Short-range order and its impact on the CrCoNi medium-entropy alloy

Abstract

Traditional metallic alloys are mixtures of elements in which the atoms of minority species tend to be distributed randomly if they are below their solubility limit, or to form secondary phases if they are above it. The concept of multiple-principal-element alloys has recently expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements. This group of materials has received much interest owing to their enhanced mechanical properties1,2,3,4,5. They are usually called medium-entropy alloys in ternary systems and high-entropy alloys in quaternary or quinary systems, alluding to their high degree of configurational entropy. However, the question has remained as to how random these solid solutions actually are, with the influence of short-range order being suggested in computational simulations but not seen experimentally6,7. Here we report the observation, using energy-filtered transmission electron microscopy, of structural features attributable to short-range order in the CrCoNi medium-entropy alloy. Increasing amounts of such order give rise to both higher stacking-fault energy and hardness. These findings suggest that the degree of local ordering at the nanometre scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of medium- and high-entropy alloys.

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Fig. 1: Energy-filtered TEM diffraction patterns, dark-field images formed with diffuse superlattice streaks and the associated high-resolution TEM images.
Fig. 2: Evidence for the three-dimensional structure of the domains and their size distribution.
Fig. 3: Dislocation analysis of both water-quenched and 1,000 °C aged samples.
Fig. 4: Comparison of mechanical properties from nanoindentation and bulk tensile tests.

Data availability

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

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Acknowledgements

This work was primarily supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract no. DE-AC02-05-CH11231 within the Damage-Tolerance in Structural Materials (KC 13) programme. R.Z., S.Z. and Y.C. acknowledge support from the US Office of Naval Research under grant nos. N00014-12-1-0413, N00014-17-1-2283 and N00014-11-1–0886, respectively. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. X-ray diffraction measurements were made at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a US Department of Energy Office of Science User Facility operated under contract no. DE-AC02-05CH11231. We thank E. Ma at Johns Hopkins University for providing a 600 °C aged alloy.

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Contributions

R.Z., S.Z., M.A., R.O.R. and A.M.M. conceived the project.; R.Z. and S.Z. conducted the energy-filtered TEM imaging and dislocation analysis; C.O. and R.Z. developed and optimized the domain recognition algorithm; R.Z. and S.Z. conducted the nanoindentation tests; R.Z., S.Z and Y.C. conducted the tensile tests. J.D. conducted the DFT simulations. T.J. conducted the XRD experiments. R.Z., S.Z., R.O.R., M.A. and A.M.M. prepared the manuscript, which was reviewed and edited by all authors. Project administration, supervision and funding acquisition was performed by R.O.R., M.A. and A.M.M.

Corresponding author

Correspondence to Andrew M. Minor.

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Peer review information Nature thanks Elena Pereloma, Christopher Woodward 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 Energy-filtered TEM diffraction patterns and dark-field images formed with diffuse superlattice streaks.

ac, Energy-filtered diffraction patterns taken from CrCoNi MEA samples that were water-quenched, aged at 600 °C for one week or aged at 1,000 °C for one week, respectively. The contrast is pseudo-coloured for better visibility. The line plots of intensity show the periodic intensity of the diffuse superlattice streaks. df, Energy-filtered dark-field images taken from water-quenched, 600 °C aged and 1,000 °C aged samples, respectively. The aperture positions are marked by the g vectors (labelled arrows). The images of the water-quenched and the 1,000 °C aged samples are the same as in Fig. 1 but are presented again here for comparison with the 600 °C aged sample.

Extended Data Fig. 2 Geometrical phase analysis strain mapping of a 1,000 °C aged sample and a water-quenched sample.

ad, 1,000 °C aged alloy; eh, water-quenched sample. a, e, Drift-corrected HRSTEM images of the 1,000 °C aged sample and the water-quenched sample, respectively. The fast Fourier transformed images are shown inset. bd, Strain maps of a showing nanometre-sized local fluctuations of strain (εxx, normal strain in the x direction; εyy, normal strain in the y direction). fh, Strain maps of e showing similar but much weaker contrast of local strain.

Extended Data Fig. 3 Results of X-ray diffraction experiments on a water-quenched sample and a 1,000 °C aged sample of the CrCoNi MEA.

The indexed (111) and (200) peaks are marked. The lattice constants a are calculated on the basis of the 2θ angles of the identified peaks..

Extended Data Fig. 4 Diffuse anti-phase boundary (DAPB) energy as a function of successive dislocation slip events from a calculated SRO model.

The data in the inset table represent different states of SRO and the plot is from the state marked blue.

Extended Data Fig. 5 Detailed ‘g·b’ analysis of partial dislocations in the CrCoNi MEA.

ae, Water-quenched sample; fj, sample aged at 1,000 °C. a, f, Diffraction references showing the diffraction conditions (g vectors) used for the analysis. b, g, Lower-magnification DC-STEM images of dislocations in the water-quenched and aged samples, respectively. ce, hj, Two-beam DC-STEM images of the boxed areas in b and g, respectively; the Burgers vectors of the visible dislocations are noted on the images.

Extended Data Fig. 6 Detailed statistical analysis of the pop-in events.

Pop-in events were observed during nanoindentation tests (see Methods section ‘Nanoindentation experiments’ for details). a, b, Distribution of the pop-in load from water-quenched and 1,000 °C aged samples, respectively. c, d, Distribution of the pop-in depth from water-quenched and 1,000 °C aged samples, respectively. The fitted normal distribution functions are listed in the panels. The results of numerical analysis are summarized in Extended Data Table 1.

Extended Data Fig. 7 Element mapping of the water-quenched and aged CrCoNi samples using EDS.

ae, Water quenched sample; fj, sample aged at 1,000 °C. a, f, Reference HAADF (high-angle annular dark field) images showing the regions of interest of a water-quenched sample and a 1,000 °C aged sample, respectively. bd, gi, Element mapping of Co, Ni and Cr of the water-quenched sample and the 1,000 °C aged sample, respectively. e, j, Quantitative results of line scans of the water-quenched sample and the 1,000 °C aged sample, respectively. The line scan directions are marked by the dashed lines in a and f.

Extended Data Table 1 Statistical results of SFE measurements and nanoindentation tests
Extended Data Table 2 Detailed steps of the Gaussian template fitting process

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Zhang, R., Zhao, S., Ding, J. et al. Short-range order and its impact on the CrCoNi medium-entropy alloy. Nature 581, 283–287 (2020). https://doi.org/10.1038/s41586-020-2275-z

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