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Bifunctional nanoprecipitates strengthen and ductilize a medium-entropy alloy

Nature volume 595, pages 245–249 (2021)Cite this article

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Abstract

Single-phase high- and medium-entropy alloys with face-centred cubic (fcc) structure can exhibit high tensile ductility1,2 and excellent toughness2,3, but their room-temperature strengths are low1,2,3. Dislocation obstacles such as grain boundaries4, twin boundaries5, solute atoms6 and precipitates7,8,9 can increase strength. However, with few exceptions8,9,10,11, such obstacles tend to decrease ductility. Interestingly, precipitates can also hinder phase transformations12,13. Here, using a model, precipitate-strengthened, Fe–Ni–Al–Ti medium-entropy alloy, we demonstrate a strategy that combines these dual functions in a single alloy. The nanoprecipitates in our alloy, in addition to providing conventional strengthening of the matrix, also modulate its transformation from fcc-austenite to body-centred cubic (bcc) martensite, constraining it to remain as metastable fcc after quenching through the transformation temperature. During subsequent tensile testing, the matrix progressively transforms to bcc-martensite, enabling substantial increases in strength, work hardening and ductility. This use of nanoprecipitates exploits synergies between precipitation strengthening and transformation-induced plasticity, resulting in simultaneous enhancement of tensile strength and uniform elongation. Our findings demonstrate how synergistic deformation mechanisms can be deliberately activated, exactly when needed, by altering precipitate characteristics (such as size, spacing, and so on), along with the chemical driving force for phase transformation, to optimize strength and ductility.

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Fig. 1: Microstructures and tensile properties of FNAT-m-47h and FNAT-47h alloys.
Fig. 2: Microstructure analysis of the FNAT-47h and FNAT-4h alloys.
Fig. 3: Room-temperature mechanical properties and deformation mechanisms of FNAT alloys.
Fig. 4: Comparison of uniform elongation versus ultimate tensile strength at room temperature of the current MEAs (FNAT-47h, FNAT-8h, and FNAT-4h) with other HEAs and steels.

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All figures have associated raw data, available on request from Y.Y.

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Acknowledgements

This research was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (testing and analysis of mechanical properties and responsible deformation mechanisms, TEM characterization of the FNAT-4h alloy, and writing of the manuscript) and by the Laboratory Directed Research and Development programme of Oak Ridge National Laboratory (ORNL) (microstructural characterization and first-principles calculations), managed by UT-Battelle, LLC, for the US Department of Energy. Y.Y. acknowledges CompuTherm for providing the phase diagram calculation software Pandat. Resources at ORNL’s High Flux Isotope Reactor for small-angle neutron scattering, Spallation Neutron Source for neutron diffraction, and Center for Nanophase Materials Sciences for atom probe tomography were used in this study, which are US DOE Office of Science User Facilities.

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  1. Tianyi Chen

    Present address: Oregon State University, Corvallis, OR, USA

Authors and Affiliations

  1. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    Ying Yang, Tianyi Chen, Lizhen Tan, Yanli Wang, German D. Samolyuk & Easo P. George

  2. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    Jonathan D. Poplawsky, Andrew R. Lupini & Albina Borisevich

  3. Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    Ke An & Ken Littrell

  4. Materials Science and Engineering Department, University of Tennessee, Knoxville, TN, USA

    Easo P. George

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Contributions

Y.Y. conceived the study, designed the alloy and supervised the project. T.C., A.R.L., A.B. and L.T. performed the TEM and STEM analyses. L.T. performed EBSD analysis. K.A. performed neutron diffraction and phase analysis. Y.W. performed and analysed tensile tests with DIC. J.D.P. performed the APT analysis. G.D.S. performed the first-principles calculations. K.L. performed the SANS analysis. Y.Y. and E.P.G. analysed and interpreted the mechanical properties and deformation mechanisms and wrote the manuscript. All authors reviewed and commented on the manuscript.

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Correspondence to Ying Yang or Easo P. George.

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Extended data figures and tables

Extended Data Fig. 1 Temperature dependence of phase equilibria and composition calculated using the CALPHAD approach, and the free energy from first-principles calculations.

a, Mole fraction of equilibrium phases in FNAT. b, Compositions of elements (at%) in the fcc phase. c, Free energy of fcc and bcc phases that have the same composition (Fe-23Ni-3.5Al-0.5Ti, at%) as the matrix of FNAT.

Extended Data Fig. 2 Information for estimating yield strength of FNAT-47h.

a, Room-temperature tensile stress–strain curve of FNAT after solutionizing at 1,100 °C and water quenching showing yield strength of ~325 MPa. b, STEM bright-field image showing dislocations cutting particles (yellow arrows) in the matrix of a plastically deformed FNAT sample. c, Corresponding HAADF image of b.

Extended Data Fig. 3 TEM/STEM analyses of interface between precipitate and matrix of the FNAT-47h alloy.

a, b, HAADF lattice images before deformation (from tab section of tensile specimen). a, A lattice image of L12/bcc interface displays the Nishiyama–Wasserman orientation \({(1\bar{1}1)}_{{{\rm{L1}}}_{2}}\parallel {(0\bar{1}1)}_{{\rm{bcc}}}\) and \({[110]}_{{{\rm{L1}}}_{2}}\parallel {[100]}_{{\rm{bcc}}}\), where the interplanar spacings, \(d{(1\bar{1}1)}_{{{\rm{L1}}}_{2}}\) = 0.2072 nm and \(d{(0\bar{1}1)}_{{\rm{bcc}}}\) = 0.2027 nm, result in a mismatch of ~2.2% and a semi-coherent interface. b, A lattice image of L12/fcc interface region exhibits full cube-on-cube lattice coherency with \({(\bar{1}11)}_{{{\rm{L1}}}_{2}}\parallel {(\bar{1}11)}_{{\rm{fcc}}}\) and \({[110]}_{{{\rm{L1}}}_{2}}\parallel {[110]}_{{\rm{fcc}}}\), where the interplanar spacings, \(d{(1\bar{1}1)}_{{{\rm{L1}}}_{2}}\) = 0.2072 nm and \(d{(1\bar{1}1)}_{{\rm{fcc}}}\) = 0.2074 nm, result in a small mismatch of ~0.1%. c–e, TEM and HAADF lattice images after deformation (from gauge section of tensile specimen). c, Dark-field TEM image showing L12 precipitates, some containing streaks (marked by arrowheads), embedded in a deformation-induced bcc grain after deformation. d, HAADF lattice image showing a stacking fault with a Burgers vector of \(\tfrac{1}{6} < 112 > \) formed in one of the L12 precipitates containing streaks. Labels A, B, C in the inset denote alternating {111} planes in the L12 structure. e, HAADF lattice image showing semi-coherent L12/bcc interface with the Kurdjumov–Sachs orientation relationship64, namely, \({(\bar{1}11)}_{{{\rm{L1}}}_{2}}\parallel {(\bar{1}10)}_{{\rm{bcc}}}\) and \({[110]}_{{{\rm{L1}}}_{2}}\parallel {[111]}_{{\rm{bcc}}}\) and a lattice mismatch of ~2.2%. The slip-transfer mechanism in c–e is common when dislocations move from fcc matrix to L12 precipitate, but is rarely seen when the matrix is bcc.

Extended Data Fig. 4 STEM HAADF image, corresponding FFT diffractogram and inverse FFT image of the as-quenched FNAT-4h alloy sample tilted to [110] zone axis condition.

a, HR STEM image. b, FFT diffractogram. c, the inverse FFT generated using the blue-circled 'extra spots' in b. The diffuse spatial distribution of the bright features in c indicates that the extra spots in b are caused by small variable local misorientations due to local internal strains as opposed to any nano-domains of a different matrix phase.

Extended Data Fig. 5 Ultimate tensile strength versus Ni or (Ni + Co) content at room temperature and 700 °C.

Open symbols, room temperature (RT); filled symbols, 700 °C. The FNAT alloy is compared with different materials whose data sources are Fe-based heat-resistant alloys including 410, 430, 302, 309, 310, 316, 321, 34765 A286 and Incoloy 80066; Ni-based super alloys66 including Inconel 706, Inconel 718, Nimonic 80A, PE 16, Inconel 625, Inconel 600, Hastelloy S, Hastelloy X, Udimet 630; and Ni–Co-based super alloys66 including Incoloy 909, Nimonic 90, Nimonic 115, Waspaloy, Udimet 720, Udimet 500, Rene 41, Rene 95, Astroloy, Inconel 617 and Haynes 230.

Extended Data Fig. 6 Textures of FNAT-47h and FNAT-m-47h after hot-rolling and annealing.

a, b, EBSD pole figures for FNAT-47h, c, d, EBSD pole figures for FNAT-m-47h. The normal direction (ND) in each case is at the centre of the circle and is either 001 or 111 as marked. TD in a–d refers to the transverse direction. e, Neutron diffraction spectra of FNAT-47h along the rolling direction (RD) and the normal direction (ND). Note that we have shifted the RD and ND spectra horizontally with respect to each other to facilitate comparison (otherwise the peaks would overlap). Both EBSD (a, b) and ND (e) results show that FNAT-47h has negligible texture.

Extended Data Fig. 7 Fracture analysis of tensile-tested FNAT-m-47h and FNAT-47h alloys.

a, FNAT-m-47h: fractograph and stress–strain curve showing considerable necking before fracture, and DIC strain maps showing strain localization in the region where fracture eventually occurs. b, FNAT-47h: fractograph and stress–strain curve showing minimal necking before fracture, and DIC strain maps showing relatively diffuse strain distribution throughout the gauge section with no evidence of localization near the fracture plane.

Extended Data Table 1 Precipitate parameters calculated from different fits to APT data
Full size table
Extended Data Table 2 Parameters of precipitates in deformed and undeformed bcc and fcc grains
Full size table

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Yang, Y., Chen, T., Tan, L. et al. Bifunctional nanoprecipitates strengthen and ductilize a medium-entropy alloy. Nature 595, 245–249 (2021). https://doi.org/10.1038/s41586-021-03607-y

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