Processing, alloy composition and phase transition effect on the mechanical and corrosion properties of high entropy alloys: a review

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w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Review

Article

Processing,

alloy

composition

and

phase

transition

effect

on

the

mechanical

and

corrosion

properties

of

high

entropy

alloys:

a

review

Kenneth

Kanayo

Alaneme

a,∗

_,

_Michael

_Oluwatosin

_Bodunrin

a,b

_,

_Samuel

_Ranti

_Oke

a

a_Department_of_{Metallurgical}_and_Materials_Engineering,_Federal_University_of_Technology,_Akure,_Nigeria

b_School_of_Chemical_and_{Metallurgical}_Engineering,_DST-NRF_Centre_of_Excellence_in_Strong_Materials,_African_Materials_Science_and

EngineeringNetwork(AMSEN),UniversityoftheWitwatersrand,Johannesburg,SouthAfrica

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received29August2015

Accepted8March2016

Availableonline12April2016

Keywords:

Highentropyalloys

Highentropyofmixing

Equimolarratio

Mechanicalproperties

Phasetransition

Corrosionbehavior

a

b

s

t

r

a

c

t

Thispaperreviews fromthe corpusof literatureson high-entropyalloys (HEAs), their

mechanicalandcorrosionbehaviorasaffectedbymetallurgicalfactorssuchasprocessing

technique,composition,phaseformationandtransition.HEAsareapromisingclassofalloys

whicharedesignedbasedontheuseofmultiplecomponentalloyingelementsinequimolar

ornearequimolarratio.Therehasbeensurginginterestinthisclassofalloysonaccountof

theiruniquepropertyrange.Theiruniquemetallurgicalcharacteristics,structures,

mechan-icalandcorrosionproperties,currentandpotentialareasofapplications,andsuggestions

forfutureresearcharediscussedinthispaper.

EditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Dr.K.K.AlanemeisanAssociate

Profes-sor in the Department of Metallurgical

and MaterialsEngineering, Federal

Uni-versity of Technology Akure. He is a

registeredengineerwithCouncilfor

Reg-ulationofEngineeringinNigeria(COREN).

He has successfully supervised

sev-eral Doctoraland MastersStudents.His

researchinterestsinclude:applied

phys-ical metallurgy; mechanical behaviour

ofmaterials: mechanicalpropertiesand

∗ _{Corresponding}_author_.

E-mail:[email protected](K.K.Alaneme).

testing; mechanics and mechanisms of strengthening,

deformation, and fracture; materials integrity and failure

analysis; composites and advanced materials; corrosion

and wear; metallurgical machines design; and materials

characterization.

M.O. Bodunrin is a Lecturer II in the

Department of Metallurgical and

Mate-rials Engineering, Federal University of

TechnologyAkure.Heisaregistered

engi-neerwithCouncilforRegulationof

Engi-neeringinNigeria(COREN)andcurrently

aPhDresearchstudentattheSchoolof

ChemicalandMetallurgicalEngineering,

http://dx.doi.org/10.1016/j.jmrt.2016.03.004

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University of Witwatersrand,

Johannes-burg,SouthAfrica.

S.R. Okeis anAssistantLecturerinthe

Department of Metallurgical and

Mate-rials Engineering, Federal University of

TechnologyAkure.HeiscurrentlyaPhD

researchstudentattheMetallurgicaland

MaterialsEngineering,FederalUniversity

ofTechnology,Akure,Nigeria.

1. Introduction

ThedesignofHEAsisarelativelynewpathinthedevelopment

ofadvancematerialswithuniquepropertiesunmatchedby

alloysproducedbyconventionalalloydevelopmentapproach

which is based on only one dominant element [1,2]. High

entropyalloysarelooselydeﬁnedassolidsolutionalloysthat

containmorethanﬁveprincipal elementsinequalornear

equalatomicpercent[3,4].

Previousstudies[5,6]haveshownthatHEAspredominantly

consistofasimplefacecenteredcubic(FCC),bodycentered

cubic(BCC),orFCC+BCCstructuresolidsolutionphaseowing

tothehighentropyofmixing,insteadofmanyintermetallic

phasesorothercomplexphases.However,smallquantities

ofintermetalliccompound phase/metastableparticles have

beenobservedinsomeHEAs[7].FromHume–Rotheryrules,

werecognizethefactorsthataffecttheformationofbinary

solidsolutions,whichincludeatomicsizedifference,valence

electronconcentration(VEC),crystalstructureofthesolute

and solvent atoms and difference in electronegativity [8].

Besides these factors,enthalpy and entropy of mixing are

themost importantphaseformationparameters forHEAs.

Higher entropy of mixing will lead to a lower Gibbs free

energy(G=Hmix−TSmix)whichtendstostabilizethe

formationofsolid-solutionphases,ratherthanintermetallic

phases.Formulti-componentsystems,theratioentropyand

enthalpyofmixing(TSmix/Hmix)wouldbemoreimportant

topredicttheformationofsolidsolutionphases, andthus,

aparameter˝canbedeﬁned˝=TmTS/HmixwhereTm

istheaveragemeltingpointofthealloysystem.Solid

solu-tionphasetendstoformaslong as˝>1issatisﬁedwhich

meansthattheeffectoftheentropyofmixingisgreaterthan

thatofthe enthalpy ofmixingatthe melting temperature

[9].

Zhangetal.[8]andGuoetal.[10]studiedtheeffectofthese

parametersonthephaseformationofHEAsandobtained

sim-ilarconclusions:theformationofsimpleorcomplexphases

dependsmainlyontheenthalpyofmixing,entropyofmixing,

andatomicsizedifferences.

Guoetal.[11]investigatedtheeffectofvalenceelectron

concentrationonstabilityofFCCorBCCphaseinhighentropy

alloysandsuggestedthevalenceelectronconcentration(VEC)

canbeusedtopredicttheBCCandFCCstructuredsolid

solu-tions ofHEAs. Fig.1summarizes the relationship between

the structure and VEC, it is observed that BCC structured

solid solution formswhen VEC<6.87;whilefor FCC

struc-tureVEC≥8,mixedFCCandBCCphaseswillco-existwhen

6.87≤VEC<8.0.

ThemetallurgicalnatureofHEAsimpacts onthem rare

property combinationswhich givesthem thepotentialsfor

use in a wide range ofengineering applications. HEAsare

known to have good thermal stability [12], high hardness

andstrength[13,14],excellentwearresistance[15],distinctive

electrical, magnetic properties [16] and impressive

corro-sion resistance [17–19]. HEAs are also reported to possess

highhardnessandhighcompressivestrengthbothatroom

temperature and elevated temperatures [20,21]; and great

integrated tensile properties, including both high ultimate

tensilestrengthandreasonableductility[22].Someofthese

spectrumsofpropertiesarerarelyobservedinconventional

alloys,makingHEAsattractiveinmanyﬁelds.Thefactthatit

canalsobeusedathightemperaturesbroadensitspotential

applicationbaseevenfurther.Forinstance,superiorstructural

alloys are in high demand for extreme and highly

sensi-tive engineering service environments, particularly in the

nuclear,turbine,andaerospaceindustries.Thepropertiesof

HEAsmakethemsuitablecandidatesforuseinsuch

environ-ments.

Overall,ithasbeenreportedthattheFCC-structuredHEAs

exhibitlowstrengthandhighplasticity,whileBCC-structured

HEAsshowhighstrengthand lowplasticity.Thus,thetype

ofcrystalstructureisadominantfactorforcontrollingthe

strengthorhardnessofHEAs[5,10].Morethan30elements

havebeenreportedly usedtoprepareover300HEAs,

form-inganexcitingnewﬁeldofmetallicmaterials[23].Thispaper

brieﬂy reviews the physical metallurgy of HEAs, including

processingroutes,areasofapplication;andeffectsof

produc-tionmethodsandalloyingelementsonthephasetransitions,

mechanicalproperties,andcorrosionbehaviorofHEAs.

2. Processing

routes

of

HEAs

Presently, the processingroutes forHEAs can be classiﬁed

basedonthestartingstatesforthealloypreparation[24].

Basi-cally,mechanicalalloyingfollowedbyisostaticpressing,arc

meltingandsurfacecoating(plasmasprayandlasercoating)

are usedforprocessingHEAs. Other processingtechniques

suchaselectrochemicalpreparationofHEAsareevolving.

2.1. Processingbymechanicalalloying

Mechanical alloying is a solid state powder processing

technique involving repeated coldwelding, fracturing, and

re-weldingofpowderparticlesinahigh-energyballmill[25].

Mechanicalalloyinghasbeenreportedtohavethecapability

ofsynthesizingavarietyofequilibriumandnon-equilibrium

alloys startingfrom blendedelemental orpre-alloyed

pow-ders [26]. Mechanical alloying ispeculiar to metal powder

processing,wheremetalpowdersaremixedtoproducesuper

alloys. Mechanical alloying occurs inthreesteps. First, the

alloy materials are combined in a ball mill and ground to

ﬁnepowders,thisisfollowedbyhotisostaticpressing(HIP)

tosimultaneouslycompressandsinterthepowders.Finally,

heattreatmentiscarriedouttoreliefexistinginternalstresses

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5.0 5.5 6.0 6.5 7.0 7.5

VRC

8.0 8.5

AlCo0.5CrCuFeNi; AlCoCr0.5CuFeNi AlCoCrCu0.5FeNi; AlCoCrCuFe0.5Ni AlCoCrCuFeNi0.5; AlCoxCrCu0.5FeNi AlCoxCrCu0.5FeNi; AlCoxCrCu0.5FeNi AlCoCr_xCu_0.5FeNi; AlCoCrCu_0.5Fe_xNi AlCoCrCu0.5FeNix; AlCoCrCu0.5FeNix AlxCrCuFeMnNi; AlxCrCuFeMnNi Al0.8CrCu1.5FeMnNi; Al0.8CrCuFe1.5MnNi Al0.8CrCuFeMn1.5Ni; AlxC0.2CuFeMnNi; AlB_xMnNiTa MoNbTaVW; MoNbTaW CrCuFeMnNi; CoCrFeMnNi 9.0 9.5

Fig.1–RelationshipbetweenVECandtheFCC,BCCphasestabilityforHEAsystems.Notes:Fullyclosedsymbolsforsole FCCphases;fullyopensymbolsforsoleBCCphase;top-halfclosedsymbolsformixesFCCandBCCphasesafter[11]with permissionfromElsevier.

processhassuccessfullybeenusedtoproducealloyssuitable

for high temperature application and notable aerospace

components [27]. HEAs produced by mechanical alloying

followed by consolidation, possess a higher pore density

than samples fabricated by casting. However, the melting

routeleadstosegregationproblems,whilebythemechanical

alloying process, homogenous chemical distribution and

solid solubility extension can be reached [26]. In addition,

mechanical alloying is a powerful solid state processing

method,whichcaneasilybeusedtoproducenano-crystalline

materialswithsuperiorproperties.

2.2. Processingbyarcmelting

Arcmeltingisthemostpopularliquidprocessingmethodfor

HEAs.TheproductionofHEAsisachievedviameltingof

var-iouselementsseverally(atleastﬁvetimes)inthearcmelting

furnace[28].Thetorchtemperatureofthearcmeltingfurnace

canbeveryhigh(>3000◦C),andcanbecontrolledbyadjusting

theelectricalpower.Hencemostofthehighmeltingelements

canbemixedintheirliquidstatebythiskind offurnaces.

However,arcmeltingmaynotbesuitableforelementswitha

lowmeltingpoint(Mg,Zn,andMn),whichevaporateseasily,

makingcompositioncontroldifﬁcult.Inthiscase,resistance

heatingorinductionheatingmaybemuchmoreappropriate.

2.3. Plasmasprayprocess

Plasmasprayprocessisaliquidprocessingmethod.The

pro-cessinvolvesplasmasprayingofHEAcoatingonapreselected

metalsubstrateatahighvelocity,givingitasmooth

protec-tivelayer[29].Inthisprocess,ﬁnelydividedHEApowdersare

initiallymeltedonpreparedsubstratesinordertoformspray

deposits.Therequiredheatisgeneratedbycombustiblegases

or electric arcsin the thermal-spraying gun.As the target

materialisgraduallyheatedup,itisconvertedtoamolten

state,andwillbeacceleratedbythecompressedgas.The

con-ﬁnedstreamofparticlesiscarriedtothesubstrate,andstrikes

thesurfacetoﬂattenandformthinplatelets.Theseplatelets

arecompatiblewiththeirregularitiesofthepreparedsurface

and to each other. Moreover, these sprayed particles are

accumulatedonthesubstratebycoolingandbuildingupone

byoneintoacohesivestructure.Thus,coatingsareformed.

2.4. Processingbylasercladding

The laser cladding process has some advantages, which

includesfastheatingandcooling,moreuniformanddense

cladding,andlessmicroscopicdefects.Alsomicro-claddingis

easilyachieved,thermalimpactonthematrixisminimaland

smallrateofdilutionisobserved[30].Thistechnologyis

simi-lartotheplasmaspraymethodinthatithasanenergysource

tomeltthefeedstockthatisbeingappliedtoasubstrate.What

differsisitusesaconcentratedlaserbeamasheatsource,and

itmeltsthesubstratethatthefeedstockisbeingappliedto.

Thistechniquenormallyresultsinametallurgicalbondthat

hassuperiorbondstrengthoverplasmaspraytechnique.One

oftheadvantagesofthelaser-claddingprocessisthatthelaser

beamcanbefocusedandconcentratedonaverysmallarea,

whichmakestheheat-affectedzoneofthesubstratevery

shal-low.Thisfeatureminimizesthechanceofcracking,distortion,

orchangeinthemetallurgyofthesubstrate.Additionally,the

lower totalheatminimizesthedilutionofthecoatingwith

materialsfromthesubstrate[31].

3. Current

and

potential

applications

of

HEAs

HEAsarecurrentlydeployedforuseasfunctionaland

struc-tural materialswithgreatpotential forselectionina wide

rangeofotherapplications.Presently:

(1) HEAareusedassolderandbrazingﬁllerforweldingpure

titanium andchromium–nickel–titaniumstainlesssteel,

cementedcarbideandsteel,respectively[24].

(2) HEAs have successfully been employed in the nuclear

industries. Their improved irradiation and high

corro-sionresistancemakeHEAspotentialcandidatesforthe

claddingofmaterialsusedinnuclearfuelsandhigh

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(3) HEAsareusedasheat-resistantorwear-resistantcoatings.

NewtechnologiesareneededtomaketheHEAscoating

moreuniformandwithhighcohesionwithsubstrates[33].

(4) High-entropycarbidesandnitridescanpotentiallybeused

asbiomedicalcoatings.Theymaypotentiallybeusableas

diffusionbarriersandhardcoatingsontoolcuttingsteels

[34].

(5) The special physical properties of HEAs, for example,

Al2.08CoCrFeNi,withnearconstantresistivitywouldmake

themusefulforelectronicapplications[35].

(6) Light-weight HEAscould beused ascasings formobile

facilities, battery anode materials, and transportation

industry.

4. Inﬂuence

of

production

route

and

alloying

elements

on

the

microstructure

and

mechanical

properties

TheattractivepropertiesofHEAsatroomandelevated

tem-peratures have been noted as a major attraction for its

considerationforuseinsomanyapplicationsincluding

tur-bine,nuclear andaerospaceindustries [36,37].Someofthe

previousstudies ontheinﬂuenceofproductiontechniques

andalloyingelementsonthemicrostructureandmechanical

propertiesofHEAarepresentedinthissection.

4.1. Inﬂuenceofproductionroute

Jithinetal.[38]comparedthemicrostructureandmechanical

propertiesofdirectlaserandarc-meltedAlxCoCrFeNi(x=0.3,

0.6and0.85)producedhighentropyalloys.Roomtemperature

compressiontestingshowedverysimilarmechanical

behav-ior and properties for the two different processing routes.

Theauthorsobservedthatirrespectiveoftheprocessing

tech-niquethestrengthofAlxCoCrFeNihighentropyalloysystem

increasedwith increase in Al concentration (Fig. 2)at the

expenseofductility.

Inanotherstudy,CoCrFeNiAlhigh-entropyalloywas

suc-cessfully synthesized by mechanical alloying [39]. It was

reportedthataBCCstructured solidsolutionwas obtained

after30h.TheBCCphaseexhibitedhighphasethermal

sta-bilityupto500◦C,andgraduallytransformedintoanFCCsolid

solutionabove500◦C.Thebulkspecimensshowedhigh

Vick-ershardnessof625HVandcompressivestrengthof1907MPa,

whichare duetothesolidsolutionstrengthening andBCC

structureultraﬁnegrains.

Weipingetal.[40]fabricatedFeNiCrCo0.3Al0.7highentropy

alloy(HEA)bymechanicalalloyingandsparkplasmasintering

process. The formation of a supersaturated solid

solu-tionwithbody-centeredcubic(BCC) structurewasreported

during mechanical alloying, which partially transformed

to face-centered cubic (FCC) structure during SPS. Bulk

FeNiCrCo0.3Al0.7 alloywithlittleporosityexhibitsmuch

bet-termechanicalpropertiesexceptcompressionratiocompared

withothertypicalHEAsofFeNiCrCoAlHEAsystem[41].The

yieldstrength,compressivestrength,compressionratioand

VickershardnessofFeNiCrCo0.3Al0.7 alloyare2033±41MPa,

2635±55MPa,8.12±0.51%and624±26HV,respectively.The

fracture mechanismofbulk FeNiCrCo0.3Al0.7 alloy is

domi-3000 2500 2000 1500 1000 T rue stress (MP a )

a

b

500 0 0.0 0.2 0.4 0.6 True strain 0.8 1.0 3000 2500 2000 1500 1000 T rue stress (MP a ) 500 0 0.0 0.2 0.4 0.6 True strain Al_0.3 Al_0.6 Al_0.85 0.8 1.0

Fig.2–Truestress–straincurveofentropyalloysystemfor Alconcentrationsof(a)directlasersinteredand(b) arc-meltedAlxCoCrFeNiHEAwithAlconcentrationsof

x=0.3,0.6,0.85after[38]withpermissionfromElsevier.

natedbyinter-crystallinefractureandquasi-cleavagefracture

[40,41].

Baldenebro-Lopezetal.[42]investigatedthesynthesisof

AlCoFeMoNiTi highentropy alloythroughsintering andarc

melting.TheyobservedtheformationofMo-richandTi-rich

phasesinthemeltedsample,whileTi-richnano-precipitates

were identiﬁedin the sintered sample. Their ﬁndings also

showthatahighermicrohardnessvaluewasachievedonthe

sinteredsamplethanforthemeltedsample.Thiswasreported

tobeonaccountof(1)thegreaterchemicalhomogeneity(less

formedphases)inthesinteredsample;(2)itslowergrainsize

aftersinteringincomparisontotheas-castsample;and(3)the

formationofnanocrystallineprecipitatesevenwithagreater

porositythan the as-castsample.Wang et al.[43]reported

ahardnessvalueof500HVforanAlCoCrFeNialloyandHsu

etal.[44]reportedahardnessof730HVforanAlCoCrFeNiMo0.5

system.Bothalloyswereproducedbyarcmeltingundera

pro-tectiveatmosphereofargon.Itisevidentthattheadditionof

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3500 AlCoCrFeNiTix 3000 2500 2000 1500 1000 500 0 0 5

Ti0 Ti0.5 Ti1 Ti1.5

10 15 20 True strain, % T rue stress (MP a) 25 30 35

Fig.3–Compressivetruestress–straincurvesof

AlCoCrFeNiTi_xhighentropyalloyafter[45]withpermission fromElsevier.

Generally,itisobservedthattheprocessingrouteemployed

forthefabricationofHEAsgreatlyaffectsthesolidsolution

phasesandmechanicalproperties.TheSPSprocessedHEAs

appearedtohavemuchbetter mechanicalproperties

com-paredtoMAandarcmeltingprocessedHEAs.

4.2. Effectofalloyingelements 4.2.1. Titanium

Zhou et al. [45] studied the effect of Ti alloying on

AlCoCrFeNiTix,(x=0,0.5,1,1.5).Itwasreportedthatthealloy

systemiscomposedprimarilyoftheBCCsolidsolutionand

possessesexcellentroom-temperaturecompressive

mechan-icalproperties.OfallthealloysystemsasobservedinFig.3,

theAlCoCrFeNiTi0.5alloyappearedtohaveasuperior

combi-nationoftheyieldstress,fracturestrength,andplasticstrain,

whicharesuperiortovaluesobtainedformosthigh-strength

alloys,suchasbulkmetallicglasses[46].

4.2.2. Aluminumandcopper

Fanetal.[47],designed(FeCrNiCo)AlxCuyhigh-entropyalloys

and studied the inﬂuences ofAl and Cu elements on the

micro-structureandmechanicalpropertiesofthealloy.They

observedthat as Alelement level increasedfrom 0.5to1,

the microstructure of the alloy system changed from FCC

structuretoduplexFCCplusBCCstructureandthenasingle

BCCstructure.IncreaseofAlconcentrationgreatlyenhanced

Young’smodulus,hardnessandyieldstrengthofthesealloys.

Cu-richphaseformedinthealloyswhenCuwasinhighlevels.

IncreaseofCuconcentrationsigniﬁcantlydecreasedfracture

strengthofthehigh-entropyalloyswhenAlwasinthelevel

x=1.

Thestudy carried out byLi et al.[48] reportedthat the

structure of as-cast FeCoNiCrCu0.5Alx high-entropy alloys

transformsfromFCCphasetoBCCphasewithincreaseinAl

content.ThestablephaseofFeCoNiCrCu0.5AlxHEAtransforms

from FCCphase toFCC+BCC duplex phaseswhen x value

increasesfrom0.5to1.5.ThehardnessofBCCphaseishigher

thanthatofFCCphase,andthecorrosionresistanceofBCC

phaseisbetterthanFCCphaseinNaClandacidmedium.They

concludedthathighhardnessandgoodcorrosionresistance

canbeobtainedinas-castFeCoNiCrCu0.5Al1.0alloy.

4.2.3. Vanadiumaddition

Dong et al.[49] investigatedthe effectsofvanadium

addi-tion on the microstructure and mechanical properties of

AlCoCrFeNiVx(xvaluesinmolarratio,x=0,0.2,0.5,0.8,1.0)

alloys.ForAlCoCrFeNiV0.2alloy,thecompressivestrengthand

plasticstrainwereashighas3297.8MPaand26.8%,

respec-tively, whichare rareinhigh entropyalloysasobservedin

Fig.4.Theﬁnenanoscalespinodaldecomposition

microstruc-ture was a key factor for the high fracture strength of

AlCo–CrFeNiV0.2 alloy[50]. TheVickers hardnessincreased

almostlinearlyfromHV534toHV648.8withtheincreaseof

Velement(Fig.3).InAl0.5CoCrCuFeNiVxalloysystem[51],it

was reportedthat, withincreaseinVcontentfrom x=0to

x=3.0,thecrystalstructureofthealloystransformsfromFCC

toFCC+sigmaphase,thentoBCCstructure,whichshowsthat

theVelementcanstabilizetheBCCstructure.Anincreasein

hardnesswithincreaseinVanadiumwasalsoreported.

Simi-larobservationswerealsoreportedbyStepanovetal.[52].

4.2.4. Niobiumaddition

InAlCoCrFeNiNbxalloysystem,it wasfoundthatadditions

ofNbcausedtheprecipitationofaLavesphasewithaHCP

structure,andastheNbcontentisincreased,theresultant

micro-structurechangedfromhypoeutectictohypereutectic

and suchtransition had a striking(increase) effect on the

compressiveyieldingstrengthandVickershardness[53].

How-ever, the presenceof Almakesit impossible toaccurately

clarifyeffectsofNbonthephasestabilityofthe

CoCrFeNi-basedalloyssincealuminumisastrongBCCstabilizerand

its additioninto thissystemisamajorcomplicationtothe

strengthening andphaseformation[54].Thisledtofurther

investigation byLiuet al.[55],theysynthesized aseriesof

ﬁve-component CoCrFeNiNbx HEAs to investigate alloying

effectsofNbonthestructureandtensileproperties.Itwas

foundthatthemicrostructurechangesfromtheinitialsingle

face-centeredcubic(FCC)toduplexFCCplushexagonal

close-packed(HCP)structurewithadditionsofNb.Thealloysystem

exhibitsahypoeutecticstructureandthevolumefractionof

theNb-enrichedLavesphasewiththeHCPstructureincreased

withincreasingtheNbcontent(Fig.5).

It is noted that both the fracture and yield strengths

increaseastheNbconcentrationincreases.Speciﬁcally,the

fracture and yield strengths of alloy Nb0 are 413MPa and

147MPa,respectively, but increasesigniﬁcantlyto1004MPa

and637MPa,respectively,inalloyNb0.412.Incontrast,the

ten-sileelongationreducedappreciablyfrom49.1%inalloyNb0to

1.3%inalloyNb0.412.

4.2.5. Molybdenumaddition

Dong et al. [56]investigated the effects of the addition of

variousamountsofMoonthemicrostructuresand

mechan-ical properties ofAlCrFeNiMox (x=0, 0.2, 0.5, 0.8,1.0) high

entropy alloys. XRD resultsrevealed that the crystal

struc-ture transformed from two BCC phases toone BCC phase

plusFeCrMo-typesigmaphase.AstheMocontentincreases

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3500 ₆₈₀ 660 640 620 600 580 Hardness(HV) 560 YHV=114.8x+534 540 520 500 3000 2500 2000 1500 Engineer ing stress (MP a) 1000 V0 V0.2 V0.5 _V0.8 V1.0 500 0 0 5 10 15 20 Engineering strain, % 25 30 35 40 0.0 0.2 x value of AlcoCrFeNiVx 0.4 0.6 0.8 1.0

Fig.4–Compressiveengineeringstress–strainandVickershardnesscurvesofAlCoCrFeNiV_xalloysafter[49]with permissionfromElsevier.

from 1406.2to 1748.6MPa.Thealloy withalowcontent of

Mo elementabout x=0.2has the highestfracture strength

of3222MPa and plasticstrain of0.287. When the valueof

xincreases from x=0.2tox=0.8,the fracturestrength and

FCC Laves phase Intensity , a.u.

a

b

Intensity , a.u. Nb0.412 Nb0.309 Nb0.206 Nb0.155 Nb0.103 Nb0.412 Nb0.309 Nb_0.206 Nb0.155 Nb0.103 Nb0 40 45 50 Minor peak 55 Nb0 20 40 60 (200)FCC 2_θ, degree 2θ, degree 80 100

Fig.5–(a)XRDpatternsoftheas-castCoCrFeNiNbx(x=0, 0.103,0.155,0.206,0.309and0.412)alloysand(b)the detailedscansforthepeakof(200)oftheFCCphaseafter [55]withpermissionfromElsevier.

10 000

a

b

BCC1 BCC2 Mo00 Mo02 Mo05 Mo08 Mo10 8000 6000 Intensity (cps) Engineer ing stress/MP a 4000 2000 3500 3000 2500 2000 1500 1000 500 0 0 5 10 15 20 25 30 Engineering strain/% 35 40 45 0 20 40 Mo00 Mo02 Mo05 Mo08 60 80 100 2θ (degree) σ phase

Fig.6–XRDandstress–strainpatternsoftheAlCrFeNiMo_x high-entropyalloysafter[56]withpermissionfrom Elsevier.

the plasticstrain decreased signiﬁcantly from 3222MPato

1512.5MPaandfrom0.287to0,respectively(Fig.6).Onthe

con-trary,thehardnessvalueofthealloysincreasedsigniﬁcantly

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4.2.6. Siliconaddition

Liuet al.[57]evaluatedthe microstructuresand properties

ofAl0.5CoCrCuFeNiSix HEA. Theauthors reported that the

additionofSidestabilizedtheFCCstructureandintroduced

thetransitionfromaclosed-packedFCCstructuretoa

loose-packedBCCstructuretorelaxthelatticedistortionenergydue

tothesmallerradiusofSi.TheyalsoobservedthatasSi

con-tentincreases,compressivestrengthofthealloysincreased,

whileductilitydecreased.

4.2.7. Tinaddition

TheeffectofSn addition onhigh-entropyFeMnNiCuCoSnx

(x=0,0.03,0.05,0.08,0.1and0.2)wasinvestigatedbyLiuetal.

[58].Theyreportedthatthealloysexhibitedgoodplasticity,

and thatthe concentration ofSn elementplayed a

signiﬁ-cantroleinthemicrostructureandtensileproperties.When

0.03<x<0.05,thealloysexhibitedhightensile strengthand

plasticity,andthemaximumelongationstrainandstrength

are 16.9%and 476.9MPa,respectively, becauseoftheir

sin-gleFCCsolutions.Anintermetalliccompound(Cu5.6Sn)inthe

interdendriticregionsformed withtheconcentration ofSn

higherthan0.05,whichdegradestheductilityofalloys.The

hardnesswasalsoobservedtoincreasewithincreaseinthe

concentrationofSn.

5. Inﬂuence

of

production

route

and

alloying

elements

on

the

corrosion

behavior

of

HEAs

HEAshavebeenreportedtoexhibitverygoodcorrosion

resis-tancecomparedtoothermetallicsystemsandalloys.Some

ofthestudiesontheinﬂuenceofproductiontechniquesand

alloyingelementsonthecorrosionbehaviorofHEAsare

dis-cussedinthissection.

5.1. Inﬂuenceofproductionroute

The corrosion behavior of laser cladding processed

AlCr-FeCuCo high-entropy alloys studied using electrochemical

workstation was reported by Qiu et al. [59]. The authors

reportedthat,withincreaseinthescanningspeed,thealloy

corrosionresistanceperformanceshowsanenhancementin

the ﬁrst and then aweakenedtrend. They concludedthat

underthesameconditionsofscanningspeed,thecorrosion

resistanceofAlCrFeCuCoHEAsin1mol/LNaClsolutionis

bet-terthanin0.5mol/LH2SO4solutions.

Inanotherstudy,thecorrosionbehavioroflasercladding

proceededAl2CrFeNiCoCuTixhigh-entropyalloycoatingswas

investigated [60]. Ti element was reported to promote the

formationofBCCstructuretoacertainextent.Theauthors

compared the corrosion behavior of the HEA with Q235

steel and observed that the free-corrosion current density

ofAl2CrFeNiCoCuTixhigh-entropyalloycoatingsisreduced

by1–2ordersofmagnitude,whichmakesthefree-corrosion

potentialtobemore“positive”.Thecorrosionresistanceof

Al2CrFeCoCuNiTixhigh-entropyalloycoatingswasenhanced

in0.5mol/LHNO3solutionswithincreasingTicontent.

Highentropyalloycoatingsweresynthesizedonaluminum

substrate by laser surface engineering [31]. Dilution from

the substrate wasminimizedwith theaid ofmulti-layered

coatings.Theyobservedthathigherlaserinputenergyduring

processingleadtouniformmixingamongstthecomponents

resultinginformationofevenlydistributedhighentropyalloy

phasesthroughoutthematrix.Thisresultedinenhanced

cor-rosionresistanceofthecoatingsinnearneutralNaClsolution.

5.2. Inﬂuenceofalloyingelements 5.2.1. Copperaddition

Hsuet al.[61]andLinet al.[62]reportedthat theaddition

of Cu to CoCrFeNi alloy leads tothe formation ofCu-rich

inter-dendritic phase, which suffers from galvanic

corro-sion and severely degrades the corrosion resistance. Hsu

et al. [61] observed that the passive ﬁlm on the Cu-rich

interdendriteregionsdoesnotoffergoodprotection,which

narrowsdownthepassivationregion.Theysuggestedthatthe

corrosionbehaviorcanbeimprovedbyreducingtheamount

ofCu-richphaseviahigh-temperatureannealing.

ThecorrosionbehaviorofCuCrFeNiMnHEAsin1MH2SO4

solution was reported by Ren et al. [63]. Theresults show

that theHEAsexhibitedagoodcorrosionresistancethatis

mainlyinﬂuencedbyCucontentandelementalsegregation

degree.ThecorrosionresistancedegradedwhenincreasingCu

contentandelementalsegregationdegree.Amongthetested

alloys,the2Mn2alloywithlowCucontentandelemental

seg-regationdegreedisplayedbettergeneralcorrosionresistance.

Onthecontrary,theCu2CrFe2NiMn2alloywithhighCu

con-tent and elemental segregation degreeexhibited the worst

generalcorrosionresistance.

5.2.2. Aluminumaddition

Linetal.[64]studiedthecorrosionpropertiesofhigh-entropy

Al0.5CoCrFeNialloy.Theauthorsreportedthattheadditionof

Alsigniﬁcantlyreduced thepitting potentialandincreased

theareaoflocalized/pittingcorrosionofthesealloysinNaCl

solution. ThisisanindicationthatAlisdetrimentaltothe

corrosionresistanceoftheAlxCrFe1.5MnNi0.5alloysinsaline

environment.Itwasalsoreported[17,65]thattheadditionof

MototheCo1.5CrFeNi1.5Ti0.5Moxalloyaffectedthecorrosion

behaviorpositively.Theirﬁndingrevealedthatthealloyhasa

widepassivationregionof1.43Vin1MNaCl(increasedpitting

potential)anddoesnotsufferfromanypittingcorrosion.

Lee et al. [66] compared the electrochemical properties

ofAl0.5CoCrCuFeNialloy withthat of304stainlesssteelin

deaerated1MH2SO4solution.Theauthorsreportedthatthe

corrosionpotential(Ecorr)oftheAl0.5CoCrCuFeNiHEA(−0.080

VSHE) is apparently higher than that of the 304 stainless

steel (−0.151 VSHE), the corrosion current density (Icorr) of

the Al0.5CoCrCuFeNi HEA (3.19␮A/cm2) is alsoobserved to

belowerthanthatofthe304stainlesssteel(33.18␮A/cm2_).

Additionally,the304stainlesssteelhasawiderregionofthe

passivepotentialthantheAl0.5CoCrCuFeNiHEA.Thisclearly

indicatesthattheAl0.5CoCrCuFeNialloyismoreresistantto

generalcorrosion than the 304stainlesssteel(higherEcorr,

lowerIcorr)inthe1MH2SO4solution.

5.2.3. Titaniumaddition

LiuandGuo[67],comparedthemicrostructureand

(8)

entropyalloyswiththatofcommercial304stainlesssteelin

0.5molH2SO4solutionand1molNaClsolution.Theresults

revealthattheAlFeCuCoNiCrTix(x=0.5,1.0,1.5)highentropy

alloysismainly composedofFCC structureand bcc

struc-ture.Polarization curvesshow that, comparedwiththat of

304stainlesssteel,thealloysexhibitlowercorrosionratein

0.5molH2SO4solution,however,thepittingcorrosion

resis-tanceissuperiortothatof304stainlesssteelin1molNaCl

solution.

5.3. CorrosionbehaviorofHEAsandstainlesssteel

Qiu [68] studied the electrochemical properties of

AlCr-FeNiCoCu HEAs, and made a comparison of corrosion

properties between the HEAs and the type-304

stain-less steel 1mol/L NaCl solution. The corrosion kinetic

parameters of the alloy are obtained by linear ﬁtting:

free-corrosion potential Ecorr=−0.012V; free-corrosion

cur-rent density Icorr=3.23×10−9A/cm2. At the same

exper-imental conditions, Qiu and Zhang [69] reported that

corrosion kinetic parameters for 304 stainless steel:

free-corrosion potential Ecorr=−0.238V; free-corrosion current

densityIcorr=3.50×10−7A/cm2.Theauthorobservedthatthe

free-corrosion potential of the AlCrFeNiCoCu high entropy

alloysis more“positive”comparedto thatof304 stainless

steel,andthefree-corrosioncurrentdensityisreducedbytwo

ordersofmagnitude.

5.4. Inﬂuenceofsolidiﬁcation

Hongbaoetal.[70]investigatedthemicrostructureand

cor-rosion behavior of FeCoNiCrAl high entropy alloy under

directional solidiﬁcationin3.5% NaClsolution. The results

showedthatwithincreasingsolidiﬁcationrate,theinterface

morphologyofthealloyevolvedfromplanartocellularand

dendritic.Theauthorsreportedthatthecorrosionproductsof

bothnon-directionallyanddirectionallysolidiﬁedFeCoNiCrAl

alloysappearedasrectangularblocksinphaseswhichCrand

Feareenriched,whileAlandNiaredepleted,suggestingthat

AlandNiaredissolvedintotheNaClsolution.Itwasobserved

thatthecorrosionresistanceofdirectionallysolidiﬁed

FeCoN-iCrAlalloyissuperiortothatofthenon-directionallysolidiﬁed

FeCoNiCrAlalloyina3.5%NaClsolution.

6. Summary

and

suggestions

for

future

research

HEAs have shown promise and demonstrated unique and

attractivepropertiesforvariousengineeringapplications.This

paperreviewed various aspects of the physical metallurgy

ofHEAs, including processing routes, areas ofapplication,

effectsofproductionmethodsandalloyingelementsonthe

microstructure,mechanicalproperties,corrosionandphysical

behavior.Thereviewissummarizedasfollows:

• Multiple-processingmethods,suchasmechanicalalloying,

arc melting, plasmaspray technique and laser cladding,

havesuccessfullybeenemployedintheproductionofHEAs.

• HEAsholdthepotentialinawiderangeofapplicationssuch

asfunctional and structuralmaterialsespeciallyturbine,

nuclearandaerospaceindustries.

• The microstructure, mechanical, corrosion and physical

properties of HEAs were greatly altered with respect to

processingroutesandadditionofalloyingelements.

Maximizing the potentials of HEAs will no doubt be a

motivating scheme for materialsscientists to pursue. The

metallurgyofHEAsshowthatthereisstillalotthatisyet

untappedandunexploredforhighperformanceapplications.

FutureresearchfocusinthescienceofHEAsshouldbedriven

inthedirectionofweightandcostreduction.Otherprocessing

routes other than those mentioned in the review can be

exploredintheproductionofHEAs.Forexample,HEAscan

beprocessedbyawroughtprocessincludinghomogenization,

hot/coldworking,andannealingtoeliminatecastingdefects

and improve microstructure. Presently, there are limited

literaturesontheplasticdeformability,fracture,creep,fatigue

and wear behavior of HEAs. Studies in this direction are

desiderataforathoroughunderstandingofitsmechanicaland

wearbehavior;andhencefunctionaluseinloadbearingand

contactapplications.

Conﬂicts

of

interest

Theauthorsdeclarenoconﬂictsofinterest.

r

e

f

e

r

e

n

c

e

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