高熵合金的微观组织和性能

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ProgressinMaterialsScience61(2014)1–93ContentslistsavailableatScienceDirectProgressinMaterialsSciencejournalhomepage:www.elsevier.com/locate/pmatsciMicrostructuresandpropertiesofhigh-entropyalloys

YongZhanga,⇑,TingTingZuoa,ZhiTangb,MichaelC.Gaoc,d,KarinA.Dahmene,PeterK.Liawb,ZhaoPingLuaaStateKeyLaboratoryforAdvancedMetalsandMaterials,UniversityofScienceandTechnologyBeijing,Beijing100083,ChinaDepartmentofMaterialsScienceandEngineering,TheUniversityofTennessee,Knoxville,TN37996,USAcNationalEnergyTechnologyLaboratory,1450QueenAveSW,Albany,OR97321,USAdURSCorporation,POBox1959,Albany,OR97321-2198,USAeDepartmentofPhysics,UniversityofIllinoisatUrbana-Champaign,1110WestGreenStreet,Urbana,IL61801-3080,USAbarticleinfoabstract

Thispaperreviewstherecentresearchanddevelopmentofhigh-entropyalloys(HEAs).HEAsarelooselydeﬁnedassolidsolutionalloysthatcontainmorethanﬁveprincipalelementsinequalornearequalatomicpercent(at.%).Theconceptofhighentropyintroducesanewpathofdevelopingadvancedmaterialswithuniqueproperties,whichcannotbeachievedbytheconventionalmicro-alloyingapproachbasedononlyonedominantelement.Uptodate,manyHEAswithpromisingpropertieshavebeenreported,e.g.,highwear-resistantHEAs,Co1.5CrFeNi1.5TiandAl0.2Co1.5CrFeNi1.5Tialloys;high-strengthbody-centered-cubic(BCC)AlCoCrFeNiHEAsatroomtemperature,andNbMoTaVHEAatelevatedtemperatures.Furthermore,thegeneralcorrosionresis-tanceoftheCu0.5NiAlCoCrFeSiHEAismuchbetterthanthatoftheconventional304-stainlesssteel.ThispaperﬁrstreviewsHEAfor-mationinrelationtothermodynamics,kinetics,andprocessing.Physical,magnetic,chemical,andmechanicalpropertiesarethendiscussed.Greatdetailsareprovidedontheplasticdeformation,fracture,andmagnetizationfromtheperspectivesofcracklingnoiseandBarkhausennoisemeasurements,andtheanalysisofser-rationsonstress–straincurvesatspeciﬁcstrainratesortestingtemperatures,aswellastheserrationsofthemagnetizationhysteresisloops.Thecomparisonbetweenconventionalandhigh-entropybulkmetallicglassesisanalyzedfromtheviewpointsofeutecticcomposition,denseatomicpacking,andentropyofArticlehistory:Received26September2013Accepted8October2013Availableonline1November2013⇑Correspondingauthor.Tel.:+8601062333073;fax:+8601062333447.E-mailaddress:drzhangy@ustb.edu.cn(Y.Zhang).0079-6425/$-seefrontmatterÓ2013ElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.pmatsci.2013.10.0012Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93mixing.Glassformingabilityandplasticpropertiesofhigh-entropybulkmetallicglassesarealsodiscussed.Modelingtech-niquesapplicabletoHEAsareintroducedanddiscussed,suchasabinitiomoleculardynamicssimulationsandCALPHADmodeling.Finally,futuredevelopmentsandpotentialnewresearchdirectionsforHEAsareproposed.Ó2013ElsevierLtd.Allrightsreserved.Contents1.Introduction..........................................................................31.1.Fourcoreeffects.................................................................41.1.1.High-entropyeffect.......................................................41.1.2.Sluggishdiffusioneffect....................................................51.1.3.Severelattice-distortioneffect...............................................61.1.4.Cocktaileffect............................................................71.2.Keyresearchtopics...............................................................91.2.1.Mechanicalpropertiescomparedwithotheralloys.............................101.2.2.Underlyingmechanismsformechanicalproperties.............................111.2.3.AlloydesignandpreparationforHEAs.......................................111.2.4.TheoreticalsimulationsforHEAs............................................12Thermodynamics.....................................................................122.1.Entropy.......................................................................132.2.Thermodynamicconsiderationsofphaseformation....................................152.3.MicrostructuresofHEAs..........................................................18Kineticsandalloypreparation...........................................................233.1.Preparationfromtheliquidstate...................................................243.2.Preparationfromthesolidstate....................................................293.3.Preparationfromthegasstate.....................................................303.4.Electrochemicalpreparation.......................................................34Properties...........................................................................344.1.Mechanicalbehavior.............................................................344.1.1.Mechanicalbehavioratroomtemperature....................................354.1.2.Mechanicalbehavioratelevatedtemperatures................................384.1.3.Mechanicalbehavioratcryogenictemperatures...............................454.1.4.Fatiguebehavior.........................................................464.1.5.Wearbehavior..........................................................484.1.6.Summary...............................................................494.2.Physicalbehavior................................................................504.3.Biomedical,chemicalandotherbehaviors...........................................53Serrationsanddeformationmechanisms..................................................555.1.SerrationsforHEAs..............................................................565.2.BarkhausennoiseforHEAs........................................................585.3.ModelingtheSerrationsofHEAs...................................................615.4.DeformationmechanismsforHEAs.................................................66Glassformationinhigh-entropyalloys....................................................676.1.High-entropyeffectsonglassformation.............................................676.1.1.Thebestglassformerislocatedattheeutecticcompositions....................676.1.2.Thebestglassformeristhecompositionwithdenseatomicpacking..............676.1.3.Thebestglassformerhashighentropyofmixing..............................676.2.GFAforHEAs...................................................................686.3.Propertiesofhigh-entropyBMGs...................................................70Modelingandsimulations..............................................................727.1.DFTcalculations................................................................737.2.AIMDsimulations...............................................................757.3.CALPHADmodeling..............................................................80Futuredevelopmentandresearch........................................................812.3.4.5.6.7.8.Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9339.8.1.FundamentalunderstandingofHEAs................................................8.2.ProcessingandcharacterizationofHEAs.............................................8.3.ApplicationsofHEAs.............................................................Summary...........................................................................Disclaimer..........................................................................Acknowledgements...................................................................References..........................................................................828383848585851.IntroductionRecently,high-entropyalloys(HEAs)haveattractedincreasingattentionsbecauseoftheiruniquecompositions,microstructures,andadjustableproperties[1–31].Theyarelooselydeﬁnedassolidsolutionalloysthatcontainmorethanﬁveprincipalelementsinequalornearequalatomicpercent(at.%)[32].Normally,theatomicfractionofeachcomponentisgreaterthan5at.%.Themulti-compo-nentequi-molaralloysshouldbelocatedatthecenterofamulti-componentphasediagram,andtheirconﬁgurationentropyofmixingreachesitsmaximum(RLnN;RisthegasconstantandNthenumberofcomponentinthesystem)forasolutionphase.ThesealloysaredeﬁnedasHEAsbyYehetal.[2],andnamedbyCantoretal.[1,33]asmulti-componentalloys.Bothrefertothesameconcept.Therearealsosomeothernames,suchasmulti-principal-elementsalloys,equi-molaralloys,equi-atomicratioalloys,substitutionalalloys,andmulti-componentalloys.Cantoretal.[1,33]pointedoutthataconventionalalloydevelopmentstrategyleadstoanenor-mousamountofknowledgeaboutalloysbasedononeortwocomponents,butlittleornoknowledgeaboutalloyscontainingseveralmaincomponentsinnear-equalproportions.Theoreticalandexperi-mentalworksontheoccurrence,structure,andpropertiesofcrystallinephaseshavebeenrestrictedtoalloysbasedononeortwomaincomponents.Thus,theinformationandunderstandingarehighlydevelopedonalloysclosetothecornersandedgesofamulti-componentphasediagram,withmuchlessknowledgeaboutalloyslocatedatthecenterofthephasediagram,asshownschematicallyforternaryandquaternaryalloysystemsinFig.1.1.Thisimbalanceissigniﬁcantforternaryalloysbutbecomesrapidlymuchmorepronouncedasthenumberofcomponentsincreases.Formostquater-naryandotherhigher-ordersystems,informationaboutalloysatthecenterofthephasediagramisvirtuallynonexistentexceptthoseHEAsystemsthathavebeenreportedveryrecently.Inthe1990s,researchersbegantoexploreformetallicalloyswithsuper-highglass-formingability(GFA).Greer[29]proposedaconfusionprinciple,whichstatesthatthemoreelementsinvolved,thelowerthechancethatthealloycanselectviablecrystalstructures,andthusthegreaterthechanceofglassformation.Maetal.[3]foundthatthebestglassformerisnotexactlyattheeutecticcompo-sition,andhasashifttowardsthehigh-entropyzoneinthephasediagram.Recently,Takeuchietal.[34]reportedahigh-entropybulkmetallicglass(BMG),whichcanhaveacriticalsizeover10mm.Zhaoetal.[35]andGaoetal.[36]reportedahigh-entropyBMG,whichcanbeplasticallydeformedatroomtemperature.However,forsomeHEAs,theirGFAisratherlow,andtheycanonlyformFig.1.1.Schematicternaryandquaternaryalloysystems,showingregionsofthephasediagramthatarerelativelywellknown(green)nearthecornersandrelativelylesswellknown(white)nearthecenter[33].4Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93solid-solutionseventhoughthecoolingrateisveryhigh,e.g.,alloysofCuCoNiCrAlFeTiV,FeCrMnNiCo,CoCrFeNiCu,AlCoCrFeNi,NbMoTaWV,etc.[1,2,12–14].Theyieldstrengthofthebody-centeredcubic(BCC)HEAscanberatherhigh[12],usuallycompa-rabletoBMGs[12].Moreover,thehighstrengthcanbekeptupto800KorhigherforsomeHEAsbasedon3dtransitionmetals[14].Incontrast,BMGscanonlykeeptheirhighstrengthbelowtheirglass-transitiontemperature.1.1.FourcoreeffectsBeingdifferentfromtheconventionalalloys,compositionsinHEAsarecomplexduetotheequi-molarconcentrationofeachcomponent.Yeh[37]summarizedmainlyfourcoreeffectsforHEAs,thatis:(1)Thermodynamics:high-entropyeffects;(2)Kinetics:sluggishdiffusion;(3)Structures:severelatticedistortion;and(4)Properties:cocktaileffects.Wewilldiscussthesefourcoreeffectsseparately.1.1.1.High-entropyeffectThehigh-entropyeffects,whichtendtostabilizethehigh-entropyphases,e.g.,solid-solutionphases,wereﬁrstlyproposedbyYeh[9].Theeffectswereverycounterintuitivebecauseitwasex-pectedthatintermetalliccompoundphasesmayformforthoseequi-ornearequi-atomicalloycom-positionswhicharelocatedatthecenterofthephasediagrams(forexample,amonocliniccompoundAlCeCoformsinthecenterofAl–Ce–Cosystem[38]).AccordingtotheGibbsphaserule,thenumberofphases(P)inagivenalloyatconstantpressureinequilibriumconditionis:P¼Cþ1ÀFð1-1ÞwhereCisthenumberofcomponentsandFisthemaximumnumberofthermodynamicdegreesoffreedominthesystem.Inthecaseofa6-componentsystematgivenpressure,onemightexpectamaximumof7equilibriumphasesataninvariantreaction.However,tooursurprise,HEAsformso-lid-solutionphasesratherthanintermetallicphases[1,2,4,17].Thisisnottosaythatallmulti-compo-nentsinequalmolarratiowillformsolidsolutionphasesatthecenterofthephasediagram.Infact,onlycarefullychosencompositionsthatsatisfytheHEA-formationcriteriawillformsolidsolutionsinsteadofintermetalliccompounds.Thesolid-solutionphase,accordingtotheclassicalphysical-metallurgytheory,isalsocalledater-minalsolidsolution.Thesolid-solutionphaseisbasedononeelement,whichiscalledthesolvent,andcontainsotherminorelements,whicharecalledthesolutes.InHEAs,itisverydifﬁculttodifferentiatethesolventfromthesolutebecauseoftheirequi-molarportions.Manyresearchersreportedthatthemulti-principal-elementalloyscanonlyformsimplephasesofbody-centered-cubic(BCC)orface-cen-tered-cubic(FCC)solidsolutions,andthenumberofphasesformedismuchfewerthanthemaximumnumberofphasesthattheGibbsphaseruleallows[9,23].Thisfeaturealsoindicatesthatthehighen-tropyofthealloystendstoexpandthesolutionlimitsbetweentheelements,whichmayfurthercon-ﬁrmthehigh-entropyeffects.Thehigh-entropyeffectismainlyusedtoexplainthemulti-principal-elementsolidsolution.Accordingtothemaximumentropyproductionprinciple(MEPP)[39],highentropytendstostabilizethehigh-entropyphases,i.e.,solid-solutionphases,ratherthanintermetallicphases.Intermetallicsareusuallyorderedphaseswithlowerconﬁgurationalentropy.Forstoichiometricintermetalliccom-pounds,theirconﬁgurationalentropyiszero.WhetheraHEAofsinglesolidsolutionphaseisinitsequilibriumhasbeenquestionedinthesci-entiﬁccommunity.Therehavebeenaccumulatedevidencestoshowthatthehighentropyofmixingtrulyextendsthesolubilitylimitsofsolidsolution.Forexample,Lucasetal.[40]recentlyreportedab-senceoflong-rangechemicalorderinginequi-molarFeCoCrNialloythatformsadisorderedFCCstruc-ture.Ontheotherhand,itwasreportedthatsomeequi-atomiccompositionssuchasAlCoCrCuFeNicontainseveralphasesofdifferentcompositionswhencoolingslowlyfromthemelt[15],andthusitiscontroversialwhethertheycanbestillclassiﬁedasHEA.TheempiricalrulesinguidingHEAfor-mationareaddressedinSection2,whichincludesatomicsizedifferenceandheatofmixing.Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9351.1.2.SluggishdiffusioneffectThesluggishdiffusioneffecthereiscomparedwiththatoftheconventionalalloysratherthanthebulk-glass-formingalloys.Recently,Yeh[9]studiedthevacancyformationandthecompositionpar-titioninHEAs,andcomparedthediffusioncoefﬁcientsfortheelementsinpuremetals,stainlesssteels,andHEAs,andfoundthattheorderofdiffusionratesinthethreetypesofalloysystemsisshownbe-low:HEAsAtomicradiusofsomeselectedelements[88,89].ElementONCBSPBeSiGeFeNiCrCoCuVMnSnNdRadius(nm)0.073000.075000.077300.082000.102000.106000.112800.115300.124000.124120.124590.124910.125100.127800.131600.135000.162000.16400ElementScMoWRePdPtGaZnSeUNbTaAlAuAgTiGdCeRadius(nm)0.164100.136260.136700.137500.137540.138700.139200.139450.140000.142000.142900.143000.143170.144200.144470.146150.180130.18247ElementMgZrCaSnLaNdScPrInMgLiPbThGdYHfSmRadius(nm)0.160130.160250.197600.162000.187900.164000.164100.165000.165900.160130.151940.174970.180000.180130.180150.157750.1810015Fig.2.3.RelationshipbetweenDelta,d(heredisampliﬁedby100timesforconvenience)andDHmixinsomeHEAs(S:indicatesthealloycontainingonlysolidsolutions;C:indicatesthealloycontainingintermetallics;S1–S8:Ref.[2];9:CrFeCoNiAlCu0.25,10:VCuFeCoNi,11:Al0.5CrFeCoNi,12:Ti2CrCuFeCoNi,13:AlTiVYZr,14:ZrTiVCuNiBe[93]).However,theaboveanalysisonlyconsidersrandomsolidsolution.Fortherealsolution,theentro-pyofmixingcanbemuchmorecomplicatedsincetheexcessentropyofmixingneedstobeconsid-ered.Theexcessentropyofmixingcomesfromexistenceofchemicalorderingorsegregation,andvibrational,magnetic,andelectroniccontributions.Itcanbenegativeorpositive,dependingoneachindividualsystem,asdetailedinareviewarticlebyOriani[87].2.2.ThermodynamicconsiderationsofphaseformationForalloydevelopmentofHEAs,achallengingquestionishowtopredictphasestability(e.g.,thenumberofequilibriumphasesandthemolefractions)asafunctionoftemperatureandcomposition.16Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.2.4.RelationshipbetweenDHmixanddvaluesofsomeHEAsystems[94].Fig.2.5.Phase-formationmapbasedontheXanddforthemulti-componentalloys.Fortheformationofsolid-solutions,X>1.1andd<6.6%.ThezonemarkedBmeansthezonemainlyformsBMGs,andmarkedImainlyformsintermetallicscompounds,alsothereisatransitionzonewhichformsboththerandomsolidsolutionandtheintermetalliccompound[28].AccordingtotheHume-Rutheryrule,theatomic-sizedifference(d)andtheenthalpyofmixing(DHmix)aretwodominantfactors.ForaHEA,thetwoparametersaredeﬁnedasfollows[88,89]:vﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ0u, !!2uXNXuN@d¼ti¼1ci1Àricirii¼1ð2-7Þð2-8ÞDHmix¼NXi¼1;i–j4DHmixABcicjY.Zhangetal./ProgressinMaterialsScience61(2014)1–9317whereriistheatomicradiusoftheithcomponent,andDHmixABistheenthalpyofmixingforthebinaryAandBelements.Bysummarizingthedatafromtheliterature,adiagramwithDHmix$dcanbeplotted,asshowninFig.2.2[17].Itisknownthattheatomicsizeofanelementisaffectedbythesurroundingatoms.HeretheGoldschmidtatomicsizewhichistheatomicsizewhenthecoordinationnumberis12,isused.ThetypicaldataformostelementsarelistedinTable1[90,91].Fortheenthalpyofmixing,onlythebinarydataisavailable.Eq.(2-8)canbeemployedtoevaluatetheenthalpyofmixingforthemulti-componentsalloysbythebinarydata,andsometypicaldataforthebinaryenthalpyofmixingareavailablefromRefs.[88,92].Fig.2.2showsthatDHmixroughlydecreasestomorenegativevalueswiththeincreaseofd.Fortherandomsolidsolutions,theDHmixisintherangefromÀ15to5kJ/mol,whiledisintherangefrom1%to5%.Renetal.[93]alsosummarizedasimilarDHmix$dplotwiththeirresults,asshowninFig.2.3whichcorrelatesverywellwithFig.2.2.ZhangandFu[94]employedvariousHEAsystemstoexplorethequantitativecriterionforphaseformation.ThevaluesofDSmix,DHmix,anddfortheseHEAsystemswerealsocalculatedfromEqs.(2-4),(2-7),and(2-8)andplottedinFig.2.4.ItisclearthatCoCrFeNiCu,CoCrFeNiMn,andCoCrFeNiVHEAscanformsimplesolid-solutionphases.TheDHmixanddvaluesofthesealloysapproachzeroastheirpositionsarelocatedattheupper-leftcornerofFig.2.4.However,theotheralloyswithDHmixanddvaluesbeyondtheupper-leftcornercontainnotonlysolid-solutionphasesbutalsointermetalliccompounds.Therefore,criterionforsimplesolidsolutionsinHEAsshouldbetheDHmixanddvaluesfallingintotheupper-leftdistrictinFig.2.4underthefulﬁllmentofhighentropyofmixing(DSmix=1.61R).Tobespeciﬁc,thequantitativecriterionfortheformationofsimplesolidsolutionsis:DSmix>13:38J=Kmol;À10kJ=mol1meansthattheeffectofthemixingentropyisgreaterthanthatoftheenthalpyofmixingatthemeltingtemperature,andthehigh-entropyphasetendstoform.Togiveadeﬁnitionforthehighentropy,weﬁrstneedabenchmark:eitherthefusionentropyofthemetallicalloysortheenthalpyofmixingatthemeltingpoint.Yehetal.[2]selectedthefusionentropyandtheentropyofmixingfortheﬁveelementsatanequi-atomicratioisgreaterthanthefusionentropy.Theessenceofthiscriterion,asshowninEq.(2-9),isthathigh-entropysolidsolutionphasemayformaslongasX>1issatisﬁed,whichimpliesthattherequirementofﬁveprincipalelementstoformHEAsmaynotbenecessary.Theoret-icallyspeaking,threeelementsmightformHEAs.Themoreelementsinanequi-molarHEA,thehigherentropyofmixingis.However,thecontentofeachelementshouldbehigherthan5at.%.TherelationbetweenXanddisshowninFig.2.5.Theplotsuggeststhattherequirementsforthesolid-solutionformationareXP1.1andd66.6%.Moreover,Xanddobeyahyperbolicrelation,whichindicatesthatdÂLnX=constant[28].2.3.MicrostructuresofHEAsHume-Rothery[95]generalizedseveralrulesonsubstitutionalsolidsolutionsinalloysystems,including:(1)thedifferenceintheatomicsizebetweenthesoluteandsolventatomsmustbelessthan15%,(2)thecrystalstructuresofthesoluteandsolventmustmatch,(3)therearethesamevalencestatesbetweenthesolventandsolute,and(4)thesoluteandsolventshouldhavesimilarelectroneg-ativity.Forgenerations,Hume-Rotheryruleshavebeenusedinthetraditionalalloyingdesign.Gener-ally,asolidsolutionisoftenobservedwhenthetwoelements(generallymetals)involvedarefromthesamefamilyintheperiodictable(i.e.,thesamecolumn).Conversely,achemicalcompoundformswhentheHume-Rotheryrulesarenotsatisﬁed.MindthatHume-Rotheryrulesonlyaddresssubstitu-tionalsolidsolution.Thesolutemaybeincorporatedintothesolventcrystallatticesubstitutionally,byreplacingasolventatominthelattice,orinterstitially,byﬁttingintothespacebetweensolventY.Zhangetal./ProgressinMaterialsScience61(2014)1–9319Fig.2.8.Therelationshipamongthemicrostructure,NieqandCreqofCuCrFeNiMnalloysystem[93].Fig.2.9.X-raydiffractionpatternsforAlxCrCuFeNi2alloys(x=0.2–1.2)[97].atoms.Bothtypesofsolidsolutionsaffectthepropertiesofthematerialbydistortingthecrystallatticeanddisruptingthephysicalandelectronichomogeneityofthesolventmaterial.ThebinaryCu–Niphasediagram,asshowninFig.2.6,isanexampleofHume-Rotheryrulesoffor-mationofanisomorphoussolidsolution[96].All4rulesaresatisﬁedinthissimplecase.ThecaseofHEAsseemstobeanexceptiontotheHume-Rotheryrules.AllHEAsreportedhaveaminimumof4–5components,andtheelementshaveamixtureofsimpleFCC,BCC,andHCPstructures.Forexample,theFCCCoCrCuFeNiHEAcontainsaBCCmetal,Cr,whilebothCuandNiareFCC.Feundergoesstruc-turalchangesfromBCCtoFCCat912°CandFCCtoBCCagainat1394°C,andCotransformstoFCCfromHCPat422°C.Furthermore,Cralsohasmuchlowerelectronegativitythantherestoftheele-mentsinthealloy.Consequently,therearefewelementsintheperiodictablethatissolubleinCrtoanappreciableamountexceptV.However,formationofaFCCsolidsolutionphasewasobservedandformationofintermetallicrphases(Co2Cr3anda-CoFe,PearsonsymboloftP30)wasdepressedintheCoCrCuFeNiandCoCrFeNiHEAs.AnotherexampleistheformationofaBCCsolidsolutionphaseintheAl3CoCrCuFeNiHEA.Inthiscase,theelement,Al,hasanFCCstructure.Therefore,itistemptingtostatethatthehighentropyofmixingcanoverweightheHume-Rotheryrulesandperhapsisthemostimportantparameterinthesolid-solutionformationthathasbeenoverlookedinthetraditionalphysicalmetallurgy.Tilltoday,allreportedHEAshaveeithertheFCCorBCCstructure,andnoHCPstructuredHEAs,asshowninFig.2.7[11].ThisobservationisnotsosurprisingbecausemostelementspreferaBCCorFCCstructure.Amongtransitionmetals,thereareatotalof9elementsthathaveaHCPstructureatroom20Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.2.10.RelationshipbetweenVECandtheFCC,BCCphasestabilityforHEAsystems.Noteonthelegend:greencolorforsoleFCCphases;redcolorforsoleBCCphase;yellowcolorformixes,FCCandBCCphases[97].temperature,i.e.Sc,Ti,Zr,Hf,Co,Tc,Ru,Re,Os,CdandZnwiththeﬁrstﬁvetransformingtoBCCathighertemperature.TcisaradioactiveelementwhileRe,RuandOsareextremelyexpensiveele-ments.CdandZnbothhaveaverylowmeltingpointandhighvaporpressure.Amongnon-transitionmetals,MgprefersaHCPstructurewhileBeundergoesHCP?BCCtransformationatT=1270°C.WhilethevastmajorityofrareearthelementspreferaHCPordoubleHCPstructureatroomtemper-ature,mostofthemtransformtoBCCatveryhightemperatures.Therefore,itislikelythatHEAsbasedonrareearthelementscanbeformedwithaHCPstructuregiventhat:(1)theyallhavesimilaratomicsizes;(2)theyallformisomorphousbinarysolidsolution.Renetal.[93]suggestedthatelementalinteractionscanaffectnotonlythedistributionofelementsindendrite(DR)andinter-dendrite(ID)regionsbutalsothemicrostructuresintheCuCrFeNiMnalloysystem.AlloyswithasingleFCCphasearealwaysrelatedtotheirhighcontentsofCu,Ni,andMn,whichissimilarinstainlesssteels.Accordingtothealloyingeffectofelementsonmicrostructuralcharacteristicsinstainlesssteels,elementscanbeclassiﬁedintotwotypes:oneisFCCstabilizer,suchasNi,Mn,Cu,CandN,andtheotherisBCCstabilizer,suchasCr,Mo,Si,andNb.TheNiequivalent(Nieq,byamassfraction)andCrequivalent(Creq,byamassfraction)areoftenusedtopredictthechangesofmicrostructuresinstainlesssteels:thehighertheNieq,theeasieritistoobtainanFCCstructure.Conversely,thehighertheCreq,theeasieritistoformaBCCstructure.Therefore,theten-dencyinformingFCCandBCCphasescanbeestimatedbymeansoftherelationshipbetweenNieqandCreqinHEAs.BasedontheexperienceandthedistinctcompositiondifferencebetweenstainlesssteelsandHEAs,theexpressionsofNieqandCreqaremodiﬁedproperlyasfollows[93]:Nieq¼Ni%þ0:5Mn%þ0:25Cu%Creq¼Cr%þFe%ð2-10Þð2-11ÞwhereNi%,Mn%,Cu%,Cr%,orFe%standsfortheatomicpercent(at.%)ofeachelement,asshowninFig.2.8.However,Coisawell-knownstrongFCCformer.OtherFCCformersthatarelessknownincludePd,Pt,Ir,andRh.OntheBCCformerside,mostrefractorymetalshaveaBCCstructureandtendtostabilizetheBCCstructure,and,consequently,HEAsofvariouselementalcombinationshavebeenreported,basedonrefractorymetals[14].Guoetal.[97]suggestedusingthevalenceelectronconcentration(VEC)topredicttheBCCandFCCstructuredsolidsolutionsofHEAs.Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9321Fig.2.11.SchematicillustrationofBCCcrystalstructure:(a)perfectlattice(takeCrasexample);(b)distortedlatticecausedbyadditionalonecomponentwithdifferentatomicradius(takeaCr–Vsolidsolutionasexample);(c)seriousdistortedlatticecausedbymanykindsofdifferent-sizedatomsrandomlydistributedinthecrystallatticewiththesameprobabilitytooccupythelatticesitesinmulti-componentsolidsolutions[20].VEC¼XCiðVECiÞð2-12ÞhereCiistheatomicpercentoftheithelement,andVECiistheVECoftheithelement.Fig.2.9showstheX-raydiffraction(XRD)patternsofGuo’salloys(AlxCrCuFeNi2)[97].ItisseenthatwiththeincreaseoftheAlcontent,thephasestructureschangefromFCCtoBCCinAlxCrCuFeNi2alloys.Fig.2.10summarizedtherelationshipbetweenthestructureandVEC,andwecanseethatfortheBCC-structuredsolidsolution,VEC<6.8;whileforFCC,VEC>8[97].22Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.2.12.PredictedphasediagramofAlxCoCrCuFeNialloysystemwithdifferentaluminumcontents(xvalues).L:liquidphase.ThephasetransitiontemperaturesofthealloysweremeasuredbyDTA(temperaturelimitofmeasurement:1400°C)andindicatedasblacksoliddotsintheﬁgure[45].Fig.3.1.Aschematicdiagramofthearcmeltingmethod[99].Zhangetal.[20]suggestedthatthephasechangefromtheFCCtoBCCintheTixCoCrFeNiCu1ÀyAlyalloysystemshouldbeduetotheatomic-levelstrainenergy,asshowninFig.2.11.AstheAlelementhasarelativelylargeratomicsize,theadditionoftheAlelementwillinducethehigheratomic-levelstress[73,74]inHEAswiththestructureofhigheratomic-packingefﬁciency(APE).TheHEAwithanY.Zhangetal./ProgressinMaterialsScience61(2014)1–9323FCCstructuregenerallyhashigherAPEthanthatwithaBCCstructure.WithmoreAladditions,theatomic-levelstresswillbeincreasedtoohightotolerate,andthus,resultinginatransitiontotheBCCstructurewithlowerAPE,whichwilldecreasethestrainenergyandreducetheGibbsfreeenergy.Tongetal.[45]plottedasimplephasediagramfortheAlxCoCrCuFeNialloy,aspresentedinFig.2.12,whichshowsthephasechangewiththevariationofAlcontentatdifferenttemperatures.Thephasediagramshowsaneutecticreactionat15at.%Al.InthelowerAlcontentregion(<15at.%),theprimaryphaseisanFCCphase.WhileinthehigherAlcontentregion(>15at.%),thepri-maryphasebecomesaBCCphase.WhentheAlcontentincreasestomorethan25at.%,theorderedB2phasedominates.3.KineticsandalloypreparationThermodynamics,asmentionedintheformersection,mainlyaddressesthestabilityofthestate,anddoesnotconcerntherate,time,androutes.Thekineticsinmaterialsscienceisdeﬁnedasthepro-cessofthematerialstransitionfromonestatetoanother,withtime.Thereisalsoanotherterm,dynamics,whichismainlyusedformechanicalbehavior,andconventionallyrelatedtotheloadingspeedandtime.Thus,thekineticsforHEAsinthissectionwillfocusonthemobilityoftheelementsduringphasetransformations.Inanotherword,themobilityoftheparticlesinthealloyisequaltotheinverseoftheviscosity,g,ofthealloy,whichisrelatedtothediffusioncoefﬁcient,D,bytheStokes–Einsteinequation[98]:D¼jT6pgcð3-1ÞEq.(3-1)isapplicableinthecaseoflowReynoldsnumbers,fordiffusionofsphericalparticlesthroughaliquid,whereDisthediffusionconstant,cistheradiusofadiffusingparticle,Tistheabsolutetem-Rperature,andj¼N,whereRisthegasconstant,andNisAvogadro’snumber.Thehighentropyofthealloyinitsliquidstatewillpotentiallyfacilitateformingthehigh-entropyphases.However,thekineticsplayaveryimportantroleforphaseformation.ThemicrostructuresofHEAscanbecontrolledbythecoolingrate,theprocessingroutesofthealloys,andplasticdeformationincombinationwithvariousheattreatment.Thus,theirpropertiescanbeoptimizedbycontrollingprocessingparameters.Fig.3.2.XRDpatternsofbothsplat-quenchedandas-castequi-atomicAlCoCrCuFeNiHEA.Thesplat-quenchedHEAshowsthepresenceofaBCCphase(aBCC=2.87Å),whereastheas-castHEalloyindicatesthepresenceofoneBCC(aBCC=2.87Å)andtwoFCCphases.TwoFCCphasesareclearlyvisiblefromtwooverlappingpeaksshownintheinset,correspondingtothetwo{111}planeswithslightlydifferentlatticeparameters(aFCC1=3.59ÅandaFCC2=3.62Å)[15].24Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.3.3.Bright-ﬁeldTEMimagesofas-castequiatomicAlCoCrCuFeNiHEalloy:(a)dendriteandinterdendriticregions;(b)microstructureofdendriticregionwithplate-likeprecipitatesorientedalongtheh110idirections.Thecorrespondingelectrondiffractionpatternofthe[001]zoneaxispresentedintheinsetexhibitsreﬂectionsoforderedBCC,B2structure(aBCC=2.88Å).ThecharactersA–DarepositionsoftheEDXmeasurements;(c)dendriticregionwithrhombohedron-shapedprecipitates(aFCC2=3.64Å).Thecorrespondingelectrondiffractionpatternoftheh110izoneaxisintheinsetdisplaysweaksuperlatticereﬂectionscorrespondingtoL12structure;(d)microstructureoftheinterdendriticregionandcorrespondingelectrondiffractionpatternoftheh112izoneaxis,showingweaksuperlatticereﬂectionscorrespondingtoL12structure(aFCC1=3.58Å)[15].Atpresent,typicalprocessingroutesforHEAscanbesummarizedaccordingtothestartingstatesforthealloypreparation,mainly(1)fromtheliquidstate,(2)fromthesolidstate,(3)fromthegasstate,and(4)fromelectrochemicalprocess.3.1.PreparationfromtheliquidstateApopularliquidprocessingmethodisarcmelting.ThetypicalfurnaceisshowninFig.3.1.Thetorchtemperatureofthearc-meltingfurnacecanbeveryhigh(>3000°C),andcanbecontrolledbyFig.3.4.SchematicrepresentationofphasesegregationobservedduringsolidiﬁcationofAlCoCrCuFeNiHEAbytwodifferentprocessingconditions:splatquenching(coolingrate106–107KsÀ1)andcasting(coolingrate10–20KsÀ1)[15].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9325adjustingtheelectricalpower.Hence,mostofthehigh-meltingelementscanbemixedintheirliquidstatebythiskindoffurnaces[99].However,forelementswithalowmeltingpoint,whichareeasytoevaporate,e.g.,Mg,Zn,andMn,thearc-meltingprocessmaynotbethebestchoice,becausethecom-positioncannotbepreciselycontrolled.Inthiscase,resistanceheatingorinductionheatingmaybemuchmoreappropriate.Singhetal.[15]carefullystudiedthedecompositionprocessoftheAlCoCrCuFeNiHEA,andtheyfoundthatahighcoolingratetendedtopromoteformationofasinglephase,aspresentedinFig.3.2.Fig.3.3showsthebright-ﬁeldTEMimagesoftheas-castAlCoCrCuFeNiHEA.Fivephaseswerepresentintheas-castsample;theinterdendriticregionconsistingoftheCu-richphaseofanL12struc-ture(FCC1),dendritescontainingCu-richplate-likeprecipitatesoftheB2type,rhombohedron-shapedCu-richprecipitatesoftheL12type(FCC2),Al–Ni-richplates(B2),andCr–Fe-richinterplates(BCC).ThestructureofthesphericalCu-richprecipitatescouldnotbedeterminedbecauseoftheirsmallsize,buttheircompositionissimilartothatoftheplate-likeCu-richprecipitates.Thus,itcanbeconsideredthatthesetwophasesareofthesametype.Nevertheless,theCu-richFCC1andFCC2phaseshavedif-ferentcompositionsandwereidentiﬁedastwoseparatephases.Thekineticsofmicrostructureforma-tioncanbesummarizedinFig.3.4,whichshowsastrongdependenceofmicrostructureonthecoolingrateduringpreparation.Itcanbefoundthathighcoolingratesfavoredtheformationofpolycrystal-linephaseswithasizeoffewnanometers.However,relativelylowcoolingratesledtotheformationoftypicaldendriticandinterdendriticmicrostructuresduetoelementalsegregation.Thisworkclearlydemonstratedthatforcertain‘‘HEAs’’,asinglesolid-solutionphasecanonlyformatrelativelyhighcoolingrates,andtheirhigh-entropystateisvalidinametastablecondition.Annealingatelevatedtemperaturesorusingslowcoolingratesinducestheformationofmultiplephases,resultinginadra-maticreductionintheconﬁgurationalentropyofmixingduetoelementalpartitioningamongthesephases.Therefore,properannealingexperimentsarecrucialtotestifyifatrueequilibriumstateexists,i.e.,asinglehigh-entropysolid-solutionphasecanform.Tongetal.[45]summarizedthekineticsoftheAlxCoCrCuFeNi(xfrom0to3.0)HEAsduringthecoolingprocess,asshowninFig.3.5.ForthealloycontainingahighAlcontent,Spinodaldecomposi-tionoccurs,leadingtoasubmicron-modulatedstructure.Fig.3.6showstheXRDpatternsforthealloyhavingalowAlcontent(i.e.,Al0.5CoCrCuFeNi)preparedunderdifferentconditions(as-annealed,as-rolled,andas-homogenized).Itisclearthatphaseformationandmicrostructuresofthealloyaretyp-Fig.3.5.DepictionofphaseformationsequenceduringcoolingofAlxCoCrCuFeNialloysystemwithdifferentaluminumcontents[45].26Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.3.6.XRDpatternsofAl0.5CoCrCuFeNialloyinthethreestates.QC:waterquench[100].icalkineticallydependent[100].Afterannealingat1173Kfor5h,forexample,thediffractionpeakcorrespondingtotheCu-richFCCphasebecomesstrong,suggestingthatformationofthisphasewasgreatlypromoted.AnotherliquidtechniqueisBridgmansolidiﬁcation,whichisalsocalledtheBridgman–Stockbargermethod[101,102].ThismethodisnamedaftertheHarvardphysicist,PercyWilliamsBridgman,andtheMassachusettsInstituteTechnology(MIT)physicist,DonaldC.Stockbarger.Thistechniqueispri-marilyusedforgrowingsingle-crystalingots,andinvolvesheatingthepolycrystallinematerialaboveitsmeltingpointandslowlycoolingitfromoneendofitscontainer,whereaseedcrystalislocated.Fig.3.7.AschematicdiagramoftheBridgmansolidiﬁcation[104].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9327Fig.3.8.AschematicdiagramoftheconstitutionalundercoolingbyGandV,hereweassumethatthesolidandliquidinterfacesareparalleltotheliquidmetalofGa–Inalloyssurface,andthedistancebetweenthetwofaceskeepsconstant[106].Asinglecrystalofthesamecrystallographicorientationastheseedmaterialisgrownontheseed,andprogressivelyformedalongthelengthofthecontainer.Suchprocesscanbecarriedoutinahorizontalorverticalgeometry.TheBridgmanmethodisapopularwayforproducingcertainsemiconductorcrystalsforwhichtheCzochralskiprocess[103]ismoredifﬁcult,suchasgalliumarsenide.Thediffer-encebetweentheBridgmantechniqueandtheStockbargermethodissubtle.WhileatemperaturegradientisalreadyinplacefortheBridgmantechnique,theStockbargermethodrequirespullingtheboatthroughatemperaturegradienttogrowthedesiredsinglecrystal.Whenseedcrystalsarenotemployed,polycrystallineingotscanalsobeproducedfromafeedstockconsistingofrods,chunks,oranyirregularly-shapedpiecesoncetheyaremeltedandallowedtore-solidify.Theresultingmicro-structuresoftheingotsasobtainedarecharacteristicofdirectionallysolidiﬁedmetalsandalloyswiththealignedgrains[90,91].Fig.3.7showsatypicalBridgmansolidiﬁcationfacilitycooledbyaliquidmetal(aGa–Ineutecticalloy,whichisaliquidatroomtemperature)[104].Thesamplesareloadedinacrucibleandmeltedbyinductiveheatingorresistantheating,andthenthemeltedalloysaregrad-uallypulleddowntotheliquidmetal.Theoutsideoftheliquidmetalisfurthercooledbywater.Zhangetal.[105]reportedthatthemorphologyofAlCoCrFeNialloychangedfromthedendritepreparedbycoppermouldcastingtotheequi-axedgrainsbyBridgmansolidiﬁcation.Thischangewasduetothehigh-temperaturegradient,G,thelowgrowthvelocity,V,andthehighratioofG/VfortheBridgmansolidiﬁcation.Forthecopper-mouldcasting,G/Visusuallylow,astheGisavariableandVisusuallyveryhigh.AsshowninFig.3.8,thehighG/Vvaluetendtodecreasetheconstitutionalundercoolingofthealloy,andthusitispossibletorestrainthedendriteformation[106].Wangetal.[107]appliedathermal-spray(TS)technologytofabricatecoatingsoftheNixCo0.6Fe0.2-CrySizAlTi0.2HEAs.AtypicalTSplasmafacilityispresentedinFig.3.9[108,109].Inthisprocess,ﬁnely-dividedHEApowdersareinitiallymeltedonpreparedsubstratesinordertoformspraydeposits.Therequiredheatisgeneratedbycombustiblegasesorelectricarcsinthethermal-sprayinggun.Asthetargetmaterialisgraduallyheatedup,itisconvertedtoamoltenstate,andwillbeacceleratedbythecompressedgas.Theconﬁnedstreamofparticlesiscarriedtothesubstrate,andstrikesthesurfacetoﬂattenandformsthinplatelets.Theseplateletsarecompatiblewiththeirregularitiesofthepre-paredsurfaceandtoeachother.Moreover,thesesprayedparticlesareaccumulatedonthesubstratebycoolingandbuildinguponebyoneintoacohesivestructure.Thus,coatingsareformed.TheresultsalsoindicatethatthehardnessoftheHEAspreparedbytheTSincombinationwithannealingat1100°C/10hissigniﬁcantlyincreasedtothatoftheas-caststate(1045HV).Thesesam-plesexhibitedexcellentcoarseningresistance,resultingfromtheCr3Siandseveralunidentiﬁedphases.ThemainfeaturesfoundinTEMarelargeamountsofnano-sizedprecipitatesanddislocations.Notethat,thisNixCo0.6Fe0.2CrySizAlTi0.2alloysystemdoesprecipitateduringcasting,whichisquitedif-ferentfrommanyotherHEAs.ThetypicalTEMcharacterizationoftheas-castNixCo0.6Fe0.2CrySizAlTi0.2alloyswiththeaddition/removalofMn,Si,andNielementsarealsocomparedinthisstudy.There-sultsshowedthatasigniﬁcantamountofnano-sizedparticles(rangingfrom5to10nm),atomicseg-regation,twinningstructures,andsub-grainstructuresweredistributedinthematrix.TheabovestudyfurtherconﬁrmedthatphaseformationandﬁnalmicrostructureofHEAsarestronglydependantontheprocessingconditions.Zhangetal.[110]reportedthelow-costHEAcoatingwithanominal28Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.3.9.PlasmaSprayProcess(AirPlasmaSprayCoating).Itisprocesssinwhichthemetalsubstrateiscoatedwithcoatinggivingitasmoothprotectivelayer.Thisincludesairplasmaspraying(ASP)particlesofthecoatingatapre-selectedparticlewithvelocityofabout500meterspersecond[108].compositionof6FeNiCoCrAlTiSipreparedbylasercladding.Atypicalschematic[111–113]isshowninFig.3.10.ThistechnologyissimilartotheTSmethodinthatithasanenergysourcetomeltthefeedstockthatisbeingappliedtoasubstrate.Whatitdiffersisthatitusesaconcentratedlaserbeamastheheatsource,anditmeltsthesubstratethatthefeedstockisbeingappliedto.ThistechniquenormallyresultsinametallurgicalbondthathasthesuperiorbondstrengthoverTS.Theresultantcoatingisdensewithnovoidsorporosity.Oneoftheadvantagesofthelaser-claddingprocessisthelaserbeamwhichcanbefocusedandconcentratedtoaverysmallareaandkeepstheheat-affectedzoneofthesubstrateveryshallow.Thisfeatureminimizesthechanceofcracking,distorting,orchangingthemet-allurgyofthesubstrate.Additionally,thelowertotalheatminimizesthedilutionofthecoatingwithmaterialsfromthesubstrate.ThecoatingpreparedbylasercladdinghasasimpleBCCsolidsolutionwithhighmicro-hardness,highresistancetosoftening,andlargeelectricalresistivity.AfterbeingannealedatT<750°C,thecoatingshowshighthermalstability,anditsresistivityslightlydecreases,butthemicro-hardnessal-mostremainsunchanged.AfterannealingatT>750°C,themicro-hardnessofthecoatingslowlyde-creaseswithincreasingthedecompositionrateoftheBCCsolidsolutions[110].Fig.3.10.Atypicalhigh-performancelasercladdingusingthelaser-inductionhybridcladdinghead[111].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93293.2.PreparationfromthesolidstateMechanicalalloying(MA)isasolid-statepowder-processingtechniqueinvolvingrepeatedcoldwelding,fracturing,andre-weldingofpowderparticlesinahigh-energyballmill[114,115].Originallydevelopedtoproduceoxide-dispersionstrengthenednickel-andiron-basesuperalloysforapplica-tionsinaerospaceindustry,MAhasnowbeenshowntobecapableofsynthesizingavarietyofequi-libriumandnon-equilibriumalloysstartingfromblendedelementalorpre-alloyedpowders.MAisakintometal-powderprocessing,wheremetalsmaybemixedtoproducesuperalloys.Mechanicalalloyingoccursinthreesteps.First,thealloymaterialsarecombinedinaballmillandgroundtoﬁnepowders.Ahot-isostatic-pressing(HIP)processisthenappliedtosimultaneouslycompressandsinterthepowders.Aﬁnalheat-treatmentstagehelpsremoveexistinginternalstressesproducedduringanycoldcompaction,whichmayhavebeenused.ThisMAprocesshassuccessfullyproducedalloyssuit-ableforhigh-heatturbinebladesandotheraerospacecomponents.TheschematicforthemechanicalalloyingtechniqueisshowninFig.3.11[116].Chenetal.[117]preparedtheBeCoMgTiandBeCoMgTiZnequi-molaralloysentirelycomposedofHCPelementsbyMA.Nocrystallinesolidsolutionsandcompoundsformedbeforefullamorphization.WeeberandBakkerhadclassiﬁedtheamorphizationreactionsofMAinbinaryalloysintothreetypesin1988[118,119].Theﬁrsttype(type-I)featuredpeakshiftingofeachelementduetotheformationofacrystallinesolid-solutionphase,andthenpeakbroadeningduetotheformationofanamorphousstructure.Thesecondtype(type-II)featuredadecreaseofelementalpeaksaccompaniedwithanin-creaseoftheamorphousbroadpeak.Theﬁnaltype(type-III)featuredtheformationofanintermetal-licorintermediatecompoundpriortoanamorphousstructure[118].Theamorphizationprocessesofthesetwoalloysconformthetype-IIamorphizationoftheclassiﬁcationproposedbyWeeberandBak-keretal.[118,119].Theinhibitionofintermetalliccompoundsbeforeamorphizationisduetochem-icalcompatibilityamongconstituentelementsincompanywithhigh-entropyanddeformationeffects,whichenhancethemutualsolubility.Directformationoftheamorphousphaseinsteadofthecrystallineoneattributestotheirlargerangeofatomicsize.Thismechanismcouldbeaguidelineforthetype-IIamorphizationofmulti-componentalloys.Varalakshmietal.[120]reportedthenanocrystallineequiatomicHEAssynthesizedbyMAintheCuNiCoZnAlTisystemfromthebinaryCuNialloytothesenaryCuNiCoZnAlTialloy.Anattempthasbeenmadetoﬁndtheinﬂuenceofnon-equiatomiccompositionsontheHEAformationbyvaryingtheCucontentupto50at.%(CuxNiCoZnAlTi;x=0%,8.33%,33.33%,and49.98%).Thephaseformationandstabilityofmechanically-alloyedpowdersatanelevatedtemperature(1073Kfor1h)werestud-ied.Thenano-crystallineequi-atomicalloyshaveFCCstructureuptothequinaryCuNiCoZnalloyandhaveaBCCstructureinthehexanaryCuNiCoZnAlTialloy.Innon-equiatomicalloys,BCCistheFig.3.11.Theschematicforthemechanicalalloying[116].30Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.3.12.Aschematicdiagramofthesputtermethod[123].dominatingphaseinthealloyscontaininglessthan8.33at.%Cu,andtheFCCphasewasobservedinalloyswithahigheramountofCu.TheVicker’sbulkhardnessandcompressivestrengthoftheequi-atomicnano-crystallinesenaryCuNiCoZnAlTiHEAafterHIPare8.79and2.76GPa,respectively.ThehardnessoftheseHEAsishigherthanthatofmostcommercialhardfacingalloys(e.g.,stellite,whosecompressivestrengthis4.94GPa).3.3.PreparationfromthegasstateTouseHEAcoatingsastribologicalapplications,Changetal.[121]preparedthe(AlCrTaTiZr)Nxmulti-componentcoatingsbyusingthesputteringtechniquewhichisdemonstratedinFig.3.12[122].Mechanicalproperties,creepbehaviors,deformationmechanisms,andinterfaceadhesionofthe(AlCrTaTiZr)NxcoatingswithdifferentNcontentswerecharacterized.WithincreasingtheN2-to-total(N2+Ar)ﬂowratio(RN)duringthesputteringdeposition,the(AlCrTaTiZr)Nxcoatingstransformedfromanamorphousphasetoanano-compositeandﬁnallyacrystallinenitridestructure.Thehardnessofthecoatingsaccordinglyincreasedfrom13GPatoahighvalueofabout30GPa,butthecreepstrainratealsoincreasedfrom1.3Â10À4to7.3Â10À4sÀ1.TheplasticdeformationoftheamorphouscoatingdepositedwithRN=0%proceededthroughtheformationandextensionofshearbands,whereasdislocationactivitiesdominatedthedeformationbehaviorofthecrystallinenitridecoatingsdepositedwithRN=10%and30%.WithincreasingtheRN,theinterfaceadhesionenergybe-tweenthecoatingsandthesubstrateswasalsoenhancedfrom6.1to22.9J/m2.OntheSisubstrates,anativeoxidelayermightexist.Consequently,thebondingbetweentheoxidelayerandthemetalliccoatingwithRN=0%wasweak.WithincreasingtheRN,moreNatomswereintroducedandalargernumberofstrongcovalentbondsbetweenN,OandSimightprobablyformattheinterfaces,thusenhancingtheinterfaceadhesionbetweenthe(AlCrTaTiZr)Nxcoatingsandthesubstrates.Themorphologyofthesurfaceandcross-sectionofthedepositedﬁlmsareshowninFig.3.13.Itisclearthattheﬁlmsareveryuniform.InFig.3.13(a)and(b),nospecialgrowthfeaturewasfoundintheamorphouscoatingdepositedwithRN=0%.WhenincreasingtheRNto10%,acolumnarstructurewithapyramid-likesurfacewasobservedinFig.3.13(c)and(d),suggestingformationofspeciﬁccrys-tallographyinthisﬁlmbyintroducinganN2ﬂow.AstheRNincreasedto30%,thesurfacemorphologyY.Zhangetal./ProgressinMaterialsScience61(2014)1–9331Fig.3.13.SEMsurfacemorphologiesandcross-sectionalmicrostructuresof(AlCrTaTiZr)NxcoatingsdepositedwithdifferentN2-to-totalﬂowratios(RN):(a)surfaceand(b)cross-sectionforRN=0%;(c)surfaceand(d)cross-sectionforRN=10%;(e)surfaceand(f)cross-sectionforRN=30%[121].transformedtoadome-likestructurecoexistedwiththecolumnarstructureasseeninFig.3.13(e)and(f),implyingthefurtherchangeinthecrystallographicstructurewithahighN2ﬂow.Linetal.[124]reportedthatthe(AlCrTaTiZr)Oxﬁlmsdepositedat623Kbydirectcurrent(DC)magnetronsputteringfromHEAtarget.Oxygenconcentrationincreaseswiththeoxygenﬂowratio(RO),andsaturatesnear67at.%.As-depositedﬁlmshaveanamorphousstructure,butthemorphologyoftheseﬁlmsaredifferentasshowninFig.3.14.Themetallicﬁlmiscomposedofgrainsabout20nm,andsmallergranuleswereseeninthegrainswithoutoxygen.Thecross-sectionimageshowscolumn-likestructurealongthegrowthdirection.Thesmallergranulesareamorphousclusters,the‘‘grains’’areagglomerationsoftheclusters,andthecolumnsarethusbundlesoftheagglomerationscaused32Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.3.14.FieldEmissionScanningElectronMicroscopy(FE-SEM)imagesoftheas-depositedﬁlms:(a)planeviewand(b)cross-sectionimagesofAlCrTaTiZrmetallicﬁlms;(c)planeviewand(d)cross-sectionimagesofﬁlmsdepositedatoxygenﬂowratio(RO)=2.5%;(e)planeviewand(f)cross-sectionimagesofﬁlmsdepositedatRO=15%;(g)planeviewand(h)cross-sectionimagesofﬁlmsdepositedatRO=50%[124].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9333fromtheﬁlmgrowthduringsputtering.Afteradditionof2.5%oxygentotheatmosphere,thecontrastoftheﬁlmsfades,themorphologybecomesunclear,andthecolumnstructuredisappears.Theﬁlmisdensewithnolowdensitycolumnargrainboundaries.However,thesurfacesoftheﬁlmspreparedatRO=15%becomerougherandshowmosaicofcracknetwork.Theaveragemosaicunitsizeisaround30nm.Thecrosssectionoftheﬁlmshowslotsofdiscretepellets,whichindicatesthattheﬁlminteg-rityispoorandeasytoinducelocalcracking.AtRO=50%,themorphologyissimilartothemosaicofcracksatRO=15%,whichisattributabletothetensilestressinducedbythethermalexpansioncoef-ﬁcientdifferencebetweentheﬁlmandsubstrateduringcooling.Thehardnessoftheseﬁlmsvariesintherangeof8–13GPa.Allamorphousoxideﬁlmsmaintaintheiramorphousstructureupto800°Cforatleast1h.Afterannealingat900°Cfor5h,crystallinephaseswiththestructuresofZrO2,TiO2,orTi2ZrO6form.Annealingenhancesmechanicalpropertiesoftheﬁlms,andthehardnessandmoduluscanreachthevaluesofabout20and260GPa,respectively.Theresistivityofthemetallicﬁlmsisaround102lXcmbutdrasticallyrisesto1012lXcmwhentheoxygenconcentrationincreases.Fig.4.1.(a)TheadditionofNbelementsintothisHEAchangestheoriginalphaseconstitution,whichyieldstheformationoforderedLavesphasebesidessolidsolutionphase.(b)Thecompressivestress–straincurvesoftheAlCoCrFeNiNbxrodsampleswithadiameterof5mm(x=0,0.1,0.25,and0.5)[135].34Y.Zhangetal./ProgressinMaterialsScience61(2014)1–933.4.ElectrochemicalpreparationYaoetal.[125]preparedtheBiFeCoNiMnHEAﬁlmusinganelectrochemicalmethod.SEMshowsthatthesurfaceoftheﬁlmwasgrained,andthenano-rodswithahighaspectratioof10wereobtainedbypotentiostaticelectrodepositionintheN,N-dimethylformamide(DMF)–CH3CNorganicsystem.AnamorphousﬁlmofBi19.3Fe20.7Co18.8Ni22.0Mn19.2wasobtainedbypotentiostaticelectrode-positionatÀ2.0V,butamainsolidsolutionphasewithaFCCstructurewasidentiﬁedbyXRDandselectedareaelectrondiffraction(SAED)patternsaftertheﬁlmswereannealedunderN2atmosphere.Also,thedifferentialthermalanalysis(DTA)curveveriﬁedthattheﬁlmwascrystallizedbytheheattreatmentat762K.Theas-depositedﬁlmsshowsoftmagneticbehavior,andhardmagneticanisot-ropywasobservedintheannealedﬁlms.LimitedworkonpreparingHEAsbyelectro-depositionwasreported,butdeﬁnitely,thisfabricationrouteprovideaninnovativeapproachtodevelopnewHEAswithuniqueproperties.4.PropertiesTheconstitutionofmaterialsscienceusuallycontainsfourcomponents,whichformsthematerialssciencetetrahedron.Theyinclude:(1)compositionsandstructures;(2)processing;(3)properties;and(4)performance.Sometimes,thecharacterizationorthemodelingandsimulationcanalsobethoughtasthecenterofthetetrahedron.Inthischapter,themechanical,physical,andchemicalpropertiesofHEAsaredescribedasfollows.4.1.MechanicalbehaviorSuperiorstructuralalloysremaininahighdemandforsomeextremeenvironmentalengineering,particularlyinthenuclear,turbine,andaerospaceindustries.Owingtomicrostructures,HEAsarere-portedwithhighhardnessandhighcompressivestrengthbothatroomtemperatureandelevatedtemperatures[2,21,100,126–130,4].HEAshaveshowngreatintegratedtensileproperties,includingbothhighultimatetensilestrengthandreasonableductility[100,131,132].Overall,ithasbeenre-portedthattheFCC-structuredHEAsexhibitlowstrengthandhighplasticity,andBCC-structuredHEAsshowhighstrengthandlowplasticity.Thus,thestructuretypesarethedominantfactorforcontrollingthestrengthorhardnessofHEAs.Inthischapter,wereviewsomeoftheHEAspapersFig.4.2.Compressivetruestress–straincurvesofAlCoCrFeNiTixalloyrodswithadiameterof5mm[12].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9335Fig.4.3.Asthecoolingratewasincreased,boththestrengthandtheplasticityareenhancedsigniﬁcantly.Compressivetruestress–truestraincurvesfortheAlCoCrFeNialloysampleswithdifferentdiameters[129].concerningmechanicalproperties:whathasbeenreported,andwhatthedeformationmechanismsare.Thus,thedesignanddevelopmentofHEAstothenextlevelissuggested.4.1.1.MechanicalbehavioratroomtemperatureForroom-temperaturemechanicalpropertiesofHEAs,theyieldstrengthcanbevariedfrom300MPafortheFCC-structuredalloys,suchasCoCrCuFeNiTixsystem,toabout3000MPafortheBCC-structuredalloys,suchasAlCoCrFeNiTixsystem[12,13].ThevaluesofVickershardnessrangefrom100to900.Thestructuretypesareoneofthedominantfactorsforcontrollingthemechanical36Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.4.CompressiveyieldstrengthofAlxCoCrCuFeNialloysystemtestedatdifferenttemperatures:(A)Al0.5CoCrCuFeNi,(B)Al1.0CoCrCuFeNi,and(C)Al2.0CoCrCuFeNialloys[2].Fig.4.5.Hardness,strengths,andelongationasafunctionoftemperatureforas-rolledsamples[100].behaviorofHEAsatroomtemperature.Here,wewouldliketodiscusstwoothereffectsonmechanicalbehavior.4.1.1.1.Alloyingeffects.Likeotherconventionalalloys,smallamountsofalloyingelementscanalsobeaddedtoHEAstoincreaseordecreasethestrength,plasticity,hardness,etc.Theadditionofonealloy-ingelementtoimproveonepropertymayhaveunintendedeffectsonotherproperties.Forexample,inordertoinvestigatethedifferencesofeffectsofvariousalloyingelementsonmechanicalpropertiesofAlCoCrFeNialloy,theeffectsofC,Mo,Nb,Si,TielementsonAlCoCrFeNialloyhavebeeninvestigatedsystematically[12,133–135].MaandZhang[135]studiedtheNballoyingeffect,ﬁndingthatthemicrostructuresandpropertiesoftheAlCoCrFeNbxNiHEAsbecametwophasesinthepreparedAlCoCrFeNbxNiHEAs:oneisbody-cen-tered-cubic(BCC)solidsolutionphase(Fig.4.1(a));theotheristheLavesphaseof(CoCr)Nbtype.TheY.Zhangetal./ProgressinMaterialsScience61(2014)1–9337Fig.4.6.Hardness,strengths,andelongationasafunctionoftemperatureforasannealedsamples[100].Fig.4.7.MicrostructureoftheAlCrCuNiFeCoHEAin(a)as-castand(b)hot-forgedconditions[131].microstructuresofthealloyseriesvaryfromhypoeutectictohypereutectic,andthecompressiveyieldstrengthandVickershardnesshaveanapproximatelylinearincreasewithincreasingNbcontentasshowninFig.4.1(b)[135].Zhouetal.[12]investigatedtheTialloyingeffectonAlCoCrFeNiTix,designedbyusingthestrategyoftheequiatomicratioandhighentropyofmixing.ThealloysystemiscomposedprimarilyoftheBCCsolidsolutionandpossessesexcellentroom-temperaturecompressivemechanicalproperties,asshowninFig.4.2.ParticularlyfortheAlCoCrFeNiTi0.5alloy,theyieldstress,fracturestrength,andplas-ticstrainareashighas2.26GPa,3.14GPa,and23.3%,respectively,whicharesuperiortomostofthehigh-strengthalloyssuchasbulkmetallicglasses[12,136].4.1.1.2.Cooling-rateeffects.Thehighcoolingratesareeffectiveforreducingtheinter-dendritecompo-sitionsegregationandmakingthemicrostructuremoreuniform,andtheductilitycanbeimprovedwhiletheyieldstrengthhasnosigniﬁcantchange.Wangetal.[129]studiedthecoolingrateseffectsonthemicrostructureandmechanicalbehaviorsofaHEAofAlCoCrFeNibypreparingas-castrodsampleswithdifferentdiameters.Smallerdiameter38Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.8.Typicalstress–straincurvesofAlCoCrCuFeNi(a)theas-castand(b)hotforgedsamplesdeformedatdifferenttemperaturesandtheinitialstrainrateof10À3sÀ1[131].Fig.4.9.PhotographsofAlCoCrCuFeNitensilesamplesafterdeformationat1000°C:(a)anon-deformedsample;(b)as-cast_¼10À3sÀ1[131].sample(d=77%);and(c)forgedsample(d=864%).erodsampleshavehighercoolingratewhenusingthesameequipment.HefoundthatthecastsampleshavethesamephaseofBCCsolidsolutionwhilehighercoolingratesleadtomoreuniformmicrostruc-tureswithreducedinter-dendritecompositionsegregationasshowninFig.4.3(a–d)[129].Withdecreasingthecastingdiameter,whichmeanstheincreasingcoolingrate,boththestrengthandtheplasticityareincreasedslightly,asshowninFig.4.3(e).Fromthefractographs,thesamplesexhibitfeaturestypicalofcleavagefracture.Thesamplewasfracturedintheverticaldirectiontotheside,whichmaybehelpfultothelargeplasticityforhighercoolingratespecimens.Thisworkmaycontrib-utetoexploringtheapplicationofHEAs,especiallywhenseekingdifferentmechanicalpropertieswiththesamechemicalcomposition.4.1.2.MechanicalbehavioratelevatedtemperaturesThehigh-temperaturepropertiesoftheHEAswerealsoextensivelystudied[14,100,127,131].Like,conventionalalloys,themicrostructuresandmechanicalpropertiescanbetunedtoachievetheopti-mum.Fig.4.4showsthattheyieldstrengthdecreaseswithincreasingthetestingtemperature[2].ItisseenthatthelowAl-contentalloyexhibitslowyieldstrength,butitdecreasesslowlywithincreasingtemperature.Fig.4.5presentsthechangeinmechanicalpropertiesoftheAl0.5CoCrCuFeNiHEAoftheY.Zhangetal./ProgressinMaterialsScience61(2014)1–9339Fig.4.10.SEMimagesoftheAlCoCrCuFeNifracturesurfacesoftensilesamplesaftertensiledeformationatroomtemperature:(aandb)as-castand(candd)hot-forgedconditions[131].rolledsampleasthetemperatureincreased[100].Fig.4.6showsmechanicalpropertiesoftheAl0.5-CoCrCuFeNiHEAofannealedsamplesasafunctionofthetestingtemperature[100].Itisshownthat,afterannealing,thestrengthandhardnessdecreasewhiletheelongationincreases,andthestrengthdecreasesslowlywithincreasingthetemperature,whichmeansthatheat-treatmentwillhavethemostimpactonthemechanicalproperties,especiallyatelevatedtemperatures.Wewouldliketodis-cusstheheat-treatmenteffectonhightemperaturepropertiesofHEAs,includinghigh-performanceHEAsatelevatedtemperatures,calledrefractoryHEAs.4.1.2.1.Heat-treatmenteffects.Fig.4.7presentsthemicrostructureoftheAlCrCuNiFeCoHEAin(a)as-castand(b)hot-forgedconditions[131].Considerablereﬁnementofthecastmicrostructurewasob-servedafterextensivemulti-stepforgingat950°C.Fig.4.8exhibitsthetypicalstress–straincurvesoftheas-cast(a)andhot-forged(b)samplesdeformedatdifferenttemperatureswithaninitialstrainrateof10À3sÀ1[131].Weﬁndthat,afterforging,thealloyisconsiderablysofterandmuchmoredeformablethantheas-castalloy.Fig.4.9exhibitsthephotographsoftensilesamplesafterdeforma-tionat1000°C:(a)anon-deformedsample;(b)anas-castsample(tensileductility,d=77%);and(c)aforgedsample(d=864%)atastrainrateof10À3sÀ1.Theforgedsampledemonstrateshighlyhomoge-neousﬂow,greatresistancetoneckformation,andexceptionallyhighelongationof864%.Fig.4.10presentsthecorrespondingSEMimagesofthefracturesurfacesofsamplesaftertensile-testingdefor-mationatroomtemperature:(aandb)as-castand(candd)hot-forgedconditions[131].Atlowmag-niﬁcation,theas-castalloysamplehasacoarse-facetedappearance[Fig.4.10(a)],whereastheforged40Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.11.Compressiveengineeringstress–straincurvesofNbMoTaWandVNbMoTaWHEAsatroomtemperatureandhightemperatures[14].Fig.4.12.Thetemperaturedependenceofthespeciﬁcyield-strengthoftheTaNbHfZrTialloyincomparisonwiththatforTaNbMoW,TaNbVMoW,andCrCoCuFeNiAl0.5castalloys[128].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9341Fig.4.13.TheSEMbackscatterimagesoftheNbMoTaW(aandb)andVNbMoTaW(candd)HEAsafterthedeformationat1,673K.[14].samplehasﬁnegranularappearance[Fig.4.10(c)].Thisobservationisconsistentwiththemuchsmal-lergrain/particlesizeoftheforgedconditionthantheas-castcondition.Highmagniﬁcationimagesconﬁrmbrittle,quasi-cleavage,fractureoftheas-castalloy,withsuchcharacteristicfeaturesasﬂatfacets,angularfacetedsteps,river-patternmarkings,cleavagefeathers,andtongues[Fig.4.10(b)].Atthesametime,higher-magniﬁcationimagesoftheforgedsampleconﬁrmamixedtypeofbrittleandductilefracture[Fig.4.10(d)].Thebrittletypefractureisreﬂectedbythepresenceofﬂatfacetswithcharacteristicriver-patternmarkingsinsidelargedimpleswhiletheductiletypefractureisre-ﬂectedinnumerousdimplesofdifferentdiameterssurroundingtheﬂatfacets.Itislikelythat,duringtensiledeformationoftheforgedsample,cracksareformedattheinterfacesoftheBCCandFCCpar-ticlesbybrittlefracture,andthenthecrackopeningintovoidsoccursbyplasticdeformationofnear-est,moreductileregions[131].4.1.2.2.RefractoryHEAs.Currently,Ni-basedsuperalloysalreadyhavethegreatcombinationofele-vatedtemperatureproperties,includingcreepresistance,corrosionresistance,anddamagetolerance,butoperatingtemperaturesarereachingthetheoreticallimitsofthesematerials.Thehighentropyalloying(HEA)approachwasusedtodevelopnewrefractoryalloys,whichcontainseveralprincipal42Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.14.Theengineeringstress–straincompressioncurvesoftheNbCrMo0.5Ta0.5TiZralloysamplesafterHIPat296K,1073K,1273K,and1473K[127].Fig.4.15.SEMsecondaryelectronimagesofthefracturesurfaceofaNbCrMo0.5Ta0.5TiZralloysamplesaftercompressiondeformationatroomtemperature[127].alloyingelementsatnearequiatomicconcentrations,usingnewmetallicmaterialswithhighermelt-ingpoints,suchasrefractorymolybdenum(Mo)andniobium(Nb)alloys[14,16,126–128].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9343Fig.4.16.Thecompressivetruestress–straincurvesoftheAlCoCrFeNiHEAat(a)298and(b)77K.Theyieldstrengthsandfracturestrengthsatcryogenictemperaturesincreasedistinguishingly,comparedtothecorrespondingmechanicalpropertiesatambienttemperature[137].Senkovetal.[14]reportedtheyieldstrengthofNbMoTaWandVNbMoTaWalloysatelevatedtemperaturesasshowninFig.4.11.FromFig.4.11,wecanseethattheVadditionisbeneﬁcialtoincreasingthestrength,butnotsuitablefortheheat-softeningresistance.Fig.4.12exhibitsthespeciﬁcyield-strengthchangewithincreasingthetemperaturefortheTaNbHfZrTi,TaNbMoW,TaNbVMoW,andCrCoCuFeNiAl0.5castalloys[128].ItcanbeseenthatthehighspeciﬁcyieldstrengthoftheCrCoCuFeNiAl0.5HEAscanbesustainedoverto1100K,andtheTaNbMoWHEAcansustainitshighspeciﬁcstrengthto1800K.Fig.4.13showsthebackscatterimagesoftheNbMoTaWandVNbMoTaWHEAsafterthedeformationat1673K[14].Lowerdensityandbetterroomtemperatureductilityarerequiredifweseekforapplicationsintheaerospaceengineering.ByreplacingV,Ta,andWintheNbMoTaWandVNbMoTaWalloyswithlighterCr,Mo,andZr,respectively,thedensityofthenewrefractoryNbCrMo0.5Ta0.5TiZrHEAsalloywasre-ducedto8.2g/cm3.Inaddition,thenewalloyshowedimprovedroomtemperatureductility,relativetotheNbMoTaWandVNbMoTaWalloys.Fig.4.14exhibitstheengineeringstress–straincompression44Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.17.ThelowandhighmagniﬁcationsforthefracturesurfacesoftheAlCoCrFeNiHEAat298Kshownin(a)and(b),respectively;ThelateralsurfaceofthedeformedsampleoftheAlCoCrFeNiHEAat298Kshownin(c);ThelowandhighmagniﬁcationsforthefracturesurfacesoftheAlCoCrFeNiHEAat77Kshownin(d)and(e),respectively;ThelateralsurfaceofthedeformedsampleoftheAlCoCrFeNiHEAat77Kshownin(f)[137].curvesoftheNbCrMo0.5Ta0.5TiZralloysamplesafterHIPat296K,1073K,1273K,and1473K[127].Duringdeformationat296K,theyieldstrengthwas1595MPaandcontinuousstrengtheningoccurreduntilthealloyfracturedbylocalizedshearingatmaximumstrengthof2046MPaaccumulatingabouta5%strain.DuringtestingatT=1073K,yieldstrengthdecreasedto983MPa,thepeakstrengthof1100MPawasachievedatastrainof4.2%,andthesamplefracturedbyshearingatastrainofabout6%,afteradecreaseinstrengthto1050MPa.Anincreaseinthetestingtemperatureto1273Kresultedinaconsiderablesofteningofthealloyafterashortstageofstrainhardening.Atthistemperature,yieldstrength(r0.2)=546MPa,compressivestrength(rp)=630MPa,andtheminimumstrengthachievedduringstrainsofteningwas393MPaafteraplasticstrainofabout22%.Atthistemperature,thesampledidnotfracture.Anincreaseinthetemperatureto1473K,ledtoafurtherdecreaseintheﬂowstressofthealloy,andr0.2was170MPa.Thepeakstressof190MPawasreachedshortlyafteryielding,followedbyweaksofteningandasteadystateﬂowattheminimumstrengthY.Zhangetal./ProgressinMaterialsScience61(2014)1–9345Fig.4.18.Atalltemperaturesto4.2K,theAl0.5CoCuCrFeNiHEApossesseshighplasticityundercompression.Attemperaturesbelow15K,thecurvesshowaserratedshape.Theinsertedﬁgureillustratesatypicalserratedstress–straincurve[138].(rmin)=135MPa.Nosamplefractureoccurredatthistemperature.Fig.4.15presentstheSEMsecond-ary-electronimagesofthefracturesurfaceofaNbCrMo0.5Ta0.5TiZralloysamplesaftercompressiondeformationatroomtemperature[127].Itshowsacombinationofplasticandbrittledeformationmechanism.Theformationofdimplesisconvincingevidenceofplasticdeformation[Fig.4.15(bandc)]whilethemicrostructureoftearedpiecesisthesymbolofquasi-cleavagefractureoftheFCC(Laves)phase,aswellasalongtheinterfaces[Fig.4.15(aandd)].Insummary,refractoryHEAisauniqueideaandrelativelynewﬁeld.Comparedtocompositionswith3delements,refractoryHEAs(4delements)showexceptionallybrightmechanicalproperties,especiallyatelevatedtemperatures.Moreresearchﬁeldsincludingnewcompositionsandfracturemechanismneedtobeexplored.4.1.3.MechanicalbehavioratcryogenictemperaturesItisfullyacknowledgedthatthelow-temperaturemechanicalpropertiesareparticularlycriticalforrealapplicationsofmetallicalloys.ThemechanicalbehaviorsofHEAsatcryogenictemperaturesareyettobeinvestigatedindetail.ItisalsoknownthatFCCmetaldoesnotshowaductile–brittletransitiontemperature(DBTT),soshouldweexpectthesameforFCCHEAs?Meanwhile,becauseDBTTisknownforBCCmetals,hasanybodydoneworkondeformationofBCCHEAsatlowT?TheaimofthischapteristoreviewthecompressivecharacteristicsofbothBCCandFCCHEAsatcryogenictemperatures.Qiaoetal.studiedthecompressivecharacteristicsofasingle-phaseBCCHEAwiththecompositionofAlCoCrFeNiat77K[137].FortheAlCoCrFeNiHEA,thereisnoobviousductiletobrittletransitioneventhetemperatureisloweredto77K.Comparingwiththecompressionpropertiesatdifferenttem-peratures,theyieldingstrengthsandfracturestrengthsoftheAlCoCrFeNiHEAincreaseby29.7%and19.9%,respectively,whenthetemperaturesdecreasefrom298to77K,asshowninFig.4.16.However,thefracturestrainschangegentlywhilethefracturemodesat298and77Kareintergranularandtransgranular,respectivelyasshowninFig.4.17[137].ThismeansthattheDBTToftheAlCoCrFeNiBCCHEAislowerthan77K.Laktionovaetal.[138]studiedthedeformationpeculiaritiesofAl0.5CoCrCuFeNialloybyinatem-peraturerangefrom300Kto4.2K.Thealloyhasbeenfoundtoprovideahighstrengthandplasticitygreaterthan30%inthistemperaturerange:theyieldstressamountsto450MPafor300Kand46Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.19.S–NcurvescomparingthefatigueratiosoftheAl0.5CoCrCuFeNiHEA,otherconventionalalloysandbulkmetallicglasses[139].750MPafor4.2KasshowninFig.4.18.Attemperaturesbelow15K,thesmoothbehaviorofthestress–straincurveschangestothejump-likeone,calledserrationbehavior.Thistrendisproposedtoberelatedtothechangeofdeformationmechanisms,whichisdiscussedindetailsinSection5.Thus,FCCHEAsdonotexhibitDBTT,likeFCCmetals.4.1.4.FatiguebehaviorManypotentialapplicationsforHEAs,suchasaircraftenginecomponents,frequentlyencountercyclicloading.Ifweseekfortheapplicationintheaerospaceindustryorotherarea,besidesmonotonicloading,thefatiguebehaviorandlifetimepredictionareoneofthemostsigniﬁcantfactors,whicharerequiredtobestudiedandexplored,yetrarelyreported.ThelimitedpublicationpublishedsofarisconcerningthefatiguebehaviorofAl0.5CoCrCuFeNiHEAs,discussedasbelow.Hemphilletal.[139]studiedfatiguebehavioroftheAl0.5CoCrCuFeNiHEAandcomparedtheresultstomanyconventionalalloys,suchassteels,titaniumalloys,andadvancedBMGs.Fig.4.19(a)showsthatatypicalstressrangevs.thenumberofcyclestofailure(S–N)curvescomparingfatigueratios[fatigueratio=fatigueendurancelimit/ultimatetensilestrength(UTS))oftheAl0.5CoCrCuFeNiHEAY.Zhangetal./ProgressinMaterialsScience61(2014)1–9347Fig.4.20.Plotscomparing(a)thefatigueendurancelimitsand(b)thefatigueratiosoftheAl0.5CoCrCuFeNiHEAasafunctionoftheUTSofotherstructuralmaterialsandBMGs[139].tootherconventionalalloys,andBMGs[140].ThelowerboundofthefatigueratiosofHEAscomparesfavorablytothoseofsteels,titanium,andnickelalloys,andoutperformszirconiumalloysaswellassomeoftheZr-basedBMGs.Moreover,forsomematerials,suchasultra-highstrengthsteelsandwroughtaluminumalloys,theirhightensilestrengthsresultinlowerfatigueratiosduetotheirbrittlenature.Thestronggroup(whichisdeﬁnedassamplesinthegroupcontainfewerfabricationdefectsandcanrevealtheintrinsicfatiguebehavioroftheHEA)ofHEAstendstooutperformthesematerialsbydisplayingagreaterfatigueratiothanmaterialswithcomparabletensilestrengthsduetothere-ducednumberofdefects.TheupperboundofthefatiguelimitofHEAsissigniﬁcantlyhigherthanthatofotherconventionalalloysandBMGs,showingthatHEAshavethepotentialtooutperformthesematerialsinstructuralapplicationswithimprovedfabricationandprocessing.Fig.4.19(a)illustratesthefatigue-endurancelimitsfortheAl0.5CoCrCuFeNiHEAasafunctionofUTS.OnereasonforthehighfatiguestrengthofHEAsisthehightensilestrengthofthesematerials.ItcanbeclearlyseenthatastheUTSincreasesthefatigue-endurancelimitalsoincreasesinalinearfashion,approximatelyequalto0.5formostmaterials[139].HEAsfollowasimilarpatternandeven48Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.21.(a)WeightgaintestofAl00Ti05(Co1.5CrFeNi1.5Ti0.5),Al02Ti05(Al0.2Co1.5CrFeNi1.5Ti0.5),Al00Ti10(Co1.5CrFeNi1.5Ti),Al02Ti10(Al0.2Co1.5CrFeNi1.5Ti),SUJ2(AISI52100)andSKH51(AISIM2)specimensat600°C(873K)and800°C(1073K)for24h.Thedataispresentedasweightincreaseperunitareaaftertest.(b)Hothardnessvs.temperatureplotsforAl00Ti10,Al02Ti10,SUJ2andSKH51specimensfromroomtemperatureto900°C(1173K)[140].exceedthisratio,withanupperboundof0.703.TobettercomparethefatigueperformanceofHEAswithothermaterialswithrespecttotheirUTS,thefatigueratiosareusedasshowninFig.4.19(b).Fig.4.20(a)illustratesthisrelationshipcomparingthefatigue-endurancelimitvs.UTS.ItshightensilestrengthmaycontributetothehighfatiguestrengthsofHEAs.ItcanbeclearlyseenthatastheUTSincreasestheendurancelimitwillalsoincreaseinalinearfashion,approximatelyequalto0.5formostmaterials[139].HEAsfollowasimilarpatternandevenexceedthisratio,withanupperboundof0.703asshowninFig.4.20(b).TheseresultsarehighlyencouragingforexcellentfatigueresistanceinHEAsandwithpotentiallongfatiguelife,evenatstressesapproachingtheultimatestress.BecauseofthelackoftheliteratureonthefatiguebehaviorofHEAs,thefocusofthecontinuingstudiesshouldbeplacedonthedatapointsthatshowanunexpectedlylongfatiguelife.Ifthenecessaryinformationonthefatigueresis-tancecanbefoundandapredictionmodelforfatiguespecimenscanbedeveloped,HEAshaveabrightfutureinvariousapplicationsforcomponentsinfatigueenvironments.4.1.5.WearbehaviorAlthoughitischeaperthanmostsuperalloysandTialloys,thecostofHEAsarestillhigherthanthatofsteel,owingtotheuseofsomeexpensiveelements,suchasCoandCu.Thus,HEAsshowmoreY.Zhangetal./ProgressinMaterialsScience61(2014)1–9349Fig.4.22.VickershardnessandwearcoefﬁcientofAlxCoCrCuFeNialloyswithdifferentaluminumcontents[141].competitiveandpotentialforuseintools,molds,andstructuralcomponents,sowearisafundamentalphenomenonintheseapplications.TestsonthewearbehaviorofHEAshavebeenconductedunderabrasiveconditionsandadhesiveconditions.Chuangetal.[140]reportedthatthewearresistanceoftheCo1.5CrFeNi1.5TiandAl0.2Co1.5CrFeNi1.5Tialloysarebetterthanthatofconventionalwear-resistantsteelswithsimilarhardnessasshowninFig.4.21.IncreasingthemolarratioofTisigniﬁcantlyincreasesthevolumefractionofgprecipitates.TheadditionofAldecreasestheamountofthecoarsegphaseintheinterdendritic(ID)regionsandtriggerstheformationoftheneedle-likegphasewithWidmanstättenstructuresintheAl-richregionsoftheID[140].ThedifﬁcultyineffectivelyrelocatingAlresultsinthisphenomenon.ThehardnessvaluesofCo1.5CrFeNi1.5TiandAl0.2Co1.5CrFeNi1.5TialloysareHV654andHV717,respectively,becauseofthestrengtheningeffectfromthehardgphase[140].Theexcellentanti-oxidationpropertyandresistancetothermalsofteningintheseHEAsareproposedtobethemainreasonsfortheexcellentwearresistance.AlloyingcanaffectthewearbehaviorofHEAs.Wuetal.[141]studiedadhesivewearbehaviorofAlxCoCrCuFeNiHEAsasafunctionofaluminumcontent.Hefoundthat,forhigherAlcontent,thewornsurfaceissmoothandyieldsﬁnedebriswithahighoxygencontent,whichgivesalargeimprovementinwearresistanceasshowninFig.4.22.Thereasonofthisimprovementisattributedtoitshighhard-ness,whichnotonlyresistsplasticdeformationanddelamination,butalsobringsabouttheoxidativewearinwhichoxideﬁlmcouldassistthewearresistance[141].4.1.6.SummaryUptonow,someHEAswithuniquepropertieshavebeenreported:e.g.,high-strengthbody-cen-tered-cubic(BCC)AlCoCrFeNiHEAsatroomtemperature,andrefractoryVNbMoTaWatelevatedtem-peratures.Forroom-temperaturemechanicalpropertiesofHEAs,theyieldstrengthcanbevariedfrom300MPafortheFCC-structuredalloystoabout3000MPafortheBCC-structuredalloys.Thealloyandcooling-rateeffectscanchangethemicrostructuresandinﬂuencemechanicalproperties.Atelevatedtemperatures,thehighspeciﬁcyieldstrengthoftheAl0.5CrCoCuFeNiHEAscanbesustainedoverto1100K,andtheTaNbMoWrefractoryHEAcansustainitshighspeciﬁcstrengthto1800K.Likecon-ventionalalloys,heat-treatmentprocessisalsooneofthemostcrucialfactorsonaffectingmechanicalproperties.Atcryogenictemperatures,fortheAl0.5CoCrCuFeNiHEA,thereisnoobviousductiletobrit-tletransitioneventhetemperatureisloweredto4.2K.Ontheotherhand,HEAsalsopossessbothhighfatigueresistanceandhighwearresistance.Forfatiguebehavior,Al0.5CoCrCuFeNiHEAswasstudiedandcomparedtomanyconventionalalloys,suchassteels,titaniumalloys,andadvancedbulkmetallicglasses.TheAl0.5CoCrCuFeNiHEAscouldhavesupremefatigueresistanceoutperformingconventionalalloysandBMGs,iftheinclusioncontents50Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93Fig.4.23.Magnetizationcurvesoftheas-castandas-annealedCoCrFeNiCuAlhighentropyalloys:(a)thehysteresisloopsand(b)thetemperaturedependenceofmagnetization(M(T))curveswhilecoolingthealloysat200Oe[142].canbereducedintheHEA.Forwearbehavior,thewearresistancesoftheCo1.5CrFeNi1.5TiandAl0.2-Co1.5CrFeNi1.5Tialloysarebetterthanthatofconventionalwear-resistantsteelswithsimilarhardness.4.2.PhysicalbehaviorMaandZhang[135]reportedthattheAlCoCrFeNbxNi(x=0,0.1,0.25,0.5,and0.75)HEAsexhibitferromagneticproperties,becausetheirpermeability(v)isintherangeof2.0Â10À2–3.0Â10À3.WhilefortheTi0.8CoCrCuFeNiandTiCoCrCuFeNialloys,theyexhibitsuperparamagneticproperties[13],whichisduetothenano-particlesforminginthealloy.Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9351Fig.4.24.Electricalconductivityr(a),thermalconductivity(b),thermal-expansion(CTE)(c),andCurietemperatureandmolecularﬁeldasafunctionofxinAlxCoCrFeNialloys(d)[144].Fig.4.23presentsthemagnetichysteresisloopsoftheas-castandas-annealedCoCrFeNiCuAlHEAsmeasuredatroomtemperature.Bothalloysshowexcellentsoftmagneticproperties.Saturatedmag-netizations(Ms),remanenceratio(Mr/Ms),andcoercivity(Hc)oftheas-castandas-annealedalloysareestimatedtobe38.18emu/g,5.98%,45Oe,and16.08emu/g,3.01%,15Oe,respectively.Zhangandco-workers[13]hadinvestigatedthemagneticpropertiesofCoCrCuFeNiTixalloys,whichexhibitedsat-uratedmagnetizationlowerthan2emu/g.Comparedwiththemagneticpropertiesofbulkmetals,boththeas-castandas-annealedCoCrFeNiCuAlalloysshowhighMsandlowHc.Thealloyspresentcomparablemagneticpropertieswiththesoftferriteandcanbeutilizedassoftmagneticmaterials.Ontheotherhand,theas-castalloyexhibitsasimilarsuper-paramagneticcurve,whichcanbeattrib-utedtothenanoscaledmicrostructure.ThedecrementofMsintheas-annealedalloyresultedfromthestructurecoarseningandphasetransformation.Fig.4.23(b)depictsthetemperaturedependenceofmagnetization[M(T)]curvesoftheas-castandas-annealedalloys,whilecoolingthemtoliquidnitro-genat200Oe.Theresultsclearlyrevealaferromagnetictransitionofbothalloysatalltemperatures.TheM(T)curvesalsoindicatethehighCurietemperature(Tc)ofbothalloys[142].ChenandKao[143]reportedthattheelectricalresistivityoftheAl2.08CoCrFeNiHEAisveryclosetoaconstantoveratemperaturerangefrom4.2to360K.Theaveragetemperaturecoefﬁcientofresis-tivity(TCR)from4.2to360Kis72ppm/K,withahalf-parabolicshapeinitsresistivity-temperaturecurvethatissimilartoManganin(ManganinisatrademarkednameforanalloyofCu86Mn12Ni2).ItwasﬁrstdevelopedbyEdwardWestonin1892.Manganinfoilsandwiresareusedinthemanufactureofresistors,particularlyammetershunts,becauseoftheirvirtuallyzerotemperaturecoefﬁcientofresistancevalueandlong-termstability.SeveralManganinresistorsservedasthelegalstandardfortheohmintheUnitedStatesfrom1901to1990.Manganinwiresarealsousedasanelectrical52Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93conductorincryogenicsystems,minimizingtheheattransferbetweenpoints,whichneedelectricalconnections.Manganinisalsousedingaugesforstudyinghigh-pressureshockwaves(suchasthosegeneratedfromthedetonationofexplosivesbecauseithaslowstrainsensitivitybuthighhydrostaticpressuresensitivity.Itindicatesthatthephononeffectonthisalloykeepsnearlythesamethroughthistemperaturerange.Chouetal.[144]reportedtheelectricalconductivity,r,asshowninFig.4.24(a),thermalconduc-tivity,inFig.4.24(b),thecoefﬁcientofthermalexpansion(CTE)inFig.4.24(c),andtheCurietemper-ature,Tc,togetherwithmolecularﬁeld,inFig.4.24(d),asafunctionofcomposition,x,intheAlxCoCrFeNialloys.Theelectricalconductivity,r,forAlxCoCrFeNicanbedividedintothreepartsaccordingtothemicrostructureofthealloys.Theelectricalconductivitydecreaseswithincreasingxinthesingle-phase(FCCorBCC)regions,whileithasaminimumatx=0.875intheduplex-phase(FCC+BCC)region.Theelectricalconductivityintheduplex-phaseregionissmallerthanthatinthesinglephase.Theelectricalresistivity,whichistheinverseofr,isinﬂuencedbytheelectron–electroninterac-tion(T1/2),magneticeffect(T2),andphonon(T3)[144].Attemperaturesverynearabsolutezerotem-perature,aKondo-typeterm,lnT,mayappearintheexpressionofelectricalresistivityfordilutemagneticatomsinnonmagneticalloys.Atintermediatetemperatures,e.g.,100–200K,TandT2maycontributetoelectricalresistivity.AttemperatureshigherthantheDebyetemperaturewherethethermaleffectissigniﬁcant,electricalresistivityisproportionaltothetemperature,T,andelectricalresistivitymaybewrittenasq¼qoþcTð4-1Þhereqoistheresidualelectricalresistivityat0K.Thermalconductivity,j(T),isgenerallyobtainedfrommeasurementsofthermaldiffusioncoefﬁ-cient,a(T),speciﬁcheat,C(T),anddensity,q(T),ofamaterialatatemperature,T,andthencalculatedaccordingtothefollowingequation[144],jðTÞ¼aðTÞÂCðTÞÂqðTÞð4-2ÞFig.4.24(b)showsj(T)asafunctionofxfortheAlxCoCrFeNiHEAs.AtaspeciﬁcT,j(x)decreaseswithxineachsingle-phaseregionforbothFCCandBCCstructures,andthermalconductivityforBCCFig.4.25.ComparisonsoftheanodicpolarizationcurvesfortheAl0.5CoCrCuFeNialloyandthe304stainlesssteelindeaerated1NH2SO4[150].Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9353structureisgreaterthanthatforFCCstructure.ThermalconductivityintheFCC+BCCduplexregionisthelowestforeachspeciﬁcT.AsshowninFig.4.24(c),CTEfortheAlxCoCrFeNialloysdecreaseswithx.TheCurietemperaturepresentedinFig.4.24(d)increaseswithxintheFCC-andBCC-single-phaseregions,andreachesamin-imumintheduplexFCC+BCCregion.Usingtheinvareffecttomeasurethissecond-orderphasetran-sition,onecanhaveaccuratedatafortheCurietemperature.Thecorrespondingmolecularﬁeldoftheorderof107Oe,showninFig.4.24(d),isalsoestimatedbyassumingthemagnetizationofeachalloybeingwithoneBohrmagneton.Yangetal.[145]alsoreportedtheelectricalresistivityofNbTiAlVTaLax,CoCrFeNiCu,andCoCrFe-NiAlHEAs.WiththeincreaseofLaaddition,theresistivitiesofNbTiAlVTaLaxalloysincrease.Withincreasingthetemperature,theresistivityoftheCoCrFeNiCualloydecreases,whiletheCoCrFeNiAlal-loyincreases.Kaoetal.[146]studiedtheelectrical,magnetic,andHallpropertiesoftheAlxCoCrFeNiHEAs,andfoundaKondo-likebehavior.Generally,theKondoeffectdescribesthescatteringofconductionelec-tronsinametalduetomagneticimpurities.Itisameasureofhowelectricalresistivitychangeswithtemperature,whichisusuallyobservedinthedilutealloysystems.TheKondo-effectlikelyappearsinalloyscontainingrare-earthelementslikecerium,praseodymium,andytterbium,andactinideele-mentslikeuranium.Lucasetal.[147]studiedthemagneticpropertiesoftheFeCoCrNiHEAs,andevaluatedtheirpoten-tialapplicationsathightemperatures,andsuggestedthattheinclusionofCr,whichisananti-ferro-magneticelement,willnotbegoodforthemagneticproperties,andPdwouldbemuchbettertosubstituteforCr.Singhetal.[148]furtherstudiedmagnetichardeningoftheAlCoCrCuFeNiHEAsbya3-dimensionatomprobe(3D-AP)andTEM,andindicatedthedecompositionofCrFeCo-richre-gionsintoFeCo-richandCr-richdomains,respectively.4.3.Biomedical,chemicalandotherbehaviorsBraicetal.[149]investigatedthebiomedicalpropertiesof(TiZrNbHfTa)Nand(TiZrNbHfTa)CHEAcoatings.ItreportedthattheHEAcarbidecoatingexhibitedhighhardnessofabout31GPa,andgoodfrictionbehaviorandwearresistancewhentestedinsimulatedbodyﬂuids.Cell-viabilitytestsprovedthattheosteoblastcellswereadherenttothecoatings,andveryhighpercentages(>80%)oflivecellswereobservedonthesamplesurfaceafter72hincubationtime.Leeetal.[150]reportedthecorrosionresistanceoftheHEAofAl0.5CoCrCuFeNi.Fig.4.25showsthatthecorrosionpotential(Ecorr)oftheAl0.5CoCrCuFeNiHEA(À0.080VSHE)isapparentlymorenoblethanthatofthe304stainlesssteel(À0.151VSHE),andthecorrosioncurrentdensity(icorr)oftheAl0.5CoCrCuFeNiHEA(3.19lA/cm2)isalsolowerthanthatofthe304stainlesssteel(33.18lA/cm2)Fig.4.26.TEMimageoftheCu/NbSiTaTiZr/Siteststructureafter800°Cannealing.InsetshowsthehighresolutionimageoftheNbSiTaTiZrbarrier[153].54Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93byanorderofmagnitudeina1NH2SO4solution.Additionally,the304stainlesssteelhasawiderregionofthepassivepotentialthantheAl0.5CoCrCuFeNiHEA.Clearly,theAl0.5CoCrCuFeNialloyismoreresistanttogeneralcorrosionthanthe304stainlesssteel(higherEcorr,lowericorr)inthe1NH2SO4solution.Chenetal.[99]studiedthemicrostructureandelectrochemicalpropertiesofCu0.5NiAlCoCrFeSiHEAs,andmadeacomparisonofcorrosionpropertiesbetweentheHEAsandthetype-304stainlesssteel.Theanodic-polarizationcurvesoftheHEAs,obtainedinaqueoussolutionsofNaClandH2SO4,clearlyindicatedthatthegeneralcorrosionresistanceoftheHEAsatambienttemperatureissuperiortothatof304stainlesssteels,irrespectiveoftheconcentrationofanelectrolyteintherangeof0.1–1M.Ontheotherhand,theHEAs’resistancetopittingcorrosioninaClÀenvironmentisinferiortothatof304stainlesssteel,asindicatedbyalowerpittingpotentialandanarrowerpassiveregionfortheHEA.Testsin1NsulfuricacidcontainingdifferentconcentrationsofchlorideionsshowedthatHEAshavetheleastresistancetogeneralcorrosionatachloride-ionconcentrationof0.5M(closetotheconcentrationinseawater).Thelackofhysteresisincyclic-polarizationtestsconﬁrmedthatHEAs,likethe304stainlesssteel,arenotsusceptibletopittingcorrosioninthechloride-free1NH2SO4.Chenetal.[151,152]reportedthattwopromisingHEAs,AlxCrFe1.5MnNi0.5(x=0.3and0.5),weredesignedfromtheAlCoCrCuFeNialloysbysubstitutingMnforexpensiveCoandexcludingCutoavoidtheCusegregation.Microstructuresandpropertieswereinvestigatedandcomparedindifferentstates:as-cast,as-homogenized,as-rolled,andas-agedstates.TheAl0.3CrFe1.5MnNi0.5alloyintheas-cast,as-homogenized,andas-rolledstateshasadual-phasestructureofBCCandFCCphases,inwhichAl,Ni-richprecipitatesoftheB2-typeBCCstructuredispersedintheBCCphase.TheAl0.5CrFe1.5-MnNi0.5alloyinthecorrespondingstateshasamatrixofaBCCphaseinwhichCr-richparticlesoftheBCCstructureandAl,Ni-richprecipitatesoftheB2-typeBCCstructuredisperse.ThesethreeBCCphaseshavethesamelatticeconstant.BothalloysareprocessableandshowahardnessrangeofHV300–500intheas-cast,as-forged,as-homogenized,andas-rolledstates.TheAl0.5CrFe1.5MnNi0.5al-loyhasahigherhardnesslevelthanAl0.3CrFe1.5MnNi0.5becauseofitsfullBCCphase.Bothalloysdis-playasigniﬁcanthigh-temperatureage-hardeningphenomenon;theas-castAl0.3CrFe1.5MnNi0.5alloycanattainthehighesthardness,HV850,at600°Cfor100h,andAl0.5CrFe1.5MnNi0.5canpossessevenhigherhardness,HV890.TheaginghardeningresultedfromtheformationofaCr5Fe6Mn8-likephase.Priorrollingonthealloysbeforeagingcouldsigniﬁcantlyenhancetheage-hardeningrateandhard-nesslevelduetointroduceddefects.TheAl0.5CrFe1.5MnNi0.5alloyexhibitstheexcellentoxidationresistanceupto800°C,whichisbetterthantheAl0.3CrFe1.5MnNi0.5alloy.Combiningthismeritwithitshighsofteningresistanceandwearresistance,ascomparedtocommercialalloys,theAl0.5CrFe1.5-MnNi0.5alloyhasthepotentialforhigh-temperaturestructuralapplications.Kaoetal.[46]studiedtheCoFeMnTiVZrHEAsystemfortheabsorptionanddesorptionofhydrogen.Pressure-composition-isotherms(PCIs)demonstratethatCoFeMnTixVZr,CoFeMnTiVyZr,andCoFeMnTiVZrzcanabsorbanddesorbhydrogenforx,y,andzthatsatisfy0.5Nb$Zr>Ta>Hf)seemstofollowtheorderoftheatomicsizeandweightoftheconstituentelements,furtherconﬁrm-inglackofstrongchemicalordering.Conversely,verystrongshort-rangeorderingofNi–P,followedbyPt–PandPd–P,werepredictedinPdPtNiCuPatT=1200K.Theorderingtendencybecomesstrongerasthetemperatureislowereddur-ingsolidiﬁcation.Asaresult,quenchingisrequiredtosuppresscrystallizationtoformanamorphousstate.ThesimulateddiffusionconstantsareCu:1.01Â10À5,Ni:8.10Â10À6,P:8.81Â10À6,Pd:7.36Â10À6,Pt:7.70Â10À6cm2/s.AlthoughPatomsare20%smallerthanCuatoms,CudiffusesY.Zhangetal./ProgressinMaterialsScience61(2014)1–9379Fig.7.7.AIMD-predictedpartialpaircorrelationfunctionsandmeansquaredisplacementplotforPdPtNiCuPatT=1200Kafter30pssimulationtime,showingstrongorderingbetweenPandNi[250].ThesimulateddiffusionconstantsareCu:1.01Â10À5,Ni:8.10Â10À6,P:8.81Â10À6,Pd:7.36Â10À6,andPt:7.70Â10À6cm2/s.thefastest.ThereasonisduetoformationofstrongshortrangeorderingofP–NipairsfollowedbyP–PtpairswhilethecorrelationsassociatedwithCuaretheweakestoverall.BasedonthepresentAIMDsimulationsofvariousHEAsincomparisonwithavailableexperiments,itistemptingtosuggestthefollowingguidelinesinsearchingfornewHEAswithimprovedpropertiesusingAIMDsimulations:(1)Promotetheformationofhomogenoussolidsolutionsbyavoidingstrongchemicalsegregationandformationofdetrimentalintermetallics,startingintheliquidstate.80Y.Zhangetal./ProgressinMaterialsScience61(2014)1–931600fccfcc+bccbcc1500LiquidL+bcc_B2L+fcc1400T [C]L+bcc_A21300L+fcc+bcc_B21200L+bcc_A2+bcc+B2fccL+fcc+L+fcc+bcc_A2+bcc_B2bcc_A2bcc_A2+bcc_B21100fcc+bcc_B2fcc+bcc_A2+bcc_B2100000.511.522.53Al ratio (x)Fig.7.8.thecalculatedisoplethoftheAlxCoCrFeNialloyswithx=0–3usingourcurrentthermodynamicdescription[248].(2)Maintainequivalentorcomparablediffusivityamongprincipalelementstocontrolthekineticsofphasetransformationaimingtowardformingmoreorlesshomogenousmicrostructuresdur-ingcooling.Theseguidelinesarecomplementarytotheempiricalrules(i.e.heatofmixingandatomicsizedif-ference)forHEAformationasaddressedinSection2.CombiningthemwillacceleratedesignofnewHEAs.7.3.CALPHADmodelingComparedtoDFTcalculationsandAIMDsimulations,theCALPHADmethodallowsuserstoper-formthermodynamicandkineticcalculationsbasedonthephenomenologicalapproach[286–288]thatareusedtoquantityGibbsfreeenergiesofindividualphasesandmobilityinasystem.Typicalthermodynamiccalculationsincludebutarenotlimitedtophasecompositions,phasefractions,andphasestabilityasafunctionofcomposition,temperature,andpressure.Veryrecently,Zhangetal.[248]developedathermodynamicdatabasefortheAl–Co–Cr–Fe–Nisys-tem.Theiremphasiswasplacedontheinteractionparametersoflower-orderconstituentsystemsthatareusuallythemosteffectiveandimportanttoobtainareliablehigher-orderthermodynamicdatabase.Therearenohigher-order(quaternaryorquinary)interactionparametersusedintheirthermodynamicdatabase[248]asaﬁrstapproximation.Fig.7.8showstheverticalsectionoftheAlxCoCrFeNifortheAlratiofrom0to3.ThisﬁgurepredictsthephasestabilitywithrespecttoAlcontentsintheAlxCoCrFeNialloy.ItisseenthattheprimarysolidiﬁedphaseisFCCwhenx<0.75,anditisBCCandB2,whichsolidiﬁesﬁrst,whenx>0.75,whichseemtobegenerallyconsistentwiththeexperimentalresults[248].Non-equilibriumsolidiﬁcationpathsimulationswasperformed[248]usingtheScheilmodel,asshowninFig.7.9toillustratethefractionofsolidsduringsolidiﬁcationintheAlxCoCrFeNialloys.ItisseenthatthecalculationpredictstheformationofaBCCphaseof2.3%whenx=0.3,asexperimen-tallyobservedbyKaoetal.[146].Fig.7.10presentstheequilibriumphaseschangewithrespecttocompositionoftheAlxCoCryFeNialloyshomogenizedat1373K.ThistrendindicatesthattheBCCphaseregionincreases,andtheFCC/FCC+BCCregiondecreaseswithincreasingtheCrconcentration,demonstratingthatCrstabilizestheBCCphase.WhentheCrratio,x,equalsto2.0,nopureFCCphaseY.Zhangetal./ProgressinMaterialsScience61(2014)1–9381(a)150014501400(b)L-->bcc_B2Temperature [C]L-->bcc_B2+bcc_A213501300125012001150L-->bcc_A2+fccL-->bcc_A2 x=0.8 x=1.0 x=1.2 x=1.4 x=1.6 x=1.8 x=2.0L-->bcc_A2+fcc+bcc_B20.00.10.20.30.40.50.60.70.80.91.0Fraction of SolidFig.7.9.SolidiﬁcationpathscalculatedbytheScheilmodelfortheAlxCoCrFeNialloysusingourcurrentthermodynamicdatabase:(a)x=0.1–0.8;(b)x=0.8–2[231].regioncanformwithintheAlxCoCr2FeNialloys.TheeffectofotherelementsinthephasestabilityofBCC/FCCisalsocalculatedinthepaper[248].8.FuturedevelopmentandresearchHEAsarebasedonthemulti-principalelementsconcept,whichhasstimulatedrisinginterestsforbasicscienceandapplications.ThepotentialapplicationsofHEAsaremainlybasedontheiruniqueproperties.Theirexcellenthigh-temperaturepropertiesmayprovidethepotentialtoreplacetheNi-basedsuperalloy,e.g.,AlCoCrFeNi,whichisexpectedtobeoflighterweightandlowercost.ItisalsoreportedthatHEAscanbeusedasthermalbarriercoatingsfortheTi-basedalloysandadiffusionbarrierbetweenCuandSiintheintegratedcircuit(IC)industries.TheexcellentwearresistancemakesHEAsusefulformoldmaterials.TherearealsoreportsusingHEAsinthe4-mode-ringlaserGyro.TheHEAscarbidesandnitridesarepotentiallyapplicableascoatingsforthebiomedicalmaterials,andthenearconstantresistivityin4.2–360KcouldmaketheAl2.08CoCrFeNiHEAusefulforelectronic82Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93AlxCoCryFeNi honogenized at 1100Co bcc bcc+fcc fccy=2.0y=1.5y=1.0y=0.50.00.20.40.60.81.01.21.41.61.82.0Al ratio (x)Fig.7.10.thecalculatedtransitionrangesoftheAlxCoCryFeNialloyshomogenizedat1100°C[248].deviceparts.Inthischapter,futuredirectionsareproposedinthebroadareasoffundamentalunder-standing,processingandapplications.8.1.FundamentalunderstandingofHEAsQuantifyingtheentropysourcesofHEAsremainstobethemostimportanttopicinfundamentalunderstandingofHEAformation.Inthisregard,inelasticneutronscatteringandnuclearresonantinelasticX-rayscatteringwillbeveryusefultechniquestomeasurephononspectraasdemonstratedbyFultzetal.inanumberoforderedanddisorderedalloys[272–276,289–291].Inrealsolidsolutionalloysorderingand/orclusteringalwaysdestroyconﬁgurationalentropyofmixing,soprecisecalcu-lationofconﬁgurationalentropyofmixinginHEAsisneeded.Typicalmethodsforthispurposein-cludeclustervariationmethod(CVM),generalalgorithms,andMonteCarlosimulations.Fromthepointofviewofmaterialsdesign,oneneedsphasediagraminformation,e.g.thephaseﬁeldofahigh-entropyphaseinthespaceofcompositionandtemperatureaswellasimportantcom-petingorderedcompoundsphase(s).Inordertoexpeditephasediagramdetermination,theapproachthatcombinesDFTcalculations,high-throughputexperiments[292,293]andCALPHADmodelingisawisechoiceasreportedpreviously[38,294–298].Establishingreliableself-consistentthermodynamicdatabaseusingtheCALPHADmethodnotonlyfacilitatesphasediagramvisualizationformorethan5componentsystemsbutalsocanbeusedtodirectlycalculatetheentropy,enthalpyandGibbsfreeen-ergyofthehigh-entropyphaseasafunctionoftemperatureandcomposition.IthasbeencriticallyclaimedthereexistseverelatticedistortioneffectandsluggishdiffusioneffectinHEAs.However,precisemeasurementstoquantifytheseeffectsarestilllacking.Computersimula-tionsinthisregardwillbeimportant.However,calculatingthediffusioncoefﬁcientofHEAsusingDFTmethodscanbeadauntingtaskduetothelargeconﬁgurationalspaceinthehighlyconcentratedalloys.AdditionaltopicsinaddressingfundamentalunderstandingofHEAsare:(1)Quantifyingtheenthalpyofthehigh-entropyphasesinceitistheGibbsfreeenergythatdeter-minesthephasestabilityatconstanttemperatureandpressure.Experimentalmeasurementssuchasheatofsolutionandheatcapacityareimportant.ComputationalstudyusingDFTandCALPHADwillbeveryusefultocalculatehowenthalpyofmixingchangeswithcomposition,whichinturncanhelpidentifyHEAcomposition.Y.Zhangetal./ProgressinMaterialsScience61(2014)1–9383(2)HEAshavethehighchemicaldisorder.Amorphousalloyshavehightopologicaldisorder.Hence,HEAsarealloysjustinbetweentheconventionalalloysandtheamorphousalloys.Theplasticdeformationstructuralunitsaretypicallydislocationsandtwinsintheconventionalcrystallinealloys,andtypicallySTZsandTTZsintheamorphousalloys.Thus,whatwouldbethedeforma-tionmechanisminHEAs?DedicatedexperimentsandsimulationsonplasticdeformationofHEAsareneeded,forexample,usingthesingle-crystalofHEAswithBCCandFCCstructures,althoughtherehaveaccumulatedreportsontension,compression,andhardnessexperimentsonHEAs.(3)Themicro-andnano-structuresofHEAsafterplasticdeformationmayrequirefurtherstudybythehigh-resolutionTEM,neutrondiffraction,andsynchrotronhigh-energyX-raydiffractions,etc.,toprovidethedeformationmechanismofHEAs.(4)LimitedresultsaboutthefatiguebehaviorsfortheHEAshavebeenobtained.MoreworkontheBCC,BCC+FCC,andFCCstructuredHEAsneedstobedone,especiallythehigh-temperaturefatigueproperties.(5)TostudytheenvironmentalpropertiesofHEAs,dedicatedexperimentsinair,moistureair,watervapor,CO,hydrogen,H2Setc.atelevatedtemperatureneedtobeconducted.(6)CreepperformanceofHEAsasstructuralmaterialsneedtobestudied.8.2.ProcessingandcharacterizationofHEAsMaterialspropertiesaredictatedbytheirmicrostructure.ThereisnoexceptionforHEAs.Themicrostructurecanbemanipulatedbyfabricationmethods(seeSection3),plasticdeformation(e.g.rolling,forging),andheattreatment.Castingandpowdermetallurgycanbeemployedtoobtainnearnetshapeproducts.ThoroughcharacterizationofthemicrostructureofHEAsin3-dimensionareimportantininterpretingmaterialspropertiessuchasfracturetoughnessandfracturemechanisms.Theprogressin3Dmicrostructurecharacterizationanddigitalconstructioncanbefoundinreviewarticles[299,300]).Inthisregard,grainboundarycharacterandchemistrydistribution(forreviewssee[299,301])areimportantfutureresearchtopicsforHEAs.Forexample,controlledthermo-mechan-icalprocessingcanbeusedtopromoteformationofpreferredgraintextureand/orgrainboundarycharacterdistribution.Inaddition,listedbelowaresomeusefultopics:(1)Thenano-sizedpillarscanbefabricatedfromtheas-preparedHEAsusingfocusionbeams(FIB),andthepillarsofsinglephasewithdesirablepropertiescanbemanufactured,andtheycanbetheGenesforthematerials.ThenthenewmaterialscanbedesignedbasedonthepropertiesoftheGenes.(2)Largeplasticdeformationcanbeusedtoreﬁnethestructuretosubmicro-ornano-scaleusingequalchannelangularpressingmethod.PorousHEAsmaterialswithcontrollableporesizesinmicro-tonano-sizedscalecanbeprocessedusingchemicalorcastingmethods.TheporousHEAswouldbeusedasrobustﬁltersforpurifyingthewaterandairorthehotsmokesatele-vatedtemperaturesandhostileenvironment.(3)Detaileddeformationandfracturemechanismshavenotbeenclearlyidentiﬁed.Thedislocationstructuresbeforeandafterthefracturewillneedtobeinvestigated.Understandingtheinterac-tionbetweendislocationsandsoluteswillprovideinsightsintotheeffectsofthealloyingele-mentsontheductilityofthealloy.Inaddition,forfatiguestudy,onlylimiteddatawerereportedandonlyatroomtemperature,butfatiguebehavioratelevatedtemperaturesneedstobeexplored.Itisbelievedthatareductioninthenumberofthedefects,suchasaluminumoxideinclusionsandmicro-cracks,mayresultinastrongfatiguebehavior.HowtoreducethesedefectsisparticularlycrucialforimprovingthefatigueresistanceofHEAs.8.3.ApplicationsofHEAsHEAsholdthepotentialinawiderangeofapplicationssuchasfunctionalandstructuralmaterials.TheconceptofHEAscanalsobeappliedtoceramics,polymerandevenliquid.AsanextensiontoSec-tion4,herearesomepromisingopportunitiesforHEAs:84Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93(1)HEAscanbeusedasthetransitionallayerbetweenthetwotypesofalloys,e.g.,theHEAsolderusedforweldingpuretitaniumandchromium–nickel–titaniumstainlesssteel[302];andHEAsbrazingﬁllermetalusedforweldingcementedcarbideandsteel,whichprovidesgoodﬂexibil-ityofthebrazingﬁllermetalandfavorablemanufactureandinstallationprocess[303].(2)HEAscanexhibitexcellentsuper-paramagneticproperties,ferromagneticproperties,andsoftmagneticproperties.(3)HEAscouldbeusedinthenuclearindustries.TheirmuchhighirradiationresistanceandhighcorrosionresistancewouldmakeHEAspotentialcandidatesforthecladdingmaterialsusedforthenuclearfuelsandhighpressurevessels.(4)Thehigh-entropyconceptcouldbeusedtosimulatetheﬁssionprocessofnuclearreactors.Thenuclearﬁssionprocessisaprocesswithentropyincreasing,becausethetypesoftheelementsareincreasingwiththenuclearﬁssionreactions.(5)HEAsmaybeusedasheat-resistantorwear-resistantcoatings.NewtechnologiesareneededtomaketheHEAscoatingmoreuniformandwithhighcohesionwithsubstrates.(6)TherefractorymetalHEAsmaybeusedasthermalbarriercoatings,whichneedstobeinvestigated.(7)HEAscarbidesandnitrideswouldalsobeinterestingfortheiruniqueproperties.Theymayhavethestructuresofamorphousorsolidsolutions,andwithhighhardnessandstrength.Theymaypotentiallybeusableasdiffusionbarriers,andhardcoatingsonthetoolcuttingsteelsorthehighspeedsteels.Recentworkalsoshowsthatthehigh-entropycarbidesandnitridescanpotentiallybeusedasbiomedicalcoatings[151](8)ThespecialphysicalpropertiesoftheHEAs,e.g.,Al2.08CoCrFeNi,withnearconstantresistivitywouldmakethemusefulforelectronicapplications.Thus,thiskindofHEAsneedstobefurtherstudied.(9)Light-weightHEAscouldbeusedascasingsforthemobilefacilities,batteryanodematerials,andtransportationindustry.9.SummaryAsanewclassofmaterialsthatcontainmulti-principal-elements,HEAshavedemonstrateduniqueandattractiveengineeringproperties.Thepresentpaperhasreviewedtheformationcriteria,thermo-dynamics,processing,kinetics,mechanicalproperties,andcomputermodelingofHEAs,assumma-rizedasfollows:ThefourcoreeffectsoftheHEAshavebeensummarizedtounderstandtheHEAs;theparameters,suchas,enthalpyofmixing,atomicsizedifference,X,VEC,havebeenusedtopredictthephasefor-mationfortheHEAs.Multiple-processingmethods,suchasarcmelting,inductivemelting,sputter,lasercladding,andelectrochemical,havebeensummarized.TheHEAswithspecialpropertieshavebeenelaboratedonebyone,e.g.,thesuperhighstrengthofBCCHEAs,andhighwearresistanceHEAs,highstrengthHEAsathightemperatures,Kondo-likebehaviorsHEAs,goodfatigueresistance,etc.TheplasticdeformationandfracturemechanismofHEAshavebeendiscussedfromthestandpointsofthecracklingnoiseandtheserrationsonthestress–straincurves.High-entropyBMGslackbothlong-rangechemicalorderandthetopologicalorder,whichenablethempossessspecialstructuresandproperties.Understandingtheformationmechanismofhigh-en-tropyBMGsishelpfultogainnewinsightsintotheformationmechanismofmetallicglassesandoth-ersinthematerials.Preliminarycomputersimulationsandmodelingworkhavebeenbrieﬂysummarized,andmoreef-fortsinthistopicneedtobecarriedoutsothatthefundamentalsandstructuresoftheHEAscanbewellunderstood.Insummary,HEAsprovideanewchallengetothematerialsscientists.Withthein-depthworkonHEAs,moreandmorespecialpropertiesofHAEswillbecharacterizedanddevelopedinthefuture.WiththeadvancedtechnologiesdevelopedfortheprocessingandcharacterizationsofHEAs,1-dimen-sional(1-d),HEAwires;2-dimensional(2-d),HEAﬁlms;and3-dimensional(3-d),bulkHEAsamplesY.Zhangetal./ProgressinMaterialsScience61(2014)1–9385willbefabricatedandstudied.Thephase-formationthermodynamics,kineticsandprocessing,andcomputermodelingandsimulations,etc.,forHEAswillbeextensivelystudied,whichwillleadthematerialsscientistsintoanewandwonderfulmaterials-scienceworld.DisclaimerThisreportwaspreparedasanaccountofworksponsoredbyanagencyoftheUnitedStatesGov-ernment.NeithertheUnitedStatesGovernmentnoranyagencythereof,noranyoftheiremployees,makesanywarranty,expressorimplied,orassumesanylegalliabilityorresponsibilityfortheaccu-racy,completeness,orusefulnessofanyinformation,apparatus,product,orprocessdisclosed,orrep-resentsthatitsusewouldnotinfringeprivatelyownedrights.Referencehereintoanyspeciﬁccommercialproduct,process,orservicebytradename,trademark,manufacturer,orotherwisedoesnotnecessarilyconstituteorimplyitsendorsement,recommendation,orfavoringbytheUnitedStatesGovernmentoranyagencythereof.Theviewsandopinionsofauthorsexpressedhereindonotnec-essarilystateorreﬂectthoseoftheUnitedStatesGovernmentoranyagencythereof.AcknowledgementsTheauthorsareindebtedtoProf.G.L.Chen,whopassedawayin2011,forhispioneeringworkinHEAsandacademicguidance.TheauthorsarealsogratefultoProf.W.K.Wang,Prof.W.H.Wang,Prof.Y.Li,Prof.H.A.Davies,Prof.Z.Q.Sun,Prof.T.G.Nieh,Prof.X.D.Hui,Prof.J.P.Lin,Dr.Y.Q.Cheng,andDr.G.Y.Wangforvaluablediscussion,commentsandadvices.Z.Y.isgratefultotheﬁnancialsupportoftheNationalNaturalScienceFoundationofChina(GrantNos.50971019,51010001and51001009),111Project(B07003)andProgramforChangjiangScholarsandInnovativeResearchTeaminUniver-sity.P.K.L.appreciatesthesupportfromtheUSNationalScienceFoundation(DMR-0909037,CMMI-0900271,andCMMI-1100080),theDepartmentofEnergy(DOE),OfﬁceofNuclearEnergy’sNuclearEnergyUniversityProgram(NEUP)00119262,andtheDOE,OfﬁceofFossilEnergy,NationalEnergyTechnologyLaboratory(DE-FE-0008855).K.A.DandP.K.LthankDOEforthesupportthroughprojectDE-FE-0011194withtheprojectmanager,S.Markovich.M.C.GandP.K.LverymuchappreciatesthesupportfromtheU.S.ArmyResearchOfﬁceproject(W911NF-13-1-0438)withtheprogrammanager,S.N.Mathaudhu.M.C.G.acknowledgessupportoftheInnovativeProcessingandTechnologiesProgramoftheNationalEnergyTechnologyLaboratory’s(NETL)StrategicCenterforCoalundertheREScon-tractDE-FE-0004000.ThisworkusedthecomputingfacilityatTexasAdvancedComputingCenter(TACC)throughAward#DMR120048bytheExtremeScienceandEngineeringDiscoveryEnvironment(XSEDE),whichissupportedbyNationalScienceFoundationgrantnumberOCI-1053575.References[1]CantorB,ChangITH,KnightP,VincentAJB.Microstructuraldevelopmentinequiatomicmulticomponentalloys.MaterSciEng,A2004;375–377:213–8.[2]YehJW,ChenSK,LinSJ,GanJY,ChinTS,ShunTT,etal.Nanostructuredhigh-entropyalloyswithmultipleprincipalelements:novelalloydesignconceptsandoutcomes.AdvEngMater2004;6(5):299–303.[3]MaD,TanH,ZhangY,LiY.Correlationbetweenglassformationandtypeofeutecticcoupledzoneineutecticalloys.MaterTrans2003;44(10):2007–10.[4]ZhangY,YangX,LiawPK.Alloydesignandpropertiesoptimizationofhigh-entropyalloys.JOM2012;64(7):830–8.[5]ZhangY,MaSG,LiawPK,TangZ,ChengYQ.Broadguidelinesinpredictingphaseformationofhigh-entropyalloys,MRSCommunications,submittedforpublication.[6]MaSG,ZhangY,LiawPK.DampingbehaviorsofAlxCoCrFeNihigh-entropyalloysbyadynamicmechanicalanalyzer,JAlloyCompd,inpreparation.[7]ZhangY,ZuoTT,LiawPK,ChengYQ.High-entropyalloyswithhighsaturationmagnetizationandelectricalresistivity.SciRep2013;3:1455.http://dx.doi.org/10.1038/srep01455.[8]YangX,ZhangY,LiawPK.MicrostructureandcompressivepropertiesofTiZrNbMoVxhigh-entropyalloys.ProcEng2012:292–8.[9]YehJW.Recentprogressinhigh-entropyalloys.PresentationatChangshameeting;2011.[10]ZhangY,WangXF,ChenGL,QiaoY.EffectofTionmicrostructureandpropertiesofCoCrCuFeNiTixhigh-entropyalloys.AnnChimSciMatér2006;31(6):699–709.[11]YehJW,LinSJ,ChinTS,GanJY,ChenSK,ShunTT,etal.FormationofsimplecrystalstructuresinCu–Co–Ni–Cr–Al–Fe–Ti–Valloyswithmultiprincipalmetallicelements.MetallMaterTransA2004;35(8):2533–6.86Y.Zhangetal./ProgressinMaterialsScience61(2014)1–93[12]ZhouYJ,ZhangY,WangYL,ChenGL.SolidsolutionalloysofAlCoCrFeNiTixwithexcellentroom-temperaturemechanicalproperties.ApplPhysLett2007;90(18):181904.[13]WangXF,ZhangY,QiaoY,ChenGL.NovelmicrostructureandpropertiesofmulticomponentCoCrCuFeNiTixalloys.Intermetallics2007;15(3):357–62.[14]SenkovON,WilksGB,ScottJM,MiracleDB.MechanicalpropertiesofNb25Mo25Ta25W25andV20Nb20Mo20Ta20W20refractoryhighentropyalloys.Intermetallics2011;19:698–706.[15]SinghS,WanderkaN,MurtyBS,GlatzelU,BanhartJ.Decompositioninmulti-componentAlCoCrCuFeNihigh-entropyalloy.ActaMater2011;59:182–90.[16]SenkovON,WilksGB,MiracleDB,ChuangCP,LiawPK.Refractoryhigh-entropyalloys.Intermetallics2010;18(9):1758–65.[17]ZhangY,ZhouYJ,LinJP,ChenGL,LiawPK.Solid-solutionphaseformationrulesformulti-componentalloys.AdvEngMater2008;10(6):534–8.[18]LiC,LiJC,ZhaoM,JiangQ.Effectofalloyingelementsonmicrostructureandpropertiesofmultiprincipalelementshigh-entropyalloys.JAlloyCompd2009;475(1–2):752–7.[19]ChangHW,HuangPK,DavisonA,YehJW,TsauCH,YangCC.NitrideﬁlmsdepositedfromanequimolarAlCrMoSiTialloytargetbyreactivedirectcurrentmagnetronsputtering.ThinSolidFilms2008;516(18):6402–8.[20]ZhangY,ChenGL,GanCL.PhasechangeandmechanicalbehaviorsofTixCoCrFeNiCu1ÀyAlyhighentropyalloys.JASTMInt2010;7(5):102527.[21]ZhangY.Mechanicalpropertiesandstructuresofhighentropyalloysandbulkmetallicglassescomposites.MaterSciForum2010;654–656:1058–61.[22]ZhangY,ZhouYJ.Solidsolutionformationcriteriaforhighentropyalloys.MaterSciForum2007;561–565:1731–9.[23]ZhangY,ZhouYJ,HuiXD,WangML,ChenGL.Minoralloyingbehaviorinbulkmetallicglassesandhigh-entropyalloys.SciChina,SerG2008;51(4):427–37.[24]CaiH,ZhangH,ZhengH.Softmagneticdevicesappliedforlowzeroexcursionfour-moderinglasergyro.IEEETransMagn2007;43(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