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Facile route to bulk ultrafine-grain steels for high strength and ductility

Abstract

Steels with sub-micrometre grain sizes usually possess high toughness and strength, which makes them promising for lightweighting technologies and energy-saving strategies. So far, the industrial fabrication of ultrafine-grained (UFG) alloys, which generally relies on the manipulation of diffusional phase transformation, has been limited to steels with austenite-to-ferrite transformation1,2,3. Moreover, the limited work hardening and uniform elongation of these UFG steels1,4,5 hinder their widespread application. Here we report the facile mass production of UFG structures in a typical Fe–22Mn–0.6C twinning-induced plasticity steel by minor Cu alloying and manipulation of the recrystallization process through the intragranular nanoprecipitation (within 30 seconds) of a coherent disordered Cu-rich phase. The rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micrometre grains but also enhances the thermal stability of the obtained UFG structure through the Zener pinning mechanism6. Moreover, owing to their full coherency and disordered nature, the precipitates exhibit weak interactions with dislocations under loading. This approach enables the preparation of a fully recrystallized UFG structure with a grain size of 800 ± 400 nanometres without the introduction of detrimental lattice defects such as brittle particles and segregated boundaries. Compared with the steel to which no Cu was added, the yield strength of the UFG structure was doubled to around 710 megapascals, with a uniform ductility of 45 per cent and a tensile strength of around 2,000 megapascals. This grain-refinement concept should be extendable to other alloy systems, and the manufacturing processes can be readily applied to existing industrial production lines.

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Fig. 1: Mechanical properties.
Fig. 2: Characterization of the microstructure of 4Cu annealed at 760 °C for 5 min.
Fig. 3: Effects of annealing temperature and time on the UFG structure.
Fig. 4: Mechanisms for effective grain refinement and high thermal stability.
Fig. 5: Deformed microstructure of UFG 0Cu and 4Cu.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

We thank J. Nutter for technical assistance with precession electron diffraction characterization. J.G. and W.M.R. acknowledge EPSRC project ‘Designing Alloys for Resource Efficiency (DARE)’ (EP/L025213/1) for financial support, and the Henry Royce Institute for Advanced Materials (EP/R00661X/1) for JEOL F200 Transmission Electron Microscope access at Royce @ Sheffield. Z.L. and S.J. acknowledge financial support from the National Natural Science Foundation of China (Nos. 51531001, 51921001, 51671018, 51971018 and 11790293), 111 Project (B07003) and Innovative Research Team in University (IRT_14R05), the Projects of SKL-AMM-USTB (2018Z-01, 2018Z-19 and 2019Z-01), the Fundamental Research Fund for the Central Universities of China (FRF-TP-18-093A1) and National Postdoctoral Program for Innovative Talents (BX20180035). H. Zhang acknowledges support from the US Department of Commerce, NIST under the financial assistance awards 70NANB17H249 and 70NANB19H138. A.V.D. acknowledges support from Materials Genome Initiative funding allocated to NIST. D.G. thanks the UKRI for a Future Leaders Fellowship, MR/T019123/1. Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

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Contributions

J.G., S.J., W.M.R. and Z.L. designed the experimental programme. J.G. carried out the main experiments. S.J. and Z.P.L. conducted the 3D-APT, synchrotron experiment and analysed the data. H. Zhang conducted the HR-STEM characterization and analysed the data. J.G. and H. Zhang conducted STEM-EDS mapping and analysed the data. Y.H. analysed XRD patterns for calculation of dislocation density. J.G., S.J., H. Zhang, Z.L. and W.M.R. wrote the manuscript and discussed the results. All authors reviewed and contributed to the final manuscript.

Corresponding authors

Correspondence to Suihe Jiang, Huairuo Zhang, Zhaoping Lu or W. Mark Rainforth.

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Peer review information Nature thanks Julie Cairney and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Mechanical properties.

Engineering stress–strain curves of 0Cu and 4Cu annealed at 760 °C for 5 min and 20 min, and 3Cu annealed at 760 °C for 5 min. Upon the addition of Cu both the yield strength and ultimate tensile strength increase markedly, with ductility comparable to that of 0Cu.

Extended Data Fig. 2 ADF-STEM analysis of 4Cu, 0Cu and 3Cu annealed at 760 °C for 5 min.

a, ADF image (left) of 4Cu showing a high density of nanoprecipiates, and the corresponding SAED pattern (right) showing only the matrix reflection of the [110]fcc zone axis without any extra reflection of the precipitates. b, ADF image of 0Cu showing an average grain size of 2.2 μm. c, ADF image of 3Cu presenting a UFG structure with a high density of nanoprecipitates. No elemental segregation at grain boundaries was detected.

Extended Data Fig. 3 Thermal stability evaluation of UFG structures.

ad, EBSD analysis of 0Cu (a1d1), 3Cu (a2d2) and 4Cu (a3d3) annealed at 760 °C (a), 810 °C (b), 860 °C (c) and 910 °C (d) for 5 min. a4, b4, EBSD maps of 0Cu annealed at 760 °C for 20 min (a4) and 60 min (b4). c4, d4, EBSD maps of 4Cu annealed at 760 °C for 20 min (c4) and 60 min (d4). Owing to their enhanced thermal stability, UFG structures of the Cu-doped alloys can be obtained over a wide range of annealing temperatures and times.

Extended Data Fig. 4 Microstructural analysis of 4Cu annealed at 760 °C for 0.5 min, 1 min and 2 min.

a–c, EBSD analysis of 4Cu annealed at 760 °C for 0.5 min (a), 1 min (b) and 2 min (c), revealing that nucleation for recrystallization occurred extensively after 0.5 min of annealing. As the annealing time extends from 1 min to 2 min, the volume fraction of the recrystallized matrix increases from 76% to 95%. d, e, ABF-STEM image (d) and the reconstruction of the APT dataset (e) of 4Cu annealed for 1 min, showing the formation of equiaxed grains of size 300 ± 150 nm and Cu-rich precipitates with an average size of 3.7 nm and a number density of 8.8 × 1023 m−3. The isoconcentration surfaces are 20 at% Cu.

Extended Data Fig. 5 Effect of the trajectory aberrations of APT on the composition analysis of particles.

The error bars are standard deviations of the mean. The isoconcentration surfaces are 30 at% Cu.

Extended Data Fig. 6 EBSD map of UFG 0Cu and calculation of individual strengthening contribution of dislocations and nanotwins of UFG 0Cu and 4Cu.

a, The EBSD map of UFG 0Cu processed by a two-step cold-rolling and flash-annealing process shows a grain size of 1.1 ± 0.5 μm. b, Twinning gradually dominates the strengthening beyond 15% strain in 4Cu, whereas dislocation multiplication governs the strengthening in the entire deformation stage of the UFG 0Cu.

Extended Data Fig. 7 Deformed microstructure analysis of 4Cu pre-strained to 15% and 45%.

a, b, The corresponding bright-field images of Fig. 5c, d, respectively, showing a high density of dislocations and nanotwins with interspacing of between 300 nm and 500 nm. c, Reconstructed APT data of 4Cu pre-strained to 15% showing some of the nanoprecipitates flattened along the loading direction. d, e, Bright-field TEM image (d) and its corresponding high-resolution misorientation map superimposed with nanotwin boundaries (e) (solid red lines, indexed nanotwin boundaries; thin dashed red lines, nonindexed nanotwin boundaries) obtained using a NanoMEGAS DigiSTAR system with a step size of 1.5 nm. Numerous small dislocation cells (blue arrows) were observed in nanotwins and their interspaces. The isoconcentration surfaces are 20 at% Cu.

Extended Data Fig. 8 Microstructure and mechanical property analyses of TRIP steels and medium entropy alloys with small amounts of Cu.

a, b, EBSD maps of the TRIP steels with composition Fe–15Mn–0.4C (a) and Fe–15Mn–0.4C–3Cu (wt%) (b) after annealing at 730 °C for 5 min. c, Tensile stress–strain curves of the TRIP steels in a, b. d, e, EBSD maps and of 33Co–33Cr–34Ni (d) and (33Co–33Cr–34Ni)0.97Cu0.03 (at%; e) after annealing at 810 °C for 10 min. f, Tensile stress–strain curves of the alloys in d, e. The alloys with minor Cu content exhibit finer microstructures and enhanced mechanical properties.

Extended Data Fig. 9 Modified Williamson–Hall plots of FWHM as a function of \(\,{\boldsymbol{K}}{\bar{{\boldsymbol{C}}}}^{{\boldsymbol{1}}/{\boldsymbol{2}}}\).

a, Analysis of peak broadening for cold-rolled 4Cu alloys. b, Analysis of peak broadening for the pre-strained UFG 0Cu and 4Cu alloys.

Extended Data Table 1 Composition (wt%) of 0Cu, 3Cu and 4Cu alloys

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Gao, J., Jiang, S., Zhang, H. et al. Facile route to bulk ultrafine-grain steels for high strength and ductility. Nature 590, 262–267 (2021). https://doi.org/10.1038/s41586-021-03246-3

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