The deformation, recrystallisation and texture of three magn(3)

recrystallisationmayoccurinthosezones.33

Recrystallisationtextures

Inversepole gurespresentedinFig.6a–cindicatethat

theunrecrystallisedgrainsregionsinAZ31,ZC71and

WE43areallorientatedwithbasalplanesand,101¯0.

directionsparalleltotheextrusionaxis.Thealignment

ofbasalplanesalongtheextrusionaxisistypicalof

behaviourreportedinmagnesiumalloys.1,2,11

ThetextureoftherecrystallisedgrainsinAZ31

(Fig.

,101¯6d)andZC71(Fig.6e)consistsofastrong

0.orientationaccompaniedbyaweaker,2¯110.

component.Thetexture

recrystallisation¯hasthereforeevolvedduring

110

the,101¯asa,ponentaccompanies

0.orientationexhibitedbytheunrecrystal-

lisedregions.TheasextrudedbulktexturesofAZ31and

ZC71(Fig.7aandb),re ecttheorientations

recrystallisedgrains:astronger¯ofthe

accompaniedbyaweaker,2¯,ponentis

ponent.In

hydrostaticallyextrudedAZ31a,101¯0.texturehas

beenassociated

structure.The,2¯withapartiallyrecrystallisedmicro-

ponentwasseento

attheexpenseof,101¯increase

0.ifstaticgraingrowthoccurred

duringpostextrusioncooling.3

TherecrystallisedgrainsinextrudedWE43havea

spreadoforientations(Fig.6f),andthetextureisalso

weakrelativetoAZ31andZC71.Theorientations

presentaredifferenttothoseoftheunrecrystallised

1178MaterialsScienceandTechnology2007VOL23NO10regions,andtothoseoftherecrystallisedgrainsinAZ31andZC71.NotonlyisthereisadistinctreorientationoftextureassociatedwithrecrystallisationandtheearlystagesofgraingrowthinWE43,butinFig.5recrystallisationisalsoassociatedwithparticles.Studiesofrecrystallisationandtextureinparticlecontainingaluminiumalloyshaveindicatedthatrecrys-tallisationnucleiatparticleshavearandomspectrumoforientations.17–20Thismayresultinthedevelopmentofaweakorrandomtextureduringrecrystallisationandgraingrowth.ArandomtextureobservedinextrudedWE54wasattributedtoPSN.2AlthoughWE43appearstoexhibitrelativelyweaktextures,thebulktextureoftheasextrudedWE43consistsoftwodistinctcomponents(Fig.7c).The rstcomponenthasabasalorientation,whichisassociatedwiththeunrecrystallisedregionsofthemicrostructure.Thesecondcomponentconsistsofbasalplanesorien-tatedy45ufromtheextrusionaxis.Thiscomponentisassociatedwiththerecrystallisedgrains.Thereis,therefore,anorientationselectionduringrecrystallisa-tionortheearlystagesofgraingrowth.InsinglecrystalstudiesPSNgrainshavebeenreportedtodevelopwithspeci corientations,whereasinstudiesofpolycrystalsPSNhasbeenassociatedwithaweakeningofthetexture.17–20AlthoughPSNappearstobearecrystallisationmechanismintheWE43alloy,andmayhavecontributedtoaweakeningofthetexture,randomlyorientated‘PSN’grainsdonotdominatethe naltexture.AnnealingtexturesDuringannealingfor24hat435uCAZ31andZC71fullyrecrystallisedandthegrainsizeannealing,2¯increased.¯During110.alsoreplaced,1010.asthedominanttexturecomponent(Fig.7dande).ThemajortexturechangeduringrecrystallisationandgraingrowthinHCPmaterialsisoftena30urotationaroundthec-axis,23andachangeofthisnatureoccursduringrecrystallisationandgraingrowthinAZ31wherearelativelyminorcomponent,,¯andZC71110.,developsatthedetrimentofamajorone,,101¯20..DuringannealingtheWE43fullyrecrystallised,andaswithAZ31andZC71thegrainsizeincreased.InthecaseofWE43,however,thebasalplanesintheannealedmaterialareorientatedaty45u,rathertheextrusionaxis(Fig.7f).The,2¯thanparallel,ponentexhibitedbyAZ31andZC71didnotgrowinWE43.Thereis,however,asimilarphenomenoninallthreealloys:orientationselectionoccursveryearlyintherecrystallisationgraingrowthprocess.Althoughthegrowingcomponentsmaydiffer,thesegrainsmaintaintheirearlyadvantageasgrowthproceedsanddominatethe nalannealingtexture.Anannealingtexturedevelopsasaconsequenceofthedistributionnucleiorientationspresentinthedeformedmicrostructure,andtherelativegrowthofthoseorientations.Undersomecircumstancesboundariesofaparticularcharacterhavehighmobility,andnucleimayhaveagrowthadvantageasaresultoftheirsize,positionorsurroundingdeformationsubstructure.25Shearbandshavebeenobservedinmagnesiumalloys,6includingWE53.21Asbasalplanesorientatedaty45utotheextrusionaxisareofsimilarorientationtobasalplanesalignedintheplaneofashearband,6theannealingtexturesinWE43maybeassociatedwith

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

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