High-entropy Alloy Composites Research Articles

Design of corrosion-resistant high-entropy alloys through valence electron concentration and new PHACOMP

Various methods can be used to predict the phase composition of conventional alloys. However, it is relatively difficult to predict high-entropy alloys (HEAs) owing to their complexity. There remain many challenges in designing HEAs with desirable properties. In this study, two strategies were applied to design corrosion-resistant non-equal-molar CoCrFeMoNi HEAs with a single phase. The two strategies were prediction for valence electron concentration dependent structural behavior to evaluate the matrix of HEAs, and average d-orbital energy level (M̅d) that was initially used to predict precipitation in superalloys under different temperatures. The validity of these two approaches was determined by thermal stability. The corrosion behaviors of the designed HEAs were investigated by polarization tests. In conclusion, the phase compositions of CoCrFeMoNi HEAs were predicted using a combination of the two approaches. The corrosion resistance was also competitive compared to that of Hastelloy C276. This research can provide a reference for the future development of corrosion-resistant HEAs with a single phase.

Обзор исследований сплавов, разработанных на основе энтропийного подхода

This paper provides a review of studies on the development and characterization of high-entropy alloys (HEAs). It is structured in the following way. Alloys’ design strategy based on entropy approach. Expectations and modern perceptions. This section describes the initial principles of multicomponent alloys design which provide stable structure and mechanical properties. It is noted that the role of high mixing entropy in the formation of disordered solid solutions and the suppression of the brittle intermetallic phases formation have been significantly reconsidered over time. Currently, obtaining a single-phase solid solution structure is not the main requirement for HEAs. The composition of HEAs. This section describes some typical multicomponent alloys having different elemental compositions. It is shown, that at present time the most studied alloys are based on 3-d transition elements. Using alloys of this group the possibility of providing both high and low values of strength and ductility is shown. Fabrication methods of HEAs. This section describes the methods for the fabrication of high-entropy alloys. It is noted that the most commonly used methods are based on the melting of the initial materials and its subsequent crystallization. Such methods of HEAs fabrication as powder metallurgy, magnetron sputtering, self-propagating high-temperature synthesis, melt spinning, and diffusion welding are also discussed. Structure of HEAs. This section provides the data on HEAs possessing multiphase structure and containing fine nanosized precipitates. Besides, the studies in which HEAs have been obtained in the form of metallic glasses, carbides, oxides, and borides are reviewed. The factors that can affect the structural state of the multicomponent alloys are discussed. The ambiguity of opinions of different research groups is noted. Properties of HEAs. This section mainly concentrates on the mechanical properties of HEAs. However, some other promising properties of HEAs like high wear resistance and reduced diffusivity are also discussed. Plastic deformation of HEAs. This section describes the evolution of the structure and properties of HEAs caused by thermal and mechanical processing. Characterization methods of HEAs. This section lists the characterization techniques, which are most frequently used to study HEAs. The structure of these alloys is mainly studied by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and optical microscopy. The methods for properties measurements are also briefly reviewed. Application of HEAs. This section describes the promising fields of HEAs application. It can be utilized in the aerospace, aircraft, and nuclear industries as well as for car manufacturing, austoelectronics, and in the design of microwave devices. Russian-language publications on HEAs. This section lists the studies, published in the Russian language as well as the thesis, done in Russian universities.

High-entropy alloys: Structure, mechanical properties, deformation mechanisms and application

The article considers a brief review of the foreign publications on the study of the structure, phase composition and properties of five-component high-entropy alloys (HEAs) in different structural states in a wide temperature range over the past two decades. HEAs attract the attention of scientists with their unique and unusual properties. The difficulties of comparative analysis and generalization of data are noted due to different methods of obtaining HEAs, modes of mechanical tests for uniaxial compression and tension, sizes and shapes of the samples, types of thermal treatments, and phase composition (bcc and fcc crystal lattices). It is noted that the HEA with a bcc lattice has mainly high strength and low plasticity, and the HEA with a fcc lattice has low strength and increased plasticity. A significant increase in the properties of the FeMnCoCrNi HEA with a fcc lattice can be achieved by alloying with boron and optimizing the parameters of thermal mechanical treatment at alloying with carbon in the amount of 1 % (at.). The deformation curves analyzed in the temperature range –196 ÷ 800 °C indicate an increase in the yield strength with a decrease in the grain size from 150 to 5 microns. As the temperature decreases, the yield strength and elongation increase. The effect of deformation rate on the mechanical properties is an increase in the ultimate strength and yield strength, which is most noticeable at high rates of 10–2 ÷ 103 s–1. The features of HEAs deformation behavior in the mono- and poly-crystalline states are noted. The complex of high operational properties of HEAs makes it possible to use them in various industries. There are good prospects of using energy treatment to modify the surface layers and further improve HEAs properties.

Calculation-driven design of off-equiatomic high-entropy alloys with enhanced solid-solution strengthening

Addition of Mo to the face-centered cubic (fcc) NiCoCrFe base alloy is an attractive method for improving the solid-solution strengthening of high-entropy alloys (HEAs). However, the low solubility of Mo in the equiatomic base alloy limits implementation. In this study, we used thermodynamic and ab initio calculations to develop an off-equiatomic NiCoCrFe-based HEA with an Mo content of up to 11.11 at%. Thermodynamic phase diagrams were constructed for the derived quinary subsystems with various Mo contents, and potential precipitate-free HEA compositions were determined. The degrees of lattice distortion (DLD) in seven selected HEAs were evaluated by statistically analyzing bond lengths determined using ab initio calculations. This approach could accurately predict the relative magnitudes of DLDs for multiple off-equiatomic HEAs in the studied system. Consequently, an off-equiatomic Ni1.8Co0.95Cr0.8Fe0.25Mo0.475 HEA was designed with enhanced lattice distortion, solid-solution strengthening, and yield strength. Microscopic analysis confirmed that the designed HEA exhibited a single fcc lattice, while excess Mo was detected at the grain boundary (GB) in a coarse-grained sample. It was deduced that slight GB segregation had a negligible influence on the lattice concentrations and solid-solution strengthening.

Understanding the Links between the Composition-Processing-Properties in New Formulations of HEAs Sintered by SPS

This work presents two new compositions of high entropy alloys (HEAs) that were designed with the aim of obtaining a body-centered cubic (BCC) phase with high hardness values and a moderate density. Sintering was performed using Spark Plasma Sintering (SPS) with different heating rates to determine the influence of the processing parameters on the phase formation. The microstructural study revealed that the presence of Ni in the composition promoted phase separation, and the mechanical study confirmed a clear influence on the mechanical properties of both the composition and heating rate. The combination of microscopy with compression and nanoindentation tests at room and high temperature made it possible to advance our understanding of the relationships between the composition, processing, and properties of this emerging group of alloys.

The effect of Cu and Mn elements on the mechanical properties of single-crystal CoCrFeNi-based high-entropy alloy under nanoindentation

Considering the impact of chemical compositions, the mechanical properties and microstructure evolution of the single-crystal FCC CoCrFeNi-based high-entropy alloys (HEAs) are investigated by using molecular dynamics simulation in nanoindentation. The addition of Cu and Mn elements would decrease the stacking faults energy resulting in the dislocation-mediated deformation. Moreover, the total number of chemical compositions of CoCrFeNi-based HEAs has critical effects on the results according to the Hertz contact theory. The plastic deformation is studied by correlating the P–h curve with the instantaneous defect structure and dominated by nucleation of Shockley partial dislocations or the motions of stacking faults. Owing to the large amount of Hirth and Stair-rod dislocations, CoCrFeNiCu HEA has the largest indentation force, next followed by CoCrFeNi and CoCrFeNiMn HEAs. Then, the influence of chemical composition on the radial distribution function is explored, and it exhibits that Cu and Mn elements are conducive to amorphization. Finally, the analysis of microstructure evolution reveals that the Mn addition would contribute to the slipping process for CoCrFeNi-based HEAs causing the decrease of indentation force and hardness. In contrast, the Cu addition results in irregular slipping mode accompanying the appearance of dislocation loops. In this simulation, the mechanical properties of single-crystal CoCrFeNi-based HEAs are strongly dependent on the chemical composition, which contributes to the composition design of high entropy alloys in the future.

Role of binder phase on the microstructure and mechanical properties of a mechanically alloyed and spark plasma sintered WC-FCC HEA composites

Mechanical behavior of the binder phase in a ceramic-metal (cermet) composite is critical for its overall properties, especially toughness and shock resistance. High entropy alloys (HEA) are promising binder phase materials owing to their excellent mechanical properties and impact on obtaining fine-grain cermet microstructure. In the current work, Co27.4Cr13.8Fe27.4Ni27.4Mo4 HEA designed via ICME approach was used as a binder to fabricate ultrafine WC-HEA composite by mechanical alloying and spark plasma sintering. The effect of an HEA binder on the microstructure and mechanical properties of WC-HEA composite are discussed in terms of experimental observation and Weibull modulus (9.7). Compared to a commercial WC-Co composite, a WC-10 wt.% HEA composite exhibits higher hardness and fracture toughness of 21 GPa and 10.5 MPa m1/2, respectively.

Development of an ANN-based generalized model for hardness prediction of SPSed AlCoCrCuFeMnNiW containing high entropy alloys

The present study reports the development of AlCrFeMnNiWx (x = 0, 0.05, 0.1, 0.5 mol) high entropy alloys (HEAs), processed by mechanical alloying (MA) cum spark plasma sintering (SPS) techniques, followed by the evaluation of the mechanical properties. Furthermore, an artificial Neural Network (ANN)-based model has been developed for the prediction of the hardness of a particular class of HEAs by using 36 HEAs available data from the literature, which stimulates the data by utilizing training, validation, and testing methods in a useful way with excellent overall regression coefficient (R) is 97.1 %. A backpropagation ANN model (9−9-1 neuron system) has been used to predict the value of the hardness with an accuracy of 95.9 %, which is based on elemental composition and sintering temperature. The predicted capability of the developed model also provides the freedom to choose the HEA composition with the required hardness of HEA without any experimental trials.

Passive film composition and stability of CoCrFeNi and CoCrFeNiAl high entropy alloys in chloride solution

In this work, CoCrFeNi and CoCrFeNiAl high entropy alloys (HEAs) were produced by arc melting. The aim was to investigate the correlation of the electrochemical behavior with microstructural features and chemical compositions of the surfaces during the first hours of immersion in 3.5 wt% NaCl solution at room temperature. Scanning electron microscopy (SEM) analysis and X-ray diffraction (XRD) patterns indicated that the CoCrFeNi alloy was comprised of one single face-centered cubic phase with distinct dendritic and interdendritic regions. The CoCrFeNiAl, in turn, was comprised of B2 and body-centered cubic (BCC) crystalline structures. The BCC phase was associated with dendritic regions while the B2+BCC structure was formed in interdendritic regions. The evolution of the electrochemical behavior was monitored by electrochemical impedance spectroscopy (EIS) up to 24 h of immersion. Al addition decreased the corrosion resistance of the CoCrFeNi alloy, as indicated by the lower charge transfer resistance values. X-ray photoelectron analysis of the passive films revealed that the oxide layer on the CoCrFeNi alloy was enriched with Cr2O3, improving its corrosion resistance. Conversely, the Al2O3-rich passive film formed on the CoCrFeNiAl alloy decreased its corrosion resistance.

High-entropy alloys and the periodic table of elements

To select the compositions of high-entropy alloys (HEA) consisting of five or more elements, it is necessary to use methods that take into account many variables and the complexity of assessing the relationships between them. Based on chemical information approaches to the analysis of Web of Science databases, data on the frequency of use of chemical elements in the described HEAs were obtained, which allow us to determine trends in the research and development of new materials.

Microstructure and strengthening mechanisms in FCC-structured single-phase TiC–CoCrFeCuNiAl0.3 HEACs with deformation twinning

Abstract This study aimed to investigate dislocation structures in CuNiCoFeCrAl0.3 alloy particles using a transmission electron microscope (TEM). A non-equiatomic CoCrFeCuNiAl0.3 high-entropy alloy composite (HEAC) reinforced with TiC5vol% nanoparticles was produced by mechanical alloying and spark plasma sintering. X-ray and TEM microanalyses confirmed the predominance of a nanotwinned single-phase face-centered cubic (FCC) solid solution with TiC nanoparticles. Further findings revealed that twin structure formation in FCC high-entropy alloys (HEAs) by the powder metallurgy technology was dependent on dislocations and the stratified structure formed in particles during mechanical alloying, low stacking-fault energy γSF, pressure, and hardness phase. The TiC5vol%–CoCrFeCuNiAl0.3 HEAC had the following excellent comprehensive mechanical properties: yield strength, 1582 MPa; fracture strength, 2185 MPa; and plastic strain, 23.60%. The dislocation glide and semi-twin expansion in HEAs provided excellent plasticity, while TiC particles and transgranular twins contributed to the material strength.

Mechanical and damping properties of the multi-layer graphenes enhanced CrMnFeCoNi high-entropy alloy composites produced by powder metallurgy

The CrMnFeCoNi-based composites with different multi-layer graphenes (MLGs) content (0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%) were prepared by powder metallurgy. The effects of the MLGs content on the mechanical and damping properties of the composites were studied specifically. The strength and hardness are obviously improved, and the tensile strength can reach 762 MPa. With the addition of MLGs, new damping mechanisms including the intrinsic damping of graphene and the interface damping are introduced, which lead the damping properties of composites to remain at a high level. As the content of MLGs increases, the Maximum internal friction (Q-1max) of the composites increases first and then decreases, and its maximum value is 0.0667.

In situ synchrotron X-ray imaging and mechanical properties characterization of additively manufactured high-entropy alloy composites

Laser beam directed energy deposition has become an increasingly popular advanced manufacturing technique for materials discovery as a result of the in situ alloying capability. In this study, we leverage an additive manufacturing enabled high throughput materials discovery approach to explore the composition space of a graded Wx(CoCrFeMnNi)100−x sample spanning 0 ≤ x ≤ 21 at%. In addition to microstructural and mechanical characterization, synchrotron high speed x-ray computer aided tomography was conducted on a W20(CoCrFeMnNi)80 composition to visualize melting dynamics, powder-laser interactions, and remelting effects of previously consolidated material. Results reveal the formation of the Fe7W6 intermetallic phase at W concentrations> 6 at%, despite the high configurational entropy. Unincorporated W particles also occurred at W concentrations> 10 at% accompanied by a dissolution band of Fe7W6 at the W/matrix interface and hardness values greater than 400 HV. The primary strengthening mechanism is attributed to the reinforcement of the Fe7W6 and W phases as a metal matrix composite. The in situ high speed x-ray imaging during remelting showed that an additional laser pass did not promote further mixing of the Fe7W6 or W phases suggesting that, despite the dissolution of the W into the Fe7W6 phase being thermodynamically favored, it is kinetically limited by the thickness/diffusivity of the intermetallic phase, and the rapid solidification of the laser-based process.

Influence of in-situ and ex-situ precipitations on microstructure and mechanical properties of additive manufacturing CoCrFeMnNi high-entropy alloys

CoCrFeMnNi high entropy alloys (HEAs) based composites have been synthesized by laser melting deposition (LMD). Through TiC and WC particle additions, ex-situ TiC and in-situ M23C6 precipitations are formed, respectively. The average grain size of the LMD-fabricated CoCrFeMnNi HEA is significantly reduced. With TiC and WC additions, appearing the combined effects of grain refinement and precipitation reinforcement, further to remarkably improve the tensile strength from 550 MPa to 610 MPa and 776 MPa for the HEAs, whereas the plasticity decreased from 52% to 47% and 37%, respectively. The HEAs with WC addition shows higher tensile strength, which probably results from the homogeneous distribution of the nano-scale M23C6 reinforcement precipitates and a single deformation pattern. This work provides insight into the effects of different introduction modes of reinforced phases when it comes to the mechanical properties for HEA composites.

Plastic deformation mechanism in crystal-glass high entropy alloy composites studied via molecular dynamics simulations

Recent experimental observations have demonstrated the possibility to develop a crystal-glass composite (CGC) with high strength and ductility by incorporating dual-phase of high entropy alloys (HEAs) and metallic glasses (MGs). In the current work, molecular dynamics simulations were performed to uncover the plastic deformation mechanism in CGC structures consisting of the CoCrFeMnNi HEA and its MG counterpart. A fully crystalline polycrystal model and two CGC models were simulated to reveal the influence of the MG phase thickness and the temperature on the plastic deformation behaviors. It is found that the MG phase can effectively inhibit deformations mediated by grain boundary and dislocations emitted from the glass-crystalline interface, which enhances the yielding stress and strain in the CGC model. At the low temperature (T ≤ 300 K) regime during which the MG phase deform via the shear transformation zone, the thinner MG phase was more effective in strengthening due to the more limited space for shear band formation compared with the thicker MG phase. At the high temperature (T ≥ 700 K) regime that the MG phase mainly deform by homogeneous viscous flow, the MG phase acts as a softer phase, which can contribute to good deformability and thermal stability of the nanograined crystalline phase. The current work may shed light on an in-depth understanding of the deformation mechanisms of the CGCs and may advance the development of metallic materials with a synergy of high strength and ductility.

Effects of W addition on the Corrosion Behaviors of FeCoNiCrMn High Entropy Alloy Composites in the 3.5 wt.% NaCl solution

The effects of W addition on the corrosion behaviors of FeCoNiCrMn high entropy alloy composites in 3.5 wt.%NaCl solution are investigated in this study. The X-ray diffraction and scanning electron microscopy suggest that the quantity of added W particles results in the increasing quantity of particles in the composites and large amount of cohesionless phases covering the matrix occur in the alloy with 1.0 molar ratio of W element. The trend of corrosion potential current density means that all W-added high entropy alloy composites have a better corrosion resistance referring to the base high entropy alloy composites and 0.8 molar ratio of W element into base generates a best corrosion resistance in the current work. The Electrochemical impedance spectra and compositional analysis of corroded surface of as-cast high entropy alloys demonstrate that W particles act as anode in the composites and Fe-Mn-Cr is as the cathode.

High-Entropy Alloys for Advanced Nuclear Applications.

The expanded compositional freedom afforded by high-entropy alloys (HEAs) represents a unique opportunity for the design of alloys for advanced nuclear applications, in particular for applications where current engineering alloys fall short. This review assesses the work done to date in the field of HEAs for nuclear applications, provides critical insight into the conclusions drawn, and highlights possibilities and challenges for future study. It is found that our understanding of the irradiation responses of HEAs remains in its infancy, and much work is needed in order for our knowledge of any single HEA system to match our understanding of conventional alloys such as austenitic steels. A number of studies have suggested that HEAs possess ‘special’ irradiation damage resistance, although some of the proposed mechanisms, such as those based on sluggish diffusion and lattice distortion, remain somewhat unconvincing (certainly in terms of being universally applicable to all HEAs). Nevertheless, there may be some mechanisms and effects that are uniquely different in HEAs when compared to more conventional alloys, such as the effect that their poor thermal conductivities have on the displacement cascade. Furthermore, the opportunity to tune the compositions of HEAs over a large range to optimise particular irradiation responses could be very powerful, even if the design process remains challenging.

Preparation, structures and properties of tungsten-containing refractory high entropy alloys

As a new type of multi-principal component solid solution alloy, high-entropy alloy has the four major effects, i.e. high entropy, lattice distortion, slow diffusion, and “cocktail” in orderly arrangement of atoms and chemical disorder. It exhibits excellent comprehensive performances and is expected to be used as a new type of high-temperature structural material, wear-resistant material, and radiation-resistant material, which is used in the areas of aerospace, mining machinery, nuclear fusion reactors and others. In this paper, the present research status, conventional preparation methods, microstructures and phase compositions of tungsten high entropy alloys are mainly introduced. In view of the excellent comprehensive properties of high-entropy alloys, the mechanical properties, friction and wear resistance, and radiation resistance of tungsten high-entropy alloys are summarized, and the future research directions of tungsten high-entropy alloys are also prospected.

Thermodynamic Prediction of Phase Composition of Transition Metals High-Entropy Alloys

Thermodynamic Prediction of Phase Composition of Transition Metals High-Entropy Alloys

High-throughput calculations based on the small set of ordered structures method for non-equimolar high entropy alloys

In this work, the small set of ordered structures (SSOS) method, which was initially developed for equimolar high entropy alloys (HEAs) in solid solution phase, is extended to deal with non-equimolar HEAs. Using quinary HEAs with face-centered cubic lattice structure as an example, we show that the SSOS method can be directly and efficiently applied to a subset of HEAs with a non-equimolar composition that can be represented by small ordered structures (SOS) containing only 6, 7, or 8 atoms. For an HEA with a non-equimolar composition that can only be represented by an SOS with more than 8 atoms, however, multiple SSOS solutions can be obtained by selecting from a combination of SOS containing 5, 6, and 7 atoms. An averaging scheme is proposed to deal with the non-uniqueness of the SSOS solutions. The accuracy of the SSOS method is validated against the SQS predictions for a selected set of non-equimolar compositions of AlTiFeCrMn HEA in terms of ground state energies, lattice constants and mass densities. We find that application of the SSOS method to non-equimolar HEAs is sufficiently fast and accurate, and thus makes it suitable for high-throughput DFT computations.