Electron backscatter diffraction (EBSD) microstructure evolution in HPT copper annealed at a low temperature

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

Original

Article

Electron

backscatter

diffraction

(EBSD)

microstructure

evolution

in

HPT

copper

annealed

at

a

low

temperature

夽

Alexander

P.

Zhilyaev

_,

_Semyon

_N.

_Sergeev

_,

_Terence

_G.

_Langdon

a_{MaterialsResearchGroup,FacultyofEngineeringandtheEnvironment,UniversityofSouthampton,Southampton,UnitedKingdom} b_{InstituteforMetalsSuperplasticityProblems,Khalturina,Ufa,Russia}

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Articlehistory:

Received6June2014 Accepted25June2014 Availableonline28July2014

Keywords:

Hardness

High-pressuretorsion Homogeneity

Severeplasticdeformation

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b

s

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t

DetailedEBSDanalysiswasperformedoncopperspecimensprocessedbyhigh-pressure torsionatP=6GPaforonewholeturnandsubsequentlyannealedatatemperatureof 100◦Cfor15,30and60min.Thebasicmicrostructuralparameters(meangrainsize,GB statistics,microtexture)wereevaluatedinthemid-radiusareasoftheHPTdisks. Micro-hardnessofallsampleswasmeasuredacrossthetwodiametersandinterlinkedtothe microstructuresobserved.SmallbutnoticeablechangesofmicrohardnessinHPTcopper afterannealingweredetected.Thechangeswereinterlinkedtomicrostructuralparameters acquiredbyEBSD.Therelationshipsobtainedarediscussedintermsofthemicrostructure andmicrotextureevolutionduringlowtemperatureannealing.

©2014BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.

1.

Introduction

Theprocessingofultrafine-grained(UFG)andnanostructured metallicmaterialsusingsevereplasticdeformation(SPD)[1] isanewandpromisingmethodofenhancingtheproperties ofmetalsandalloysforadvancedstructuralandfunctional applications [2,3]. Traditionally, there have been two main techniquesforproducingUFGmaterialsusingeither equal-channelangularpressing(ECAP)[4]orhigh-pressuretorsion (HPT)[5,6] but other techniques are now availablesuch as accumulative roll bonding (ARB) [7], multiaxial forging [8],

夽_{PaperpresentedintheformofanabstractaspartoftheproceedingsofthePanAmericanMaterialsConference,SãoPaulo,Brazil,July}

21st_to25th_2014. ∗ _{Correspondingauthor.}

E-mailaddresses:[email protected],[email protected](A.P.Zhilyaev).

twistextrusion[9],plainstrainmachining(PSM)[10]and oth-ers[11].SPDprocessingisattractivebecausethestrainingis practicallyunlimitedduetotheunchangingsamplegeometry andshape.However,thereisageneraltendencyfora satura-tioningrainrefinementforhighmeltingtemperaturemetals [12]andrecovery(andevenrecrystallization)forlowmelting materials[13]processedbycontinuingSPDmethods.

ThereisaconsiderableinterestincreatingUFGstructure in purecopper and its alloys inorder to getthe optimum combination oflow wearrate and high conductivity. How-ever,purecopperhassomedistinctdrawbacks,suchaslow strengthandlowthermostability,whichtendstorestrictits

http://dx.doi.org/10.1016/j.jmrt.2014.06.008

2238-7854/©2014BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevierEditoraLtda.

Este é um artigo Open Access sob a licença de CC BY-NC-ND

applications. There are numerous reports on the applica-tion of SPD for grain refinement of pure copper by ECAP andHPTwherethethermostabilityofultrafine-grained cop-perwasanalyzed. Probablythefirst comprehensivereports on copper were published in 1999 for ECAP [14] and 2000 forHPT [15]. In the latter report, the microhardnessof an HPT disk of 98.5% purity Cu (P=5GPa, N=5 turns) drops downatanannealingtemperatureofabout180–200◦C.From DSC data with a heating rate of 40◦/min, the most pro-nouncedpeakwasdetectedfortheHPTsamplestrainedfor

N=1turnat220◦C,whichcorrespondstothemicrohardness measurements[15].Asimilartemperature(∼190◦_C)of

recov-erywasalsoreportedforhigher purityHPTcopper(99.9%). Highpuritycooper(99.99%)[16]showssimilarbehaviorwhen annealing at 134, 269 and 405◦C (0.3, 0.4and 0.5of melt-ingpoint)for1h.Themicrohardnessdropsdownfrom130 to80Hvastheannealingtemperatureincreasesfrom134to 269◦C.

TherearesomereportsonlowthermostabilityofUFG cop-per atroom temperature in which recoveryprocesses and graingrowth havebeen detectedinaperiodofonemonth andoneyear[17].Otherreports[18]shownosignificantgrain refinementincoppersubjectedtorollingatliquid nitrogen temperature suggesting that recoveryprocesses take place duringrolling.

Inpractice,thesimplestwaytoachieveUFGcopperisby HPT,whichpermitstheprocessingofdiskssuitablefor exper-imentsonhighlystrainedmaterials.Inanearlierreport[19], itwasdemonstratedthatthereisaslightincreaseofVickers microhardnessinHPTcoppersubjectedtolow-temperature annealing. Careful monitoring of the microhardness and microstructureinHPTaluminumandcopperstoredatroom temperatureforlongperiodsoftime[20]hasnotrevealedany significant changesofthese parameters. Thus,the present reportwasundertakentostudy microstructureevolutionof HPTcopperspecimenssubjectedtoannealingat100◦Cfor15, 30and60min.

2.

Experimental

materials

and

procedure

Disks of copper were used as the starting material. The material was purchased from Goodfellow Cambridge Ltd., Huntingdon,UK,andthetypicalchemicalcompositionwas givenas(inppm):Ag500,Bi<10,Pb<50,O400,othermetals <300. TheHPTspecimenswereintheformofdiskshaving diameters of10mmandthicknessesofabout 1mm. These disks were processed at room temperature by HPT for a total of N=1 turn under an applied pressure of P=6GPa. Theprocessingwasconductedunderquasi-constrained con-ditions where thereis a small outflow ofmaterial around the periphery ofeach disk during processing. Parts of the processedspecimenswereannealedat100◦Cforaperiodof 15,30or60min.Duringannealing,theHPTdiskswereplaced ontheflatsurfaceofathermocoupleandhencethe temper-ature wasstable towithin±2◦. Priorto all measurements, thespecimensweremechanicallypolished on1000gritSiC paperfollowedbyafinalpolishusingadiamondsuspension containingmonocrystallinediamondwithasizeof3␮mwith subsequentelectro-chemicalpolishingatroomtemperature usinganelectrolyteofHNO3:CH3OH=1:3withavoltageof10V.

Thesesampleswereemployedformicrohardness measure-mentsandelectronbackscatterdiffraction(EBSD)analysis.

Themicrohardnesswas measuredwith stepsof0.5mm along the diameters of the disks using an FM-300 tester equippedwithaVickersindenterusingaloadof100gfand adwellingtimeof15s.

TheEBSDanalysiswasperformedusingaTESCANMIRA 3LMHFEG scanning electronmicroscope equippedwithan EBSDanalyzer“CHANNEL5”,andarectangulargridwithscan stepof50nmwas used.TheEBSDanalysiswasperformed forregionslocatednearthecenterofthedisk,nearthe mid-radius(2.5mmfromthecenter)andneartheedge(∼4.5mm fromthecenter.)Theacquireddataweresubjectedtostandard clean-upproceduresinvolvingagraintoleranceangleof5◦and

001 001 TD TD RD RD 111 RD TD

Texture name: harmonic: L=16, HW=5.0 Calculation method: harmonic series expansion Series rank (1): 16

Gaussian smoothing: 5.0є Sample symmetry: triclinic

max=4.193 3.302 2.600 2.048 1.613 1.270 1.000 0.787 25 µm

a

b

Misorientation angle

Number Fraction

Misorientation angle [degrees] 0.00 0.02 0.04 0.06 0.08 0.10 10 20 30 40 50 60 2 µm

a

b

Fig.2–Microstructure(a)andGBmisorientationdistribution(b)atmid-radiusofCudiskafterHPT(P=6GPa,N=1).

aminimumgrainsizeofthreepixels.Thegrainsizeswere measuredusingthelinearinterceptasthedistancesbetween high-angleboundarieswithmisorientationsabove15◦.

3.

Experimental

results

Fig.1presents(a)aninitialmicrostructureand(b)the micro-textureofacopperspecimenpriortohigh-pressuretorsion. TheaveragegrainsizebyEBSDwaslargerthan 25␮mwith well-defined grain boundaries and twins in the interior of grains.ThepolefiguresinFig.1(b)reflecttheinherenttexture ofextrudedrodsoftheprimarymaterial.

Fig.2showstheinversepolefiguremapandgrainboundary misorientationdistributionofHPTcopperprocessedundera loadofP=6GPaforonewholerevolutionwheretheEBSDwas takenatthemid-radiusofthedisk.Significantgrain refine-mentwithgrainselongatedalongthe torsionaldirectionis observed.ThemeangrainsizebyEBSDwasabout200–300nm which is a typical grain size for HPT copper. The fraction oflow-angleboundarieswasbelow10%,whichisconsistent withearlierreports[3].Fig.3representstheintegratedVickers microhardnesstakenatadistanceofr/2,whereristhe mid-radiusoftheHPTdisk.AslightincreaseinHvisobservedfor HPTcopperannealedfor15min.Althoughitisintherangeof

140 150

130

120

110

Annealing time (min)

Integrated Hv

0 10 20 30 40 50 60

Fig.3–Integratedmicrohardnessatx=r/2asafunctionof annealingtime.

theexperimentalerror,thegaininmicrohardnessisabout4% anditwassystematicallydetected.

AnX-rayanalysisofHPT-processedcopperinFig.4givesa sizeofcoherentdomainsof287.5±6.3nm,whichisingood agreementwiththeEBSDdata.Fig.5showsaTaylorfactormap

60.0 80.0 100.0 40.0 2-Theta [degrees] Cu 200.0 400.0 Intensity 1/2 [Count 1/2 ]

2 µm 2 µm 2 µm 5 µm

A

B

C

D

Fig.5–TaylorfactormapofHPTcopper:(a)initial,andafterannealingatT=100◦Cfor(b)15min,(c)30minand(d)60min.

2 µm

2 µm

2 µm 2 µm

a

b

c

d

Fig.6–KernelaveragemisorientationofHPTcopper:(a)initialandafterannealingatT=100◦Cfor(b)15min,(c)30minand (d)60min.

Table1–ExperimentalparametersofHPTcoppersubjectedtoannealingat100◦Cfor15,30and60min. Specimen d(␮m) KAM(◦) Taylorfactor LAB(%) d(nm) Microstrain

‹

_›

Dislocationdensity (m−2,×1014₎

EBSD EBSD EBSD EBSD X-ray X-ray EBSD X-ray

HPTCu 0.33 0.537 3.318 20.4 193.8 2.125 7.322 1.484

HPTCu+ann.15 0.36 0.553 3.295 23.1 185.0 2.150 7.540 1.573

HPTCu+ann.30 0.39 0.563 3.359 21.1 210.0 2.300 7.677 1.482

HPTCu+ann.60 _{0.34 0.549 3.317 20.1 200.0 2.250 7.486 1.522}

forHPTcopperafterprocessinginFig.5(a)andafterannealing at100◦Cfor15mininFig.5(b),30mininFig.5(c)and60minin Fig.5(d).TheaveragevalueoftheTaylorfactor(TF)decreases slightlyfrom3.318forHPTcopperto3.295fortheHPTdisk annealedfor15min.Forthesampleannealedfor30min,the TFincreasestoavalueof3.359andthenitdecreasesagainto 3.317forthespecimenannealedfor60min.Thesevariations oftheTaylorfactors are notinagreementwithchanges in theVickersmicrohardness.TheKernelaveragemisorientation (KAM)mapsforcopperspecimensareshowninFig.6.The averageKAMalsoincreasesforthespecimensannealedfor 15and30minandthenslightlydecreasesfortheHPTcopper annealedfor60min.

Table1providesacomprehensivesummaryofall experi-mentaldataobtainedinthisinvestigation.Theareaweighted grainsizeobtainedfromEBSDshowsagradualincreasewith annealingtime.Also,thereisanoticeableincreaseinthe frac-tionoflow-angleboundaries(LAB)forthespecimenannealed for15min. X-rayanalysisrevealed asmall decrease inthe coherentdomainsizefrom193.8nm(HPTcopper)to185nm (after annealing for 15min) and a gradual increase in the microstrainlevel.

4.

Microstructure

evolution

during

low

temperature

annealing

Inpuremetals,therearealimitednumberoffactors influenc-inghardening:theHall–Petchrelationshipandthedislocation hardeningandtexture(correspondingtoachangeinthe Tay-lorfactor).Thetexturecanchangeduringsignificantchanging ofthemicrostructureasinrecrystallizationandgraingrowth. Duringlowtemperature,annealingat100◦Citisdifficultto expectsignificantchangesintextureandtheaveragevalues oftheTaylorfactorforallspecimensclearlysupportthis.On thebasisoftheexperimentalparameters,itispossibleto eval-uatethe dislocation density either usingthe average KAM (=/(b·h), where istheaverageangleinradians, bisthe Burgersvectorandhisthestepsizeof50nminEBSD),orusing themicrostrainlevelmeasuredbyX-rays(=2√3

‹␧

_›

where

‹ε

_›

size.ThetwofinalcolumnsinTable1representthedislocation densitiescalculatedonthebasisofX-rayanalysisandEBSD experiments,respectively.Thedislocationdensitycalculated fromKAMcorrespondstothegeometricallynecessary dislo-cations(GND)anditisabout3timeshigherthanthedensity ofthe statisticallystoreddislocations(SSD)estimatedfrom X-rayanalysis.BothvaluesshowaslightincreaseintheCu specimenannealedfor15min.

5.

Summary

Integrated microhardnessmeasurements atthe mid-radius positionshowasystematicincreasefortheHPTcopper sub-jectedtolowtemperatureannealingat100◦Cfor15min.The mostcorrelatedparametersthatmayberesponsibleforthis changearethefractionoflow-anglegrainboundariesandthe sizeofthecoherentdomains.

Conflict

of

interest

Theauthorsdeclarenoconflictsofinterest.

Funding

ThisworkwassupportedinpartbytheEuropeanResearch CouncilunderERCGrantAgreementNo.267464-SPDMETALS (APZ&TGL).

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