PAGB) represents prior austenite GB. (E) Corresponding selected area diffraction pattern in the marked area in (D) showing the near Nishiyama-Wassermann relationship between α′ and γ on both sides of the CBs due to having the same parent austenite. (F) Mn concentration profile obtained by TEM-EDS line scan along the white line in (D).

  • Fig. 4 Mechanical properties of the investigated steels.

    (A) Mechanical properties of the 0.18C-8Mn steel and (B) 0.2C-8Mn-0.2Mo-0.05Nb steel subjected to various thermomechanical treatments. (C) Comparison of mechanical properties of the CBE-processed steels with other low carbon advanced high-strength steels (more details in fig. S4) (13, 2325). To allow a fair comparison, all data are from tensile tests with a gauge geometry obeying the ASTM E8/E8M standard. The stress jumps and serrations in the tensile curves are due to the Portevin-Le Chatelier (PLC) effect, which is frequently observed in medium Mn steels (36).

  • Fig. 5 Microstructural change of the CBE sample during deformation.

    Electron channeling contrast imaging (ECCI) of the sample (A) before deformation and (B) after being stressed to ~94% of the yield point. The contrast in the lower region of the austenite grain changes after deformation, indicating that dislocations are generated in this grain and pile up at the α′-γ interface. EBSD kernel average misorientation (KAM) mappings of austenite in the sample (C) before deformation and (D) after being stressed to ~94% of the yield point. The EBSD data show an increase in local misorientation near interface regions after deformation, suggesting a higher number of geometric necessary dislocations in these areas. These results support that when the CBE sample is stressed to near-macroscopic yield point, microyielding takes place inside the nanotwined austenite. The ultrafine martensitic network isolates austenite in submicron domains, which limits dislocation pileups at austenite-martensite interfaces.

  • Fig. 6 Strain-induced martensitic transformation of metastable austenite and work hardening behavior in ART and CBE steels.

    (A) Strain hardening behaviors of ART and CBE steels. (B) Kinetics of strain-induced martensitic transformation in ART and CBE steels measured by in situ magnetic induction, along with the microstructure as observed by EBSD near the fracture site, where austenite is marked in red in the EBSD maps. The step-like transformation is due to PLC bands passing through the measured volume. The transient increase and decrease in the transformed austenite fraction at the beginning of each step are artifacts caused by the PLC bands entering and leaving the finite magnetic probing area.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/13/eaay1430/DC1

    Fig. S1. Dilatometry study and inverse pole figure maps of austenite.

    Fig. S2. Nanohardness of each phase in ART and CBE steels.

    Fig. S3. Microstructures of refined CBE steel and microalloyed CBE steel.

    Fig. S4. DICTRA simulation of the spatial stability of a CB inside austenite at 800°C.

    Fig. S5. Extended mechanical properties.

    Table S1. Chemical composition of the studied alloys in weight %.

    Reference (37)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Dilatometry study and inverse pole figure maps of austenite.
    • Fig. S2. Nanohardness of each phase in ART and CBE steels.
    • Fig. S3. Microstructures of refined CBE steel and microalloyed CBE steel.
    • Fig. S4. DICTRA simulation of the spatial stability of a CB inside austenite at 800°C.
    • Fig. S5. Extended mechanical properties.
    • Table S1. Chemical composition of the studied alloys in weight %.
    • Reference (37)

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