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Fig.6.SEMfractographsofthixoformedZA27alloyformedat470?Cheatedfor90min(dietemperature:300?C).Showing(a)porosityand(b)fracturedinclusion.
Figs.3(b),9(b)and(c),thattheeutecticstructureiscom-posedofmanyeutecticcellswithdifferentorientations.Dur-ingpreparationofthestartingingots,someotherimpurities,similartotheFeelement,mightalsobeintroducedintothealloyfromtheagentsofC2Cl6andK2F6Zr,orcoatingsonsometools.Theseimpuritiesarealsorejectedbysolid/liquidinterfaceduringeutecticsolidificationaftermouldfillingandfinallyagglomerateattheboundariesbetweentheeutecticcellsalthoughvisibleinclusions(underthepresentobserva-
tionconditions)arenotformed.Inaddition,smallporosi-tiesalsopreferentiallyformatthesesites.Thus,thebondingstrengthbetweentheseboundariesisimpairedandtheyareeasytobeseparatedduringtensiletesting.
3.1.2. Effectofreheatingtemperature
Thetensilepropertiesofspecimenformedatreheatingtemperatureof473?Carecomparativetothatofthespeci-menformedat470?C.However,thetensilepropertiesob-viouslydecrease with the further increase of reheating tem-perature.Table1showsthatboththeprimaryparticlesizeandparticlefractionslightlydecreasewithincreasingthereheatingtemperature.Theformercanimprovethetensileproperties.Thelatterenhancestheinhomogeneityofthemi-crostructurebecauseoftheincreaseofthecorrespondingmatrixfraction,andthenimpairstheproperties[10].Itisalsofoundthat,similartothatshownbyFig.4,theliquidsegregationbecomeseriouswithincreasingthereheatingtemperature.Furthermore,thehigherthereheatingtemper-atureis,thelesstheadvantagesofthixoformingprocessis,especiallyforthedecreaseofporosities.Thatistosaythatthebenefitforimprovingtensilepropertiesfromthedecreaseofprimaryparticlesizeisinferiortotheharmforimpairingtensilepropertiesfromthemoreseriousliquidsegregationandtheincreaseofporositiesasthereheatingtemperatureincreased.
Fig.10representstheSEMfractographsofthetensile-fracturedspecimensformedatdifferentreheatingtemper-aturesof475and480?C.TogetherwithFig.5(c),itcanbeseenthatthenumberofdimplesincreaseswiththein-creaseofreheatingtemperature.Thisdemonstratesthattheporositiesincreasewiththereheatingtemperature.Inaddi-tion,wecanalsoseethatthelaminarcharacteristicsgrad-uallydiminishasthereheatingtemperatureincreases.In
Fig.7.SEMmicrographsofthixoformedZA27alloyformedatreheatingtemperatureof470?C,reheatingdurationof80minanddietemperatureof300?C.Showingplate-likeFe-richintermetallicexistedineutecticsbetweenprimaryparticles.
T.J.Chenetal./MaterialsScienceandEngineeringA382(2004)90–103
Fig.8.SEMsideviewsoftensilefracturedareasofthixoformedZA27alloysformedatdifferentreheatingdurationsof:(a)70min;(b)75min;(c)90minand(d)120min(reheatingtemperature:470?C;dietemperature:300?C).
spiteofthat,thecrackpathsstilltendtofollowthema-trixbetweenprimaryparticles,regardlessofthereheatingtemperature.Itcanbeexpectedthatthehigherthereheat-ingtemperature,thehigherthetemperatureofthedieafterbeingfilledupbythesemi-solidalloy,andthustheslowerthesolidificationrateoftheliquidphase.TheeutecticsoftheZA27alloyresultsfromnon-equilibriumsolidificationandits amount decreaseswith the decrease
ofsolidificationrate[11].Thisstandpointissupportedbythepresentre-sultshownbyFig.11:theamountofeutecticsbetweenpri-maryparticlesatreheatingtemperatureof470?Cismorethanthatat480?C.Correspondingly,theamountofthesec-ondaryprimarydendrites(primaryphasesolidifiedfromtheliquidphaseinthesemi-solidstate)at470?Cislessthanthatat480?C.Itiseasilyexpectedthatthecracksmayfre-quentlytransversethesecondaryprimarydendritesduringgrowingalongthematrixofthespecimenformedathigherreheatingtemperature.Inaddition,theeutecticsinthespec-imenformedathigherreheatingtemperaturetendtopresentarod-shapestructure,notawell-definedlaminarstructureinthespecimenformedatlowertemperature(comparingFig.11(a)with11(b)).Therefore,thelaminar
characteristics
ofthefracturesurfacegraduallydisappearwiththeincreaseofreheatingtemperature.
3.1.3. Effectofdietemperature
Threedietemperatures,250,300and350?Careem-ployedinthiswork.Thetensilepropertiesofboththespeci-mensformedat250and350?Carelowerthoseofthespec-imenformedat300?C.Themicrostructuralexaminationin-dicatesthatboththeprimaryparticlesizeandtheprimaryparticle fractionincrease withtheraising ofthedie temper-ature(Table1).Theincreaseofprimaryparticlesizecom-monlyimpairsthetensileproperties.Theincreaseofthepri-maryparticlefraction,tosomeextent,enhancestheunifor-mityofthemicrostructure,whichcanbedemonstratedbycomparingthemicrostructuresshowninFig.12.Therefore,thecontributionsofthesetwovariationstotensileproper-tiesarecontraryandthefinalpropertiesaredeterminedbytheircontributiondegrees.
Fig.13showstheSEMfractographsofthetensile-fracturedspecimensformedatdifferentdietemperaturesof250and350?C.ComparedwithFig.5(c),itcanbeseenthatthenumbersofdimplesinboththespecimensformedat250
T.J.Chenetal./MaterialsScienceandEngineeringA382(2004)90–103
Fig.9.SEMsideviewsneartensilefracturedareasofthixoformedZA27alloysformedatreheatingtemperatureof470?C,reheatingdurationof80minanddietemperatureof300?C.Showingcracksinitiatedfrom:(a)void;(b)inclusionand(c)eutectics.
and350?Caregreaterthanthatinthespecimenformedat300?C.Atlowtemperatureof250?C,theremaybenoenoughtimeforthesolidificationshrinkagetobefullyfedduetorapidsolidification,resultinginformationofporosi-ties,sothepropertiesarelower.Atthehightemperatureof350?C,thesecondaryprimaryphaselayersattachedtotheprimaryparticlesurfacesarequitethick(Fig.12(b)).Thismayeasilyresultintheprimaryparticlesconnectingeachother,prematurelyformingasolidskeleton,similartothedendriticsolidskeletonformedduringtraditionalcasting.Thus,theshrinkagealsocannotbefullyfed.
Fig.13alsoshowsthatthelaminarfeatureofthefracturesurfacesalsograduallydisappearswithincreasingthedietemperature.Similartothechangewiththereheatingtem-perature,thisisalsoattributedtotheamountandstructureoftheeutecticsbetweenprimaryparticles,whichdepends
onthesolidificationratedeterminedbydietemperature.In-corporatedwithFig.11(a),Fig.12showsthattheamountofeutecticsdecreasesandthelaminarcharacteristicsdisappearwiththeincreaseofdietemperature.Atthehightempera-tureof350?C,theprimaryparticlesareconnectedtogetherbythethicksecondaryprimaryphaselayersattachedtotheirsurfaces(seeFig.12(b)).Itcanbeeasilyexpectedthatthetensilecracksmustfrequentlytraversethesecondarypri-maryphase,andofcause,alsotraversetheformerprimaryparticles(seeFig.14).Itmustbenotedthatthisphenomenoncanalsobeobservedintheotherfracturesurfaces,butthefrequencyisrelativelylow.Fig.12(a)alsoshowsthatsomethickboundariesexistbetweeneutecticcellsinsomeregionsoftheeutecticstructure.AsdiscussedinSection3.1.1,a
largeamountofimpuritiesandsmallporositiesmayagglom-erateatthesesitesandthenimpairsthebondingstrength