Abstract
Metals with nanocrystalline grains have ultrahigh strengths approaching two gigapascals. However, such extreme grain-boundary strengthening results in the loss of almost all tensile ductility, even when the metal has a face-centred-cubic structure—the most ductile of all crystal structures1,2,3. Here we demonstrate that nanocrystalline nickel–cobalt solid solutions, although still a face-centred-cubic single phase, show tensile strengths of about 2.3 gigapascals with a respectable ductility of about 16 per cent elongation to failure. This unusual combination of tensile strength and ductility is achieved by compositional undulation in a highly concentrated solid solution. The undulation renders the stacking fault energy and the lattice strains spatially varying over length scales in the range of one to ten nanometres, such that the motion of dislocations is thus significantly affected. The motion of dislocations becomes sluggish, promoting their interaction, interlocking and accumulation, despite the severely limited space inside the nanocrystalline grains. As a result, the flow stress is increased, and the dislocation storage is promoted at the same time, which increases the strain hardening and hence the ductility. Meanwhile, the segment detrapping along the dislocation line entails a small activation volume and hence an increased strain-rate sensitivity, which also stabilizes the tensile flow. As such, an undulating landscape resisting dislocation propagation provides a strengthening mechanism that preserves tensile ductility at high flow stresses.
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Data availability
The data generated during and/or analysed during the present study are available from the corresponding authors upon reasonable request.
Code availability
Molecular dynamics simulations were performed using the LAMMPS software package, which is available at https://lammps.sandia.gov.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China (grant numbers 51931004, 51601067, 52171011, 51571120, 51604156 and 91963112), 111 Project 2.0 (grant number BP2018008), the Science and Technology Development Program of Jilin Province (grant number 20160520007JH) and the Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09). X.L. was supported by the Australian Research Council (DP190102243). H.L. acknowledges the support by the China Scholarship Council, and USYD and XJTU for hosting her as a visiting student. E.M. acknowledges XJTU for hosting his research at the Center for Alloy Innovation and Design. The scientific and technical input and support from the Microscopy Australia node at the University of Sydney (Sydney Microscopy & Microanalysis) is appreciated. 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. We are grateful to G. X. Sun for his assistance in nanoindentation tests and data analysis, Y. Yang and S. Sun for their help with artwork, and Z. J. Fang for his help with processing the synchrotron XRD data.
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S.H., X.D. and J.S. proposed the study and supervised the research. E.M. directed the flow of ideas and interpretation of the results, H.L. synthesized the alloys and conducted the mechanical testing with input from Y.C. (Jilin University) and Y.C. (Xi’an Jiaotong University), under the guidance of J.L. H.L., Q.H. and M.J.C. performed the TKD, TEM and STEM characterizations and analysis, under the guidance of X.L. S.J. and G.S. performed APT measurements as well as processed and interpreted the data. H.Z., S.L. and B.C. carried out the molecular dynamics simulations. Y.R. conducted in situ synchrotron XRD tests, and processed and interpretated the data with assistance from K.Y. The writing of the paper was led by E.M., based on the draft by S.H. with input from H.Z. and S.L. All authors contributed to the discussion of the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 A representative X-ray diffraction pattern.
The single-phase fcc structure is clearly confirmed for the as-deposited NC NiCo solid solution. The orientation indices are also included.
Extended Data Fig. 2 TEM microstructure of the as-deposited NC NiCo alloy.
a, A typical bright-field TEM image at a low magnification. The corresponding selected area electron diffraction pattern is given in the inset. b, c, HRTEM images showing a solid solution structure without any precipitate. c is the close-up image of a local region (white square) in b.
Extended Data Fig. 3 APT results of the as-deposited NC NiCo alloy.
Combined and individual elemental maps of one studied alloy with chemical composition in atomic per cent (at%) according to APT analysis. For this sample region, the average composition is Ni56Co44. Metallic impurities, such as Fe, Cu and Zn in the deposited samples originate from the impurities in the anode material and from the chemicals used for the electrolytic bath. C and S mainly come from surfactants.
Extended Data Fig. 4 Compositional inhomogeneities on nanometre scale in the NC NiCo alloy.
High-resolution atom map of a, x-y plane and b, x-z plane, both of which were reconstructed based on the computer-assisted data mining of the APT data sets. Compositional inhomogeneities with varying nanometre length scales can be seen clearly. c, A representative STEM-HAADF image of the NC NiCo alloy. Circles superimposed on each atom column with different colours represent the magnitude of d, intensity ratio and e, correlation coefficient. The intensity ratio range is clipped to 0.9–1.1 to better reveal the presence of agglomeration of an element. A perfectly ordered structure has the correlation coefficient of unity and the totally random structure has zero correlation. The correlation coefficient reaches a maximum of approximately 0.6 in the present NC NiCo alloys. There appears to be appreciable chemical inhomogeneities, in local regions with length scales in nano-meter range.
Extended Data Fig. 5 Tensile engineering stress-strain curves of NC NiCo alloys.
Multiple tests were conducted with samples from different plates (batches) with comparable grain size but slightly varying compositions around 50 at% Co, all of which having compositionally undulated nanostructures. The room-temperature tensile tests were performed at strain rates of 5 × 10−3 s−1 and 5 × 10−4 s−1. The results indicate reproducible properties, that is, excellent combination of strength and ductility.
Extended Data Fig. 6 Fractographs of NC NiCo samples.
a, Optical micrographs of the gauge section of representative specimens. The red and blue arrows mark the locations beyond which obvious plastic localization and localized necking is seen, respectively. SEM images of fracture surfaces at b, low and c, high magnifications.
Extended Data Fig. 7 Tensile yield strength vs elongation to failure of the present NC NiCo alloy, in comparison with all previous single-phase NC metals and alloys.
Our NC NiCo alloys are clearly separated from the general trend of conventional NC metals and alloys, including previously electrodeposited concentrated solutions (ECS), for which the elongation to failure was unfortunately not recorded via extensometer (see footnote of Supplementary Table 1).
Extended Data Fig. 8 Atomic strain and stress fields in NC NiCo alloys.
a, HAADF image and corresponding maps of b, horizontal normal strain (εxx), c, vertical normal strain (εyy) and d, shear strain (εxy).
Extended Data Fig. 9 Spatial correlation between local stress and local chemical composition in NC NiCo alloy.
The pre-set wavelength of compositional undulation is 10 nm. Pxx, Pyy and Pzz is the local stress along three different directions while the red curve presents local concentration of Ni.
Extended Data Fig. 10 Representative hardness value (H) from nanoindentation as a function of indentation strain rate (\(\dot{{\boldsymbol{\varepsilon }}}\)) for NC NiCo alloy.
The strain rate sensitivity exponent, m, was estimated to be about 0.04 from the slope of the double logarithmic line fitting the H versus \(\dot{\varepsilon }\) data.
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This file contains Supplementary Figs. 1–17, Notes 1–5, Table 1 and refs. 1–43.
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Li, H., Zong, H., Li, S. et al. Uniting tensile ductility with ultrahigh strength via composition undulation. Nature 604, 273–279 (2022). https://doi.org/10.1038/s41586-022-04459-w
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DOI: https://doi.org/10.1038/s41586-022-04459-w
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