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A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: closed (31 December 2018) | Viewed by 23220

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Prof. Dr. Michael Regev

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Department of Mechanical Engineering, Braude College of Engineering, Karmiel 2161401, Israel
Interests: metallurgy; magnesium alloys; amorphous alloys; friction stir welding and processing of Al, Cu, Mg and Ti alloys; high temperature mechanical properties; joining processes
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Dear Colleagues,

The search for new materials with improved properties, as we all know, never stops. High Entropy Alloys (HEAs) are among the new and promising material groups, alloys, that have been attracting attention since the beginning of the 21st Century.

Historically, alloys have consisted of one, sometimes two, or even three, major elements, together with minor ones. HEAs, in contrast, contain at least five principal elements in equal, or near equal atomic percentage.

HEAs are known for their attractive physical and mechanical properties. It should be noted that some HEAs show unique properties, such as high-strength at room temperature, as well as at elevated temperatures. Their room temperature yield strength can vary from 300 MPa for FCC-structured alloys to about 3000 MPa for BCC-structured alloys. As for elevated temperatures, some refractory HEAs can sustain their high specific strength up to 1800 K. Certain HEAs also possess both high fatigue resistance and high wear resistance. Other HEAs exhibit excellent paramagnetic, ferromagnetic and soft magnetic properties together with high irradiation resistance and high corrosion resistance. Four core effects are responsible for the superior properties of some HEAs: the high mixing entropy effect, the sluggish diffusion effect, the lattice distortion effect and the cocktail effect, which says that the properties of HEAs cannot just be taken from averaging the properties of the constituting elements.

HEAs have potentially a wide range of applications, such as in functional and structural materials. Among many other applications one can mention their potential in the nuclear and at the aerospace industries, as well as in the production of heat-resistant and wear-resistant coatings and joining processes.

It is my pleasure to invite you to submit manuscripts on the subject of HEAs for this Special Issue. Full papers, communications, and reviews are all welcome.

Prof. Michael Regev
Guest Editor

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Keywords

High Entropy Alloys
Advanced Materials
Mechanical Properties
Physical Properties
Characterization
Microstructure
Elevated temperature properties

Published Papers (5 papers)

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Research

11 pages, 6955 KiB

Open AccessArticle

Compositional Dependence of Phase Selection in CoCrCu_0.1FeMoNi-Based High-Entropy Alloys

by Ning Liu, Chen Chen, Isaac Chang, Pengjie Zhou and Xiaojing Wang

Materials 2018, 11(8), 1290; https://doi.org/10.3390/ma11081290 - 25 Jul 2018

Cited by 25 | Viewed by 3431

Abstract

To study the effect of alloy composition on phase selection in the CoCrCu_0.1FeMoNi high-entropy alloy (HEA), Mo was partially replaced by Co, Cr, Fe, and Ni. The microstructures and phase selection behaviors of the CoCrCu_0.1FeMoNi HEA system were investigated. Dendritic, inter-dendritic, and eutectic microstructures were observed in the as-solidified HEAs. A simple face centered cubic (FCC) single-phase solid solution was obtained when the molar ratio of Fe, Co, and Ni was increased to 1.7 at the expense of Mo, indicating that Fe, Co, and Ni stabilized the FCC structure. The FCC structure was favored at the atomic radius ratio δ ≤ 2.8, valence electron concentration (VEC) ≥ 8.27, mixing entropy ΔS ≤ 13.037, local lattice distortion parameter α₂ ≤ 0.0051, and ΔS/δ² > 1.7. Mixed FCC + body centered cubic (BCC) structures occurred for 4.1 ≤ δ ≤ 4.3 and 7.71 ≤ VEC ≤ 7.86; FCC or/and BCC + intermetallic (IM) mixtures were favored at 2.8 ≤ δ ≤ 4.1 or δ > 4.3 and 7.39 < VEC ≤ 8.27. The IM phase is favored at electronegativity differences greater than 0.133. However, ΔS, α₂, and ΔS/δ² were inefficient in identifying the (FCC or/and BCC + IM)/(FCC + BCC) transition. Moreover, the mixing enthalpy cannot predict phase structures in this system. Full article

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

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8 pages, 2752 KiB

Open AccessArticle

Enhanced Strength of a Mechanical Alloyed NbMoTaWVTi Refractory High Entropy Alloy

by Yan Long, Kai Su, Jinfu Zhang, Xiaobiao Liang, Haiyan Peng and Xiaozhen Li

Materials 2018, 11(5), 669; https://doi.org/10.3390/ma11050669 - 25 Apr 2018

Cited by 52 | Viewed by 4943

Abstract

A NbMoTaWVTi refractory high entropy alloy (HEA) has been successfully synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). The microstructure and mechanical properties of this alloy are investigated. It is observed that only two types of body-centered cubic (BCC) solid solutions are formed in the powders after ball milling for 40 h. However, a new face-centered cubic (FCC) precipitated phase is observed in the BCC matrix of bulk material consolidated by SPS. The FCC precipitated phase is identified as TiO, due to the introduction of O during the preparing process of HEA. The compressive yield strength, fracture strength, and total fracture strain of the consolidated bulk HEA are 2709 MPa, 3115 MPa, and 11.4%, respectively. The excellent mechanical properties can be attributed to solid solution strengthening and grain boundary strengthening of the fine-grained BCC matrix, as well as the precipitation strengthening owing to the formation of TiO particles. Full article

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

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13 pages, 4223 KiB

Open AccessFeature PaperArticle

Microstructural Evolution and Phase Formation in 2nd-Generation Refractory-Based High Entropy Alloys

by Eyal Eshed, Natalya Larianovsky, Alexey Kovalevsky, Vladimir Popov Jr., Igor Gorbachev, Vladimir Popov and Alexander Katz-Demyanetz

Materials 2018, 11(2), 175; https://doi.org/10.3390/ma11020175 - 23 Jan 2018

Cited by 20 | Viewed by 4914

Abstract

Refractory-based high entropy alloys (HEAs) of the 2nd-generation type are new intensively-studied materials with a high potential for structural high-temperature applications. This paper presents investigation results on microstructural evolution and phase formation in as-cast and subsequently heat-treated HEAs at various temperature-time regimes. Microstructural examination was performed by means of scanning electron microscopy (SEM) combined with the energy dispersive spectroscopy (EDS) mode of electron probe microanalysis (EPMA) and qualitative X-ray diffraction (XRD). The primary evolutionary trend observed was the tendency of Zr to gradually segregate as the temperature rises, while all the other elements eventually dissolve in the BCC solid solution phase once the onset of Laves phase complex decomposition is reached. The performed thermodynamic modelling was based on the Calculation of Phase Diagrams method (CALPHAD). The BCC A2 solid solution phase is predicted by the model to contain increasing amounts of Cr as the temperature rises, which is in perfect agreement with the actual results obtained by SEM. However, the model was not able to predict the existence of the Zr-rich phase or the tendency of Zr to segregate and form its own solid solution—most likely as a result of the Zr segregation trend not being an equilibrium phenomenon. Full article

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

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10087 KiB

Open AccessArticle

Microstructures, Hardness and Corrosion Behaviors of FeCoNiNb_0.5Mo_0.5 and FeCoNiNb High-Entropy Alloys

by Chun-Huei Tsau and Wei-Li Wang

Materials 2018, 11(1), 16; https://doi.org/10.3390/ma11010016 - 23 Dec 2017

Cited by 11 | Viewed by 3673

Abstract

This study investigates the effects of niobium and molybdenum on FeCoNi alloy, including on the microstructures and hardness of FeCoNiNb_0.5Mo_0.5 and FeCoNiNb alloys, and the polarization behaviors of these alloys in 1 M sulfuric acid and 1 M sodium chloride solutions. The results in this study indicate that both FeCoNiNb_0.5Mo_0.5 and FeCoNiNb alloys had a dual-phased dendritic microstructure; all of the phases in these alloys were solid solution phases, and no ordering was observed. Therefore, the solid solution effect significantly increased the hardness of these two alloys; in particular, FeCoNiNb alloy had the highest hardness of the alloys of interest. The corrosion resistance of FeCoNiNb_0.5Mo_0.5 and FeCoNiNb alloys was less than that of FeCoNi alloy because of their dual-phased dendritic microstructures. The corrosion resistance of the FeCoNiNb_0.5Mo_0.5 alloy exceeded that of the FeCoNiNb alloy in these solutions. However, FeCoNiNb_0.5Mo_0.5 and FeCoNiNb alloys exhibited a favorable combination of corrosion resistance and hardness. Full article

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

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13758 KiB

Open AccessArticle

Microstructure, Tensile and Creep Properties of Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀ High Entropy Alloy

by Natalya Larianovsky, Alexander Katz-Demyanetz, Eyal Eshed and Michael Regev

Materials 2017, 10(8), 883; https://doi.org/10.3390/ma10080883 - 31 Jul 2017

Cited by 19 | Viewed by 4964

Abstract

This paper examines the microstructure and mechanical properties of Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀. Two casting processes, namely, gravity casting and suction-assisted casting, were applied, both followed by Hot Isostatic Pressing (HIP). The aim of the current study was to investigate the creep and tensile properties of the material, since the literature review revealed no data whatsoever regarding these properties. The main findings are that the HIP process is responsible for the appearance of a Hexagonal Close Packed (HCP) phase that is dispersed differently in these two castings. The HIP process also led to a considerable increase in the mechanical properties of both materials under compression, with values found to be higher than those reported in the literature. Contrary to the compression properties, both materials were found to be highly brittle under tension, either during room temperature tension tests or creep tests conducted at 282 °C. Fractography yielded brittle fracture without any evidence of plastic deformation prior to fracture. Full article

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

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