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