Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

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Abstract

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands1,2,3. Their outstanding strength originates from semi-coherent precipitates4,5, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load6,7,8. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.

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Figure 1: Mechanical properties and typical STEM images of the solution-annealed (15 min at 950 °C) and of the aged (3 h at 500 °C) Ni(Al,Fe)-maraging steels.
Figure 2: High-resolution HAADF STEM images and three-dimensional reconstruction of an APT dataset confirming the B2 nature of the precipitates with full lattice coherence.
Figure 3: Atom probe analysis showing tomography and composition of the precipitates.
Figure 4: Synchrotron XRD results showing the overall phase transformation and extremely low lattice misfit between bcc matrix and B2 precipitates.

Change history

  • 27 April 2017

    The y-axis units were corrected in Fig. 1a; an additional statement was included in the Acknowledgements.

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Acknowledgements

This research was supported by the National Natural Science Foundation of China (grant numbers 51531001, 51422101, 51671018, 51271212 and 51371003), the 111 Project (grant number B07003), the International S&T Cooperation Program of China (grant number 2015DFG52600), the Program for Changjiang Scholars and Innovative Research Team in University (grant number IRT_14R05) and the Projects of SKL-AMM-USTB (grant numbers 2016Z-04, 2016-09, 2016Z-16). Y.W. acknowledges the Top-Notch Young Talents Program and the Fundamental Research Funds for the Central Universities. We thank X. Xu at Tongling University, Z. Fan at the China Academy of Engineering Physics, Y. Qiao, L. You and G. Liu at the University of Science and Technology Beijing, and Y. Tian at Tohoku University for help with sample preparation and discussions. M.Y. and B.G. are grateful to U. Tezins and A. Sturm for their support of the focused ion beam and APT facilities at the Max-Planck-Institut für Eisenforschung. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.

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Contributions

Z.L. designed the study. S.J., H.C., H.W, X.L. and Y. Wu carried out the main experiments. S.J., H.W., Z.L. and D.R. analysed the data and wrote the main draft of the paper. M.Y., B.G., D.P. and D.R. prepared the APT samples and interpreted the results. M.C. and A.H. conducted the STEM characterization. Y.Wa. conducted the synchrotron experiments. All authors contributed to the discussion of the results, and commented on the manuscript.

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Correspondence to Zhaoping Lu.

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Reviewer Information Nature thanks M. Moody, J. Morris Jr 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 Figure 1 Comparison of ultimate tensile stress and raw material cost between the typical ultrahigh-strength steels and the Ni(Al,Fe)-maraging steels.

The typical ultrahigh-strength steels include the commercially successful 18 wt% Ni maraging steels (Ni18Co9Mo5, wt%)1, Co-free maraging steels (Ni18.5Mo4Ti, wt%)2, W-strengthened steel (Ni19W4.2Ti1.2, wt%)29, and some commercial ultrahigh-strength steels (for example, Ni11Co13Cr3, wt%)30.

Extended Data Figure 2 The chemical compositions of the precipitates with different shapes and sites.

a, b, The selected precipitates (a) and corresponding chemical compositions (b). Precipitate A: round precipitates far away from Mo/C-decorated dislocations. Precipitate B: rod precipitates far away from Mo/C-decorated dislocations. Precipitate C: round/rod precipitates attached to Mo/C-decorated dislocations. The errors are standard deviations of the mean.

Extended Data Figure 3 Mass spectrum for the 40 precipitates from a subset.

Extended Data Figure 4 NbC precipitated in the solution-annealed specimen.

a, Scanning electron microscope image of the solution-annealed steel showing the presence of coarse primary NbC (0.5–2 μm; yellow circles). b, TEM image of the aged steel showing co-existence of NbC and Ni(Al,Fe) precipitates. The inset highlights the Ni(Al,Fe) precipitates in a local region of b. c, Energy dispersive spectroscopy and SAED patterns (the inset in c) of the large precipitate in b, confirming that the precipitate is MC-type (that is, a face-centred cubic metal carbide with metal-to-carbon ratio 1) NbC. Owing to its small volume fraction and large size, NbC was not observed in the APT characterization.

Extended Data Figure 5 Lattice parameters of the matrix and the B2 versus f(θ).

f(θ) = cos2(θ) × (1/sinθ + 1/θ)/2, where θ is the Bragg angle for each peak34. The best lattice parameter a was obtained at f(θ) = 0. The errors of the lattice parameters are associated with the errors in experimental measurements of θ.

Extended Data Table 1 Diffusion coefficients of Fe, Ni, Al and Mo in α-Fe at 500 °C

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Jiang, S., Wang, H., Wu, Y. et al. Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation. Nature 544, 460–464 (2017). https://doi.org/10.1038/nature22032

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