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Excellent ductility and serration feature of metastable CoCrFeNi high-entropy alloy at extremely low temperatures

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  • ReceivedSep 16, 2018
  • AcceptedNov 9, 2018
  • PublishedDec 6, 2018

Abstract


Funded by

the National Natural Science Foundation of China(51471025,51671020,51471024,11771407)

Office of Fossil Energy

National Energy Technology Laboratory(DE-FE-0011194)

S.N. Mathaudhu

and D.M. Stepp; the support from the National Science Foundation(DMR-1611180,1809640)

Drs G. Shiflet and D. Farkas.


Acknowledgment

This work was supported in part by the National Natural Science Foundation of China (51471025, 51671020, 51471024 and 11771407); the Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (DE-FE-0011194) with the program manager Dr. J. Mullen; the support from the US Army Research Office project (W911NF-13-1-0438) with the program managers Drs. M.P. Bakas, S.N. Mathaudhu, and D.M. Stepp; the support from the National Science Foundation (DMR-1611180 and 1809640) with the program directors, Drs G. Shiflet and D. Farkas. We also thank Prof. WH Wang at the Institute of Physics, Chinese Academy of Sciences, Prof. XL Wang at the City University of Hong Kong, and Prof. K Samwer at the University of Göttingen for their insightful and constructive comments on this paper. We appreciate Dr. RJ Huang, SF Li, and Z Zhang at the Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, for their help on mechanical tests.


Interest statement

These authors declare no conflict of interest.


Contributions statement

Zhang Y conceived the overall project. Liu J, He Z, and Li L performed the experiments. He Z conducted the TEM characterization. Liao X, Lin Q, and An X performed the STEM characterization. Ren J, Guo X and Yu L carried out the calculations of the serration feature. Lin J, Guo X, He Z, Liaw P, Ren J and Zhang Y wrote the paper. All authors discussed the results and commented on the manuscript.


Author information

Junpeng Liu received his PhD degree from the University of Science and Technology Beijing (USTB) in 2018. Currently, he is a Post-doctoral Fellow at the State Key Laboratory of Non-linear Mechanics, Institute of Mechanics, Chinese Academy of Sciences. His current research interest focuses on the deformation and strengthening/ductilization of advanced structural materials, such as high-entropy alloys, and high-performance steels.


Xiaoxiang Guo received his Master’s degree from Zhengzhou University in 2017. Currently, he is a PhD candidate at the School of Mathematics and Statistics, Zhengzhou University, China. His current research interest focuses on the mathematical applications in materials science.


Zhanbing He received his PhD degree in materials science from Dalian University of Technology in 2005. He did scientific research at Stockholm University, Swiss Federal Institute of Technology in Lausanne (EPFL), and Ecole Polytechnique from 2005 to 2013. He joined the State Key Laboratory for Advanced Metals and Materials at the University of Science and Technology Beijing in 2013 as a full Professor. His research interest is in TEM, quasicrystals, and high-entropy alloys.


Yong Zhang received his PhD degree in materials science from USTB in 1998. He did post-doctoral work at the Institute of Physics, Chinese Academy of Sciences. Then, he joined NUS-MIT SMA in 2000. He has been a Professor of materials science at USTB since 2004. His research interest is in high-throughput preparation of high-entropy alloys, and theory prediction of the phase formation for the multicomponent materials.


Supplement

Supplementary information

Supporting data are available in the online version of this paper.


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  • Figure 1

    Typical microstructure of the CoCrFeNi HEA. (a) Optical metallographic image of the sample, displaying roughly equiaxed grains with an average size of 13 μm. (b) XRD pattern shows the alloy has a single FCC phase. (c) TEM image and electron-diffraction patterns (d), presenting high-density dislocations and some twins in the original sample.

  • Figure 2

    Tensile properties of the CoCrFeNi alloy at different temperatures. (a) Engineering stress-strain curves and photograph of the dog-bone-shaped samples, before and after tensile tests. The enlarged stress-strain curve at 4.2 K is displayed in the inset. This HEA exhibits increasing strength with decreasing temperature, and the ductility reaches the maximum at 77 K, as shown in (b). In addition, at 20 K and below, the stress drop occurs in the tensile curve. Moreover, the ductility of this alloy decreases abruptly below 77 K. (c) Θ vs. true strain (εT) curves at different temperatures. Inset: an enlarged view of the black-dashed box. The strain-hardening rate is adequate, including at 4.2 K, to sustain large true strains before necking instability.

  • Figure 3

    Ashby map showing the UTS versus percent elongation to failure for HEAs, compared with a wide range of cryogenic materials at 4.2 K. The oval-shaped pink region represents the HEA, showing its excellent combination of high strength and great ductility.

  • Figure 4

    The dynamic analysis of the serration feature. (a) The mutual information, I(τ), as a function of time delay, τ, for the serration at 4.2 and 20 K. The insets show the magnified curves. When I(τ) first reaches its local minimum, the corresponding time decay, τ, is the suitable value, τ0 [32]. (b) E1(m) and E2(m) as a function of the embedding dimension, m, at 4.2 and 20 K. The inset gives a clear view of the state at 4.2 and 20 K, respectively. E1(m) reflects the change in the mean value of the distance fluctuation, and the E2(m) is used to distinguish the deterministic signals from stochastic signals. When E1(m) tends to be steady and E2(m) approaches 1, the appropriate embedding dimension, m0, is obtained [33].

  • Figure 5

    SEM images of fracture surfaces of the CoCrFeNi samples after the tensile tests at 293 (a), 200 (b), 77 (c), and 4.2 K (d), respectively. It shows a totally ductile fracture with a plenty of micrometer-sized or nanometer-sized dimples. The extent of micrometer dimples decreases with the temperature decreasing to 77 K, but increases at 4.2 K, consistent with the ductility (elongation failure) observed in tensile tests. A similar trend of the variation of elongation with the average dimple diameter was also reported for austenitic stainless steel [38].

  • Figure 6

    TEM and high-resolution STEM images of the CoCrFeNi sample with nanoscale twins and HCP phases after the tensile test at the liquid-helium temperature. (a) TEM image of deformation twins. (b) A close-up TEM observation of (a), and the SAED pattern (c), indicating that the typical deformation twins are parallel to each other on {111} planes. (d) TEM image and the electron-diffraction patterns in (f), showing that twins in two orientations intersect each other. (e) The blue parallelogram and the red triangle present different morphologies due to intersecting twins. All the TEM images indicate that high-density nano-twins drastically refine the grains. (g) Bright-field TEM image with SAED patterns of [110]FCC||[112¯0]HCP showing the FCC twinning and HCP phase in the sample. (h) High-resolution STEM image containing the HCP stacking, stacking fault [SF], and nano-twins [T], demonstrate that the FCC→HCP transition occurs. (i) The enlarged image of the red rectangle in (h) shows the ABABAB HCP stacking.

  • Table 1   Mechanical properties of the CoCrFeNi high-entropy alloy at different temperatures

    Tension properties

    Temperature

    293 K

    200 K

    77 K

    50 K*

    20 K*

    4.2 K

    σy (MPa)

    446

    488

    590

    ---

    ---

    680

    σu (MPa)

    664

    790

    1,070

    1,152±8

    1,240±40

    1,251±10

    εf

    63%

    72%

    78%

    68.6%±3.3%

    62%±0.6%

    61.6%±1.6%

    E (GPa)

    189

    194

    204

    --

    --

    221

    Work-hardeningcapability, σuσy (MPa)

    218

    302

    480

    --

    --

    580

  • Table 2   The relevant parameters of the serrations at 4.2 and

    Temperature

    Parameter

    Fractal dimension, D

    Suitable time delay, τ0

    Suitable embedding dimension, m0

    Largest Lyapunov exponent, λ

    20 K

    1.22

    5

    10

    0.001

    4.2 K

    1.23

    14

    15

    0.050

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