The deformation, recrystallisation and texture of three magn(7)

Mackenzieetal.Thedeformation,recrystallisationandtextureofthreemagnesiumalloyextrusions

thosedeformationfeatures.Itmustbenoted,however,

thatarelativelyhighextrusiontemperatureof360uCis

expectedtopromotehomogeneousdeformation,and

hencelimittheevolutionofshearbands.

WE43hascharacteristicsthatgiverisetoanenergy

advantageforgrowthofthenon-basalcomponents.

Soluteelementscanin uenceboundarymobilityand

recrystallisation.25,34Inparticular,soluteelementscan

changegrainboundarybehaviourandorientation

relationshipsforhighmobility.Solutedraghasbeen

associatedwithweak22texturesinrareearthcontainingmagnesiumalloys.Variationsindeformationsub-

structuremayalsoevolveasaresultofsoluteelements,

andasubstructurecan25in uencenucleiorientationdistributionandgrowth.

AlthoughtheMLIgrainsizeoftheannealedWE43

was100mm,somegrainsgrewupto,350mm.Dataof

EBSDindicatedthattheseabnormallylargegrainswere

notresponsiblefortheunusualevolutionoftextureinthis

alloy.Abnormalgraingrowthhasbeenassociatedtexture

reorientationsinsomemagnesiumalloys,24butthisdoes

notappeartohavebeenthecasewiththeWE43

investigatedinthisstudy.Instead,thenon-basalorienta-

tionsinWE43developasaconsequenceofitsparticular

recrystallisationandearlygraingrowthcharacteristics.

Mechanicalproperties

Table2indicatesthatinAZ31andZC71thetensile

strengthishigherthanthecompressivestrengthparallel

totheextrusionaxis,particularlyinthespecimens

annealedfor24hat435uC.Microstructuralinvestiga-

tionsrevealedthattheannealedAZ31andZC71

twinnedduringcompression,butnotduringtension.

Inbothalloysthebasalplanesarealignedparalleltothe

extrusionaxis,andwhenthisisthecase,duringtensile

andcompressivedeformationparalleltotheextrusion

axisslipcannotoccureasilyonthebasalplanes.4,5

Compressionparalleltotheextrusionaxisinvolves

axisextension,whichcanbeaccommodatedby{101¯c-

2}

twinning,whilsttensiletestinginvolvesc-axiscompres-

sion,whichismoredif cultasitisaccommodatedby

pyramidal,35cza.slipthathasahighcriticalresolvedshearstress.Deformationbytwinningiseasierduring

c-axisextension,sothetensilestrengthparalleltothe

extrusionaxisishigherthanthecompressivestrengthin

alloyswithbasalplanesparalleltotheextrusionaxis.

Thetensile/compressiveratiosinTable2indicatethat

themechanicalasymmetryinAZ31andZC71increases

afterannealing.Theincreaseingrainsizeduring

annealingisprobablysigni cant,becausetwinningtends

toincreaseattheexpenseofslipasgrainsizeincreases.36,37

Astwinningisresponsibleformechanicalasymmetryin

magnesiumextrusionswithbasalplanesparalleltothe

extrusionaxis,itislikelythattheasymmetryincreasesasa

resultofgraingrowthduringannealing.

TheWE43exhibitsverylittlemechanicalasymmetry

ineithertheasextrudedorannealedcondition

(Table2).Italsotwinnedverylittleduringtensionor

compression.Mechanicalasymmetryis

ofthe{101¯avoidediflittle

2}twinningthatcausesasymmetryoccurs,or

alternatively,ifequalamountsoftwinningoccursin

tensionandcompression.InthepresentstudyofWE43,

slipisabletooccureasilyonthebasalplanes,because

theyareorientatedaty45utotheextrusiondirection

and,astwinningisreduced,asymmetryisreducedas

well.TheWE54extrusioninvestigatedbyBallandPrangnell2alsoexhibitedsimilartensileandcompressivestrengths.If,aswasreported,theextrusionhadarandomtexture,thenequalamountsoftwinningmayhaveoccurredintensionandcompression.Summary1.Themicrostructures,texturesandmechanicalpropertiesofmagnesiumAZ31,WE43andZC71alloyshavebeencharacterisedfollowingextrusionandsub-sequentannealing.2.Allthreealloyspartiallyrecrystallisedduringextrusion.Theunrecrystallisedregionscontainedadeformationsubstructure;recrystallisedgrainswereassociatedwithboundaries,deformationfeaturesandparticles.3.Unrecrystallisedregionspossessedan,101¯0.orientation;newtexturecomponentsevolvedduringrecrystallisation.InAZ31andZC71thecomponentassociatedwiththerecrystallisedgrainswas,2¯110.;inWE43therecrystallisedgrainswereorientatedwithbasalplanesaty45utotheextrusionaxis.Ineachalloythenewcomponentgrewtodominatethe naltextureduringannealing.4.Theevolutionoftexturewasassociatedwithanorientationselectionintheearlystagesofrecrystallisa-tionandgraingrowth.ItappearsthatalloyingelementsinWE43changerecrystallisationbehaviourandtheorientationrelationshipsforhighboundarymobility.Thisleadstothedevelopmentofanon-basaltextureinWE43.5.InAZ31andZC71thebasalplaneswerealignedparalleltotheextrusionaxis,whichfavouredtwinningincompression,andresultedinhighertensilethancompressivestrengths.InWE43thebasalplanesorientatedat45utotheextrusionaxiswerefavourablyorientatedfordislocationglide.Thisreducedtheneedfordeformationtwinningthatcausesasymmetry.AcknowledgementsTheauthorswishtothankMagnesiumElektronLtdandtheEPSRCfortheirsupportofthiswork.ThecommentsofProf.B.Hutchinsonwereofparticularvalue.References1.S.KleinerandP.J.Uggowitzer:Mater.Sci.Eng.A,2004,A379,258–263.2.E.BallandP.Prangnell:Scr.Metall.Mater.,1994,31,111–116.3.J.Bohlen,S.B.Yi,J.Swiostek,D.Letzig,H.G.BrockmeierandK.U.Kainer:Scr.Mater.,2005,53,259–264.4.E.W.KellyandW.F.Hosford:TransAIME,1968,242,5–13.5.E.C.BurkeandW.R.Hibbard:Trans.AIME,1952,194,295–303.6.M.R.Barnett,M.D.NaveandC.J.Bettles:Mater.Sci.Eng.A,2004,A386,205–211.7.A.Stycynski,Ch.Hartig,J.BohlenandD.Letzig:ScrMater.,2004,50,943–947.8.S.R.Agnew,M.H.YooandC.N.Tome´:ActaMater.,2001,49,4277–4289.9.M.T.Perez-PradoandO.A.Ruano:Scr.Mater.,2002,46,149–155.10.M.T.Perez-PradoandO.A.Ruano:Scr.Mater.,2003,48,59–64.11.S.B.Yi,H.-G.Brockmeier,J.Bohlen,D.LetzigandK.U.Kainer:PhysicaB,2004,350B,507–509.12.S.R.Agnew,J.A.Horton,T.M.LilloandD.W.Brown:Scr.Mater.,2004,50,377–381.13.S.R.Agnew,P.Mehrotra,T.M.Lillo,G.M.StoicaandP.K.

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