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A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: closed (31 July 2017) | Viewed by 117092

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Dr. Michael C. Gao

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Guest Editor

AECOM|Contractor for the US DEPT of ENERGY, National Energy Technology Laboratory (Albany), 1450 Queen Ave SW, Albany, OR 97321, USA

Prof. Junwei Qiao

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Laboratory of Applied Physics and Mechanics of Advanced Materials, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Interests: special high-entropy alloys; mechanical behavior of high-entropy alloys and amorphous alloys; serration in metals and alloys
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Special Issue Information

Dear Colleagues,

Originated from the idea of multi-principal-element solid solution, the field of “high-entropy alloys (HEAs)” has attracted intense and increasing interest from academia and industries worldwide. Outstanding (physical, mechanical, and functional) properties have been reported for a variety of HEAs. In order to balance the properties for targeted applications, the microstructure of HEAs can be a single phase or composite, and traditional physical metallurgy principles have been applied to develop unique HEAs, including high-entropy stainless steels, high-entropy superalloys, high-entropy refractory alloys, high-entropy light-weight alloys, high-entropy oxides, high-entropy metallic compounds, etc. As presented in a recent comprehensive review on HEAs (High-Entropy Alloys: Fundamentals and Applications (2016). 1st edn. Gao MC, Yeh JW, Liaw PK, Zhang Y (eds). Springer International Publishing, Cham, Switzerland. doi:10.1007/978-3-319-27013-5), great challenges remain in fundamental understanding of HEAs formation and their properties, and potential high-performance HEAs are yet to be explored. The objective of this Special Issue is to timely disseminate the rapid progress in fundamental understanding and applications of HEAs.

Specific topics of interest include (but are not limited to):

Accelerated alloy design using combinatorial approach, models or others
Structure/processing/properties relationships
Advanced characterization including neutron scattering, 3-dimension scanning electron microscopy, high-resolution transmission electron microscopy, etc.
Mechanical, electrochemical, physical, anti-irradiation, thermodynamic, kinetic, and other properties
Computational modeling: first-principles density functional theory, molecular dynamics, Monte Carlo simulations, phase field, finite element methods, CALPHAD, etc.

Dr. Michael C. Gao
Professor Junwei Qiao
Guest Editor

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Keywords

high-entropy alloys
computational modeling
mechanical properties
alloy design
thermodynamics

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Published Papers (12 papers)

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Editorial

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3 pages, 162 KiB

Open AccessEditorial

High-Entropy Alloys (HEAs)

by Michael C. Gao and Junwei Qiao

Metals 2018, 8(2), 108; https://doi.org/10.3390/met8020108 - 6 Feb 2018

Cited by 28 | Viewed by 5621

Abstract

High-entropy alloys (HEAs) [1,2] loosely refer to multi-principal-element solid solution alloys due to their high configurational entropy, in contrast to traditional alloys, which focus on the edge or corner of phase diagrams with one principal component[...] Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

Research

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5406 KiB

Open AccessArticle

Virtual Testing of Composite Structures Made of High Entropy Alloys and Steel

by Victor Geantă, Tudor Cherecheș, Paul Lixandru, Ionelia Voiculescu, Radu Ștefănoiu, Daniel Dragnea, Teodora Zecheru and Liviu Matache

Metals 2017, 7(11), 496; https://doi.org/10.3390/met7110496 - 11 Nov 2017

Cited by 14 | Viewed by 5932

Abstract

High entropy alloys (HEA) are metallic materials obtained from a mixture of at least five atomic-scale chemical elements. They are characterized by high mechanical strength, good thermal stability and hardenability. AlCrFeCoNi alloys have high compression strength and tensile strength values of 2004 MPa, respectively 1250 MPa and elongation of about 32.7%. These materials can be used to create HEA-steel type composite structures which resist to dynamic deformation during high speed impacts. The paper presents four different composite structures made from a combination of HEA and carbon steel plates, using different joining processes. The numerical simulation of the impact behavior of the composite structures was performed by virtual methods, taking into account the mechanical properties of both materials. For analyzing each constructive variant, three virtual shootings were designed, using a 7.62 × 39 mm cal. incendiary armor-piercing bullet and different impact velocities. The best ballistic behavior was provided by the composite structures obtained by welding and brazing that have good continuity and rigidity. The other composite structures, which do not have good surface adhesion, show high fragmentation risk, because the rear plate can fragment on the axis of shooting due to the combination between the shock waves and the reflected ones. The order of materials in the composite structure has a very important role in decreasing the impact energy. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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3765 KiB

Open AccessArticle

The Phase Composition and Microstructure of Al_xCoCrFeNiTi Alloys for the Development of High-Entropy Alloy Systems

by Thomas Lindner, Martin Löbel, Thomas Mehner, Dagmar Dietrich and Thomas Lampke

Metals 2017, 7(5), 162; https://doi.org/10.3390/met7050162 - 9 May 2017

Cited by 28 | Viewed by 7415

Abstract

Alloying aluminum offers the possibility of creating low-density high-entropy alloys (HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination. The effect of aluminum on the phase composition and microstructure of the compositionally complex alloy (CCA) system Al_xCoCrFeNiTi was studied with variation in aluminum content (molar ratios x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the different domains in the microstructure. The crystal structure was determined using X-ray diffraction (XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with associated orientation distribution was performed using electron backscatter diffraction. This made it possible to correlate the chemical and structural conditions of the phases. The phase formation strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum amounts. The hard η-(Ni, Co)₃Ti phase in the x = 0.2 alloy was detected via metallographic investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc) with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural and chemical properties as well as microstructure formation contribute to a better understanding of the alloying effects concerning the aluminum content in CCAs. Especially in the context of current developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach to the development of new alloy systems. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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4479 KiB

Open AccessArticle

Investigating the Wear Behavior of Fe-Based Amorphous Coatings under Nanoscratch Tests

by Yixuan Wu, Qiang Luo, Jin Jiao, Xianshun Wei and Jun Shen

Metals 2017, 7(4), 118; https://doi.org/10.3390/met7040118 - 29 Mar 2017

Cited by 17 | Viewed by 5054

Abstract

The wear behavior of two coatings (Fe_49.7Cr₁₈Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4 and Fe₄₀Cr₂₃Mo₁₄C₁₅B₆Y₂) sprayed by high-velocity air fuel technology was [...] Read more.

The wear behavior of two coatings (Fe_49.7Cr₁₈Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4 and Fe₄₀Cr₂₃Mo₁₄C₁₅B₆Y₂) sprayed by high-velocity air fuel technology was investigated through nanoscratch tests under ramping loads. Compared with the substrate, the Fe-based amorphous coatings exhibit lower penetration depth, higher elastic recovery, and lower wear volume, indicating the excellent wear resistance of the coatings. This behavior is related to the high hardness and high hardness/elastic modulus ratio (H/E) of the Fe-based amorphous coatings. From the scanning electron microscopy images of the scratch grooves, it is found out that ploughing governs the wear behavior of the coatings and substrate. In addition, spalling wear easily occurs in the pore regions of the coatings. Full article

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11114 KiB

Open AccessArticle

Deformation Behavior of Al_0.25CoCrFeNi High-Entropy Alloy after Recrystallization

by Jinxiong Hou, Min Zhang, Huijun Yang and Junwei Qiao

Metals 2017, 7(4), 111; https://doi.org/10.3390/met7040111 - 24 Mar 2017

Cited by 36 | Viewed by 6860

Abstract

Cold rolling with subsequent annealing can be used to produce the recrystallized structure in high entropy alloys (HEAs). The Al_0.25CoCrFeNi HEAs rolled to different final thickness (230, 400, 540, 800, 1000, 1500 μm) are prepared to investigate their microstructure evolutions and mechanical behaviors after annealing. Only the single face-centered cubic phase was obtained after cold rolling and recrystallization annealing at 1100 °C for 10 h. The average recrystallized grain size in this alloy after annealing ranges from 92 μm to 136 μm. The annealed thin sheets show obviously size effects on the flow stress and formability. The yield strength and tensile strength decrease as t/d (thickness/average grain diameter) ratio decreases until the t/d approaches 2.23. In addition, the stretchability (formability) decreases with the decrease of the t/d ratio especially when the t/d ratio is lower than about 6. According to the present results, yield strength can be expressed as a function of the t/d ratio. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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3678 KiB

Open AccessArticle

The BCC/B2 Morphologies in Al_xNiCoFeCr High-Entropy Alloys

by Yue Ma, Beibei Jiang, Chunling Li, Qing Wang, Chuang Dong, Peter K. Liaw, Fen Xu and Lixian Sun

Metals 2017, 7(2), 57; https://doi.org/10.3390/met7020057 - 15 Feb 2017

Cited by 146 | Viewed by 13828

Abstract

The present work primarily investigates the morphological evolution of the body-centered-cubic (BCC)/B2 phases in Al_xNiCoFeCr high-entropy alloys (HEAs) with increasing Al content. It is found that the BCC/B2 coherent morphology is closely related to the lattice misfit between these two phases, which is sensitive to Al. There are two types of microscopic BCC/B2 morphologies in this HEA series: one is the weave-like morphology induced by the spinodal decomposition, and the other is the microstructure of a spherical disordered BCC precipitation on the ordered B2 matrix that appears in HEAs with a much higher Al content. The mechanical properties, including the compressive yielding strength and microhardness of the Al_xNiCoFeCr HEAs, are also discussed in light of the concept of the valence electron concentration (VEC). Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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4135 KiB

Open AccessArticle

Phase Evolution of the Al_xNbTiVZr (x = 0; 0.5; 1; 1.5) High Entropy Alloys

by Nikita Y. Yurchenko, Nikita D. Stepanov, Mikhail A. Tikhonovsky and Gennady A. Salishchev

Metals 2016, 6(12), 298; https://doi.org/10.3390/met6120298 - 25 Nov 2016

Cited by 28 | Viewed by 5775

Abstract

Al_xNbTiVZr (x = 0; 0.5; 1; 1.5) high entropy alloys were fabricated by vacuum arc melting and annealed at 1200 °C for 24 h. The NbTiVZr alloy had single body centered cubic (bcc) solid solution phase after annealing at 1200 °C, while, in the Al-containing alloys, C14 Laves and Zr₂Al-type phases are found. The alloys were subjected to annealing at 800 °C and 1000 °C. It was shown that annealing temperature (800 °C or 1000 °C) weakly affected the produced phases but the Al content had pronounced effect on structure of the annealed alloys. The NbTiVZr alloy decomposed into bcc, Zr-rich hexagonal close-packed (hcp), and C15 Laves phases. In the Al_0.5NbTiVZr alloy, the bcc matrix phase also decomposed into a mixture of bcc and C14 Laves phases. In the AlNbTiVZr alloy, annealing resulted in an increase of volume fraction of Zr₂Al-type phase. Finally, in the Al_1.5NbTiVZr alloy, formation of AlNb₂-type phase was observed. The highest fraction of second phases appeared after annealing in the NbTiVZr alloy. It is demonstrated that the strong chemical affinity and high enthalpy of formation of intermetallic phases in Al-Zr atomic pair govern the intermetallic phase formation in the alloys at 1200 °C. Increase of volume fraction of second phases in the alloys due to annealing at 800 °C and 1000 °C is in proportion to the decrease of Zr concentration in the bcc matrix phase. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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3249 KiB

Open AccessArticle

Hot Deformation Behavior of As-Cast and Homogenized Al_0.5CoCrFeNi High Entropy Alloys

by Yu Zhang, Jinshan Li, Jun Wang, Sizhe Niu and Hongchao Kou

Metals 2016, 6(11), 277; https://doi.org/10.3390/met6110277 - 15 Nov 2016

Cited by 30 | Viewed by 5510

Abstract

The hot deformation behavior of as-cast and homogenized Al_0.5CoCrFeNi high entropy alloys (HEAs) during isothermal compression was investigated as a function of temperature and strain rate. Results indicated that flow stress in a homogenized state was always higher than that in an as-cast state under the same deformation conditions. Moreover, the optimum thermo-mechanical processing (TMP) conditions for the hot working of the homogenized state were identified as 945–965 °C and 10^−1.7–10^−1.1 s⁻¹ and were easier to determine in practice. Constitutive equations, for both states, correlating the flow stress of Al_0.5CoCrFeNi with strain rate and deformation temperature were also determined. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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3401 KiB

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Influence of Annealing on the Microstructures and Oxidation Behaviors of Al₈(CoCrFeNi)₉₂, Al₁₅(CoCrFeNi)₈₅, and Al₃₀(CoCrFeNi)₇₀ High-Entropy Alloys

by Todd M. Butler and Mark L. Weaver

Metals 2016, 6(9), 222; https://doi.org/10.3390/met6090222 - 12 Sep 2016

Cited by 42 | Viewed by 6872

Abstract

The understanding of the oxidation behaviors of as-cast and annealed high-entropy alloys (HEAs) is currently limited. This work systematically investigates the influence of annealing on the microstructures and oxidation behaviors of AlCoCrFeNi-based HEAs. Annealing was found to alter the distribution of Al-rich phases which caused a change in the oxidation mechanisms. In general, all three of the investigated HEAs displayed some degree of transient oxidation at 1050 °C that was later followed by protective, parabolic oxide growth. The respective oxidation behaviors are discussed relative to existing oxide formation models for Ni–Cr–Al alloys. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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4197 KiB

Open AccessArticle

Serration Behavior in Pd_77.5Cu₆Si_16.5 Alloy

by Zhong Wang, Jiaojiao Li, Bo Yuan, Rongfeng Wu, Jing Fan and Baocheng Wang

Metals 2016, 6(8), 191; https://doi.org/10.3390/met6080191 - 17 Aug 2016

Cited by 4 | Viewed by 4690

Abstract

The strain-rate-dependent plasticity under uniaxial compression at the strain rates of 2 × 10⁻³, 2 × 10⁻⁴ and 2 × 10⁻⁵ s⁻¹ in a Pd_77.5Cu₆Si_16.5 alloy is investigated. At different strain rates, the serration events exhibit different amplitudes and time scales. The intersection effects take place obviously, and the loading time is much longer than the relaxation time in the serration event at three strain rates. However, the time intervals between two neighboring serrations lack any time scale, and the elastic energy density displays a power-law distribution at the strain rate of 2 × 10⁻³ s⁻¹, which means that the self-organized critical (SOC) behavior emerges with increasing strain rates. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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Review

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2256 KiB

Open AccessFeature PaperReview

Corrosion-Resistant High-Entropy Alloys: A Review

by Yunzhu Shi, Bin Yang and Peter K. Liaw

Metals 2017, 7(2), 43; https://doi.org/10.3390/met7020043 - 5 Feb 2017

Cited by 688 | Viewed by 33657

Abstract

Corrosion destroys more than three percent of the world’s gross domestic product. Therefore, the design of highly corrosion-resistant materials is urgently needed. By breaking the classical alloy-design philosophy, high-entropy alloys (HEAs) possess unique microstructures, which are solid solutions with random arrangements of multiple elements. The particular locally-disordered chemical environment is expected to lead to unique corrosion-resistant properties. In this review, the studies of the corrosion-resistant HEAs during the last decade are summarized. The corrosion-resistant properties of HEAs in various aqueous environments and the corrosion behavior of HEA coatings are presented. The effects of environments, alloying elements, and processing methods on the corrosion resistance are analyzed in detail. Furthermore, the possible directions of future work regarding the corrosion behavior of HEAs are suggested. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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961 KiB

Open AccessFeature PaperReview

An Insight into Evolution of Light Weight High Entropy Alloys: A Review

by Amit Kumar and Manoj Gupta

Metals 2016, 6(9), 199; https://doi.org/10.3390/met6090199 - 26 Aug 2016

Cited by 139 | Viewed by 14263

Abstract

High Entropy Alloys (HEAs) are the most recently developed new class of materials, which are known for their unique structural properties. Lightweight materials are currently in excessive demand for transportation and energy saving applications. In this review, efforts have been made to summarize the work done towards the development of HEAs targeting lightweight applications. Some new synthesis techniques are suggested for the fabrication of lightweight HEAs (LWHEAs). The concept of porous structure fabrication, microwave sintering of green compact, casting by disintegration melt deposition and advanced manufacturing using additive manufacturing are discussed as future directions of LWHEAs synthesis. LWHEAs for potential biomedical applications have also been addressed. Full article

(This article belongs to the Special Issue High-Entropy Alloys (HEAs))

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