Relationship between the unique microstructures and behaviors of high-entropy alloys

High-entropy alloys(HEAs),which were introduced as a pioneering concept in 2004,have captured the keen interest of nu-merous researchers.Entropy,in this context,can be perceived as representing disorder and randomness.By contrast,elemental composi-tions within alloy systems occupy specific structural sites in space,a concept referred to as structure.In accordance with Shannon entropy,structure is analogous to information.Generally,the arrangement of atoms within a material,termed its structure,plays a pivotal role in dictating its properties.In addition to expanding the array of options for alloy composites,HEAs afford ample opportunities for diverse structural designs.The profound influence of distinct structural features on the exceptional behaviors of alloys is underscored by numer-ous examples.These features include remarkably high fracture strength with excellent ductility,antiballistic capability,exceptional radi-ation resistance,and corrosion resistance.In this paper,we delve into various unique material structures and properties while elucidating the intricate relationship between structure and performance.

Effect of hafnium and molybdenum addition on inclusion characteristics in Co-based dual-phase high-entropy alloys

Specific grades of high-entropy alloys(HEAs)can provide opportunities for optimizing properties toward high-temperature applications.In this work,the Co-based HEA with a chemical composition of Co_(47.5)Cr_(30)Fe_(7.5)Mn_(7.5)Ni_(7.5)(at%)was chosen.The refractory metallic elements hafnium(Hf)and molybdenum(Mo)were added in small amounts(1.5at%)because of their well-known positive effects on high-temperature properties.Inclusion characteristics were comprehensively explored by using a two-dimensional cross-sectional method and extracted by using a three-dimensional electrolytic extraction method.The results revealed that the addition of Hf can reduce Al_(2)O_(3)inclusions and lead to the formation of more stable Hf-rich inclusions as the main phase.Mo addition cannot influence the inclusion type but could influence the inclusion characteristics by affecting the physical parameters of the HEA melt.The calculated coagulation coefficient and collision rate of Al_(2)O_(3)inclusions were higher than those of HfO_(2)inclusions,but the inclusion amount played a larger role in the agglomeration behavior of HfO_(2)and Al_(2)O_(3)inclusions.The impurity level and active elements in HEAs were the crucial factors affecting inclusion formation.

Comprehensive insights into recent innovations:Magnesium-inclusive high-entropy alloys

This review focuses on thermodynamic and physical parameters,synthesis methods,and reported phases of Magnesium(Mg)containing high-entropy alloys(HEAs).Statistical data of publications concerning Mg-containing HEAs were collected and analyzed.Data on the chemical elements included in Mg-containing HEAs,their theoretical end experimental densities,thermodynamic parameters,physical parameters,fabricated techniques and reported phases were also collected and discussed.On the basis of this information,a new classification for HEAs was proposed.It is also shown that the existing thermodynamic parameters cannot accurately predict the formation of a single phase solid solution for Mg-containing HEAs.The physical parameters of Mg-containing HEAs are within a wide range,and most of the synthesized Mg-containing HEAs have a complex multiphase structure.

Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

A series of high-entropy alloys(HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method.In order to study the irradiation evolution of helium bubbles in the FeCoNiCrbased HE As with γ' precipitates,these samples are irradiated by 100-keV helium ions with a fluence of 5 × 10^(20) ions/m^(2) at 293 K and 673 K,respectively.And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles.Transmission electron microscope(TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles.Experimental results reveal that nanosized,spherical,dispersed,coherent,and ordered L1_(2)-type Ni_(3)Ti γ' precipitations are introduced into FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K.Under the ageing treatment conditions adopted in this work,γ' nanoparticles are precipitated in FeCoNiCr(Ni_(3)Ti)_(0.1) HE As,with average diameters of 15.80 nm,37.09 nm,and 62.50 nm,respectively.The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm,1.65 nm,and 1.58 nm,respectively.The improvement in the irradiation resistance of FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs is evidenced by the diminution in bubbles size.Furthermore,the FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs containing γ' precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles,which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries.Subsequently,annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy(E_(a)) for helium bubbles,resulting in larger helium bubble size.This study provides guidance for improving the irradiation resistance of L1_(2)-strengthened high-entropy alloy.

Effect of Mn content on microstructure and properties of AlCrCuFeMnx high-entropy alloy

AlCrCuFeMnx(x=0,0.5,1,1.5,and 2)high-entropy alloys were prepared using the vacuum arc melting technology.The microstructure and mechanical properties of AlCrCuFeMnxwere analyzed and tested by XRD,SEM,TEM,nanoindentation,and electronic universal testing.The results indicate that the AlCrCuFeMnxhigh-entropy alloy exhibits a dendritic structure,consisting of dendrites with a BCC structure,interdendrite regions with an FCC structure,and precipitates with an ordered BCC structure that form within the dendrite.Manganese(Mn)has a strong affinity for dendritic,interdendritic,and precipitate structures,allowing it to easily enter these areas.With an increase in Mn content,the size of the precipitated nanoparticles in the dendritic region initially increases and then decreases.Similarly,the area fraction initially decreases and then increases.Additionally,the alloy’s strength and wear resistance decrease,while its plasticity increases.The Al Cr Cu Fe Mn1.5alloy boasts excellent mechanical properties,including a hardness of 360 HV and a wear rate of 2.4×10^(-5)mm^(3)·N^(-1)·mm^(-1).It also exhibits impressive yield strength,compressive strength,and deformation rates of 960 MPa,1,700 MPa,and 27.5%,respectively.

Atomistic evaluation of tension–compression asymmetry in nanoscale body-centered-cubic AlCrFeCoNi high-entropy alloy

The tension and compression of face-centered-cubic high-entropy alloy(HEA) nanowires are significantly asymmetric, but the tension–compression asymmetry in nanoscale body-centered-cubic(BCC) HEAs is still unclear. In this study,the tension–compression asymmetry of the BCC Al Cr Fe Co Ni HEA nanowire is investigated using molecular dynamics simulations. The results show a significant asymmetry in both the yield and flow stresses, with BCC HEA nanowire stronger under compression than under tension. The strength asymmetry originates from the completely different deformation mechanisms in tension and compression. In compression, atomic amorphization dominates plastic deformation and contributes to the strengthening, while in tension, deformation twinning prevails and weakens the HEA nanowire.The tension–compression asymmetry exhibits a clear trend of increasing with the increasing nanowire cross-sectional edge length and decreasing temperature. In particular, the compressive strengths along the [001] and [111] crystallographic orientations are stronger than the tensile counterparts, while the [110] crystallographic orientation shows the exactly opposite trend. The dependences of tension–compression asymmetry on the cross-sectional edge length, crystallographic orientation,and temperature are explained in terms of the deformation behavior of HEA nanowire as well as its variations caused by the change in these influential factors. These findings may deepen our understanding of the tension–compression asymmetry of the BCC HEA nanowires.

High-entropy alloys in thermoelectric application:A selective review

Since the superior mechanical,chemical and physical properties of high-entropy alloys(HEAs)were discovered,they have gradually become new emerging candidates for renewable energy applications.This review presents the novel applications of HEAs in thermoelectric energy conversion.Firstly,the basic concepts and structural properties of HEAs are introduced.Then,we discuss a number of promising thermoelectric materials based on HEAs.Finally,the conclusion and outlook are presented.This article presents an advanced understanding of the thermoelectric properties of HEAs,which provides new opportunities for promoting their applications in renewable energy.

Co-Free High-Entropy Alloys Powders Immobilized by Electrospray and Microf luidics for Decolorization of Azo Dye

In recent years,high-entropy alloys(HEAs)have received more and more attention due to their unique microstructure and properties.Several researchers have reported that some ball-milled(BM)HEAs powders possess prominent decolorization performance for azo dyes.Three kinds of Co-free HEA powders(AlCrFeMn,AlCrFeNi and FeCrNiMn)have been synthesized by ball milling in this work,of which AlCrFeMn shows the best decolorization efficiency for DB6 aqueous solution.However,at this time,the BM HEAs are in powder state and not easy to be reused,so the loss rate of the powders is high during the reaction.Sometimes,the reaction between reacted the powders and the dye solution is too fast to control.While,in order to solve these problems,this work proposes to immobilize bare BM AlCrFeMn HEA powders in calcium alginate beads(CAB s)by electrospray and microfluidics.Through four cycles of reaction,the loss rate of the AlCrFeMn powders can be reduced from 40 to 5 wt%if the powders are immobilized by CABs with an average diameter of 0.55 mm obtained at the DC voltage of 30 kV.In addition,in the four cycles of experiment,the AlCrFeMn HEA-CABs with an average diameter of0.55 mm shows better stability and easier separation than that of the bare AlCrFeMn powders.These findings provide new ideas for HEAs to decolorize azo dyes and are of great significance for protecting freshwater resources.

A Nitride-Reinforced NbMoTaWHfN Refractory High-Entropy Alloy with Potential Ultra-High-Temperature Engineering Applications

Refractory high-entropy alloys(RHEAs)have promising applications as the new generation of hightemperature alloys in hypersonic vehicles,aero-engines,gas turbines,and nuclear power plants.This study focuses on the microstructures and mechanical properties of the NbMoTaW(HfN)_(x)(x=0,0.3,0.7,and 1.0)RHEAs.The alloys consist of multiple phases of body-centered cubic(BCC),hafnium nitride(HfN),or multicomponent nitride(MN)phases.As the x contents increase,the grain size becomes smaller,and the strength gradually increases.The compressive yield strengths of the NbMoTaWHfN RHEA at ambient temperature,1000,1400,and 1800℃ were found to be 1682,1192,792,and 288 MPa,respectively.The high-temperature strength of this alloy is an inspiring result that exceeds the high temperature and strength of most known alloys,including high-entropy alloys,refractory metals,and superalloys.The HfN phase has a significant effect on strengthening due to its high structural stability and sluggish grain coarsening,even at ultra-high temperatures.Its superior properties endow the NbMoTaWHfN RHEA with potential for a wide range of engineering applications at ultra-high temperatures.This work offers a strategy for the design of high-temperature alloys and proposes an ultra-high-temperature alloy with potential for future engineering applications.

High-Entropy Alloys to Activate the Sulfur Cathode for Lithium-Sulfur Batteries

Sulfur element possesses an ultrahigh theoretical specific capacity,while the utilization of sulfur in the whole cathode is lower obviously owing to the sluggish kinetics of sulfur and discharged products,limiting the enhancement on energy density of lithium-sulfur batteries.Herein,for the first time,Fe_(0.24)Co_(0.26)Ni_(0.10)Cu_(0.15)Mn_(0.25)high-entropy alloy is introduced as the core catalytic host to activate the electrochemical performance of the sulfur cathode for lithium-sulfur batteries.It is manifested that Fe_(0.24)Co_(0.26)Ni_(0.10)Cu_(0.15)Mn_(0.25)high-entropy alloy nanocrystallites distributed on nitrogen-doped carbon exhibit high electrocatalytic activity toward the conversion of solid sulfur to solid discharged products across soluble intermediate lithium polysulfides.In particular,benefiting from the accelerated kinetics by high-entropy alloy nanocrystallites and synergistic adsorption by nitrogen-doped carbon,the cathode exhibits high reversible capacity of 1079.5 mAh g_(-cathode)^(-1)(high utilization of 89.4%)with the whole cathode as active material,instead of sulfur element.Moreover,under both lean electrolyte(3μmg^(-1))and ultrahigh sulfur loading(27.0 mg cm^(-2))condition,the high discharge capacity of 868.2 mAh g_(-cathode)^(-1)can be still achieved for the sulfur cathode.This strategy opens up a new path to explore catalytic host materials for enhancing the utilization of sulfur in the whole cathode for lithium-sulfur batteries.

Uncertainty quantification of predicting stable structures for high-entropy alloys using Bayesian neural networks

High entropy alloys(HEAs)have excellent application prospects in catalysis because of their rich components and configuration space.In this work,we develop a Bayesian neural network(BNN)based on energies calculated with density functional theory to search the configuration space of the CoNiRhRu HEA system.The BNN model was developed by considering six independent features of Co-Ni,Co-Rh,CoRu,Ni-Rh,Ni-Ru,and Rh-Ru in different shells and energies of structures as the labels.The root mean squared error of the energy predicted by BNN is 1.37 me V/atom.Moreover,the influence of feature periodicity on the energy of HEA in theoretical calculations is discussed.We found that when the neural network is optimized to a certain extent,only using the accuracy indicator of root mean square error to evaluate model performance is no longer accurate in some scenarios.More importantly,we reveal the importance of uncertainty quantification for neural networks to predict new structures of HEAs with proper confidence based on BNN.

On the Superconductivity in High-Entropy Alloy (NbTa)1-X(HfZrTi)X

The superconductivity in (NbTa)1-X(HfZrTi)X high-entropy alloy is analyzed using the theory of strong-coupled superconductor. It is concluded that (NbTa)1-X(HfZrTi)X is a strong coupled superconductor. The variation in the superconducting transition temperature from 7.9 K to 4.6 K as x increases from 0.2 to 0.84 arises because of the decrease in electronic band width due to localization and broadening of the band. It is suggested that the decrease in electronic band width is due to crystalline randomness which gives rise to the mobility edge.

Enhancing the mechanical properties of casting eutectic high -entropy alloys via W addition

The effect of W element on the microstructure evolution and mechanical properties of Al_(1.25)CoCrFeNi3 eutectic high-entropy alloy and Al_(1.25)CoCrFeNi_(3-x)W_(x)(x=0,0.05,0.1,0.3,and 0.5;atomic ratio)high-entropy alloys(HEAs)were explored.Results show that the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs are composed of face-centered cubic and body-centered cubic(BCC)phases.As W content increases,the microstructure changes from eutectic to dendritic.The addition of W lowers the nucleation barrier of the BCC phase,decreases the valence electron concentration of the HEAs,and replaces Al in the BCC phase,thus facilitating the nucleation of the BCC phase.Tensile results show that the addition of W greatly improves the mechanical properties,and solid-solution,heterogeneous-interface,and second-phase strengthening are the main strengthening mechanisms.The yield strength,tensile strength,and elongation of the Al_(1.25)CoCrFeNi2.95W0.05 HEA are 601.44 MPa,1132.26 MPa,and 15.94%,respectively,realizing a balance between strength and plasti-city.The fracture mode of the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs is ductile–brittle mixed fracture,and the crack propagates and initiates in the BCC phase.The eutectic lamellar structure impedes crack propagation and maintains plasticity.

A novel immiscible high entropy alloy strengthened via L1_(2)-nanoprecipitate

The low-cost Fe-Cu,Fe-Ni,and Cu-based high-entropy alloys exhibit a widespread utilization prospect.However,these potential applications have been limited by their low strength.In this study,a novel Fe_(31)Cu_(31)Ni_(28)Al_(4)Ti_(3)Co_(3) immiscible high-entropy alloy(HEA)was developed.After vacuum arc melting and copper mold suction casting,this HEA exhibits a unique phase separation microstructure,which consists of striped Cu-rich regions and Fe-rich region.Further magnification of the striped Cu-rich region reveals that it is composed of a Cu-rich dot-like phase and a Fe-rich region.The aging alloy is further strengthened by a L1_(2)-Ni_(3)(AlTi)nanoprecipitates,achieving excellent yield strength(1185 MPa)and uniform ductility(~8.8%).The differential distribution of the L1_(2) nanoprecipitate in the striped Cu-rich region and the external Fe-rich region increased the strength difference between these two regions,which increased the strain gradient and thus improved hetero-deformation induced(HDI)hardening.This work provides a new route to improve the HDI hardening of Fe-Cu alloys.

Microstructures and micromechanical behaviors of high -entropy alloys investigated by synchrotron X-ray and neutron diffraction techniques: A review

High-entropy alloys(HEAs)possess outstanding features such as corrosion resistance,irradiation resistance,and good mechan-ical properties.A few HEAs have found applications in the fields of aerospace and defense.Extensive studies on the deformation mech-anisms of HEAs can guide microstructure control and toughness design,which is vital for understanding and studying state-of-the-art structural materials.Synchrotron X-ray and neutron diffraction are necessary techniques for materials science research,especially for in situ coupling of physical/chemical fields and for resolving macro/microcrystallographic information on materials.Recently,several re-searchers have applied synchrotron X-ray and neutron diffraction methods to study the deformation mechanisms,phase transformations,stress behaviors,and in situ processes of HEAs,such as variable-temperature,high-pressure,and hydrogenation processes.In this review,the principles and development of synchrotron X-ray and neutron diffraction are presented,and their applications in the deformation mechanisms of HEAs are discussed.The factors that influence the deformation mechanisms of HEAs are also outlined.This review fo-cuses on the microstructures and micromechanical behaviors during tension/compression or creep/fatigue deformation and the application of synchrotron X-ray and neutron diffraction methods to the characterization of dislocations,stacking faults,twins,phases,and intergrain/interphase stress changes.Perspectives on future developments of synchrotron X-ray and neutron diffraction and on research directions on the deformation mechanisms of novel metals are discussed.

Effect of process parameters on the microstructure and properties of laser-clad FeNiCoCrTi0.5 high-entropy alloy coating

FeNiCoCrTi0.5 coatings with different process parameters were fabricated by laser cladding. The macro-morphology, phase, microstructure, hardness, and wear resistance of each coating were studied. The smoothness and dilution rate of the FeNiCoCrTi0.5 coating generally increased with the increase of specific energy(Es), which is the laser irradiation energy received by a unit area. FeNiCoCrTi0.5 coatings at different parameters had bcc, fcc, and Ti-rich phases as well as equiaxed, dendritic, and columnar structures. When Es increased, the size of each structure increased and the distribution area of the columnar and dendritic structures changed. The prepared FeNiCoCrTi0.5 coating with the Es of 72.22 J·mm-2 had the highest hardness and the best wear resistance, the highest hardness of the coating reached HV 498.37, which is twice the substrate hardness. The average hardness of the FeNiCoCrTi0.5 coating with the Es of 72.22 J·mm-2 was 15.8% higher than the lowest average hardness of the coating with the Es of 108.33 J·mm-2. The worn surface morphologies indicate that the FeNiCoCrTi0.5 coatings exhibited abrasive wear.

Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy

An AlCoCuCrFeNiTi high-entropy alloy(HEA) was prepared by mechanical alloying and sintering to study the effect of Ti addition to the widely studied AlCoCuCrFeNi system. The structural and microstructural characteristics were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The formation of four micrometric phases was detected: a Cu-rich phase with a face-centered cubic(fcc) structure, a body-centered cubic(bcc) solid solution with Cu-rich plate-like precipitates(fcc), an ordered bcc phase, and a tetragonal structure. The XRD patterns corroborate the presence of a mixture of bcc-, fcc-, and tetragonal-structured phases. The Vickers hardness of the alloy under study was more than twice that of the AlCoCuCrFeNi alloy. Nanoindentation tests were performed to evaluate the mechanical response of the individual phases to elucidate the relationship between chemical composition, crystal structure, and mechanical performance of the multiphase microstructure of the AlCoCuCrFeNiTi HEA.

Effect of Ti content on microstructure and properties of TixZrVNb refractory high-entropy alloys

This study aimed to investigate the microstructure and mechanical properties of TixZrVNb(x=1,1.5,2)refractory high-entropy alloys at room and elevated temperatures.The TiZrVNb alloy consisted of the body-centered cubic(bcc)matrix with a small amount of V2Zr phase.The Ti1.5ZrVNb and Ti2ZrVNb alloys exhibited a single-phase bcc structure.At room temperature,the tensile ductility of the as-cast alloys increased from 3.5%to 12.3%with the increase in the Ti content.The TixZrVNb alloys exhibited high yield strength at 600°C,and the ultimate yield strength was more than 900 MPa.Softening occurred at 800°C,but the ultimate yield strength could still exceed 200 MPa.Moreover,the TixZrVNb alloys displayed low densities but high specific yield strengths(SYSs).The lowest density of TixZrVNb alloys was only 6.12 g/cm^3,but the SYS could reach about 180 MPa·cm^3·g^−1,which is better than those of most reported high-entropy alloys(HEAs).

Thermodynamic analysis of the simple microstructure of AlCrFeNiCu high-entropy alloy with multi-principal elements

AlCrFeNiCu high-entropy alloy （THA） was synthesized by the arc melting and casting method. The alloy exhibits simple FCC and BCC solid solution phases rather than intermetallic compounds. The reason is that the Gibbs free energy of mixing of the equimolar A1CrFeNiCu alloy is smaller than that of inter-metallic compounds by calculation according to the Miedema model .

Effects of substrate temperature and deposition time on the morphology and corrosion resistance of FeCoCrNiMo0.3 high-entropy alloy coating fabricated by magnetron sputtering

The effects of substrate temperature and deposition time on the morphology and corrosion resistance of FeCoCrNiMo0.3 coating fabricated by magnetron sputtering were investigated by scanning electron microscopy and electrochemical tests.The FeCoCrNiMo0.3 coating was mainly composed of the face-centered cubic phase.High substrate temperature promoted the densification of the coating,and the pitting resistance and protective ability of the coating in 3.5wt%NaCl solution was thus improved.When the deposition time was prolonged at 500℃,the thickness of the coating remarkably increased.Meanwhile,the pitting resistance improved as the deposition time increased from 1 to 3 h;however,further improvement could not be obtained for the coating sputtered for 5 h.Overall,the pitting resistance of the FeCoCrNiMo0.3 coating sputtered at 500℃for 3 h exceeds those of most of the reported high-entropy alloy coatings.