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7 pages, 197 KiB

Open AccessEditorial

New Advances in High-Entropy Alloys

by Yong Zhang and Ruixuan Li

Entropy 2020, 22(10), 1158; https://doi.org/10.3390/e22101158 - 15 Oct 2020

Cited by 18 | Viewed by 4998

Abstract

Exploring new materials is an eternal pursuit in the development of human civilization [...] Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

Research

Jump to: Editorial, Review

10 pages, 2321 KiB

Open AccessArticle

Processing and Characterization of Refractory Quaternary and Quinary High-Entropy Carbide Composite

by Hanzhu Zhang and Farid Akhtar

Entropy 2019, 21(5), 474; https://doi.org/10.3390/e21050474 - 06 May 2019

Cited by 35 | Viewed by 4913

Abstract

Quaternary high-entropy ceramic (HEC) composite was synthesized from HfC, Mo₂C, TaC, and TiC in pulsed current processing. A high-entropy solid solution that contained all principal elements along with a minor amount of a Ta-rich phase was observed in the microstructure. The [...] Read more.

Quaternary high-entropy ceramic (HEC) composite was synthesized from HfC, Mo₂C, TaC, and TiC in pulsed current processing. A high-entropy solid solution that contained all principal elements along with a minor amount of a Ta-rich phase was observed in the microstructure. The high entropy phase and Ta-rich phase displayed a face-centered cubic (FCC) crystal structure with similar lattice parameters, suggesting that TaC acted as a solvent carbide during phase evolution. The addition of B₄C to the quaternary carbide system induced the formation of two high-entropy solid solutions with different elemental compositions. With the increase in the number of principal elements, on the addition of B₄C, the crystal structure of the HEC phase transformed from FCC to a hexagonal structure. The study on the effect of starting particle sizes on the phase composition and properties of the HEC composites showed that reducing the size of solute carbide components HfC, Mo₂C, and TiC could effectively promote the interdiffusion process, resulting in a higher fraction of a hexagonal structured HEC phase in the material. On the other hand, tuning the particle size of solvent carbide, TaC, showed a negligible effect on the composition of the final product. However, reducing the TaC size from −325 mesh down to <1 µm resulted in an improvement of the nanohardness of the HEC composite from 21 GPa to 23 GPa. These findings suggested the possibility of forming a high-entropy ceramic phase despite the vast difference in the precursor crystal structures, provided a clearer understanding of the phase transformation process which could be applied for the designing of HEC materials. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 2615 KiB

Open AccessArticle

Prediction of Strength and Ductility in Partially Recrystallized CoCrFeNiTi_0.2 High-Entropy Alloy

by Hanwen Zhang, Peizhi Liu, Jinxiong Hou, Junwei Qiao and Yucheng Wu

Entropy 2019, 21(4), 389; https://doi.org/10.3390/e21040389 - 11 Apr 2019

Cited by 12 | Viewed by 3761

Abstract

The mechanical behavior of a partially recrystallized fcc-CoCrFeNiTi_0.2 high entropy alloys (HEA) is investigated. Temporal evolutions of the morphology, size, and volume fraction of the nanoscaled L1₂-(Ni,Co)₃Ti precipitates at 800 °C with various aging time were quantitatively evaluated. [...] Read more.

The mechanical behavior of a partially recrystallized fcc-CoCrFeNiTi_0.2 high entropy alloys (HEA) is investigated. Temporal evolutions of the morphology, size, and volume fraction of the nanoscaled L1₂-(Ni,Co)₃Ti precipitates at 800 °C with various aging time were quantitatively evaluated. The ultimate tensile strength can be greatly improved to ~1200 MPa, accompanied with a tensile elongation of ~20% after precipitation. The temporal exponents for the average size and number density of precipitates reasonably conform the predictions by the PV model. A composite model was proposed to describe the plastic strain of the current HEA. As a consequence, the tensile strength and tensile elongation are well predicted, which is in accord with the experimental results. The present experiment provides a theoretical reference for the strengthening of partially recrystallized single-phase HEAs in the future. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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14 pages, 3985 KiB

Open AccessArticle

Effect of Mn Addition on the Microstructures and Mechanical Properties of CoCrFeNiPd High Entropy Alloy

by Yiming Tan, Jinshan Li, Jun Wang and Hongchao Kou

Entropy 2019, 21(3), 288; https://doi.org/10.3390/e21030288 - 16 Mar 2019

Cited by 4 | Viewed by 3723

Abstract

CoCrFeNiPdMn_x (x = 0, 0.2, 0.4, 0.6, 0.8) high entropy alloys (HEAs) were prepared and characterized. With an increase in Mn addition, the microstructures changed from dendrites (CoCrFeNiPd with a single face-centered-cubic (FCC) phase) to divorced eutectics (CoCrFeNiPdMn_0.2 and CoCrFeNiPdMn [...] Read more.

CoCrFeNiPdMn_x (x = 0, 0.2, 0.4, 0.6, 0.8) high entropy alloys (HEAs) were prepared and characterized. With an increase in Mn addition, the microstructures changed from dendrites (CoCrFeNiPd with a single face-centered-cubic (FCC) phase) to divorced eutectics (CoCrFeNiPdMn_0.2 and CoCrFeNiPdMn_0.4), to hypoeutectic microstructures (CoCrFeNiPdMn_0.6), and finally to seaweed eutectic dendrites (CoCrFeNiPdMn_0.8). The addition of Mn might change the interface energy anisotropy of both the FCC/liquid and MnPd-rich intermetallic compound/liquid interfaces, thus forming the seaweed eutectic dendrites. The hardness of the FCC phase was found to be highly related to the solute strengthening effect, the formation of nanotwins and the transition from CoCrFeNiPd-rich to CoCrFeNi-rich FCC phase. Hierarchical nanotwins were found in the MnPd-rich intermetallic compound and a decrease in either the spacing of primary twins or secondary twins led to an increase in hardness. The designing rules of EHEAs were discussed and the pseudo binary method was revised accordingly. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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14 pages, 19432 KiB

Open AccessArticle

Microstructure and Mechanical Properties of Precipitate Strengthened High Entropy Alloy Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ with Additions of Hafnium and Molybdenum

by Sebastian Haas, Anna M. Manzoni, Fabian Krieg and Uwe Glatzel

Entropy 2019, 21(2), 169; https://doi.org/10.3390/e21020169 - 12 Feb 2019

Cited by 32 | Viewed by 6209

Abstract

High entropy or compositionally complex alloys provide opportunities for optimization towards new high-temperature materials. Improvements in the equiatomic alloy Al₁₇Co₁₇Cr₁₇Cu₁₇Fe₁₇Ni₁₇ (at.%) led to the base alloy for this work with the chemical [...] Read more.

High entropy or compositionally complex alloys provide opportunities for optimization towards new high-temperature materials. Improvements in the equiatomic alloy Al₁₇Co₁₇Cr₁₇Cu₁₇Fe₁₇Ni₁₇ (at.%) led to the base alloy for this work with the chemical composition Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ (at.%). Characterization of the beneficial particle-strengthened microstructure by scanning electron microscopy (SEM) and observation of good mechanical properties at elevated temperatures arose the need of accomplishing further optimization steps. For this purpose, the refractory metals hafnium and molybdenum were added in small amounts (0.5 and 1.0 at.% respectively) because of their well-known positive effects on mechanical properties of Ni-based superalloys. By correlation of microstructural examinations using SEM with tensile tests in the temperature range of room temperature up to 900 °C, conclusions could be drawn for further optimization steps. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 5535 KiB

Open AccessArticle

Effect of Ti/Ni Coating of Diamond Particles on Microstructure and Properties of High-Entropy Alloy/Diamond Composites

by Wei Zhang, Mingyang Zhang, Yingbo Peng, Fangzhou Liu, Yong Liu, Songhao Hu and Yang Hu

Entropy 2019, 21(2), 164; https://doi.org/10.3390/e21020164 - 10 Feb 2019

Cited by 13 | Viewed by 3769

Abstract

In this study, an effective way of applying Ti/Ni deposited coating to the surface of diamond single crystal particles by magnetron sputtering was proposed and novel high-entropy alloy (HEA)/diamond composites were prepared by spark plasma sintering (SPS). The results show that the interfacial [...] Read more.

In this study, an effective way of applying Ti/Ni deposited coating to the surface of diamond single crystal particles by magnetron sputtering was proposed and novel high-entropy alloy (HEA)/diamond composites were prepared by spark plasma sintering (SPS). The results show that the interfacial bonding state of the coated diamond composite is obviously better than that of the uncoated diamond composite. Corresponding mechanical properties such as hardness, density, transverse fracture strength and friction properties of the coated diamond composite were also found to be better than those of the uncoated diamond composite. The effects of interface structure and defects on the mechanical properties of HEA/diamond composites were investigated. The research directions for further improving the structure and properties of high-entropy alloy/diamond composites were proposed. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 3734 KiB

Open AccessArticle

High Strength and Deformation Mechanisms of Al_0.3CoCrFeNi High-Entropy Alloy Thin Films Fabricated by Magnetron Sputtering

by Wei-Bing Liao, Hongti Zhang, Zhi-Yuan Liu, Pei-Feng Li, Jian-Jun Huang, Chun-Yan Yu and Yang Lu

Entropy 2019, 21(2), 146; https://doi.org/10.3390/e21020146 - 04 Feb 2019

Cited by 34 | Viewed by 5426

Abstract

Recently, high-entropy alloy thin films (HEATFs) with nanocrystalline structures and high hardness were developed by magnetron sputtering technique and have exciting potential to make small structure devices and precision instruments with sizes ranging from nanometers to micrometers. However, the strength and deformation mechanisms [...] Read more.

Recently, high-entropy alloy thin films (HEATFs) with nanocrystalline structures and high hardness were developed by magnetron sputtering technique and have exciting potential to make small structure devices and precision instruments with sizes ranging from nanometers to micrometers. However, the strength and deformation mechanisms are still unclear. In this work, nanocrystalline Al_0.3CoCrFeNi HEATFs with a thickness of ~4 μm were prepared. The microstructures of the thin films were comprehensively characterized, and the mechanical properties were systematically studied. It was found that the thin film was smooth, with a roughness of less than 5 nm. The chemical composition of the high entropy alloy thin film was homogeneous with a main single face-centered cubic (FCC) structure. Furthermore, it was observed that the hardness and the yield strength of the high-entropy alloy thin film was about three times that of the bulk samples, and the plastic deformation was inhomogeneous. Our results could provide an in-depth understanding of the mechanics and deformation mechanism for future design of nanocrystalline HEATFs with desired properties. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 4256 KiB

Open AccessArticle

Phases, Microstructures and Mechanical Properties of CoCrNiCuZn High-Entropy Alloy Prepared by Mechanical Alloying and Spark Plasma Sintering

by Yuchen Sun, Boren Ke, Yulin Li, Kai Yang, Mingqi Yang, Wei Ji and Zhengyi Fu

Entropy 2019, 21(2), 122; https://doi.org/10.3390/e21020122 - 29 Jan 2019

Cited by 17 | Viewed by 3437

Abstract

In the study, an equiatomic CoCrNiCuZn high-entropy alloy (HEA) was prepared by mechanical alloying (MA) and the phases, microstructures, and thermal properties of the alloy powder were explored. The results suggest that a solid solution with body-centered cubic (BCC) phase and a crystalline [...] Read more.

In the study, an equiatomic CoCrNiCuZn high-entropy alloy (HEA) was prepared by mechanical alloying (MA) and the phases, microstructures, and thermal properties of the alloy powder were explored. The results suggest that a solid solution with body-centered cubic (BCC) phase and a crystalline size of 10 nm formed after 60 h of milling. Subsequently, the alloy powder was consolidated by spark plasma sintering (SPS) at different temperatures (600 °C, 700 °C, 800 °C, and 900 °C). Two kinds of face-centered cubic (FCC) phases co-existed in the as-sintered samples. Besides, Vickers hardness and compressive strength of the consolidated alloy sintered at 900 °C were respectively 615 HV and 2121 MPa, indicating excellent mechanical properties. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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17 pages, 8924 KiB

Open AccessArticle

Microstructure and Room Temperature Mechanical Properties of Different 3 and 4 Element Medium Entropy Alloys from HfNbTaTiZr System

by Jiří Zýka, Jaroslav Málek, Jaroslav Veselý, František Lukáč, Jakub Čížek, Jan Kuriplach and Oksana Melikhova

Entropy 2019, 21(2), 114; https://doi.org/10.3390/e21020114 - 26 Jan 2019

Cited by 48 | Viewed by 5771

Abstract

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with [...] Read more.

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 4743 KiB

Open AccessArticle

Effects of Silicon Content on the Microstructures and Mechanical Properties of (AlCrTiZrV)-Si_x-N High-Entropy Alloy Films

by Jingrui Niu, Wei Li, Ping Liu, Ke Zhang, Fengcang Ma, Xiaohong Chen, Rui Feng and Peter K. Liaw

Entropy 2019, 21(1), 75; https://doi.org/10.3390/e21010075 - 16 Jan 2019

Cited by 8 | Viewed by 4600

Abstract

A series of (AlCrTiZrV)-Si_x-N films with different silicon contents were deposited on monocrystalline silicon substrates by direct-current (DC) magnetron sputtering. The films were characterized by the X-ray diffractometry (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and nano-indentation techniques. [...] Read more.

A series of (AlCrTiZrV)-Si_x-N films with different silicon contents were deposited on monocrystalline silicon substrates by direct-current (DC) magnetron sputtering. The films were characterized by the X-ray diffractometry (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and nano-indentation techniques. The effects of the silicon content on the microstructures and mechanical properties of the films were investigated. The experimental results show that the (AlCrTiZrV)N films grow in columnar grains and present a (200) preferential growth orientation. The addition of the silicon element leads to the disappearance of the (200) peak, and the grain refinement of the (AlCrTiZrV)-Si_x-N films. Meanwhile, the reticular amorphous phase is formed, thus developing the nanocomposite structure with the nanocrystalline structures encapsulated by the amorphous phase. With the increase of the silicon content, the mechanical properties first increase and then decrease. The maximal hardness and modulus of the film reach 34.3 GPa and 301.5 GPa, respectively, with the silicon content (x) of 8% (volume percent). The strengthening effect of the (AlCrTiZrV)-Si_x-N film can be mainly attributed to the formation of the nanocomposite structure. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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19 pages, 2236 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Configurational Entropy in Multicomponent Alloys: Matrix Formulation from Ab Initio Based Hamiltonian and Application to the FCC Cr-Fe-Mn-Ni System

by Antonio Fernández-Caballero, Mark Fedorov, Jan S. Wróbel, Paul M. Mummery and Duc Nguyen-Manh

Entropy 2019, 21(1), 68; https://doi.org/10.3390/e21010068 - 15 Jan 2019

Cited by 24 | Viewed by 9712

Abstract

Configuration entropy is believed to stabilize disordered solid solution phases in multicomponent systems at elevated temperatures over intermetallic compounds by lowering the Gibbs free energy. Traditionally, the increment of configuration entropy with temperature was computed by time-consuming thermodynamic integration methods. In this work, [...] Read more.

Configuration entropy is believed to stabilize disordered solid solution phases in multicomponent systems at elevated temperatures over intermetallic compounds by lowering the Gibbs free energy. Traditionally, the increment of configuration entropy with temperature was computed by time-consuming thermodynamic integration methods. In this work, a new formalism based on a hybrid combination of the Cluster Expansion (CE) Hamiltonian and Monte Carlo simulations is developed to predict the configuration entropy as a function of temperature from multi-body cluster probability in a multi-component system with arbitrary average composition. The multi-body probabilities are worked out by explicit inversion and direct product of a matrix formulation within orthonomal sets of point functions in the clusters obtained from symmetry independent correlation functions. The matrix quantities are determined from semi canonical Monte Carlo simulations with Effective Cluster Interactions (ECIs) derived from Density Functional Theory (DFT) calculations. The formalism is applied to analyze the 4-body cluster probabilities for the quaternary system Cr-Fe-Mn-Ni as a function of temperature and alloy concentration. It is shown that, for two specific compositions (Cr

_{25}

Fe

_{25}

Mn

_{25}

Ni

_{25}

and Cr

_{18}

Fe

_{27}

Mn

_{27}

Ni

_{28}

), the high value of probabilities for Cr-Fe-Fe-Fe and Mn-Mn-Ni-Ni are strongly correlated with the presence of the ordered phases L1

_{2}

-CrFe

_{3}

and L1

_{0}

-MnNi, respectively. These results are in an excellent agreement with predictions of these ground state structures by ab initio calculations. The general formalism is used to investigate the configuration entropy as a function of temperature and for 285 different alloy compositions. It is found that our matrix formulation of cluster probabilities provides an efficient tool to compute configuration entropy in multi-component alloys in a comparison with the result obtained by the thermodynamic integration method. At high temperatures, it is shown that many-body cluster correlations still play an important role in understanding the configuration entropy before reaching the solid solution limit of high-entroy alloys (HEAs). Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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14 pages, 2492 KiB

Open AccessArticle

Effects of Mo, Nb, Ta, Ti, and Zr on Mechanical Properties of Equiatomic Hf-Mo-Nb-Ta-Ti-Zr Alloys

by Ko-Kai Tseng, Chien-Chang Juan, Shuen Tso, Hsuan-Chu Chen, Che-Wei Tsai and Jien-Wei Yeh

Entropy 2019, 21(1), 15; https://doi.org/10.3390/e21010015 - 25 Dec 2018

Cited by 91 | Viewed by 8620

Abstract

Nowadays refractory high-entropy alloys (RHEAs) are regarded as great candidates for the replacement of superalloys at high temperature. To design a RHEA, one must understand the pros and cons of every refractory element. However, the elemental effect on mechanical properties remains unclear. In [...] Read more.

Nowadays refractory high-entropy alloys (RHEAs) are regarded as great candidates for the replacement of superalloys at high temperature. To design a RHEA, one must understand the pros and cons of every refractory element. However, the elemental effect on mechanical properties remains unclear. In this study, the subtraction method was applied on equiatomic HfMoNbTaTiZr alloys to discover the role of each element, and, thus, HfMoNbTaTiZr, HfNbTaTiZr, HfMoTaTiZr, HfMoNbTiZr, HfMoNbTaZr, and HfMoNbTaTi were fabricated and analyzed. The microstructure and mechanical properties of each alloy at the as-cast state were examined. The solid solution phase formation rule and the solution strengthening effect are also discussed. Finally, the mechanism of how Mo, Nb, Ta, Ti, and Zr affect the HfMoNbTaTiZr alloys was established after comparing the properties of these alloys. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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13 pages, 9065 KiB

Open AccessArticle

Fabrication of AlCoCrFeNi High-Entropy Alloy Coating on an AISI 304 Substrate via a CoFe₂Ni Intermediate Layer

by Wenyuan Cui, Sreekar Karnati, Xinchang Zhang, Elizabeth Burns and Frank Liou

Entropy 2019, 21(1), 2; https://doi.org/10.3390/e21010002 - 20 Dec 2018

Cited by 32 | Viewed by 5860

Abstract

Through laser metal deposition, attempts were made to coat AlCoCrFeNi, a high-entropy alloy (HEA), on an AISI 304 stainless steel substrate to integrate their properties. However, the direct coating of the AlCoCrFeNi HEA on the AISI 304 substrate was found to be unviable [...] Read more.

Through laser metal deposition, attempts were made to coat AlCoCrFeNi, a high-entropy alloy (HEA), on an AISI 304 stainless steel substrate to integrate their properties. However, the direct coating of the AlCoCrFeNi HEA on the AISI 304 substrate was found to be unviable due to cracks at the interface between these two materials. The difference in compositional change was suspected to be the source of the cracks. Therefore, a new transition route was performed by coating an intermediate layer of CoFe₂Ni on the AISI 304 substrate. Investigations into the microstructure, phase composition, elemental composition and Vickers hardness were carried out in this study. Consistent metallurgical bonding was observed along both of the interfaces. It was found that the AlCoCrFeNi alloy solidified into a dendritic microstructure. The X-ray diffraction pattern revealed a transition of the crystal structure of the AISI 304 substrate to the AlCoCrFeNi HEA. An intermediate step in hardness was observed between the AISI 304 substrate and the AlCoCrFeNi HEA. The AlCoCrFeNi alloy fabricated was found to have an average hardness of 418 HV, while the CoFe₂Ni intermediate layer had an average hardness of 275 HV. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 3421 KiB

Open AccessArticle

Effect of Atomic Size Difference on the Microstructure and Mechanical Properties of High-Entropy Alloys

by Chan-Sheng Wu, Ping-Hsiu Tsai, Chia-Ming Kuo and Che-Wei Tsai

Entropy 2018, 20(12), 967; https://doi.org/10.3390/e20120967 - 14 Dec 2018

Cited by 31 | Viewed by 6131

Abstract

The effects of atomic size difference on the microstructure and mechanical properties of single face-centered cubic (FCC) phase high-entropy alloys are studied. Single FCC phase high-entropy alloys, namely, CoCrFeMnNi, Al_0.2CoCrFeMnNi, and Al_0.3CoCrCu_0.3FeNi, display good workability. The recrystallization [...] Read more.

The effects of atomic size difference on the microstructure and mechanical properties of single face-centered cubic (FCC) phase high-entropy alloys are studied. Single FCC phase high-entropy alloys, namely, CoCrFeMnNi, Al_0.2CoCrFeMnNi, and Al_0.3CoCrCu_0.3FeNi, display good workability. The recrystallization and grain growth rates are compared during annealing. Adding Al with 0.2 molar ratio into CoCrFeMnNi retains the single FCC phase. Its atomic size difference increases from 1.18% to 2.77%, and the activation energy of grain growth becomes larger than that of CoCrFeMnNi. The as-homogenized state of Al_0.3CoCrCu_0.3FeNi high-entropy alloy becomes a single FCC structure. Its atomic size difference is 3.65%, and the grain growth activation energy is the largest among these three kinds of single-phase high-entropy alloys. At ambient temperature, the mechanical properties of Al_0.3CoCrCu_0.3FeNi are better than those of CoCrFeMnNi because of high lattice distortion and high solid solution hardening. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 3607 KiB

Open AccessArticle

First-Principles Design of Refractory High Entropy Alloy VMoNbTaW

by Shumin Zheng and Shaoqing Wang

Entropy 2018, 20(12), 965; https://doi.org/10.3390/e20120965 - 13 Dec 2018

Cited by 19 | Viewed by 4441

Abstract

The elastic properties of seventy different compositions were calculated to optimize the composition of a V–Mo–Nb–Ta–W system. A new model called maximum entropy approach (MaxEnt) was adopted. The influence of each element was discussed. Molybdenum (Mo) and tungsten (W) are key elements for [...] Read more.

The elastic properties of seventy different compositions were calculated to optimize the composition of a V–Mo–Nb–Ta–W system. A new model called maximum entropy approach (MaxEnt) was adopted. The influence of each element was discussed. Molybdenum (Mo) and tungsten (W) are key elements for the maintenance of elastic properties. The V–Mo–Nb–Ta–W system has relatively high values of C₄₄, bulk modulus (B), shear modulus (G), and Young’s modulus (E), with high concentrations of Mo + W. Element W is brittle and has high density. Thus, low-density Mo can substitute part of W. Vanadium (V) has low density and plays an important role in decreasing the brittleness of the V–Mo–Nb–Ta–W system. Niobium (Nb) and tantalum (Ta) have relatively small influence on elastic properties. Furthermore, the calculated results can be used as a general guidance for the selection of a V–Mo–Nb–Ta–W system. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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13 pages, 5191 KiB

Open AccessArticle

Intermediate-Temperature Creep Deformation and Microstructural Evolution of an Equiatomic FCC-Structured CoCrFeNiMn High-Entropy Alloy

by Chengming Cao, Jianxin Fu, Tongwei Tong, Yuxiao Hao, Ping Gu, Hai Hao and Liangming Peng

Entropy 2018, 20(12), 960; https://doi.org/10.3390/e20120960 - 12 Dec 2018

Cited by 30 | Viewed by 4030

Abstract

The tensile creep behavior of an equiatomic CoCrFeNiMn high-entropy alloy was systematically investigated over an intermediate temperature range (500–600 °C) and applied stress (140–400 MPa). The alloy exhibited a stress-dependent transition from a low-stress region (LSR-region I) to a high-stress region (HSR-region II). [...] Read more.

The tensile creep behavior of an equiatomic CoCrFeNiMn high-entropy alloy was systematically investigated over an intermediate temperature range (500–600 °C) and applied stress (140–400 MPa). The alloy exhibited a stress-dependent transition from a low-stress region (LSR-region I) to a high-stress region (HSR-region II). The LSR was characterized by a stress exponent of 5 to 6 and an average activation energy of 268 kJ mol⁻¹, whereas the HSR showed much higher corresponding values of 8.9–14 and 380 kJ mol⁻¹. Microstructural examinations on the deformed samples revealed remarkable dynamic recrystallization at higher stress levels. Dislocation jogging and tangling configurations were frequently observed in LSR and HSR at 550 and 600 °C, respectively. Moreover, dynamic precipitates identified as M₂₃C₆ or a Cr-rich σ phase were formed along grain boundaries in HSR. The diffusion-compensated strain rate versus modulus-compensated stress data analysis implied that the creep deformation in both stress regions was dominated by stress-assisted dislocation climb controlled by lattice diffusion. Nevertheless, the abnormally high stress exponents in HSR were ascribed to the coordinative contributions of dynamic recrystallization and dynamic precipitation. Simultaneously, the barriers imposed by these precipitates and severe initial deformation were referred to so as to increase the activation energy for creep deformation. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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21 pages, 5160 KiB

Open AccessArticle

A Novel Low-Activation VCrFeTa_xW_x (x = 0.1, 0.2, 0.3, 0.4, and 1) High-Entropy Alloys with Excellent Heat-Softening Resistance

by Weiran Zhang, Peter K. Liaw and Yong Zhang

Entropy 2018, 20(12), 951; https://doi.org/10.3390/e20120951 - 11 Dec 2018

Cited by 55 | Viewed by 4639

Abstract

The microstructure, Vickers hardness, and compressive properties of novel low-activation VCrFeTa_xW_x (x = 0.1, 0.2, 0.3, 0.4, and 1) high-entropy alloys (HEAs) were studied. The alloys were fabricated by vacuum-arc melting and the characteristics of these alloys were explored. [...] Read more.

The microstructure, Vickers hardness, and compressive properties of novel low-activation VCrFeTa_xW_x (x = 0.1, 0.2, 0.3, 0.4, and 1) high-entropy alloys (HEAs) were studied. The alloys were fabricated by vacuum-arc melting and the characteristics of these alloys were explored. The microstructures of all the alloys exhibited a typical morphology of dendritic and eutectic structures. The VCrFeTa_0.1W_0.1 and VCrFeTa_0.2W_0.2 alloys are essentially single phase, consisting of a disordered body-centered-cubic (BCC) phase, whereas the VCrFeTa_0.2W_0.2 alloy contains fine, nanoscale precipitates distributed in the BCC matrix. The lattice parameters and compositions of the identified phases were investigated. The alloys have Vickers hardness values ranging from 546 HV_0.2 to 1135 HV_0.2 with the x ranging from 0.1 to 1, respectively. The VCrFeTa_0.1W_0.1 and VCrFeTa_0.2W_0.2 alloys exhibit compressive yield strengths of 1341 MPa and 1742 MPa, with compressive plastic strains of 42.2% and 35.7%, respectively. VCrFeTa_0.1W_0.1 and VCrFeTa_0.2W_0.2 alloys have excellent hardness after annealing for 25 h at 600–1000 °C, and presented compressive yield strength exceeding 1000 MPa with excellent heat-softening resistance at 600–800 °C. By applying the HEA criteria, Ta and W additions into the VCrFeTaW are proposed as a family of candidate materials for fusion reactors and high-temperature structural applications. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 2947 KiB

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Effect of Binding and Dispersion Behavior of High-Entropy Alloy (HEA) Powders on the Microstructure and Mechanical Properties in a Novel HEA/Diamond Composite

by Mingyang Zhang, Wei Zhang, Fangzhou Liu, Yingbo Peng, Songhao Hu and Yong Liu

Entropy 2018, 20(12), 924; https://doi.org/10.3390/e20120924 - 04 Dec 2018

Cited by 14 | Viewed by 2994

Abstract

This study reports the results of the addition of diamonds in the sintering process of a FCC-structured CoCrFeNiMo high-entropy alloy. The effect of raw powder states such as elemental mixed (EM) powder, gas atomization (GA) powder and mechanical alloying (MA) powder on the [...] Read more.

This study reports the results of the addition of diamonds in the sintering process of a FCC-structured CoCrFeNiMo high-entropy alloy. The effect of raw powder states such as elemental mixed (EM) powder, gas atomization (GA) powder and mechanical alloying (MA) powder on the uniformity of constituent phase was also investigated. Examination of microstructure and evaluation of mechanical properties of the composites depending on the mixing processes were performed. As a result, GA+MA powder composite showed the highest mechanical properties. The experimental results indicated that the powder manufacturing method was an essential parameter to determine the quality of HEA/diamond composites such as the uniformity of phase and binding behavior. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 3102 KiB

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Effect of Cold Rolling on the Phase Transformation Kinetics of an Al_0.5CoCrFeNi High-Entropy Alloy

by Jun Wang, Haoxue Yang, Tong Guo, Jiaxiang Wang, William Yi Wang and Jinshan Li

Entropy 2018, 20(12), 917; https://doi.org/10.3390/e20120917 - 30 Nov 2018

Cited by 11 | Viewed by 3409

Abstract

The solid state phase transformation kinetics of as-cast and cold rolling deformed Al_0.5CoCrFeNi high-entropy alloys have been investigated by the thermal expansion method. The phase transformed volume fractions are determined from the thermal expansion curve using the lever rule method, and [...] Read more.

The solid state phase transformation kinetics of as-cast and cold rolling deformed Al_0.5CoCrFeNi high-entropy alloys have been investigated by the thermal expansion method. The phase transformed volume fractions are determined from the thermal expansion curve using the lever rule method, and the deformed sample exhibits a much higher transformation rate. Two kinetic parameters, activation energy (E) and kinetic exponent (n) are determined using Kissinger– Akahira–Sunose (KAS) and Johnson–Mehl–Avrami (JMA) method, respectively. Results show that a pre-deformed sample shows a much lower activation energy and higher kinetic exponent compared with the as-cast sample, which are interpreted based on the deformation induced defects that can promote the nucleation and growth process during phase transformation. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 2637 KiB

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Wear and Corrosion Resistance of Al_0.5CoCrCuFeNi High-Entropy Alloy Coating Deposited on AZ91D Magnesium Alloy by Laser Cladding

by Kaijin Huang, Lin Chen, Xin Lin, Haisong Huang, Shihao Tang and Feilong Du

Entropy 2018, 20(12), 915; https://doi.org/10.3390/e20120915 - 30 Nov 2018

Cited by 32 | Viewed by 3791

Abstract

In order to improve the wear and corrosion resistance of an AZ91D magnesium alloy substrate, an Al_0.5CoCrCuFeNi high-entropy alloy coating was successfully prepared on an AZ91D magnesium alloy surface by laser cladding using mixed elemental powders. Optical microscopy (OM), scanning electron [...] Read more.

In order to improve the wear and corrosion resistance of an AZ91D magnesium alloy substrate, an Al_0.5CoCrCuFeNi high-entropy alloy coating was successfully prepared on an AZ91D magnesium alloy surface by laser cladding using mixed elemental powders. Optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction were used to characterize the microstructure of the coating. The wear resistance and corrosion resistance of the coating were evaluated by dry sliding wear and potentiodynamic polarization curve test methods, respectively. The results show that the coating was composed of a simple FCC solid solution phase with a microhardness about 3.7 times higher than that of the AZ91D matrix and even higher than that of the same high-entropy alloy prepared by an arc melting method. The coating had better wear resistance than the AZ91D matrix, and the wear rate was about 2.5 times lower than that of the AZ91D matrix. Moreover, the main wear mechanisms of the coating and the AZ91D matrix were different. The former was abrasive wear and the latter was adhesive wear. The corrosion resistance of the coating was also better than that of the AZ91D matrix because the corrosion potential of the former was more positive and the corrosion current was smaller. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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18 pages, 8508 KiB

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Brute Force Composition Scanning with a CALPHAD Database to Find Low Temperature Body Centered Cubic High Entropy Alloys

by T. P. C. Klaver, D. Simonovic and M. H. F. Sluiter

Entropy 2018, 20(12), 911; https://doi.org/10.3390/e20120911 - 29 Nov 2018

Cited by 15 | Viewed by 3516

Abstract

We used the Thermo-Calc High Entropy Alloy CALPHAD database to determine the stable phases of AlCrMnNbTiV, AlCrMoNbTiV, AlCrFeTiV and AlCrMnMoTi alloys from 800 to 2800 K. The concentrations of elements were varied from 1–49 atom%. A five- or six-dimensional grid is constructed, with [...] Read more.

We used the Thermo-Calc High Entropy Alloy CALPHAD database to determine the stable phases of AlCrMnNbTiV, AlCrMoNbTiV, AlCrFeTiV and AlCrMnMoTi alloys from 800 to 2800 K. The concentrations of elements were varied from 1–49 atom%. A five- or six-dimensional grid is constructed, with stable phases calculated at each grid point. Thermo-Calc was used as a massive parallel tool and three million compositions were calculated, resulting in tens of thousands of compositions for which the alloys formed a single disordered body centered cubic (bcc) phase at 800 K. By filtering out alloy compositions for which a disordered single phase persists down to 800 K, composition ‘islands’ of high entropy alloys are determined in composition space. The sizes and shapes of such islands provide information about which element combinations have good high entropy alloy forming qualities as well as about the role of individual elements within an alloy. In most cases disordered single phases are formed most readily at low temperature when several elements are almost entirely excluded, resulting in essentially ternary alloys. We determined which compositions lie near the centers of the high entropy alloy islands and therefore remain high entropy islands under small composition changes. These island center compositions are predicted to be high entropy alloys with the greatest certainty and make good candidates for experimental verification. The search for high entropy islands can be conducted subject to constraints, e.g., requiring a minimum amount of Al and/or Cr to promote oxidation resistance. Imposing such constraints rapidly diminishes the number of high entropy alloy compositions, in some cases to zero. We find that AlCrMnNbTiV and AlCrMoNbTiV are relatively good high entropy alloy formers, AlCrFeTiV is a poor high entropy alloy former, while AlCrMnMoTi is a poor high entropy alloy former at 800 K but quickly becomes a better high entropy alloy former with increasing temperature. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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7 pages, 1291 KiB

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Elemental Phase Partitioning in the γ-γ″ Ni₂CoFeCrNb_0.15 High Entropy Alloy

by Bin Han, Jie Wei, Feng He, Da Chen, Zhi Jun Wang, Alice Hu, Wenzhong Zhou and Ji Jung Kai

Entropy 2018, 20(12), 910; https://doi.org/10.3390/e20120910 - 28 Nov 2018

Cited by 10 | Viewed by 3449

Abstract

The partitioning of the alloying elements into the γ″ nanoparticles in a Ni₂CoFeCrNb_0.15 high entropy alloy was studied by the combination of atom probe tomography and first-principles calculations. The atom probe tomography results show that the Co, Fe, and Cr [...] Read more.

The partitioning of the alloying elements into the γ″ nanoparticles in a Ni₂CoFeCrNb_0.15 high entropy alloy was studied by the combination of atom probe tomography and first-principles calculations. The atom probe tomography results show that the Co, Fe, and Cr atoms incorporated into the Ni₃Nb-type γ″ nanoparticles but their partitioning behaviors are significantly different. The Co element is much easier to partition into the γ″ nanoparticles than Fe and Cr elements. The first-principles calculations demonstrated that the different partitioning behaviors of Co, Fe and Cr elements into the γ″ nanoparticles resulted from the differences of their specific chemical potentials and bonding states in the γ″ phase. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 4073 KiB

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Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys

by Wenrui Wang, Jieqian Wang, Honggang Yi, Wu Qi and Qing Peng

Entropy 2018, 20(12), 908; https://doi.org/10.3390/e20120908 - 28 Nov 2018

Cited by 40 | Viewed by 3973

Abstract

The present work investigates the influence of micro-alloyed Mo on the corrosion behavior of (CoCrFeNi)_100−xMo_x high-entropy alloys. All of the (CoCrFeNi)_100−xMo_x alloys exhibit a single face-centered cubic (FCC) solid solution. However, the (CoCrFeNi)₉₇Mo [...] Read more.

The present work investigates the influence of micro-alloyed Mo on the corrosion behavior of (CoCrFeNi)_100−xMo_x high-entropy alloys. All of the (CoCrFeNi)_100−xMo_x alloys exhibit a single face-centered cubic (FCC) solid solution. However, the (CoCrFeNi)₉₇Mo₃ alloy exhibits an ordered sigma (σ) phase enriched in Cr and Mo. With the increase of x (the Mo content) from 1 to 3, the hardness of the (CoCrFeNi)_100−xMo_x alloys increases from 124.8 to 133.6 Vickers hardness (HV), and the compressive yield strength increases from 113.6 MPa to 141.1 MPa, without fracture under about a 60% compressive strain. The potentiodynamic polarization curve in a 3.5% NaCl solution indicates that the addition of Mo has a beneficial effect on the corrosion resistance to some certain extent, opposed to the σ phase. Furthermore, the alloys tend to form a passivation film in the 0.5 M H₂SO₄ solution in order to inhibit the progress of the corrosion reaction as the Mo content increases. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 1773 KiB

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Lattice Distortion and Phase Stability of Pd-Doped NiCoFeCr Solid-Solution Alloys

by Fuxiang Zhang, Yang Tong, Ke Jin, Hongbin Bei, William J. Weber and Yanwen Zhang

Entropy 2018, 20(12), 900; https://doi.org/10.3390/e20120900 - 25 Nov 2018

Cited by 26 | Viewed by 4328

Abstract

In the present study, we have revealed that (NiCoFeCr)_100−xPd_x (x= 1, 3, 5, 20 atom%) high-entropy alloys (HEAs) have both local- and long-range lattice distortions by utilizing X-ray total scattering, X-ray diffraction, and extended X-ray absorption fine [...] Read more.

In the present study, we have revealed that (NiCoFeCr)_100−xPd_x (x= 1, 3, 5, 20 atom%) high-entropy alloys (HEAs) have both local- and long-range lattice distortions by utilizing X-ray total scattering, X-ray diffraction, and extended X-ray absorption fine structure methods. The local lattice distortion determined by the lattice constant difference between the local and average structures was found to be proportional to the Pd content. A small amount of Pd-doping (1 atom%) yields long-range lattice distortion, which is demonstrated by a larger (200) lattice plane spacing than the expected value from an average structure, however, the degree of long-range lattice distortion is not sensitive to the Pd concentration. The structural stability of these distorted HEAs under high-pressure was also examined. The experimental results indicate that doping with a small amount of Pd significantly enhances the stability of the fcc phase by increasing the fcc-to-hcp transformation pressure from ~13.0 GPa in NiCoFeCr to 20–26 GPa in the Pd-doped HEAs and NiCoFeCrPd maintains its fcc lattice up to 74 GPa, the maximum pressure that the current experiments have reached. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 2035 KiB

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About the Reliability of CALPHAD Predictions in Multicomponent Systems

by Stéphane Gorsse and Oleg N. Senkov

Entropy 2018, 20(12), 899; https://doi.org/10.3390/e20120899 - 24 Nov 2018

Cited by 59 | Viewed by 4791

Abstract

This study examines one of the limitations of CALPHAD databases when applied to high entropy alloys and complex concentrated alloys. We estimate the level of the thermodynamic description, which is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems, by comparing [...] Read more.

This study examines one of the limitations of CALPHAD databases when applied to high entropy alloys and complex concentrated alloys. We estimate the level of the thermodynamic description, which is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems, by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from complete binary and ternary interaction descriptions. Our analysis has shown that the thermodynamic properties of a quaternary alloy can be correctly predicted by direct extrapolation from the respective fully assessed binary systems (i.e., without ternary descriptions) only when (i) the binary miscibility gaps are not present, (ii) binary intermetallic phases are not present or present in a few quantities (i.e., when the system has low density of phase boundaries), and (iii) ternary intermetallic phases are not present. Because the locations of the phase boundaries and possibility of formation of ternary phases are not known when evaluating novel composition space, a higher credibility database is still preferable, while the calculations using lower credibility databases may be questionable and require additional experimental verification. We estimate the level of the thermodynamic description which would be still sufficient to correctly predict thermodynamic properties of quaternary alloy systems. The main factors affecting the accuracy of the thermodynamic predictions in quaternary alloys are identified by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from ternary system descriptions. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 4513 KiB

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Mechanical Properties and Microstructure of a NiCrFeCoMn High-Entropy Alloy Deformed at High Strain Rates

by Bingfeng Wang, Xianrui Yao, Chu Wang, Xiaoyong Zhang and Xiaoxia Huang

Entropy 2018, 20(11), 892; https://doi.org/10.3390/e20110892 - 21 Nov 2018

Cited by 23 | Viewed by 4466

Abstract

The equiatomic NiCrFeCoMn high-entropy alloy prepared by arc melting has a single crystallographic structure. Mechanical properties and microstructure of the NiCrFeCoMn high-entropy alloy deformed at high strain rates (900 s⁻¹ to 4600 s⁻¹) were investigated. The yield strength of the [...] Read more.

The equiatomic NiCrFeCoMn high-entropy alloy prepared by arc melting has a single crystallographic structure. Mechanical properties and microstructure of the NiCrFeCoMn high-entropy alloy deformed at high strain rates (900 s⁻¹ to 4600 s⁻¹) were investigated. The yield strength of the NiCrFeCoMn high-entropy alloy is sensitive to the change of high strain rates. Serration behaviors were also observed on the flow stress curves of the alloy deformed at the strain rates ranging from 900 s⁻¹ to 4600 s⁻¹. The Zerilli–Armstrong constitutive equation can be used to predict the flow stress curves of the NiCrFeCoMn high-entropy alloy. Large amounts of deformation bands led to obvious serration behaviors of the NiCrFeCoMn high-entropy alloy under dynamic loading. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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6 pages, 3463 KiB

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Small-Scale Plastic Deformation of Nanocrystalline High Entropy Alloy

by Sanghita Mridha, Mageshwari Komarasamy, Sanjit Bhowmick, Rajiv S. Mishra and Sundeep Mukherjee

Entropy 2018, 20(11), 889; https://doi.org/10.3390/e20110889 - 20 Nov 2018

Cited by 8 | Viewed by 3924

Abstract

High entropy alloys (HEAs) have attracted widespread interest due to their unique properties at many different length-scales. Here, we report the fabrication of nanocrystalline (NC) Al_0.1CoCrFeNi high entropy alloy and subsequent small-scale plastic deformation behavior via nano-pillar compression tests. Exceptional strength [...] Read more.

High entropy alloys (HEAs) have attracted widespread interest due to their unique properties at many different length-scales. Here, we report the fabrication of nanocrystalline (NC) Al_0.1CoCrFeNi high entropy alloy and subsequent small-scale plastic deformation behavior via nano-pillar compression tests. Exceptional strength was realized for the NC HEA compared to pure Ni of similar grain sizes. Grain boundary mediated deformation mechanisms led to high strain rate sensitivity of flow stress in the nanocrystalline HEA. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 4308 KiB

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Magnetic Properties and Microstructure of FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) High-Entropy Alloys

by Zhong Li, Chenxu Wang, Linye Yu, Yong Gu, Minxiang Pan, Xiaohua Tan and Hui Xu

Entropy 2018, 20(11), 872; https://doi.org/10.3390/e20110872 - 13 Nov 2018

Cited by 22 | Viewed by 4736

Abstract

The present work exhibits the effects of Sn addition on the magnetic properties and microstructure of FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) high-entropy alloys (HEAs). The results show all the samples consist of a mixed structure of face-centered-cubic (FCC) phase [...] Read more.

The present work exhibits the effects of Sn addition on the magnetic properties and microstructure of FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) high-entropy alloys (HEAs). The results show all the samples consist of a mixed structure of face-centered-cubic (FCC) phase and body-centered-cubic (BCC) phase. The addition of Sn promotes the formation of BCC phase, and it also affects the shape of Cu-rich nano-precipitates in BCC matrix. It also shows that the Curie temperatures (T_c) of the FCC phase and the saturation magnetization (M_s) of the FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) HEAs increase greatly while the remanence (B_r) decreases after the addition of Sn into FeCoNi(CuAl)_0.8 HEA. The thermomagnetic curves indicate that the phases of the FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) HEAs will transform from FCC with low T_c to BCC phase with high T_c at temperature of 600–700 K. This work provides a new idea for FeCoNi(CuAl)_0.8Sn_x (0 ≤ x ≤ 0.10) HEAs for their potential application as soft magnets to be used at high temperatures. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 8827 KiB

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Effect of Annealing on Microstructure and Tensile Behavior of CoCrNi Medium Entropy Alloy Processed by High-Pressure Torsion

by Praveen Sathiyamoorthi, Jae Wung Bae, Peyman Asghari-Rad, Jeong Min Park, Jung Gi Kim and Hyoung Seop Kim

Entropy 2018, 20(11), 849; https://doi.org/10.3390/e20110849 - 06 Nov 2018

Cited by 40 | Viewed by 6851

Abstract

Annealing of severely plastic deformed materials is expected to produce a good combination of strength and ductility, which has been widely demonstrated in conventional materials. In the present study, high-pressure torsion processed CoCrNi medium entropy alloy consisting of a single face-centered cubic (FCC) [...] Read more.

Annealing of severely plastic deformed materials is expected to produce a good combination of strength and ductility, which has been widely demonstrated in conventional materials. In the present study, high-pressure torsion processed CoCrNi medium entropy alloy consisting of a single face-centered cubic (FCC) phase with a grain size of ~50 nm was subjected to different annealing conditions, and its effect on microstructure and mechanical behavior was investigated. The annealing of high-pressure torsion processed CoCrNi alloy exhibits partial recrystallization and near full recrystallization based on the annealing temperature and time. The samples annealed at 700 °C for 2 min exhibit very fine grain size, a high fraction of low angle grain boundaries, and high kernel average misorientation value, indicating partially recrystallized microstructure. The samples annealed for a longer duration (>2 min) exhibit relatively larger grain size, a low fraction of low angle grain boundaries, and low kernel average misorientation value, indicating nearly full recrystallized microstructure. The annealed samples with different microstructures significantly influence the uniform elongation, tensile strength, and work hardening rate. The sample annealed at 700 °C for 15 min exhibits a remarkable combination of tensile strength (~1090 MPa) and strain to failure (~41%). Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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12 pages, 3503 KiB

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Evaluation of Radiation Response in CoCrFeCuNi High-Entropy Alloys

by Yang Wang, Kun Zhang, Yihui Feng, Yansen Li, Weiqi Tang and Bingchen Wei

Entropy 2018, 20(11), 835; https://doi.org/10.3390/e20110835 - 31 Oct 2018

Cited by 20 | Viewed by 4246

Abstract

CoCrFeCuNi high-entropy alloys (HEAs) prepared by arc melting were irradiated with a 100 keV He⁺ ion beam. Volume swelling and hardening induced by irradiation were evaluated. When the dose reached 5.0 × 10¹⁷ ions/cm², the Cu-rich phases exhibited more [...] Read more.

CoCrFeCuNi high-entropy alloys (HEAs) prepared by arc melting were irradiated with a 100 keV He⁺ ion beam. Volume swelling and hardening induced by irradiation were evaluated. When the dose reached 5.0 × 10¹⁷ ions/cm², the Cu-rich phases exhibited more severe volume swelling compared with the matrix phases. This result indicated that the Cu-rich phases were favorable sites for the nucleation and gathering of He bubbles. X-ray diffraction indicated that all diffraction peak intensities decreased regularly. This reduction suggested loosening of the irradiated layer, thereby reducing crystallinity, under He⁺ ion irradiation. The Nix-Gao model was used to fit the measured hardness in order to obtain a hardness value H₀ that excludes the indentation size effect. At ion doses of 2.5 × 10¹⁷ ions/cm² and 5.0 × 10¹⁷ ions/cm², the HEAs showed obvious hardening, which could be attributed to the formation of large amounts of irradiation defects. At the ion dose of 1.0 × 10¹⁸ ions/cm², hardening was reduced, owing to the exfoliation of the original irradiation layer, combined with recovery induced by long-term thermal spike. This study is important to explore the potential uses of HEAs under extreme irradiation conditions. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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12 pages, 5492 KiB

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Effect of Annealing on Microstructure and Mechanical Properties of Al_0.5CoCrFeMo_xNi High-Entropy Alloys

by Yan-Xin Zhuang, Xiu-Lan Zhang and Xian-Yu Gu

Entropy 2018, 20(11), 812; https://doi.org/10.3390/e20110812 - 23 Oct 2018

Cited by 13 | Viewed by 3702

Abstract

The effect of annealing temperature on the microstructure, phase constituents and mechanical properties of Al_0.5CoCrFeMo_xNi high-entropy complex alloys has been investigated at a fixed annealing time (10 h). The 600 °C-annealing has no obvious effect on their microstructures, while [...] Read more.

The effect of annealing temperature on the microstructure, phase constituents and mechanical properties of Al_0.5CoCrFeMo_xNi high-entropy complex alloys has been investigated at a fixed annealing time (10 h). The 600 °C-annealing has no obvious effect on their microstructures, while the annealing at 800–1200 °C enhances the precipitation of (Al,Ni)-rich ordered BCC phase or/and (Cr,Mo)-rich σ phase, and thereby greatly affects the microstructure and mechanical properties of the alloys. All the annealed Al_0.5CoCrFeNi alloys are composed of FCC and (Al,Ni)-rich ordered BCC phases; the phase constituent of the Al_0.5CoCrFeMo_0.1Ni alloy changes from FCC + BCC (600 °C) to FCC + BCC + σ (800 °C) and then to FCC + BCC (1100 °C); the phase constituents of the Al_0.5CoCrFeMo_0.2Ni and Al_0.5CoCrFeMo_0.3Ni alloys change from FCC + BCC + σ to FCC + BCC with the annealing temperature rising from 600 to 1200 °C; while all the annealed Al_0.5CoCrFeMo_0.4Ni and Al_0.5CoCrFeMo_0.5Ni alloys consist of FCC, BCC and σ phases. The phase constituents of most of the alloys investigated are in good agreement with the calculated results from Thermo-Calc program. The alloys annealed at 800 °C under current investigation conditionshave relative fine precipitations and microstructure, and thereby higher hardness and yield stress. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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12 pages, 6383 KiB

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Effect of Zr Addition on the Microstructure and Mechanical Properties of CoCrFeNiMn High-Entropy Alloy Synthesized by Spark Plasma Sintering

by Hongling Zhang, Lei Zhang, Xinyu Liu, Qiang Chen and Yi Xu

Entropy 2018, 20(11), 810; https://doi.org/10.3390/e20110810 - 23 Oct 2018

Cited by 21 | Viewed by 3620

Abstract

As a classic high-entropy alloy system, CoCrFeNiMn is widely investigated. In the present work, we used ZrH₂ powders and atomized CoCrFeNiMn powders as raw materials to prepare CoCrFeNiMnZr_x (x = 0, 0.2, 0.5, 0.8, 1.0) alloys by mechanical alloying (MA), [...] Read more.

As a classic high-entropy alloy system, CoCrFeNiMn is widely investigated. In the present work, we used ZrH₂ powders and atomized CoCrFeNiMn powders as raw materials to prepare CoCrFeNiMnZr_x (x = 0, 0.2, 0.5, 0.8, 1.0) alloys by mechanical alloying (MA), followed by spark plasma sintering (SPS). During the MA process, a small amount of Zr (x ≤ 0.5) can be completely dissolved into CoCrFeNiMn matrix, when the Zr content is above 0.5, the ZrH₂ is excessive. After SPS, CoCrFeNiMn alloy is still as single face-centered cubic (FCC) solid solution, and CoCrFeNiMnZr_x (x ≥ 0.2) alloys have two distinct microstructural domains, one is a single FCC phase without Zr, the other is a Zr-rich microstructure composed of FCC phase, B2 phase, Zr₂Ni₇, and σ phase. The multi-phase microstructures can be attributed to the large lattice strain and negative enthalpy of mixing, caused by the addition of Zr. It is worth noting that two types of nanoprecipitates (body-centered cubic (BCC) phase and Zr₂Ni₇) are precipitated in the Zr-rich region. These can significantly increase the yield strength of the alloys. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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8 pages, 1289 KiB

Open AccessArticle

Unconventional Deformation Behaviours of Nanoscaled High-Entropy Alloys

by Yeqiang Bu, Shenyou Peng, Shiwei Wu, Yujie Wei, Gang Wang, Jiabin Liu and Hongtao Wang

Entropy 2018, 20(10), 778; https://doi.org/10.3390/e20100778 - 11 Oct 2018

Cited by 9 | Viewed by 3576

Abstract

The bulk high-entropy alloys (HEAs) exhibit similar deformation behaviours as traditional metals. These bulk behaviours are likely an averaging of the behaviours exhibited at the nanoscale. Herein, in situ atomic-scale observation of deformation behaviours in nanoscaled CoCrCuFeNi face-centred cubic (FCC) HEA was performed. [...] Read more.

The bulk high-entropy alloys (HEAs) exhibit similar deformation behaviours as traditional metals. These bulk behaviours are likely an averaging of the behaviours exhibited at the nanoscale. Herein, in situ atomic-scale observation of deformation behaviours in nanoscaled CoCrCuFeNi face-centred cubic (FCC) HEA was performed. The deformation behaviours of this nanoscaled FCC HEA (i.e., nanodisturbances and phase transformations) were distinct from those of nanoscaled traditional FCC metals and corresponding bulk HEA. First-principles calculations revealed an obvious fluctuation of the stacking fault energy and stability difference at the atomic scale in the HEA. The stability difference was highlighted only in the nanoscaled HEA and induced unconventional deformation behaviours. Our work suggests that the nanoscaled HEA may provide more chances to discover the long-expected essential distinction between the HEAs and traditional metals. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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10 pages, 399 KiB

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Impact of Chemical Fluctuations on Stacking Fault Energies of CrCoNi and CrMnFeCoNi High Entropy Alloys from First Principles

by Yuji Ikeda, Fritz Körmann, Isao Tanaka and Jörg Neugebauer

Entropy 2018, 20(9), 655; https://doi.org/10.3390/e20090655 - 30 Aug 2018

Cited by 66 | Viewed by 7527

Abstract

Medium and high entropy alloys (MEAs and HEAs) based on 3d transition metals, such as face-centered cubic (fcc) CrCoNi and CrMnFeCoNi alloys, reveal remarkable mechanical properties. The stacking fault energy (SFE) is one of the key ingredients that controls the underlying deformation mechanism [...] Read more.

Medium and high entropy alloys (MEAs and HEAs) based on 3d transition metals, such as face-centered cubic (fcc) CrCoNi and CrMnFeCoNi alloys, reveal remarkable mechanical properties. The stacking fault energy (SFE) is one of the key ingredients that controls the underlying deformation mechanism and hence the mechanical performance of materials. Previous experiments and simulations have therefore been devoted to determining the SFEs of various MEAs and HEAs. The impact of local chemical environment in the vicinity of the stacking faults is, however, still not fully understood. In this work, we investigate the impact of the compositional fluctuations in the vicinity of stacking faults for two prototype fcc MEAs and HEAs, namely CrCoNi and CrMnFeCoNi by employing first-principles calculations. Depending on the chemical composition close to the stacking fault, the intrinsic SFEs vary in the range of more than 150 mJ/m

^{2}

for both the alloys, which indicates the presence of a strong driving force to promote particular types of chemical segregations towards the intrinsic stacking faults in MEAs and HEAs. Furthermore, the dependence of the intrinsic SFEs on local chemical fluctuations reveals a highly non-linear behavior, resulting in a non-trivial interplay of local chemical fluctuations and SFEs. This sheds new light on the importance of controlling chemical fluctuations via tuning, e.g., the annealing condition to obtain the desired mechanical properties for MEAs and HEAs. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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12 pages, 3192 KiB

Open AccessArticle

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range

by Sebastian Haas, Mike Mosbacher, Oleg N. Senkov, Michael Feuerbacher, Jens Freudenberger, Senol Gezgin, Rainer Völkl and Uwe Glatzel

Entropy 2018, 20(9), 654; https://doi.org/10.3390/e20090654 - 30 Aug 2018

Cited by 49 | Viewed by 9111

Abstract

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to [...] Read more.

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, c_p-measurements are carried out from −170 °C to the materials’ solidus temperatures T_S. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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14 pages, 5529 KiB

Open AccessArticle

The Effects of Mo and Nb on the Microstructures and Properties of CrFeCoNi(Nb,Mo) Alloys

by Chun-Huei Tsau and Meng-Chi Tsai

Entropy 2018, 20(9), 648; https://doi.org/10.3390/e20090648 - 29 Aug 2018

Cited by 14 | Viewed by 4233

Abstract

The effects of niobium and molybdenum additions on the microstructures, hardness and corrosion behaviors of CrFeCoNi(Nb,Mo) alloys were investigated. All of the CrFeCoNi(Nb,Mo) alloys displayed dendritic microstructures. The dendrites of CrFeCoNiNb and CrFeCoNiNb_0.5Mo_0.5 alloys were a hexagonal close packing (HCP) [...] Read more.

The effects of niobium and molybdenum additions on the microstructures, hardness and corrosion behaviors of CrFeCoNi(Nb,Mo) alloys were investigated. All of the CrFeCoNi(Nb,Mo) alloys displayed dendritic microstructures. The dendrites of CrFeCoNiNb and CrFeCoNiNb_0.5Mo_0.5 alloys were a hexagonal close packing (HCP) phase and the interdendrites were a eutectic structure of HCP and face-centered cubic (FCC) phases. Additionally, the dendrites of CrFeCoNiMo alloys were a simple cubic (SC) phase and the interdendrites were a eutectic structure of SC and FCC phases. The volume fraction of dendrites and interdendrites in these alloys were calculated. The influences of the volume fraction of dendrite in the alloys on the overall hardness were also discussed. The CrFeCoNiNb alloy had the larger volume fraction of dendrite and thus had the highest hardness among these alloys. The CrFeCoNi(Nb,Mo) alloys also showed better corrosion resistances in 1 M H₂SO₄ and 1 M NaCl solutions by comparing with commercial 304 stainless steel. The CrFeCoNiNb_0.5Mo_0.5 alloy possessed the best corrosion resistances in these solutions among the CrFeCoNi(Nb,Mo) alloys. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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20 pages, 7702 KiB

Open AccessArticle

Tensile Behavior and Evolution of the Phases in the Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ Compositionally Complex/High Entropy Alloy

by Anna Maria Manzoni, Sebastian Haas, Haneen Daoud, Uwe Glatzel, Christiane Förster and Nelia Wanderka

Entropy 2018, 20(9), 646; https://doi.org/10.3390/e20090646 - 29 Aug 2018

Cited by 22 | Viewed by 5442

Abstract

Compositionally complex alloys, or high entropy alloys, are good candidates for applications at higher temperatures in gas turbines. After their introduction, the equiatomic Al₁₇Co₁₇Cr₁₇Cu₁₇Fe₁₇Ni₁₇ (at.%) served as a starting material and a [...] Read more.

Compositionally complex alloys, or high entropy alloys, are good candidates for applications at higher temperatures in gas turbines. After their introduction, the equiatomic Al₁₇Co₁₇Cr₁₇Cu₁₇Fe₁₇Ni₁₇ (at.%) served as a starting material and a long optimization road finally led to the recently optimized Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ (at.%) alloy, which shows promising mechanical properties. Investigations of the as-cast state and after different heat treatments focus on the evolution of the microstructure and provide an overview of some mechanical properties. The dendritic solidification provides two phases in the dendritic cores and two different ones in the interdendritic regions. Three of the four phases remain after heat treatments. Homogenization and subsequent annealing produce a γ-γ’ based microstructure, similar to Ni-based superalloys. The γ phase is Co-Cr-Fe rich and the γ’ phase is Al-Ni-Ti rich. The understanding of the mechanical behavior of the investigated alloy is supported and enhanced by the study of the different phases and their nanohardness measurements. The observations are compared with mechanical and microstructural data from commercial Ni-based superalloys, Co-based alloys, and Co-Ni-based alloys at the desired application temperature of ~800 °C. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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12 pages, 6434 KiB

Open AccessArticle

Effects of Nitrogen Content on the Structure and Mechanical Properties of (Al_0.5CrFeNiTi_0.25)N_x High-Entropy Films by Reactive Sputtering

by Yong Zhang, Xue-Hui Yan, Wei-Bing Liao and Kun Zhao

Entropy 2018, 20(9), 624; https://doi.org/10.3390/e20090624 - 21 Aug 2018

Cited by 61 | Viewed by 5519

Abstract

In this study, (Al_0.5CrFeNiTi_0.25)N_x high-entropy films are prepared by a reactive direct current (DC) magnetron sputtering at different N₂ flow rates on silicon wafers. It is found that the structure of (Al_0.5CrFeNiTi_0.25)N_x [...] Read more.

In this study, (Al_0.5CrFeNiTi_0.25)N_x high-entropy films are prepared by a reactive direct current (DC) magnetron sputtering at different N₂ flow rates on silicon wafers. It is found that the structure of (Al_0.5CrFeNiTi_0.25)N_x high-entropy films is amorphous, with x = 0. It transforms from amorphous to a face-centered-cubic (FCC) structure with the increase of nitrogen content, while the bulk Al_0.5CrFeNiTi_0.25 counterpart prepared by casting features a body-centered-cubic (BCC) phase structure. The phase formation can be explained by the atomic size difference (δ). Lacking nitrogen, δ is approximately 6.4% for the five metal elements, which is relatively large and might form a BCC or ordered-BCC structure, while the metallic elements in this alloy system all have a trend to form nitrides like TiN, CrN, AlN, and FeN. Therefore, nitride components are becoming very similar in size and structure and solve each other easily, thus, an FCC (Al-Cr-Fe-Ni-Ti)N solid solution forms. The calculated value of δ is approximately 23% for this multicomponent nitride solid solution. The (Al_0.5CrFeNiTi_0.25)N_x films achieve a pronounced hardness and a Young’s modulus of 21.45 GPa and 253.8 GPa, respectively, which is obviously much higher than that of the as-cast Al_0.5CrFeNiTi_0.25 bulk alloys. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 2586 KiB

Open AccessArticle

Microstructure and Mechanical Properties of Particulate Reinforced NbMoCrTiAl High Entropy Based Composite

by Tianchen Li, Bin Liu, Yong Liu, Wenmin Guo, Ao Fu, Liangsheng Li, Nie Yan and Qihong Fang

Entropy 2018, 20(7), 517; https://doi.org/10.3390/e20070517 - 10 Jul 2018

Cited by 18 | Viewed by 4459

Abstract

A novel metal matrix composite based on the NbMoCrTiAl high entropy alloy (HEA) was designed by the in-situ formation method. The microstructure, phase evolution, and compression mechanical properties at room temperature of the composite are investigated in detail. The results confirmed that the [...] Read more.

A novel metal matrix composite based on the NbMoCrTiAl high entropy alloy (HEA) was designed by the in-situ formation method. The microstructure, phase evolution, and compression mechanical properties at room temperature of the composite are investigated in detail. The results confirmed that the composite was primarily composed of body-centered cubic solid solution with a small amount of titanium carbides and alumina. With the presence of approximately 7.0 vol. % Al₂O₃ and 32.2 vol. % TiC reinforced particles, the compressive fracture strength of the composite (1542 MPa) was increased by approximately 50% compared with that of the as-cast NbMoCrTiAl HEA. In consideration of the superior oxidation resistance, the P/M NbMoCrTiAl high entropy alloy composite could be considered as a promising high temperature structural material. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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11 pages, 2915 KiB

Open AccessArticle

Influence of Titanium on Microstructure, Phase Formation and Wear Behaviour of AlCoCrFeNiTi_x High-Entropy Alloy

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

Entropy 2018, 20(7), 505; https://doi.org/10.3390/e20070505 - 02 Jul 2018

Cited by 69 | Viewed by 4899

Abstract

The novel alloying concept of high-entropy alloys (HEAs) has been the focus of many recent investigations revealing an interesting combination of properties. Alloying with aluminium and titanium showed strong influence on microstructure and phase composition. However, detailed investigations on the influence of titanium [...] Read more.

The novel alloying concept of high-entropy alloys (HEAs) has been the focus of many recent investigations revealing an interesting combination of properties. Alloying with aluminium and titanium showed strong influence on microstructure and phase composition. However, detailed investigations on the influence of titanium are lacking. In this study, the influence of titanium in the alloy system AlCoCrFeNiTi_x was studied in a wide range (molar ratios x = 0.0; 0.2; 0.5; 0.8; 1.0; 1.5). Detailed studies investigating the microstructure, chemical composition, phase composition, solidification behaviour, and wear behaviour were carried out. Alloying with titanium showed strong influence on the resulting microstructure and lead to an increase of microstructural heterogeneity. Phase analyses revealed the formation of one body-centred cubic (bcc) phase for the alloy without titanium, whereas alloying with titanium caused the formation of two different bcc phases as main phases. Additional phases were detected for alloys with increased titanium content. For x ≥ 0.5, a minor phase with face-centred cubic (fcc) structure was formed. Further addition of titanium led to the formation of complex phases. Investigation of wear behaviour revealed a superior wear resistance of the alloy AlCoCrFeNiTi_0.5 as compared to a bearing steel sample. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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16 pages, 6625 KiB

Open AccessArticle

The Effect of Scandium Ternary Intergrain Precipitates in Al-Containing High-Entropy Alloys

by Sephira Riva, Shahin Mehraban, Nicholas P. Lavery, Stefan Schwarzmüller, Oliver Oeckler, Stephen G. R. Brown and Kirill V. Yusenko

Entropy 2018, 20(7), 488; https://doi.org/10.3390/e20070488 - 22 Jun 2018

Cited by 8 | Viewed by 4849

Abstract

We investigate the effect of alloying with scandium on microstructure, high-temperature phase stability, electron transport, and mechanical properties of the Al₂CoCrFeNi, Al_0.5CoCrCuFeNi, and AlCoCrCu_0.5FeNi high-entropy alloys. Out of the three model alloys, Al₂CoCrFeNi adopts a [...] Read more.

We investigate the effect of alloying with scandium on microstructure, high-temperature phase stability, electron transport, and mechanical properties of the Al₂CoCrFeNi, Al_0.5CoCrCuFeNi, and AlCoCrCu_0.5FeNi high-entropy alloys. Out of the three model alloys, Al₂CoCrFeNi adopts a disordered CsCl structure type. Both of the six-component alloys contain a mixture of body-centered cubic (bcc) and face centered cubic (fcc) phases. The comparison between in situ high-temperature powder diffraction data and ex situ data from heat-treated samples highlights the presence of a reversible bcc to fcc transition. The precipitation of a MgZn₂-type intermetallic phase along grain boundaries following scandium addition affects all systems differently, but especially enhances the properties of Al₂CoCrFeNi. It causes grain refinement; hardness and electrical conductivity increases (up to 20% and 14% respectively) and affects the CsCl-type → fcc equilibrium by moving the transformation to sensibly higher temperatures. The maximum dimensionless thermoelectric figure of merit (ZT) of 0.014 is reached for Al₂CoCrFeNi alloyed with 0.3 wt.% Sc at 650 °C. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 2285 KiB

Open AccessArticle

Effect of Co and Gd Additions on Microstructures and Properties of FeSiBAlNi High Entropy Alloys

by Sicheng Zhai, Wen Wang, Juan Xu, Shuai Xu, Zitang Zhang and Yan Wang

Entropy 2018, 20(7), 487; https://doi.org/10.3390/e20070487 - 22 Jun 2018

Cited by 5 | Viewed by 3379

Abstract

FeSiBAlNi (W5), FeSiBAlNiCo (W6-Co), and FeSiBAlNiGd (W6-Gd) high entropy alloys (HEAs) were prepared using a copper-mold casting method. Effects of Co and Gd additions combined with subsequent annealing on microstructures and magnetism were investigated. The as-cast W5 consists of BCC solid solution and [...] Read more.

FeSiBAlNi (W5), FeSiBAlNiCo (W6-Co), and FeSiBAlNiGd (W6-Gd) high entropy alloys (HEAs) were prepared using a copper-mold casting method. Effects of Co and Gd additions combined with subsequent annealing on microstructures and magnetism were investigated. The as-cast W5 consists of BCC solid solution and FeSi-rich phase. The Gd addition induces the formation of body-centered cubic (BCC) and face-centered cubic (FCC) solid solutions for W6-Gd HEAs. Whereas, the as-cast W6-Co is composed of the FeSi-rich phase. During annealing, no new phases arise in the W6-Co HEA, indicating a good phase stability. The as-cast W5 has the highest hardness (1210 HV), which is mainly attributed to the strengthening effect of FeSi-rich phase evenly distributed in the solid solution matrix. The tested FeSiBAlNi-based HEAs possess soft magnetism. The saturated magnetization and remanence ratio of W6-Gd are distinctly enhanced from 10.93 emu/g to 62.78 emu/g and from 1.44% to 15.50% after the annealing treatment, respectively. The good magnetism of the as-annealed W6-Gd can be ascribed to the formation of Gd-oxides. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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9 pages, 2114 KiB

Open AccessArticle

Al-Ti-Containing Lightweight High-Entropy Alloys for Intermediate Temperature Applications

by Minju Kang, Ka Ram Lim, Jong Woo Won, Kwang Seok Lee and Young Sang Na

Entropy 2018, 20(5), 355; https://doi.org/10.3390/e20050355 - 09 May 2018

Cited by 25 | Viewed by 6224

Abstract

In this study, new high-entropy alloys (HEAs), which contain lightweight elements, namely Al and Ti, have been designed for intermediate temperature applications. Cr, Mo, and V were selected as the elements for the Al-Ti-containing HEAs by elemental screening using their binary phase diagrams. [...] Read more.

In this study, new high-entropy alloys (HEAs), which contain lightweight elements, namely Al and Ti, have been designed for intermediate temperature applications. Cr, Mo, and V were selected as the elements for the Al-Ti-containing HEAs by elemental screening using their binary phase diagrams. AlCrMoTi and AlCrMoTiV HEAs are confirmed as solid solutions with minor ordered B2 phases and have superb specific hardness when compared to that of commercial alloys. The present work demonstrates the desirable possibility for substitution of traditional materials that are applied at intermediate temperature to Al-Ti-containing lightweight HEAs. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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Review

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17 pages, 7602 KiB

Open AccessReview

Welding of High Entropy Alloys—A Review

by Jing Guo, Cong Tang, Glynn Rothwell, Lisa Li, Yun-Che Wang, Qingxiang Yang and Xuejun Ren

Entropy 2019, 21(4), 431; https://doi.org/10.3390/e21040431 - 24 Apr 2019

Cited by 41 | Viewed by 7486

Abstract

High-entropy alloy (HEA) offers great flexibility in materials design with 3–5 principal elements and a range of unique advantages such as good microstructure stability, mechanical strength over a broad range of temperatures and corrosion resistance, etc. Welding of high entropy alloy, as a [...] Read more.

High-entropy alloy (HEA) offers great flexibility in materials design with 3–5 principal elements and a range of unique advantages such as good microstructure stability, mechanical strength over a broad range of temperatures and corrosion resistance, etc. Welding of high entropy alloy, as a key joining method, is an important emerging area with significant potential impact to future application-oriented research and technological developments in HEAs. The selection of feasible welding processes with optimized parameters is essential to enhance the applications of HEAs. However, the structure of the welded joints varies with material systems, welding methods and parameters. A systemic understanding of the structures and properties of the weldment is directly relevant to the application of HEAs as well as managing the effect of welding on situations such as corrosion that are known to be a service life limiting factor of welded structures in conditions such as marine environments. In this paper, key recent work on welding of HEAs is reviewed in detail focusing on the research of main HEA systems when applying different welding techniques. The experimental details including sample preparation, sample size (thickness) and welding conditions reflecting energy input are summarized and key issues are highlighted. The microstructures and properties of different welding zones, in particular the fusion zone (FZ) and the heat affected zones (HAZ), formed with different welding methods are compared and presented in details and the structure-property relationships are discussed. The work shows that the weldability of HEAs varies with the HEA composition groups and the welding method employed. Arc and laser welding of AlCoCrFeNi HEAs results in lower hardness in the FZ and HAZ and reduced overall strength. Friction stir welding results in higher hardness in the FZ and achieves comparable/higher strength of the welded joints in tensile tests. The welded HEAs are capable of maintaining a reasonable proportion of the ductility. The key structure changes including element distribution, the volume fraction of face centered cubic (FCC) and body centered cubic (BCC) phase as well as reported changes in the lattice constants are summarized and analyzed. Detailed mechanisms governing the mechanical properties including the grain size-property/hardness relationship in the form of Hall–Petch (H–P) effect for both bulk and welded structure of HEAs are compared. Finally, future challenges and main areas to research are highlighted. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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16 pages, 2783 KiB

Open AccessReview

High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review

by Fei Zhang, Hongbo Lou, Benyuan Cheng, Zhidan Zeng and Qiaoshi Zeng

Entropy 2019, 21(3), 239; https://doi.org/10.3390/e21030239 - 02 Mar 2019

Cited by 24 | Viewed by 6281

Abstract

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show [...] Read more.

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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20 pages, 4526 KiB

Open AccessReview

Deformation Behavior of Bulk Metallic Glasses and High Entropy Alloys under Complex Stress Fields: A Review

by Shunhua Chen, Jingyuan Wang, Lei Xia and Yucheng Wu

Entropy 2019, 21(1), 54; https://doi.org/10.3390/e21010054 - 11 Jan 2019

Cited by 14 | Viewed by 4429

Abstract

The plastic deformation of bulk metallic glasses (BMGs) depends significantly on applied stress states, and more importantly, in practical applications of BMGs as structural materials, they always deform under complex stress fields. The understanding of deformation behavior of BMGs under complex stress fields [...] Read more.

The plastic deformation of bulk metallic glasses (BMGs) depends significantly on applied stress states, and more importantly, in practical applications of BMGs as structural materials, they always deform under complex stress fields. The understanding of deformation behavior of BMGs under complex stress fields is important not only for uncovering the plastic deformation mechanisms of BMGs, but also for developing BMG components with excellent mechanical performance. In this article, we briefly summarize the recent research progress on the deformation behavior of BMGs under complex stress fields, including the formation and propagation of shear bands, tunable macroscopic plasticity, and serrated plastic flows. The effect of complex stress fields on the plastic deformation mechanisms of BMGs is discussed from simple stress gradient to tailored complex stress fields. The deformation behavior of high entropy alloys (HEAs) under complex stress states has also been discussed. Challenges, potential implications and some unresolved issues are proposed. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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18 pages, 9190 KiB

Open AccessReview

Additive Manufacturing of High-Entropy Alloys: A Review

by Shuying Chen, Yang Tong and Peter K. Liaw

Entropy 2018, 20(12), 937; https://doi.org/10.3390/e20120937 - 06 Dec 2018

Cited by 145 | Viewed by 12876

Abstract

Owing to the reduced defects, low cost, and high efficiency, the additive manufacturing (AM) technique has attracted increasingly attention and has been applied in high-entropy alloys (HEAs) in recent years. It was found that AM-processed HEAs possess an optimized microstructure and improved mechanical [...] Read more.

Owing to the reduced defects, low cost, and high efficiency, the additive manufacturing (AM) technique has attracted increasingly attention and has been applied in high-entropy alloys (HEAs) in recent years. It was found that AM-processed HEAs possess an optimized microstructure and improved mechanical properties. However, no report has been proposed to review the application of the AM method in preparing bulk HEAs. Hence, it is necessary to introduce AM-processed HEAs in terms of applications, microstructures, mechanical properties, and challenges to provide readers with fundamental understanding. Specifically, we reviewed (1) the application of AM methods in the fabrication of HEAs and (2) the post-heat treatment effect on the microstructural evolution and mechanical properties. Compared with the casting counterparts, AM-HEAs were found to have a superior yield strength and ductility as a consequence of the fine microstructure formed during the rapid solidification in the fabrication process. The post-treatment, such as high isostatic pressing (HIP), can further enhance their properties by removing the existing fabrication defects and residual stress in the AM-HEAs. Furthermore, the mechanical properties can be tuned by either reducing the pre-heating temperature to hinder the phase partitioning or modifying the composition of the HEA to stabilize the solid-solution phase or ductile intermetallic phase in AM materials. Moreover, the processing parameters, fabrication orientation, and scanning method can be optimized to further improve the mechanical performance of the as-built-HEAs. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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19 pages, 4884 KiB

Open AccessReview

Liquid Phase Separation in High-Entropy Alloys—A Review

by Nicholas Derimow and Reza Abbaschian

Entropy 2018, 20(11), 890; https://doi.org/10.3390/e20110890 - 20 Nov 2018

Cited by 35 | Viewed by 7726

Abstract

It has been 14 years since the discovery of the high-entropy alloys (HEAs), an idea of alloying which has reinvigorated materials scientists to explore unconventional alloy compositions and multicomponent alloy systems. Many authors have referred to these alloys as multi-principal element alloys (MPEAs) [...] Read more.

It has been 14 years since the discovery of the high-entropy alloys (HEAs), an idea of alloying which has reinvigorated materials scientists to explore unconventional alloy compositions and multicomponent alloy systems. Many authors have referred to these alloys as multi-principal element alloys (MPEAs) or complex concentrated alloys (CCAs) in order to place less restrictions on what constitutes an HEA. Regardless of classification, the research is rooted in the exploration of structure-properties and processing relations in these multicomponent alloys with the aim to surpass the physical properties of conventional materials. More recent studies show that some of these alloys undergo liquid phase separation, a phenomenon largely dictated by low entropy of mixing and positive mixing enthalpy. Studies posit that positive mixing enthalpy of the binary and ternary components contribute substantially to the formation of liquid miscibility gaps. The objective of this review is to bring forth and summarize the findings of the experiments which detail liquid phase separation (LPS) in HEAs, MPEAs, and CCAs and to draw parallels between HEAs and the conventional alloy systems which undergo liquid-liquid separation. Positive mixing enthalpy if not compensated by the entropy of mixing will lead to liquid phase separation. It appears that Co, Ni, and Ti promote miscibility in HEAs/CCAs/MPEAs while Cr, V, and Nb will raise the miscibility gap temperature and increase LPS. Moreover, addition of appropriate amounts of Ni to CoCrCu eliminates immiscibility, such as in cases of dendritically solidifying CoCrCuNi, CoCrCuFeNi, and CoCrCuMnNi. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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23 pages, 4448 KiB

Open AccessReview

Coherent Precipitation and Strengthening in Compositionally Complex Alloys: A Review

by Qing Wang, Zhen Li, Shujie Pang, Xiaona Li, Chuang Dong and Peter K. Liaw

Entropy 2018, 20(11), 878; https://doi.org/10.3390/e20110878 - 15 Nov 2018

Cited by 105 | Viewed by 8463

Abstract

High-performance conventional engineering materials (including Al alloys, Mg alloys, Cu alloys, stainless steels, Ni superalloys, etc.) and newly-developed high entropy alloys are all compositionally-complex alloys (CCAs). In these CCA systems, the second-phase particles are generally precipitated in their solid-solution matrix, in which the [...] Read more.

High-performance conventional engineering materials (including Al alloys, Mg alloys, Cu alloys, stainless steels, Ni superalloys, etc.) and newly-developed high entropy alloys are all compositionally-complex alloys (CCAs). In these CCA systems, the second-phase particles are generally precipitated in their solid-solution matrix, in which the precipitates are diverse and can result in different strengthening effects. The present work aims at generalizing the precipitation behavior and precipitation strengthening in CCAs comprehensively. First of all, the morphology evolution of second-phase particles and precipitation strengthening mechanisms are introduced. Then, the precipitation behaviors in diverse CCA systems are illustrated, especially the coherent precipitation. The relationship between the particle morphology and strengthening effectiveness is discussed. It is addressed that the challenge in the future is to design the stable coherent microstructure in different solid-solution matrices, which will be the most effective approach for the enhancement of alloy strength. Full article

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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