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Metals
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High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys
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High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Entropic Alloys and Meta-Metals".

Deadline for manuscript submissions: closed (15 July 2022) | Viewed by 16987

Special Issue Editors

Prof. Dr. Eun Soo Park

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Guest Editor
Department of Materials Science and Engineering, College of Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
Interests: tailor-made materials design; phase transformation; microstructural characterization; microstructure-property relationships
Special Issues, Collections and Topics in MDPI journals
Dr. Hyunseok Oh

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Guest Editor
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, USA
Interests: alloy design (complex concentrated and high entropy alloys, advanced high strength steel, composite materials); mechanical properties; structure analysis (SEM techniques, synchrotron); additive manufacturing

Special Issue Information

Dear Colleagues,

The goal of this Special Issue is to discuss major materials issues for complex concentrated alloys (CCAs) and high entropy alloys (HEAs), from property-targeted design to process optimization, from structures to properties, and from the fundamental science to viable industrial applications. CCAs have been reported to have useful performances, including great toughness, high-temperature strength, corrosion resistance, as well as a good irradiation resistance. In addition, the concept of CCAs shifts the focus away from the corners of alloy phase diagrams toward their centers, vastly increasing the number of possible alloy systems with an unexplored property realm. Thus, CCAs have attracted worldwide attention as a new generation of alloys to resolve the challenges of modern industries in the fields of transportation, energy, safety, and infrastructure with remarkable properties never seen before.

Much of the interest in CCAs and HEAs is predicated on the belief that the maximized chemical complexity with multiprincipal elements would produce profound intrinsic core properties, such as the high entropy effect, the lattice distortion effect, the sluggish diffusion effect, the solid-solution strengthening effect, and the cocktail effect. Thus, early design strategies focused on increasing the number of principal elements and the configurational entropy to maximize the benefits of chemical complexity. Recently, however, several studies have shown that the nature of elements is more important for the complexity-related properties than their mere numbers. This introduces a new challenge, because not every combination of elements would be successful in achieving beneficial properties. As a result, the advantage of so many degrees of freedom for alloy design of CCAs is diminished by a lack of mixing rules, rendering alloy design an empirical try-and-error undertaking. Therefore, to make a useful guide for the new CCA and HEA design, careful guidelines are required to consider the atomic environments of CCAs in a physically meaningful way. In this Special Issue, we hope to present the design guidelines of new CCAs and HEAs, as well as highlight some challenging fundamental and application issues for their properties.

Prof. Dr. Eun Soo Park
Dr. Hyunseok Oh
Guest Editors

Manuscript Submission Information

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • High-entropy and complex-concentrated alloys
  • Material fabrication and processing
  • Theoretical modeling and simulation
  • Properties (mechanical, physical, magnetic, electric, thermal, and corrosion)
  • Industrial application

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Published Papers (4 papers)

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Research

12 pages, 5915 KiB  
Article
Manipulation of Microstructure and Mechanical Properties in N-Doped CoCrFeMnNi High-Entropy Alloys
by Jing Zhang, Kook Noh Yoon, Min Seok Kim, Heh Sang Ahn, Ji Young Kim, Wook Ha Ryu and Eun Soo Park
Metals 2021, 11(9), 1487; https://doi.org/10.3390/met11091487 - 18 Sep 2021
Cited by 13 | Viewed by 2963
Abstract
Herein, we carefully investigate the effect of nitrogen doping in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) on the microstructure evolution and mechanical properties. After homogenization (1100 °C for 20 h), cold-rolling (reduction ratio of 60%) and subsequent annealing (800 °C for 1 h), [...] Read more.
Herein, we carefully investigate the effect of nitrogen doping in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) on the microstructure evolution and mechanical properties. After homogenization (1100 °C for 20 h), cold-rolling (reduction ratio of 60%) and subsequent annealing (800 °C for 1 h), a unique complex heterogeneous microstructure consisting of fine recrystallized grains, large non-recrystallized grains, and nanoscale Cr2N precipitates, were obtained in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA. The yield strength and ultimate tensile strength can be significantly improved in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA with a complex heterogeneous microstructure, which shows more than two times higher than those compared to CoCrFeMnNi HEA under the identical process condition. It is achieved by the simultaneous operation of various strengthening mechanisms from the complex heterogeneous microstructure. Although it still has not solved the problem of ductility reduction, as the strength increases because the microstructure optimization is not yet complete, it is expected that precise control of the unique complex heterogeneous structure in nitrogen-doped CoCrFeMnNi HEA can open a new era in overcoming the strength–ductility trade-off, one of the oldest dilemmas of structural materials. Full article
(This article belongs to the Special Issue High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys)
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19 pages, 6352 KiB  
Article
Mechanical Performance and Microstructural Evolution of (NiCo)75Cr17Fe8Cx (x = 0~0.83) Medium Entropy Alloys at Room and Cryogenic Temperatures
by Jae Sook Song, Byung Ju Lee, Won Jin Moon and Sun Ig Hong
Metals 2020, 10(12), 1646; https://doi.org/10.3390/met10121646 - 6 Dec 2020
Cited by 11 | Viewed by 3633
Abstract
We investigated the effects of the addition of Co and carbon on the deformation behavior of new medium-entropy alloys (MEAs) designed by increasing the entropy of the conventional NiCrFe-type Alloy 600. The strength/ductility combination of carbon-free (NiCo)75Cr17Fe8 MEA [...] Read more.
We investigated the effects of the addition of Co and carbon on the deformation behavior of new medium-entropy alloys (MEAs) designed by increasing the entropy of the conventional NiCrFe-type Alloy 600. The strength/ductility combination of carbon-free (NiCo)75Cr17Fe8 MEA was found to be 729 MPa/81% at 298 K and it increased to a remarkable 1212 MPa/106% at 77 K. The excellent strength and ductility of (NiCo)75Cr17Fe8 at cryogenic temperature is attributed to the increased strain hardening rate caused by the interaction between dislocation slip and deformation twins. Strength/ductility combinations of carbon-doped (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at cryogenic temperature were observed to be 1321 MPa/96% and 1398 MPa/66%, respectively, both of which are superior to those of other high-entropy alloys (HEAs). Strength/ductility combinations of (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at room temperature were found to be 831 MPa/72% and 942 MPa/55%, respectively and both are far superior to 676 MPa/41% of the commercial Alloy 600. Yield strengths of carbon-free and carbon-doped alloys comprised strengthening components from the friction stress, grain size strengthening, carbide strengthening and interstitial strengthening and excellent agreement between the predictions and the experiments was obtained. A design strategy to develop new MEAs by increasing the entropy of the conventional alloys was found to be effective in enhancing the mechanical performance. Full article
(This article belongs to the Special Issue High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys)
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10 pages, 4426 KiB  
Article
GTA Weldability of Rolled High-Entropy Alloys Using Various Filler Metals
by Hyunbin Nam, Seonghoon Yoo, Junghoon Lee, Youngsang Na, Nokeun Park and Namhyun Kang
Metals 2020, 10(10), 1371; https://doi.org/10.3390/met10101371 - 14 Oct 2020
Cited by 7 | Viewed by 2849
Abstract
Gas tungsten arc (GTA) weldability of rolled CoCrFeMnNi high-entropy alloys (HEAs) was conducted using stainless steel (STS) 308L and HEA fillers. Microstructure and mechanical properties of the welds were examined to determine GTA weldability of the rolled HEA. The welds had no macro-defects, [...] Read more.
Gas tungsten arc (GTA) weldability of rolled CoCrFeMnNi high-entropy alloys (HEAs) was conducted using stainless steel (STS) 308L and HEA fillers. Microstructure and mechanical properties of the welds were examined to determine GTA weldability of the rolled HEA. The welds had no macro-defects, and component behaviour between base metal (BM) and weld metal (WM) showed significant differences in the weld using the STS 308L filler. Macro-segregation of Fe components was confirmed in the central region in the WM using the STS 308L filler. Because the columnar grain sizes of all the WMs were larger than those of the rolled HEA BM irrespective of the filler metals, the tensile properties of the GTA welds were lower than those of the rolled HEA BM, and the tensile fracture occurred in the centreline of each weld. In particular, the tensile properties of the weld using the STS 308L filler deteriorated more than those of the HEA weld. This was induced by the formation of macro-segregation and severe martensite transformation in the centreline of WM. To enhance the weldability of the rolled HEA, the formation of macro-segregation and coarse grains in the WM of GTA welds must be prevented. Full article
(This article belongs to the Special Issue High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys)
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17 pages, 4932 KiB  
Article
Machine Learning Enabled Prediction of Stacking Fault Energies in Concentrated Alloys
by Gaurav Arora and Dilpuneet S. Aidhy
Metals 2020, 10(8), 1072; https://doi.org/10.3390/met10081072 - 9 Aug 2020
Cited by 25 | Viewed by 6190
Abstract
Recent works have revealed a unique combination of high strength and high ductility in certain compositions of high-entropy alloys (HEAs), which is attributed to the low stacking fault energy (SFE). While atomistic calculations have been successful in predicting the SFE of pure metals, [...] Read more.
Recent works have revealed a unique combination of high strength and high ductility in certain compositions of high-entropy alloys (HEAs), which is attributed to the low stacking fault energy (SFE). While atomistic calculations have been successful in predicting the SFE of pure metals, large variations up to 200 mJ/m2 have been observed in HEAs. One of the leading causes of such variations is the limited number of atoms that can be modeled in atomistic calculations; as a result, due to random distribution of elements in HEAs, various nearest neighbor environments may not be adequately captured in small supercells resulting in different SFE values. Such variation further increases with the increase in the number of elements in a given composition. In this work, we use machine learning to overcome the limitation of smaller system sizes and provide a methodology to significantly reduce the variation and uncertainty in predicting SFEs. We show that the SFE can be accurately predicted across the composition ranges in binary alloys. This capability then enables us to predict the SFE of multi-elemental alloys by training the model using only binary alloys. Consequently, SFEs of complex alloys can be predicted using a binary alloys database, and the need to perform calculations for every new composition can be circumvented. Full article
(This article belongs to the Special Issue High-Entropy and Complex Concentrated Alloys: A New Generation of Alloys)
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