The deformation, recrystallisation and texture of three magn(6)

Mackenzieetal.Thedeformation,recrystallisationandtextureofthreemagnesiumalloyextrusions

askinkbands,havebeenobservedtoforminAZ31

followingcompressionattemperaturesbetween250and

400uC.26

Recrystallisation

IntheEBSDmicrographofextrudedAZ31(Fig.2),

smallgrainscircledalongtheedgeoftheunrecrystallised

regionareassociatedwithadeformationsubstructure.A

deformationsubstructurepresentsahighdensityof

potentialrecrystallisationnuclei,which,inturn,canlead

tothenucleationofrecrystallisation.27InWE43

(Fig.4),recrystallisedgrainsareassociatedwith

kinkbandsthatcrosstheunrecrystallisedregion.

Recrystallisationoccursatkinkbandsbecausenuclea-

tion27occursinregionswithhighmisorientationgradi-ents.Recrystallisedgrainshavebeenobservedat

deformationfeatures,includingtwinsand3,15kinkbands,inhotcompressedandextrudedAZ31.

InZC71(Fig.3),theunrecrystallisedregioncontains

subgrains.InAZ31thesmallestgrainsare,5mmin

diameter,andinZC71thesubgrainsareofsimilarsizes

totherecrystallisedgrains,whichtogethersuggestthat

littleboundarymigrationisinvolvedinrecrystallisation.

InFig.3smallrecrystallisedgrainsarealsoassociated

withHABsthatcrosstheunrecrystallisedregions.The

nucleationofrecrystallisationisfrequentlyassociated

withboundariesinmagnesium.28–31

InFig.5smallgrainsareassociatedwithsecond

phaseparticles.Thisindicatesthatparticlesnucleate

recrystallisationinWE43.Ithasbeensuggestedthat

PSNoccurs2,16duringextrusionofbothWE54andZC71–SiCMMC.Ifsecondphaseparticlesdonotdeform

duringdeformationthenstrainconcentrationoccursin

thevicinityoftheparticlesastheplasticallydeforming

matrix owsaroundthe‘hard’particle.32Theseregions

areknownas‘deformationzones’.Iftheconditionsof

strain,temperature,strainrateandparticlesizearesuch

thatdeformationzonesformthennucleationof

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,http://www.77cn.com.cnponentaccompanies

0.orientationexhibitedbytheunrecrystal-

lisedregions.TheasextrudedbulktexturesofAZ31and

ZC71(Fig.7aandb),re ecttheorientations

recrystallisedgrains:astronger¯ofthe

accompaniedbyaweaker,2¯,http://www.77cn.com.cnponentis

http://www.77cn.com.cnponent.In

hydrostaticallyextrudedAZ31a,101¯0.texturehas

beenassociated

structure.The,2¯withapartiallyrecrystallisedmicro-

http://www.77cn.com.cnponentwasseento

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,http://www.77cn.com.cnponentexhibitedbyAZ31andZC71didnotgrowinWE43.Thereis,however,asimilarphenomenoninallthreealloys:orientationselectionoccursveryearlyintherecrystallisationgraingrowthprocess.Althoughthegrowingcomponentsmaydiffer,thesegrainsmaintaintheirearlyadvantageasgrowthproceedsanddominatethe nalannealingtexture.Anannealingtexturedevelopsasaconsequenceofthedistributionnucleiorientationspresentinthedeformedmicrostructure,andtherelativegrowthofthoseorientations.Undersomecircumstancesboundariesofaparticularcharacterhavehighmobility,andnucleimayhaveagrowthadvantageasaresultoftheirsize,positionorsurroundingdeformationsubstructure.25Shearbandshavebeenobservedinmagnesiumalloys,6includingWE53.21Asbasalplanesorientatedaty45utotheextrusionaxisareofsimilarorientationtobasalplanesalignedintheplaneofashearband,6theannealingtexturesinWE43maybeassociatedwith

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