非等原子高熵合金的强韧性：设计,加工,微观结构和机械性能

StrongandDuctileNon-equiatomicHigh-EntropyAlloys:

Design,Processing,Microstructure,andMechanicalPropertiesChangRuobinCONTENTS1.Briefintroductionofhighentropyalloy2.Compositionaldesignofstrongandductilenon-equiatomichigh-entropyalloys3.Processingofstrongandductilebulknon-equiatomichigh-entropyalloys4.Microstructureandmechanicalpropertiesofnon-equiatomichigh-entropyalloys5.SummaryandoutlookBriefintroductiontotheHistoryofEngineeringMaterialsConventionalalloydesignoverthepastcenturieshasbeenconstrainedbytheconceptofoneortwoprevalentbaseelements.Asabreakthroughofthisrestriction,theconceptofhigh-entropyalloys(HEAs)containingmultipleprincipalelementshasdrawngreatattentionoverthelast13yearsduetothenumerousopportunitiesforinvestigationsinthehugeunexploredcompositionalspaceofmulticom-ponentalloys.Murty,Yeh,Ranganathan,Butterworth-Heinemann,2014.Fig.1.BriefintroductiontotheHistoryofEngineeringMaterialsFig.2.（1）Highentropyeffect（2）Schematicdiagramshowingthecompositionalspaceofnon-equiatomichigh-entropyalloys(HEAs),whichissignificantlylargerthanthatofconventionalalloysorequiatomicHEAs.Themoreelementsare,thehighertheentropyvalue.AsillustratedschematicallyinFig.2,comparedwithconventionalalloyswithoneortwoprincipalelementsplusminoralloyingcomponents,aswellasequiatomicHEAswithequimolarratiosofallalloyelements,non-equiatomicHEAsgreatlyexpandthecompositionalspacethatcanbeprobed.Pradeep,etal.MSEA,648(2015)183-192.2.Compositionaldesignofstrongandductilenon-equiatomichigh-entropyalloysFig.3.DifferencesintheGibbsfreeenergiesof(metastable)equiatomic,binaryfccsolidsolutionsandtheirrespectivethermodynamicequilibriumstates.(a)ThebaseCoCrFeMnNialloyat1123and1273K;(b–e)changesduetothesubstitutionof(b)CrwithMoorV,(c)FewithV,(d)CowithTiand(e)NiwithCu.Thusasignificantrelaxationofthephasestabilitytrendsseeninbinarysystemsduetopossibleentropyincreasesresultingfromanincreaseinthenumberofalloyingelementsisnotobservedinhigher-ordersystems.Formationofsingle-phasesolidsolutionsinHEAsshowsweakdependenceonmaximizationoftheconfigurationalentropythroughequiatomicratiosofelements.F.Otto,etal.ActaMaterialia61(2013)2628–2638Fig.4.Theconfigurationalentropy(Sc)ofthenon-equiatomiccompositionalHEAs(FexMn62-xNi30Co6Cr2)asafunctionofx(atomicfractionofFe).ThehorizontaldashedlineistheScoftheHEAattheequiatomiccomposition(Fe20Mn20Ni20Co20Cr20).ScisinkB(Boltzmannconstant)peratom.Thermodynamicinvestigationsofnon-equiatomic

HEAsshowedthattheconfigurationalentropycurveofthesealloysisratherflat,indicatingthatawiderangeofcompositionsalongsidetheequiatomicconfigurationassumesimilarentropyvalues.Thenon-equiatomicHEAconceptprovidespossibilitiesfortheunificationofvariousstrengtheningandtougheningmechanisms,enablingsignificantimprovementofstrain-hardeningcapacityandstrength

ductilitycombinations.D.Ma,etal.ActaMater.98,288(2015).

Athighstrains(>10%truestrain),deformationtwinningisactivatedasanadditionalmechanism,causingatransitioninthestrainhardeningratesimilarasinsomeTWIPsteels.Itwasevenfoundthatmaximumentropyisnotthemostessentialparameterwhendesigningmulticomponentalloyswithsuperiorproperties.Inthiscontext,non-equiatomicHEAswithsingle-structurehaverecentlybeenproposedtoexploretheflexibilityofHEAdesignandovercomethelimitationsoftheoriginalHEAdesignconcept.SinglephaseFCCY.Deng,etal.ActaMaterialia94(2015)124–133Fig.5Compositionaldesignofstrongandductilenon-equiatomichigh-entropyalloys

Thelimitedhardeningmechanismsavailableinsingle-phaseHEAs,i.e.,primarilydislocationinteractionandsolid-solutionstrengthening,restricttheirstrain-hardeningcapacityaswellastheattainablestrength–ductilitycombination.However,thefactthathighductilityofstrongmetallicalloyscanbeobtainedwhendifferentdeformationmechanismsareactivatedsequentiallyduringongoingloading,suchastheadditionalactivationoftwinningandphasetransformationathigherdeformationsknownfromtwinning-inducedplasticity(TWIP)andTRIPsteels.TheTWIPandTRIPphenomenaaremainlydeterminedbythevalueofthestackingfaultenergy,i.e.,theenergycarriedbytheinterruptionofthenormalstackingsequence.TheintrinsicstackingfaultenergyγIofFCC-structuredalloyscanbeexpressedas:Fig.6.Freeenergydifferences(∆G)betweentheFCCandHCPstructuresoftypicalalloysystemsat300KderivedbythermodynamiccalculationsusingtheCalphadapproach(Thermo-Calc,databaseTCFE7):(a)quaternaryFe80-xMnxCo10Cr10(x=45at.%,40at.%,35at.%,and30at.%)and(b)quinaryCo20Cr20Fe40-yMn20Niy(y=20at.%,15at.%,10,5at.%,and0at.%).ThisindicatesthattheTRIP-DPeffectintroducedintotheformerquaternaryalloycanalsoberealizedinquinaryalloyswithhighermixingentropyvalue.FreeenergydifferencesbetweentheFCCandHCPstructuresoftwotypicalalloysystemsZHIMINGLI,DIERKRAABE,JOM,Vol.69,No.11,2017Whendesigningthecompositionofstrongandductilenon-equiatomicdual-ormultiphaseHEAs,itisalsoessentialtonotethatthemultipleprincipalelementsselectedshouldbedistributeduniformlyinthemicrostructure,oratleastpartitioninsuchawaythatallofthecoexistingphaseshaveahighsolid-solutioneffectandhighmixingentropy.Furthermore,minorinterstitialelementfractionscanalsobeintroducedintostrongandductilenon-equiatomicdual-ormultiphaseHEAstofurtherimprovetheirmechanicalproperties.Z.Li,etal.Sci.Rep.7,40704(2017).Thus-preparedinterstitialHEA(referredtoasiHEA)wasindeedcharacterizedbyacombinationofvariousstrengtheningmechanismsFig.73.Processingofstrongandductilebulknon-equiatomichigh-entropyalloysFig.8.Processingroutesandrelatedparametersaswellasresultantcompositionalhomogeneitystatesfor3dtransition-metalhigh-entropyalloys.Processingofstrongandductilebulknon-equiatomichigh-entropyalloysThedistributionofthemulti-maincomponentintheblockHEAsisnotuniformbythehomogenizationtreatment.SincehomogenizedHEAsheetsexhibithugegrainsize(>30lm),cold-rollingandannealingprocessesaregenerallyrequiredtorefinethegrainstoachievebettermechanicalproperties.Annealingwasconductedtoobtainfullrecrystallizationofthemicrostructureandtocontrolthegrainsizes.Fig.9.VariationsinFCCgrainsizeandHCPphasefractionindual-phaseFe50Mn30Co10Cr10alloywithincreasingannealingtimeat900°C.Annealingtimeof0minreferstothecold-rolledstateofthesampleswithoutannealing.Interestingly,forthedesignedTRIP-assisteddual-phaseHEAs,annealingtreatmentscanbeusednotonlytocontrolthegrainsize,butalsotomodifythephasefractionsinthemicrostructure.ThevariationsintheFCCgrainsizeandHCPphasefractionofthequaternarydual-phaseZ.Li,etal.ActaMater.131,323(2017).4.Microstructureandmechanicalpropertiesofnon-equiatomichigh-entropyalloysFig.10.TypicalmicrostructuresofFe50Mn30Co10Cr10andFe49.5Mn30Co10Cr10C0.5alloysafterrecrystallizationannealingfor3min:(a1)EBSDphasemapand(a2)ECCimageofdual-phaseFe50Mn30Co10Cr10alloy;(b1)EBSDphasemap,(b2)ECCimage,(b3)APTtipreconstruction,(b4)elementalprofilesacrossaninterfaceofmatrixandcarbide,(b5)TEMbright-fieldimage,and(b6)selected-areadiffractionpatternofinterstitialFe49.5Mn30Co10Cr10C0.5alloy.Diffractionspotsmarkedbyredcirclesin(b6)showtheFCCstructureoftheM23C6carbides(Colorfigureonline).TheslightincreaseofstackingfaultenergyandcorrespondinglyhigherFCCphasestabilitywithadditionofC.ThefractionofHCPephaseintheiHEAissignificantlyreducedafterannealing(Fig.10b1)comparedwiththereferencealloywithoutC(Fig.10a1).Z.Li,etal.ActaMater.131,323(2017).Fig.11.Overviewofultimatetensilestrengthandtotalengineeringelongationobtainedforvariousnon-equiatomichigh-entropyalloys.Forcomparison,dataoftheequiatomicCo20Cr20Fe20Mn20Ni20alloy(#2)arealsoshown.Allalloysproducedin-houseusingsimilarprocessingroutesshowninFig.6forfullcontroloftheexperimentalsetup.Allthesedatastemfromuniaxialtensiletestsconductedonbulksampleswithidenticaldimensionsatroomtemperatureatstrainrateof1x10-3s-1.Withadditionofinterstitialelementcarbonintothedual-phasemicrostructure,thegrain-refinedFe49.5Mn30Co10Cr10C0.5alloy(#8)showsfurtherincreasedultimatestrengthuptonearly1GPawithtotalelongationof~60%.Thesesuperiormechanicalpropertiesareattributedtothejointactivityofvariousstrengtheningmechanismsincludinginter-stitialandsubstitutionalsolidsolution,TWIP,TRIP,nanoprecipitates,dislocationinteractions,stackingfaults,andgrainboundaries.Fig.12.Overviewofdeformationmechanismsinvariousmulticomponenthigh-entropyalloysshowingthattuningdeformationmechanismsiskeytodevelopmentofstrongandductilenon-equiatomichigh-entropyalloys(NE-HEAs).ThestrengthandductilityofthesealloysaregiveninFig.11.SS:solidsolution.Tofurtherclarifythemechanismsresponsiblefortheabovemicrostructure–propertyrelations,Fig.12providesanoverviewofthevariousdeformationmechanismsindifferentmulticomponentHEAspresentedinFig.11.Thisclearlyshowsthattuningdeformationmechanismsviacompositionadjustmentiskeytothedesignofstrongandductilenon-equiatomicHEAs.5.SummaryandoutlookThestrengthandductilityofthevariousnon-equiatomicHEAsatlowandelevatedtemperaturesarestillunknown,andnew(non-equiatomic)HEAswithexcellentstrength–ductilitycombinationsatlowandelevatedtemperaturescanbedesignedandstudied.Forthewidelystudiedtransitio