Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin
Too cold to fracture
Finding structural materials that have good fracture properties at very low temperatures is challenging but is important for fields such as space exploration. Liu et al. discovered a high-entropy chromium-cobalt-nickel alloy that has an incredibly high fracture toughness at 20 kelvin (see the Perspective by Zhang and Zhang). This behavior is caused by an unexpected phase transformation that, when combined with other microstructures, prevents crack formation and propagation. The fracture toughness of this alloy makes it potentially useful for a range of cryogenic applications. —BG
Abstract
CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters½ (MPa·m½) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.
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Science
Volume 378 | Issue 6623
2 December 2022
2 December 2022
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Copyright © 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.
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Received: 27 February 2022
Accepted: 7 October 2022
Published in print: 2 December 2022
Acknowledgments
We thank M. J. Paul for support in analyzing the tensile stress-strain data.
Funding: The research was primarily supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, through the Damage-Tolerance in Structural Materials program (KC13) at the Lawrence Berkeley National Laboratory (LBNL) under contract DE-AC02-CH11231, and the Multiscale Mechanical Properties and Alloy Design program (ERKCM06) at the Oak Ridge National Laboratory. The authors acknowledge the use of the ENGIN-X, ISIS Facility, at the Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK, for the mechanical testing in liquid helium environment, and the microscopy facilities in the National Center for Electron Microscopy, in the Molecular Foundry at LBNL, which is supported by the Office of Science, Office of Basic Energy Sciences of the US Department of Energy under contract DE-AC02-05CH11231. D.L., M.J., and P.F.-K. acknowledge support from the UK Engineering and Physical Sciences Research Council (EP/N004493/2, EP/T000368/1). B.G. acknowledges support from the ARC Future Fellowship (project FT190100484) and the UNSW Scientia Fellowship schemes.
Author contributions: R.O.R., B.G., and E.P.G. formulated the original idea. E.P.G. supervised the synthesis and processing of alloys. D.L. identified the testing equipment and method, formulated the neutron beamline proposal, and conducted the in situ and ex situ fracture toughness experiments at ENGIN-X with S.K. and P.F.-K. The fracture toughness data were analyzed by Q.Y., B.G., and D.L. under the supervision of R.O.R. The neutron diffraction data were analyzed by M.J. under the supervision of D.L. S.K. performed the uniaxial tensile tests, with guidance from E.P.G. and B.G. The structural characterization was performed by Q.Y., M.J. and D.L (EBSD), and R.Z and M.P. (TEM), the latter under the supervision of A.M.M. Simulation studies were conducted by F.W. under the supervision of M.A. R.O.R. drafted the manuscript, with all authors contributing. R.O.R. supervised the project.
Competing interests: The authors declare no competing interests, financial or otherwise.
Data and materials availability: All data that support the findings of this study are reported in the main paper and supplementary materials.
License information: Copyright © 2022 the authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original US government works. https://www.science.org/about/science-licenses-journal-article-reuse
Authors
Funding Information
U.S. Department of Energy: DE-AC02-05CH11231.
U.S. Department of Energy: ERKCM06
University of New South Wales: EP/T000368/1
University of New South Wales: FT190100484
University of New South Wales: DE-AC02-CH11231
ARC Future Fellowship: project FT190100484
U.K. Engineering and Physical Sciences Research Council: EP/N004493/2, EP/T000368/1
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