室温多道次ECAP变形对7075铝合金组织性能及沉淀相分布的影响

时间：TIME\@"yyyy'年'M'月'd'日'"2022年3月29日学海无涯页码：第1-页共1页室温多道次ECAP变形对7075铝合金组织性能及沉淀相分布的影响1Introduction

Aluminiumalloysarewidelyusedinaerospaceengineering,marinevehiclesandmilitaryapplicationsduetotheirhighspecificstrength,lowdensityandeasyprocessing.Moreover,thesealloysarethedrivingforcebehindtheglobaldevelopmentofmakingobjectsanddevicesmorelightweight[1-5].Atpresent,theresearchanddevelopmentofaluminiumalloyshavebecomeakeydevelopmenttargetfordefencescienceandtechnology[6-7].Amongstthesealloys,7075aluminiumalloyisoneofthestrongestalloysforcommercialuseandhashighutilisationvalueasastructuralmaterial.Withthedevelopmentofthemanufacturingindustry,thereisnowarequirementfortraditional7075aluminiumalloystoexhibitplasticity.

Finegrainstrengtheningisoneoftheeffectivemeansbywhichtoimprovethepropertiesofmetallicmaterials,asrepresentedbyHall-Petchtheory[8-11]:

σy=σ0+k/d12

(1)

wheredisthegraindiameter;σ0isthefrictionalforcethatactsonthedislocation;σyistheyieldlimitofthematerialandkisaconstant.Usingtheaboveequation,itcanbeseenthatthestrengthofamaterialcanbeimprovedbyrefiningitsgrains.However,therearesomelimitationstothisthatwhenthegrainsarerefinedtoclosetothephysicallimitofthedislocationitself,theaboveequationisnotapplicable.Severeplasticdeformation(SPD)isfavoredbyresearchersasauniquedeformationmethodwithcontrollablemicrostructurecharacteristics,andcanbeperformedatroomorlowtemperaturetoobtainamaterialthatexhibitsafinegrainstructure.Iftheamountofdeformationisincreased,superfinegrainmaterialscanbeobtained.Equal-channelangularpressing(ECAP)isaneffectiveSPDmethodthatobtainsbulksubmicronandnanomaterialsandisconsideredtobethemostpromisingdeformationprocessinSPDprocessingtechnology,withtheadvantagesofstabledeformation,multi-passprocessing,andproductionoflargebulkfine-grainedmetallicmaterialscomparedtootherprocesses[12-15].DuringtheECAPdeformationprocess,thematerialispassedthroughtwoequalcross-sectionalpipemoldswithacertainanglebythetopforce,duringwhichpuresheardeformationoccurstobreakthecoarsecrystalsintofinegrains.

Todate,someprogresshasbeenreportedontheECAPdeformationof7075aluminiumalloy.ZHAOetal[16]studiedtherelationshipbetweentheprecipitationbehaviouroftheprecipitationphaseandthemechanicalpropertiesofthe7075aluminiumalloyafterECAPdeformationbysolidsolutiontreatmentat480℃for5handECAPdeformationat250℃.GHALEHBANDIetal[17]performedthesinglepassECAPdeformationof7075aluminiumalloyatroomtemperatureaftersolidsolutiontreatment,whereitwasfoundthatthefracturetoughnessofthematerialwassignificantlyimprovedafterECAPdeformationwithageingtreatment.

WhenperformingECAP,thedeformationenvironmenttemperaturehasagreatinfluenceonthemechanicalpropertiesofthealloy[18-21].The7075aluminiumalloyexhibitsexcellentstrengthandstiffnessproperties,butpoorductility,makingitdifficulttoperformmulti-passECAPatroomtemperature,whichiswhyithasreceivedlessresearchattention.

Inthisstudy,7075aluminiumalloysweresubjectedtomultiplepassECAPdeformationatroomtemperaturetoinvestigatetheeffectsoftheprocessonthemicrostructuretransformation,enhancementofthepropertiesandtheprecipitationbehaviouroftheprecipitationphaseoftheoriginalmaterial.

2Experimentalmethods

Commercial7075aluminiumalloywasusedasthematerialofstudy,thecompositionofwhichisshowninTable1.Thisalloywascutintoblocksamplesof30mm×18mm×170mmsizeandsubjectedtoa30minsolidsolutionwatercoolingtreatmentat477℃.Thesurfacefinishofthetreatedsampleswasachievedusingaverticalmillingmachine.

Table1Chemicalcompositionof7075aluminumalloy(wt%)

SiFeCuMnMgCrZnTiAl

0.080.271.510.062.500.205.520.03Bal.

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PriortoECAPdeformation,homemadelubricatingfluid(graphitepowderandoilmixture)wasevenlyappliedtothesamplesandtheextrusionrod.TheprocessparametersforECAPwereasfollows:adieangleofΦ=135°,anouterangleψ=20°,theextrusionspeedwas2.5mm/s,andtheextrusionmethodwasrouteC(thesamplewasrotated180°inthedieforthenextdeformationaftereachprocess),asshowninFigure1.Themicrostructureofthe7075aluminiumalloybeforeandafterdeformationwasstudiedbyelectronbackscatterdiffraction(EBSD,NORDLYSNANO)andhighresolutiontransmissionelectronmicroscope(HRTEM,FEITalosF200X)toobservethegrainsize,grainboundarycharacteristics,structuralevolutionandprecipitationphaseprecipitationbehaviourofthealloy.ThespecificsamplinglocationisthecentreoftheEDsurface.ThespecificpolishingparametersareshowninTable2.TheresultswereanalyzedusingChannel5softwareaftercompletingtheEBSDtesting.X-raydiffractometry(XRD,D8ADVANCEA25)wasusedtoexaminetheprecipitatedphasesbeforeandafterdeformation,andthephysicalphaseanalysiswasperformedusingtheJadesoftware.Stress-straintensileexperiments(INSTRON8801universaltestingmachine)wereconducted,andthefracturesinthematerialwereobservedbyscanningelectronmicroscopy(SEM).AHX-1000TMmicro-VickershardnesstesterwasusedtoexaminethehardnessoftheEDsurfacebeforeandafterdeformationandtoconstructacloudchart.ThehardnesscloudpointandtensilesamplesizeareshowninFigure2.

Figure1PrinciplediagramofECAP

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Table2Parametersrequiredforelectrolyticpolishing

ProcessconditionParameter

Polishingsolution

10%perchloricacid

alcoholsolution

Polishingtemperature/℃-30

Polishingtime/min3

Polishingvoltage/V25

Polishingcurrent/A0.4-0.5

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Figure2(a)TherequiredmeasuringpointsfortheEDsurfacehardnesscloudchartand(b)thesizeofthetensilesample(Unit:mm)

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3Resultsanddiscussion

Figure3showsthegrainorientationdiagrambeforeandafterfourpassesofECAPatroomtemperature,fromwhichitcanbeseenthatthegrainshapeandsizechangedsignificantlyafterdeformation.Theoriginalbasemetalwasatypicalrolledstripstructure(Figure3(a)),andthegraindistributionwascoarsestrip(withanaveragegrainsizeofaround26μm).AfterECAPsheardeformation,thestripgrainwasbrokenintoafineequiaxedstructurewithanaveragegrainsizeofaround4.5μmandthedistributionwasuniform(Figure3(b)).Figure4showsthegrainboundarydistributionbeforeandaftertheECAPandthedistributionfrequencyofthegrainboundarysize.AfterfourpassesofECAPdeformation,alargenumberofstress-induceddislocationsweregeneratedduetotheadjustmentofthelatticestructureduetostressconcentration,andalargenumberofsub-grainabsorptiondislocationsweregraduallytransformedfromalow-anglegrainboundary(LAGB)tohighanglegrainboundary(HAGB)[22].ByobservingthegrainmorphologybeforeandafterECAPinFigure3andsomeHAGBfragmentsinFigure4(b),itcanbeseenthatcontinuousdynamicrecrystallisation(CDRX)occurredinthematerialafterECAP.Therefore,theproportionofHAGBincreasedsignificantly,from8.1%fortheoriginalbasemetalto41.8%.AfterECAPdeformation,bimodaldistributionwasobservedatgrainboundaries,andthepeakwasattheLAGB(15°)andHAGB(50°-62°).Accordingtoanumberofstudies,bimodaldistributioncontributestothemicrostructureofthematerialexhibitinghighstrengthandhightoughness[23].

Figure3GrainorientationbeforeandafterECAP:(a)Originalbasemetal;(b)AfterECAPdeformation

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Figure4GrainboundarydistributionbeforeandafterECAPdeformation:(a)Grainboundaryoftheoriginalbasemetal;(b)GrainboundarydiagramafterECAP;(c)Grainmisorientationdistributionoftheoriginalbasemetal;(d)GrainmisorientationdistributionafterECAPdeformation

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Figure5showsthedistributionoftherecrystallisationbeforeandafterECAPdeformationbasedonEBSD,fromwhichthedegreeofcompletionofrecrystallisationbeforeandafterECAPdeformationcanbeobtained,whereblueindicatestherecrystallisationarea,yellowisthesub-grainlineareaandredisthedeformationarea.Thepercentageoftherecrystallisationareaincreasedsignificantlyafterfour-passesofECAPdeformation,whichindicatesthattherecrystallisationeffectisverysignificant.UndertheeffectofthepuresheardeformationofECAP,alargenumberofdislocationsaccumulatedinthedeformedgrainstoformdislocationcellsandgraduallytransformedintosub-structuraltissues.

Figure5RecrystallizationdistributionbeforeandafterECAPdeformation:(a)Originalbasemetal;(b)AfterfourpassesofECAP;(c)Recrystallizationfractionplotoftheoriginalbasemetal;(d)RecrystallizationfractionplotafterfourpassesofECAP

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Figure6showsthelocalorientationdifference(KAM)andtherelativeprobabilitycurveofthealloybeforeandafterECAPdeformation,inwhichahighKAMvaluerepresentshighdislocationdensityintheregion(accordingtotheanalysisoftheEBSDdata,thedislocationdensityindicatedbytheKAMplotismainlygeometricallynecessarydislocations).ByobservingthedatashowninFigure6,itcanbeconcludedthattheKAMvaluesarereducedafterECAPdeformationcomparedtotheoriginalmaterial,whichhasthehighestpercentageat0.6and0.5afterECAPdeformation.ThisismainlyduetotheoccurrenceofCDRXinthematerial,whichmakesthedeformedgrainsgraduallyabsorbdislocationsandtransformintorecrystallisdgrains,aphenomenonthatisconsistentwiththeabovedescription.

Figure6LocalorientationdifferencedistributionbeforeandafterECAPdeformation:(a)Originalbasemetal;(b)AfterfourpassesofECAP;(c)Relativeprobabilitycurveoftheaverageorientationdifferences

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Figure7showsthepolardiagramanalysisbeforeandafterECAPdeformation,fromwhichitcanbeseenthatthepreferredorientationchangedafterfour-passesofECAPdeformation.Figure7(a)showsthattheoriginalmaterialexhibitsatypicalrolledtexture(β-fibretexture),andafterECAPdeformation,thestrongestpointisobservedforthe{111}crystalplaneandderivesmorestrongpoints,mainlyfeaturingaC-typesheartexture,whichisduetotheidealpuresheardeformationofECAP,thec-axisistiltedintheEDandNDdirections,andthetextureorientationistilted45°alongthedeformationsystem.ThedecreaseinpolardensitycomparedtotheparentmaterialafterfourpassesofECAPdeformationmaybeexplainedbythefactthatthefactorsthatchangethecrystaldegreephaseduringtheECAPdeformationprocessmainlyincludedislocationaccretionandtwinning,andthedislocationcellcausedbydislocationaccretionhasrelativelyrandomphasestatistics[24].

Figure7PolediagramanalysisbeforeandafterECAPdeformation:(a)Originalmaterial;(b)AfterECAPdeformation

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Figure8showstheXRDpatternanalysisoftheoriginalbasematerialandthealloyaftertheECAPdeformation.Itcanbeseenthattheprecipitatedphasesinprecipitationspeciesarebasicallythesamebeforeandafterdeformation,mainlyfeaturingamixtureofηphase(MgZn2),Sphase(Al2CuMg)andGPregion(Al2Cu).AfterECAPdeformation,theAl2Cuphasepeakcanbeobservedata2θangleof41.49°,andthecontentoftheMgZn2andAl2CuMgphasesdecreaseat2θanglesof44.87°and65.20°,whichmaybeduetothecoarseningofsomeinsolubleprecipitationphasesafterthesolidsolutionandnaturalageingoftheoriginalbasematerial,andthefragmentationandrefinementofprecipitatingandco-griddingwiththematrixduetoECAPdeformation[25].ThemainpeaksoftheprecipitatedphaseswereshiftedindifferentdirectionsafterECAPdeformation,indicatingthatbothprecipitationphasegenerationanddissolutionoccurred.

Figure8XRDanalysisbeforeandafterECAPdeformation

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The7075aluminumalloyisaprecipitation-reinforcedaluminumalloy,thetypeandformationofthesecondphaseofwhichhaveagreatinfluenceonitsmaterialsmatrix[26-29].Figure9showstheTEManalysisoftheoriginalbasematerialandthesecondphaseafterECAPdeformation.InFigure9(a),itcanbeseenthatthecoarsesecondphases(indicatedbythewhitearrowsinthefigure),whicharemainlyFe-containingprecipitatedphases,appearintheoriginalbasematerial,andthesesecondphaseseasilygeneratelargestraingradientsaroundthemduetotheirlargesize,thusundergoingparticlestimulatednucleation(PSN).PSNnucleationcanproducerotationalcubictextureandPtexturearoundit,whichisoneofthereasonsforthelowstrengthofthetextureduetothefragmentationofthecoarsesecondphaseaftertheECAPdeformation.Thediffuselydistributedsecondphaseplaysakeyroleintherecoveryandrecrystallisationofthe7075aluminumalloy.FromFigure9(a),itcanalsobeobservedthatadiffusephasebandappearsatthegrainboundary(shownbytheyellowarrowinthefigure),andthefinesecondphaseinthedispersionphasebandcanproducepinningeffectongrainboundarymigration,sothemigrationofthegrainboundaryalongthewidthofthediffusephasebandexertsalargepinningforce,whichinhibitstheoccurrenceofreversionandrecrystallisationandthereforethedislocationdensityishigh[30].Figure9(b)showsthediffractioncalibrationoftheprecipitatedphasetypesinthepristinebasematerial,whichmainlyincludelongrod-likeMgZn2andfinesphericalAl2CuMg.InFigure9(c),itcanbeseenthattherolledpristinebasematerialcontainsalargenumberofdislocationsaccumulatedasdislocationwalls(shownbyblackarrowsinthefigure),andtheinteractionbetweenthesecondphaseanddislocationscanbeobservedinthebasematerial,whichshowsthatthemainstrengtheningmodesofthebasematerialaresecondphaseanddislocationreinforcements.AsshowninFigure9(d),comparedwiththebasematerial,thegrainsareelongatedinthe45°directionafterfourpassesofECAPdeformation.Moreover,thedislocationdensitydecreasesandthedislocationmorphologyisgreatlychanged.ThisisbecauseafterECAPdeformation,thedislocationsaregraduallytransformedintodislocationcellsundertheeffectofshearanddynamicrecovery,andfurtherevolveintosub-grainsuptotheHAGB.ItcanbeseenfromFigure9(d)thatthesecondphaseparticlesaremoreuniformlydistributedandfinerinsize,whichaccordingtotheOrowandispersionstrengtheningtheorycancontributetothemechanicalpropertiesofthematrix.

Figure9TEManalysisofbasemetalandalloysafterECAP:(a)Distributionofinsolublephaseintheoriginalbasemetal;(b)Typeoftheprecipitatedphaseofthe7075aluminiumalloy;(c)Dislocationdistributionoftheoriginalbasemetal;(d)DislocationdistributionafterECAPdeformation

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Figure10showstheanalysisofdifferentprecipitationphasesin7075aluminiumalloyafterECAPdeformationusingHRTEMtodeterminethevariousprecipitationphasetypes.Thepositionselectedinthegreenboxinthefigureisthecircularprecipitationphase,andthelatticeconstantd=0.214nmcanbeknownbyinverseFFTtransformation,andtheprecipitationphasecanbeknownasAl2Cuphasebycomparingitslatticeconstantandmorphology.ThepositionselectedinthewhiteboxinthefigureistheAlmatrix,andbymeasuringthelatticeconstant,itisknownthatitslatticeconstantd=0.247nm,whichishigherthantheoriginallatticeconstantd=0.234nm,soitispresumedthattheAlmatrixlatticeconstantisdistortedinthevicinityoftheAl2Cuphase.Noticethattheyellowboxcheckedpositionisthebarprecipitationphase,andthelatticeconstantisknownbymeasurement.Thelatticeconstantd=0.845nm,andcombinedwiththephasemorphologycanbeknownastheMgZn2phase.Theareaselectedbytheblueboxisanellipsoidalprecipitationphase,andthelatticeconstantismeasuredtobed=0.247nm,andthephasecanbeidentifiedasAl2CuMgphaseincombinationwiththemorphology.

Figure10HRTEManalysisofdifferentprecipitationphases:(a)Almatrixlatticestripes;(b)Disc-shapedprecipitatedphase;(c)Disc-shapedprecipitatedphaselatticestripes;(d)Rod-likeprecipitatedphase;(e)Rod-likeprecipitatedphaselatticestripes;(f)Ellipticalprecipitatedphase;(g)Ellipticalprecipitatedphaselatticestripes

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Figure11showsthehardnesscloudchartbeforeandafterECAPdeformation.ItcanbeseenfromthediagramthatthehardnessissignificantlyimprovedafterfourpassesofECAPdeformation,fromtheoriginalaverageofHV115toHV145,anincreaseof26.1%.ItcanthusbeseenthattheECAPdeformationprocesshasanobviouseffectontheimprovementinhardness.AfterfourpassesofECAPdeformation,themicrohardnessdistributionisrelativelyuniform,buttheaveragehardnessoftheuppersurfaceisslightlyhigherthanthatofthelowersurface.ThisisduetothelargesheareffectontheuppersurfaceduringECAPdeformation,andthelowersurfaceisnotuniformbecausethereisadeadzoneatthecornerofthedie.Accordingtopreviousresearch,theformationoftheGPzoneincreasessignificantlyafterECAPdeformation,whichisbeneficialtoimprovingthehardnessofthealloy.AfterECAPdeformation,thegrainsareeffectivelyrefined.TherelationshipbetweengrainsizeandhardnesscanbedescribedaccordingtotheequationbyCABIBBOetal[31]:

Figure11AnalysisofhardnesscloudchartbeforeandafterECAPdeformation:(a)Basemetal;(b)AfterECAPdeformation

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H=H0+kd−1/2

(2)

whereHisthecurrenthardnessofthematerial;H0andkareconstants;anddisthegraindiameter.Thisequationshowsthatthesmallerthegrainsize,thegreaterthehardness.

Figure12showstheroomtemperaturetensilecurvesbeforeandafterECAPdeformation,withthemechanicalpropertiesshowninTable3.Theyieldstrength(YS),tensilestrength(UTS)andelongation(EL)ofthebasematerialare118MPa,153MPaand16.2%,respectively,whileafterECAPdeformation,theYS,UTSandELare314MPa,346MPaand6.5%,respectively.Comparedtothebasematerial,theYSandUTSwereenhancedby166.1%and126.1%,respectively,afterfour-passofECAPdeformation,buttheplasticitydecreasedsignificantly,from16.2%to6.5%,whichshowsthattheECAPdeformationprocessisaneffectivemeanstoimprovethestrengthofthemetal.TheeffectiveimprovementinYSandUTSafterECAPdeformationmaybetheresultofthecouplingeffectoffinegrainstrengtheningandworkhardening.TheplasticityofECAPisreducedduetotheincreaseingrainboundarydistortionanddefectsafterintenseplasticdeformation,whichreadilyleadstodislocationpluggingandisnotconducivetodislocationopening.Accordingtothetheoryoffinegrainstrengthening,afterECAPdeformation,thegrainsizedecreasesandtheproportionofrelativegrainboundariesincreases,andthustheresistancetodislocationmovementincreases.Itisdifficulttocrossthegrainboundaries;therefore,thestrengthiseffectivelyimproved.

Figure12Stress-straincurvesbeforeandafterECAPdeformation

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Table3MechanicalpropertiesbeforeandafterECAPdeformation

MaterialYS/MPaUTS/MPaEL/%

BM11815316.2

ECAP3143466.5

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DuringtheECAPdeformationprocess,the7075aluminiumalloymatrixproducesnumerousstrengtheningmechanisms,suchassecond-phasestrengthening,finegrainstrengtheninganddislocationstrengthening.7075aluminiumalloyhasalargenumberofprecipitatedphases,whichhavebeendeterminedasMgZn2phase,Al2CuMgphaseandAl2CuphasebyXRDandHRTEMabove,so,thesecond-phasestrengtheningmodelrelatedtothevolumefractionisusedtocalculateitscontributiontotheyieldstrengthof7075aluminumalloy.ThemeasuredprecipitationphasevolumefractionpictureisshowninFigure9(b).ThemeasurementsoftwareisImage-Proplus,andtheequationfollowedis[32]:

σs=σm[Vp(S+2)2+Vm]

(3)

whereσmisthematrixyieldstrength;Vpisthevolumefractionoftheprecipitatedphase;Vmisthematrixvolumefraction;Sistheaspectratiooftheprecipitatedphase.Theincrementaleffectofprecipitationphasestrengtheningontheyieldstrengthofthematerialis[32]:

Δσs=σs−σi

(4)

ThevolumefractionVpoftheprecipitatedphasewas0.155measuredbyImage-Proplussoftware;matrixvolumefractionVmis0.845;theaspectratioSoftheprecipitatedphaseis1.35.Therefore,itcanbecalculatedthattheprecipitationphasereinforcementcontributes32.85MPatotheyieldstrengthduringECAP.Finegrainstrengtheninghasasignificanteffectontheyieldstrengthimprovementofthematerial.Toinvestigatethespecificvalueofitscontributiontotheyieldstrengthofthematerial,theeffectofgrainsizeontheyieldstrengthofECAPafterdeformationwasthereforeinvestigatedaccordingtotheHall-Petchrelationship(Eq.(1)).AndthestrengtheningeffectoffinegrainsontheyieldstrengthincrementofthematerialduringECAPdeformationis[33]:

Δσs=σ1−σ0

(5)

Δσs=K0(d−1/21−d−1/20)

(6)

Inthealuminiumalloy,K0is0.06-0.28MPa/m1/2.Therefore,itcanbeconcludedthatthemaximumcontributiontothematerialyieldstrengthbyfinegrainstrengtheningis76.7MPa.AccordingtotheaboveKAMdiagramanalysis,itcanbeseenthatthedislocationdistributioninsidethematerialmatrixisrelativelyuniformafterECAPdeformation,sothecontributionofdislocationstrength(Δσd)anddislocationdensity(ρ)totheyieldstrengthhasbeenstudiedwithTaylorformula[34]:

Δσd=Mα1Gbρ−−√

(7)

whereM=3.06istheTaylorfactor;α1=0.3isaconstant;G=27GPaistheshearmodulusofAl;b=0.286nmistheBurgersvectorofdislocationsintheAlalloy,andtheinternaldislocationdensityafterECAPdeformationcanbecalculatedas6.2×1013m-2bytheKAMplot.Thecalculatedcontributiontotheyieldstrengthofthematerialmatrixbydislocationstrengtheningis54.7MPa.Therefore,itcanbededucedthatduringECAPdeformation,finegrainstrengtheningcontributesmosttotheyieldstrength,followedbydislocationstrengthening,andprecipitationphasestrengtheningthe