b
,
Science
National
form
19
dendritespacingthroughcontrollingpreparationroutes
andalloycompositions,Johnsonetal.[13,14]wereable
toachievelargetensileductilityinbothZr-andTi-based
intoBMGhavebeenmade,anddeformation-induced
phasetransformationhasbeenobservedmainlyinthe
Cu
47.5
Zr
47.5
Al
5
BMGcompositesconsistingofaCuZr
phaseasreinforcement[17–21].TheCuZrphasehasaB2
crystalstructureathightemperatures>988K,buttends
todecomposeintoCu
10
Zr
7
andZr
2
Cuatlowtemperatures
?
Correspondingauthor.Tel.:+861082375387;fax:+861062333447.
E-mailaddress:luzp@ustb.edu.cn(Z.P.Lu).
Availableonlineatwww.sciencedirect.com
ActaMaterialia59(2011)2928–2936
1.Introduction
Bulkmetallicglasses(BMGs)haveattractedmuch
attentionowingtotheirhighstrengthandlargeelasticlimit
[1–4].However,theroom-temperaturebrittlenessand
strain-softeningnaturelimittheirrealstructuralapplica-
tions[5–7].Amongmanyapproachesthathavebeenpro-
posedtoovercomethisissue,formingBMG–matrix
compositeshasprovedtobee?ectiveinenhancingcom-
pressiveplasticityandtoughness[8–12].Nevertheless,no
obvioustensileductilityhadbeenobserveduntiltherecent
workofJohnson’sgroup[13,14].Byproperlyadjusting
BMGcomposites.Unfortunately,becauseofthelackof
work-hardeningmechanisms,theseBMGcompositessu?er
frommacroscopicstrainsoftening,whichcausesunstable
deformationbehavior,withanearlyonsetofneckingnear
theyieldpoint.Tomaketheseglassyalloysviableforengi-
neeringapplications,work-hardeningcapabilityanduni-
formtensileductilityarenecessary,whichrequiresanew
methodologyfordesigningnovelBMGcomposites.
Indesigningcrystallineceramicsandsteels,theconcept
oftransformation-inducedplasticity(TRIP)hasbeen
adoptedtoincreasetoughnessandwork-hardeningproper-
ties[15,16].Similarly,attemptstointroducetheTRIPe?ect
Abstract
Thedependenceofmicrostructureonthealloycompositionandcoolingrateofaseriesof(Zr
0.5
Cu
0.5
)
100C0x
Al
x
(x=1,2,
3,...,10at.%)alloyswasinvestigatedindetailandexplainedintheframeworkoftime–temperature–transformationdiagrams.The
relationshipbetweenthemicrostructuresofbulkmetallicglass(BMG)compositesandtheirmechanicalpropertieswascharacterized
systematically.ItwasfoundthattheadditionofaluminumcanpromotetheformationofthemetastableausteniticCuZrphase,and
compositestructureswithB2–CuZrparticlescanbeformedinalloyscontaining3–8%Al.Boththevolumefractionanddistribution
ofthereinforcedB2phasecouldgreatlya?ectthedeformationbehavior,andtheBMGcompositeswithhomogeneouslydistributedsin-
gleB2–CuZrphaseexhibitedstabletensileductility.AnalysisindicatesthattheB2–CuZraustenitetransformedintotheB19
0
martensite
duringdeformation(i.e.,stress-inducedmartensitictransformation),whichaccountsfortheobservedsuperiormechanicalpropertiesof
thecurrentBMGcomposites.
C2112011ActaMaterialiaInc.PublishedbyElsevierLtd.Allrightsreserved.
Keywords:Bulkamorphousmaterials;Composites;Metastablephase;Mechanicalproperties;Martensiticphasetransformation
FormationofCu–Zr–Albulk
withimprovedtensile
Y.Wu
a
,H.Wang
a
,H.H.Wu
a
,Z.Y.Zhang
X.L.Wang
a
StateKeyLaboratoryforAdvancedMetalsandMaterials,Universityof
b
NeutronScatteringScienceDivision,OakRidge
Received15September2010;receivedinrevised
Availableonline
1359-6454/$36.00C2112011ActaMaterialiaInc.PublishedbyElsevierLtd.All
doi:10.1016/j.actamat.2011.01.029
metallicglasscomposites
properties
a
,X.D.Hui
a
,G.L.Chen
a
,D.Ma
b
,
Z.P.Lu
a,?
andTechnologyBeijing,Beijing100083,People’sRepublicofChina
Laboratory,OakRidge,TN37831,USA
13January2011;accepted17January2011
February2011
www.elsevier.com/locate/actamat
rightsreserved.
3.Results
3.1.Composition–coolingrate–microstructurerelationship
Toinvestigatethestructuraldependenceonthecompo-
sitionandcoolingratein(Cu
0.5
Zr
0.5
)
100C0x
Al
x
(x=1,
2,...,10at.%)alloys,sampleswithdi?erentAlcontents
andcastingdiameterswerefabricatedandexamined.For
thebinaryCu
50
Zr
50
alloy,afullyamorphousstructurecan-
notbeobtainedinas-castrodswithdiameter>2mm,and
themartensiteCuZrphaseappearedintheamorphous
matrix,whichisconsistentwiththepreviousreport[29].
WithadditionsofAl,thestructurewassignificantlychan-
ged,andXRDpatternsofthe(Cu
0.5
Zr
0.5
)
96
Al
4
samples
withvariousdiametersareshowninFig.1asexamples.
Whenthecastingdiameterdis2mm,thecorresponding
XRDtraceshowsasinglehump,indicatingthefullyamor-
phousnatureoftheas-castrods.Withthedecreaseinthe
Y.Wuetal./ActaMaterialia59(2011)2928–29362929
[22].Athighcoolingrates,themetastableB2structurecan
beretainedand,duringsubsequentdeformation,thestress
willinducethephasetransformationfromtheparentB2to
asupersaturatedmartensite[23,24],thusresultinginwork-
hardeningandplasticity.Previousworkconfirmedthatthe
TRIPe?ectoriginatingfromtheCuZrphaseise?ectivein
improvingthecompressivepropertiesofBMG[25–27]but,
undertension,distincttensileplasticityhasseldombeen
reported[28].Itisknownthatthemorphologyandvolume
fractionoftheCuZrphasearestronglydependentonthe
fabricationprocess(e.g.,coolingrates)andalloycomposi-
tions[29,30].Assuch,understandingtheformationkinetics
oftheBMGcompositeswithproperlydistributedB2–
CuZrphasesisthekeytoattainingwork-hardeningand
uniformtensileductility.Inthepresentwork,therefore,a
thoroughinvestigationofthemicrostructuredependence
onalloycompositionandcoolingrateinthe
(Zr
0.5
Cu
0.5
)
100C0x
Al
x
(x=1,2,...,10at.%)systemwas
conducted.ThemainpurposesweretodevelopBMGcom-
positeswithstabledeformationbehaviorandappreciable
plasticityundertensionandtounderstandtherelated
deformationmechanisms.
2.Experimental
Alloyingotswithnominalcompositionsof
(Zr
0.5
Cu
0.5
)
100C0x
Al
x
(x=1,2,...,10at.%)wereprepared
byarc-meltingamixtureofconstituentelementswithpur-
ity>99.9%inaTi-getteredatmosphere.Thealloyingots
weremeltedsixtimestoensurecompositionalhomogene-
ity.Cylindersampleswithdi?erentdiameterswerefabri-
catedbysuctioncastingusingcoppermolds.Toreduce
theporosity,thesamplelengthwascontrolledintherange
70–80mm,usingacopperstopper.Thestructuralnatureof
theas-castrodswasexaminedbyX-raydi?raction(XRD)
usingCuKaradiationandtransmissionelectronmicros-
copy(TEM)usingaJEM2010Finstrumentwithafield
emissiongun.TheTEMsampleswerefirstmechanically
groundtoa50lmthickplateandthentwin-jetelectropo-
lishedusingasolutionmixedintheratioHNO
3
:-
CH
4
O=1:3.Longitudinalandcross-sectionsurfacesof
theas-castandstrainedsampleswereexaminedbyscan-
ningelectronmicroscopy(SEM)inaZEISSSUPRA55
instrument.Volumefractionsofcrystallinephaseswere
estimatedfromtheSEMimagesofthecross-sectionsur-
faceswithImagetoolsoftware.Thermalpropertieswere
analyzedbydi?erentialscanningcalorimetry(DSC)(Net-
zschSTA449C)atarateof10Kmin
C01
.Finiteelement
analysiswasconductedusingABAQUS/Explicitv.6.6
(Providence,RI)software,andthematerialconstantswere
setaccordingtopreviousliterature[25].Tensiletestswere
carriedoutinaWDW-200Dmachinewithamaximum
loadof200kNatanengineeringstrainrateof
2C210
C04
s
C01
,andasmallstraingaugewasusedtocalibrate
andmeasurethestrainduringloading.
coolingrate(i.e.,dincreasesto4mm),sharpcrystalline
peaksofB2–CuZrappearontopoftheamorphoushump.
Afurtherdecreaseinthecoolingrate(i.e.,dincreasesto
5mm)leadstotheformationofanothercrystallinephase
whichcanberecognizedasAl
2
Zr[31].Itcanbeseenthat
themicrostructureoftheas-castsamplesforthisparticular
alloyvariesasfollows:fullyamorphous!amor-
phous+B2–CuZr!amorphous+B2–CuZr+Al
2
Zr,as
thecoolingratedecreases.Asimilartrendwasobserved
fortheotheralloyswithAlcontentupto8%butwithdif-
ferentcriticaldiameters.Forinstance,afullyamorphous
structurecanbeformedinrodswithdiameter6mm,while
thecompositestructureofamorphous+B2–CuZrcanbe
obtainedforadiameterof7mminthealloycontaining
8%Al.TheAl
2
Zrphaseprecipitateswhenthecoolingrate
decreasesfurther,i.e.,diameter>7mm.Nevertheless,when
theAlcontentinthealloyreaches10%orabove,theglass-
formingability(GFA)ofthealloysstartstodecreaseanda
fullyamorphousstructurecanonlybeobtainedintheas-
20406080100
?-Al
2
Zr
?
??
?
φ5mm
φ4mm
φ3mm
?
?
RelativeIntensity(a.u.)
2θ(degree)
φ2mm
?-B2-CuZr
?
Fig.1.XRDpatternsof(Cu
0.5
Zr
0.5
)
96
Al
4
alloyfabricatedatdi?erent
coolingrates.
r
48
Al
4
alloyswithdi?erentcastingdiameters.Thefully
amorphousspecimenshowscatastrophicfracturewithout
Fig.2.SEMimagesofCu
48
Zr
48
Al
4
alloywithdi?erentcastingdiameters:
(a)d=3mm,(b)d=4mmand(c)TEMimagesofnano-sizedcrystalsin
Cu
48
Zr
48
Al
4
alloy;insetistheselectedareaelectrondi?ractionpatternofa
nanocrystal.
castrodswithdiameters55mm.Surprisingly,composites
withasingleB2–CuZrphasecannolongerform.Instead,
Al
2
Zrandotherunknownphasesprecipitateoutofthe
amorphousmatrixwhenthecastingdiameterexceeds
5mm.
Inaddition,themorphologycharacteristics(e.g.,shape
anddistribution)ofthecrystallinephaseshaveastrong
dependenceontheAlcontentandcastingsize,asdemon-
stratedbycentralpartsoftheas-castCu
48
Zr
48
Al
4
alloy
showninFig.2.Whencastina3mmrod,thealloyexhib-
itsacompositestructurewithsphericalprecipitatesembed-
dedintheglassymatrix.Thecrystallinephasesidentifiedas
body-centeredcubic(bcc)B2–CuZrphasearehomoge-
neouslydistributedwithavolumefractionofC2415%and
size20–100lm.Asthecastingsizeisraisedto4mm,the
B2–CuZrphasestillappears,butcombinesintopatch-like
shapesandnolongerdistributeshomogeneouslyacrossthe
amorphousmatrix,anditsvolumefractionisincreasedto
C2445%.DuringtherapidcoolingofBMGmaterials,alarge
temperaturegradientwillbegeneratedwhichcouldcause
compositioninhomogeneityalongtheradiusdirectionas
aresultofthe“Sorete?ect”[32,33].Thelargerthecasting
size,theslowerthecoolingrateandthelongerthetime
availableforthethermaldi?usion,thuskineticallyfavoring
largercompositionalinhomogeneity,whichmayaccount
forthepatch-likeshapesoftheB2–CuZrphase.Itisalso
worthytonotethat,otherthanthemicrometer-sizedB2–
CuZrphases,nanometer-sizedCuZrparticleswerealso
observedinsidetheglassymatrix,asshowninFig.2c.
Bythoroughlyinvestigatingthemicrostructuresof
alloyswithdi?erentAlcontentsandcastingdiameters,
thedependenceofstructureonthecompositionandcool-
ingrate(castingdiameter)ofthepresent
(Cu
0.5
Zr
0.5
)
100C0x
Al
x
alloysisillustratedinFig.3.Itcan
beseenthatGFAofthealloysincreasesinitiallywiththe
Alcontentandreachesthemaximumatx=8%Al,and
thendecreasesastheAladditionsincreasefurther.The
alloys,inwhichBMGcompositeswithsingleB2–CuZr
phasecanbeformedatsuitablecoolingrates,contain3–
8%Al,asboundedbythedashedlineinFig.3.Thecom-
positionswithaluminumcontentof5–6%havethelargest
rangeofcoolingratesforformingsuchspecialcomposites.
3.2.Tensileproperties
ForBMGmaterialswhichusuallylacktensileductility,
uniaxialcompressionhasbeenwidelyusedtostudythe
mechanicalproperties.Recently,itwasreportedthatmany
extrinsicfactorssuchassamplegeometrymaya?ectthe
testingresults,andthecompressivepropertiesmaycontain
artifacts[34–39].Thisstudyfocusedonthetensilebehavior
ofthecurrentBMGcompositesystem,althoughexcellent
compressivedeformabilityhasalsobeenobservedcom-
paredwithmonolithicBMG[19].
ThetensilepropertiesofthecurrentBMGcomposite
2930Y.Wuetal./ActaMaterialia
systemwereinvestigatedindetail.Asanexample,Fig.4a
showsthetensiletruestress–staincurvesoftheCu
48
Z-
59(2011)2928–2936
adistinctyieldingphenomenon,whichisconsistentwith
thecharacteristicofmostBMGsamplesintension.For
10
Amor+M-CuZr
Amorphous
Amor+B2-CuZr
Amor+B2+Al
2
Zr
Y.Wuetal./ActaMaterialia
thesampleswithC2415%homogeneouslydistributedspheri-
calB2crystallites(i.e.,d=3mm),atensileductility>2%
wasachieved.Furthermore,thetensiletruestress–strain
curvedoesnotshowthestress-dropphenomenonafter
yielding,whichwaspreviouslyreportedinBMGcompos-
2
4
6
8
1086
42
Amor+B2+Al
2
Zr
+Uknownphase
Diameter,mm
Alcontent,%
0
Fig.3.MicrostructuredependenceonAlcontentandcoolingratein
(Cu
0.5
Zr
0.5
)
100C0x
Al
x
alloysystem.
0
500
1000
1500
2000
Al=4%
d=5mm
d=4mm
d=3mm
TrueStress,MPa
TrueStrain
1%
d=2mm
1000
2000
x=3
d=5mm
x=7
d=8mm
x=6
=6mm
x=6
d=4mm
x=3
d=3mm
TrueStress,MPa
TrueStrain
2%
(a)
(b)
d
Fig.4.(a)Tensiletruestress–straincurvesofCu
48
Zr
48
Al
4
alloywith
di?erentcastingdiametersand(b)representativetensilestress–strain
curvesoftheothercompositesinthecurrentalloysystem.
itesreinforcedwithcoarsedendrites[13,14],indicatingthat
thestrain-softeningphenomenonhasbeensuppressedin
thecurrentBMGcomposite.Forthesampleswiththe
B2–CuZrphaseprecipitatedinhomogeneouslyand/or
coarsenedtogether(i.e.,d=4mm),however,thetrue
stress–straincurvealsoshowsdistinctyieldingatalower
strength,butwithasmallductilityofC240.5%.Asthecasting
sizeisfurtherincreasedto5mm,thebrittleAl
2
Zrphasein
additiontoB2–CuZrprecipitates,andthespecimenfrac-
turescatastrophicallyatamuchlowerstrengthwithnodis-
tinctyielding.
Toverifywhetherthetensileductilityandwork-harden-
ingcapabilitycanbeobservedintheotheralloys,tension
testsforalltheothercompositesinthecurrentsystemwere
conducted,andseveralrepresentativetruestress–strain
curvesareshowninFig.4b.Similarly,yieldingandstable
tensiledeformationbehaviorcanbeseeninthecomposites
reinforcedbythesingleB2–CuZrphase,althoughactual
valuesoftheyieldstrengthandductilitymayvarywith
Alcontentsandcoolingrates(i.e.,castingdiameters).
Therefore,itisevidentthattensileductilitycanbe
obtainedinBMGcompositesreinforcedwithpropervol-
umefractionanddistributionofthesingleB2–CuZrphase,
whilethecompositeswithanybrittlephasesuchasAl
2
Zr
shownotensileductility.
3.3.Phasetransformationduringdeformation
Owingtothelimitintensileductility,confinedcompres-
siontestswereusedtoscrutinizestructuralchangesduring
deformation.Forexample,loading–unloadingcompres-
sionexperimentsoftheas-castCu
48
Zr
48
Al
4
alloywitha
castingsizeof3mmandanaspectratioof1werecon-
ducted,andXRDmeasurementsforthespecimenatdi?er-
entdeformationstages(i.e.,di?erentcompressionstrains)
areshowninFig.5a.Fortheas-castspecimen,onlycrys-
tallinepeaksrelatedtotheB2–CuZrphasesuperimposed
ontheamorphoushumpareseen,whichisconsistentwith
theresultsshowninFig.1.Afterbeingdeformedto10%
strain,thespecimenshowsextrapeaks,whichcanbe
indexedasaB19
0
–CuZrphase,indicatingtheoccurrence
ofphasetransformationfromB2toB19
0
.Asthecompres-
sivestrainincreasesto15%,thepeakscorrespondingto
B19
0
becomedominant,whilethosecorrespondingtoB2
arediminishing,indicatingthatmostoftheB2–CuZrphase
hastransformedtoB19
0
afterlargedeformation.Thus,it
canbeconcludedthattheB2–CuZrphasecontinuesto
transformtoB19
0
asthedeformationproceeds,andthe
higherthedegreeofdeformation,themorethemartensite
B19
0
phaseisformed.Withfurtherstrainingto32%,adif-
fusehumpsurprisinglyappearsinsteadofsharpcrystalline
peaks,whichseeminglysuggestsafullyamorphousstruc-
tureinthisdeformedsample.However,thecrystalline
phasescouldstillbeseenfromtheSEMimagesofthesam-
pleunloadedatthisstrain,asshowninFig.5b,although
59(2011)2928–29362931
theyarenolongerspherical.
12020406080100
?
?
?
anneal
32%
15%
10%
as-cast
?
?
RelativiteIntensity,a.u.
2θ,degree
-B19′CuZr?
?-B2CuZr
(a)
(c)
2932Y.Wuetal./ActaMaterialia
Toinvestigatethisfurther,thepre-strainedsampleswere
annealedatatemperatureof683K(C2410Kbelowtheglass
transitiontemperature)for2handthenwater-quenched.
AlthoughthecorrespondingSEMimageisexactlythe
sameasthatshowninFig.5b,crystallinepeakscorre-
spondingtoB19
0
andB2–CuZrphasesreappearonthe
XRDcurveofthisannealedsample.Therefore,itseems
thatthedi?usepeakofthelargelydeformedsampledoes
notresultfromafullyamorphousstructure.Usually,
broadeningoftheXRDpeaksoccursbecauseofeitherlim-
itationsinthespatialextentofthecoherentscatteringvol-
umes(i.e.,grainsize)orthepresenceofinhomogeneous
residualstress[40,41].Inthepresentcase,thepeakbroad-
eningontheXRDtraceisseeminglyduetothelatter,
causedbyconfineddeformation.Toconfirmthishypothe-
sis,TEManalysiswasconductedtoinvestigatethestruc-
turechangeinthisheavilypre-strainedsamplebeforeand
aftertheannealing,asdemonstratedinFig.5candd.In
Fig.5c,thelargelydeformedsampleshowstortuousmar-
tensiticplates,indicatingtheexistenceoflargeresidual
stress,whicheventuallygivesrisetobroadeningofthe
XRDpeak.Afterbeingannealedat683K,thetortuous
platesbecomestraightand/ordisappear,implyingthat
Fig.5.(a)XRDpatternsofpre-strainedandannealedsamplesofCu
48
Zr
48
Al
4
pre-strainedto32%.(c)TEMimagesofthemartensiticplatesinsamplespre-strained
(b)
(d)
59(2011)2928–2936
theannealingprocessrelaxestheresidualstressandthus
leadstothereappearanceofthecrystallinepeaks.
4.Discussion
4.1.FormationofBMGcompositeswithsingleB2–CuZr
phase
Fig.4explicitlyshowsthedeterminativerolesofthepre-
cipitatedphasesinthemechanicalbehaviorofthepresent
BMGcomposites.Toobtaindesirablemechanicalproper-
ties,characteristicsoftheBMGcomposites,includingmor-
phology,distribution,volumefractionandtypesof
reinforcementcrystallinephases,havetobeproperlycon-
trolledviacarefullyadjustingthecompositionandcasting
diameter,asdemonstratedinFig.3.Fig.5confirmsthe
occurrenceofthemartensitictransformationduringdefor-
mationofthecurrentcomposites,althoughthee?ectsofAl
additionsonthemartensitictransformationarenotyetelu-
cidated.Toinvestigatethecharacteristictemperaturesof
themartensitictransformationinthepresentalloys,DSC
measurementsofthemotheralloysundercontinuousheat-
ingandcoolingwereconducted,andthecorresponding
alloycompositecastinadiameterof3mm.(b)SEMimageofthesample
to32%and(d)subsequentlyannealed.
200400600800
M
s
A
s
Al6
Al4
Al0
Al6
Al4
Al0
Cooling
HeatFlow
Temperature,K
Heating
Al
2
Zr
B2
T
L
(a)
(b)
Y.Wuetal./ActaMaterialia
curvesofthreealloysareshowninFig.6a.Themartensite
starttemperature(M
s
)isC24415KforthebinaryCu
50
Zr
50
alloy,whichisconsistentwiththeliterature[42].Forthe
alloyswithadditionof4%and6%Al,M
s
isreducedto
C24320and270K,respectively,confirmingthatadditionof
Alcanremarkablysuppressthemartensitetransformation
andstabilizethemetastableausteniteCuZrphase.From
theheatingprocessoftheDSCcurves,thereversemartens-
itetransformation(i.e.,theaustenitetransformation)tem-
peratureforalloysdopedwith0%,4%and6%Alare558,
484and415K,respectively,whichimpliesthatadditionof
Alcanpromoteaustenitetransformationanddestabilize
themartensiteCuZrphase.Assuch,thecombinede?ects
ofcoolingratesandAladditionscanbeschematicallyana-
lyzedfromtheframeworkofthetime–temperature–trans-
formation(TTT)diagrambasedonthecompetitive
formationmechanism[43],asdiscussedbelow.
Forthealloyswith<2%Al,themaincompetingcrystal-
linephaseisB2–CuZr,whichisstableattemperatures
>988K[19].Toformafullyglassystructure,thecooling
ratehastobefastenoughtobypassthenoseoftheB2–
CuZrTTTcurve(i.e.,thecriticalcoolingrateR
c
).Once
theappliedcoolingrateisslowerthanR
c
,theB2–CuZr
phasewouldprecipitateoutoftheliquidathightempera-
Glass+Al
2
ZrGlass+B2
Time,s
Temperature,K
T
g
M
s
Fig.6.(a)DSCheatingandcoolingcurvesof(Cu
0.5
Zr
0.5
)
100C0x
Al
x
(x=0,
4,6)alloyingotsshowingmartensiticandaustenitictransformationand
(b)schematicTTTdiagramof(Cu
0.5
Zr
0.5
)
100C0x
Al
x
alloys.
tures.BecausetheM
s
valueisrelativelyclosetotheglass
transitiontemperatureT
g
[42],thesuper-cooledparent
B2–CuZrphasecaneasilytransformintothemartensitic
CuZrphaseviamartensitictransformation,resultingina
compositestructureconsistingofthemartensiticCuZr
phases,ratherthantheausteniteB2–CuZrphase,embed-
dedintheamorphousmatrix.
Forthealloyswith3–8%Al,formationoftheB2–CuZr
phaseisretardedbecauseofnecessaryredistributionofthe
Alatomsduringsolidification[43].Asaresult,theTTT
curveoftheB2phasemovestowardsthelongertime,lead-
ingtoreductioninthecriticalcoolingrateandhencehigh
GFA(asindicatedbysolidlinesinFig.6b).Meanwhile,
formationoftheAl
2
Zrphaseathightemperatures
becomespossible,owingtothelargeamountofAl,
althoughB2–CuZrisstilltheprimarycompetingphase.
Moreover,themartensitictransformationisalsosup-
pressedbecauseofthestabilizationoftheparentB2–CuZr
phase,i.e.,adecreasedM
s
value(seeFig.6a).Therefore,at
asuitablecoolingrate,thesupercooledB2–CuZrphasecan
beretainedintheamorphousmatrixwithoutoccurrenceof
matensitictransformation.
WithfurtherincreaseinAlcontentto>8%,formation
oftheAl
2
Zrphasebecomespredominant,andprecipita-
tionoftheB2–CuZrphaseisfurthersuppressed(see
dashedlinesinFig.6b).Consequently,glassformationis
reduced,owingtothestrongformingtendencyoftheAl
2
Zr
phase.Uponslowcooling,Al
2
Zralwaysprecipitatesfirst,
andBMGcompositescontainingsingleB2–CuZrphase
asreinforcementcannolongerbeobtained.
4.2.Originsoftheenhancedmechanicalproperties
Aselaboratedabove,thecurrentBMGcompositesrein-
forcedwithsingleB2–CuZrhaveexcellentmechanicalproper-
ties.Next,theoriginsoftheenhancedpropertiesareexplained.
4.2.1.“Blockinge?ect”fromheterogeneitiesatmultiple
lengthscales
Alateralsurfaceimageofatensilefracturedsampleof
Cu
48
Zr
48
Al
4
BMGcompositeisshowninFig.7a,andmul-
tipleshearbandsthroughouttheentiregaugelengthcanbe
seen.Fromthedetailofthelateralsurface(Fig.7b),theso-
called“blockinge?ect”canbeclearlyobserved[44].Shear
bandsdeflectattheinterfacebetweenthecrystalsandthe
glassymatrixandthendivideintomanytinysecondary
shearbandsaroundthesphericalphases.Thecrystalline
spheresactasstrongbarriersfortherapidpropagation
ofshearbandsandthuse?ectivelyenhancetheplasticity
ofthereinforcedcomposites.Moreover,the“blocking
e?ect”wasobservedforB2–CuZrphasesnotonlyof
micrometerscale,butalsonanometerscale.Fig.7cshows
theinteractionofamicro-crackwithanano-sizedB2
phase,anditcanbeseenthatthemicro-crackisremark-
ablydeflectedintoawindingpropagation,whichshould
59(2011)2928–29362933
alsoretardtherapidpropagationofthemicro-crack.
Therefore,itisclearthatthe“blockinge?ect”fromboth
(a)
(b)
2934Y.Wuetal./ActaMaterialia
micrometerandnanometersphericalphaseswould
enhancetheplasticityofthecompositeremarkably.
Aselaboratedearlier,itwasverifiedthathomogeneous
distributionofthereinforcingB2phasesintheBMGcom-
positesisbeneficialforthemechanicalproperties.To
understandfurtherthee?ectsofdistributionoftherein-
forcingB2phase,thestrainfieldforsampleswithdi?erent
dispersioncharacteristicstensionedtothesamestrainof
4%wasanalyzedbyfiniteelementanalysis(FEA),and
thecalculatedresultsareshowninFig.8.Forthecompos-
itewithinhomogeneouslydistributedcrystallinephases,a
mainshearbandisformed,exhibitingtypicallocalized
sheardeformation.Forthecompositewithhomogeneously
dispersedcrystals,thestrainfieldismoreuniform,and
shearbandsareinteractedandmultiplied,whichshould
bebeneficialforplasticity.
4.2.2.Contributionfrommartensitictransformationof
homogeneouslydistributedB2–CuZrphases
The“blockinge?ect”isanormalphenomenoninBMG
composites,andithasbeenprovedthatinteractionand
Load
direction
Load
direction
Fig.7.(a)SEMimageofthelateralsurface,(b)blowupofthelateralsurface,
Cu
48
Zr
48
Al
4
alloy.
59(2011)2928–2936
multiplicationofshearbandswithcrystallinephasesis
capableofenhancingplasticity,butnotsu?cienttoover-
comeearlyneckingofBMGcompositesundertension
[13,14].
Ithasbeenrecognizedthatthedeformation-induced
martensitictransformationcanincreasethestrain-harden-
ingrateandsuppressearlyneckinginTRIPsteels
[15,16].Stress-inducedphasetransformationhasbeen
observedinthepresentB2–CuZrreinforcedcomposites
(Fig.5),thusitisreasonabletoconjecturethatthephase
transformationfromB2–CuZrtoB19
0
–CuZrisresponsible
forsuppressingthetensilework-softeninganda?ording
ductilityofthepresentcomposites.Accordingtotheliter-
ature[45],phasetransformationoftheausteniteB2crystals
duringdeformationcouldreleasethestressconcentration
aroundthemandrestrictsfreevolumeaccumulation.As
aresult,therapidpropagationofshearbandscanbehin-
dered,whichrequiresfurtherstresstomovetheshear
bandsandconsequentlyinhibitstheearlyneckingand
work-softening.Totestthishypothesis,anotherZr
55
Cu
29-
Ni
8
Al
8
BMGcompositewithasimilarvolumefractionof
and(c)TEMimageshowingcrackpropagationinafracturedsampleof
Y.Wuetal./ActaMaterialia
thesphericalZr
2
(CuNi)crystallinephase,whichdoesnot
undergoanyphasetransformationduringdeformation,
wassynthesizedasreportedpreviously[46].Tensileexper-
imentsonthisreferenceBMGcompositeshownowork-
hardeningandductility,whichmanifeststhatthestress-
inducedmartensitictransformationisresponsibleforthe
improvedtensilepropertiesinthecurrentcomposites.
Fig.8.(aandc)FEAmodelsusedforcalculationofcompositesreinforcedby
Strainfieldofcompositesafterbeingtensionedto4%strain,correspondingto
59(2011)2928–29362935
5.Conclusions
Bysystematicinvestigationofprimaryphaseprecipita-
tionin(Zr
0.5
Cu
0.5
)
100C0x
Al
x
alloys,therelationshipbetween
thecoolingrate,Alcontent,microstructureandmechani-
calpropertieswasestablished.ItwasfoundthatBMG
compositescontainingonlythebccB2–CuZrphasewhich
thesamevolumefractionbutdi?erentdistributionofcrystals.(bandd)
(aandc),respectively.
undergoesamartensitictransformationuponloadingcan
formundersuitablecoolingratesinthecompositionrange
3–8%Al.Compositesreinforcedbyhomogeneouslydis-
tributed,singlesphericalB2–CuZrphaseshoweduniform
tensileductilitywithnostrain-softening(e.g.,3mmas-cast
Zr
48
Cu
48
Al
4
alloy).Boththe“blockinge?ect”fromthe
heterogeneitiesatdi?erentlengthscalesandthestress-
inducedmartensitictransformationoftheprecipitated
B2–CuZrphaseareresponsiblefortheobservedsuperior
mechanicalproperties.Theconceptsappliedinthecurrent
studyarebelievednottobelimitedtotheCuZrAlalloy
system,butalsohavestrongimplicationsforthedevelop-
[15]ZackayVF,ParkerER,FahrD.TransASM1967;60:252.
[16]JacquesPJ,FumemontQ,LaniF,PardoenT,DelannayF.Acta
Mater2007;55:3681.
[17]DasJ,TangMB,KimKB,TheissmannR,BaierF,WangWH,etal.
PhysRevLett2005;94:205501.
[18]KimKB,DasJ,VenkataramanS,YiS,EckertJ.ApplPhysLett
2006;89:071908.
[19]PaulyS,DasJ,BednarcikJ,MatternN,KimKB,KimDH,etal.
ScriptaMater2009;60:431.
[20]SunYF,WeiBC,WangYR,LiWH,CheungTL,ShekCK.Appl
PhysLett2005;87:051905.
[21]PaulyS,DasJ,DuhamelC,EckertJ.AdvEngMater2007;9:487.
[22]AbeT,ShimonoM,OdeM,OnoderaH.ActaMater2006;54:909.
2936Y.Wuetal./ActaMaterialia59(2011)2928–2936
mentofotherBMGcompositeswithlargeplasticityand
work-hardeningability.
Acknowledgements
ThisresearchwassupportedbyNationalNaturalSci-
enceFoundationofChina(Nos.50725104and51010001)
andthe973program(No.2007CB613903).Y.W.acknowl-
edgessupportfromNSFC(GrantNo.51001009),China
PostdoctoralScienceFoundation(GrantNo.
20100470208)andtheFundamentalResearchFundsfor
theCentralUniversities.
References
[1]JohnsonWL.MRSBull1999;24:42.
[2]InoueA.ActaMater2000;48:279.
[3]SchroersJ.AdvMater2009;21:1566–97.
[4]Lo?erJF.Intermetallics2003;11:529.
[5]SchuhCA,HufnagelTC,RamamurtyU.ActaMater2007;55:4067.
[6]ChenMW.AnnuRevMaterRes2008;38:14.
[7]AshbyMF,GreerAL.ScriptaMater2006;54:321.
[8]ConnerRD,DandlikerRB,JohnsonWL.ActaMater1998;46:6089.
[9]FanC,LiHQ,KecskesLJ,TaoKX,ChooH,LiawPK,etal.Phys
RevLett2006;96:145506.
[10]HaysCC,KimCP,JohnsonWL.PhysRevLett2000;84:2901.
[11]QiaoJW,WangS,ZhangY,LiawPK,ChenGL.ApplPhysLett
2009;94:151905.
[12]WuFF,ZhangZF,MaoSX,PekerA,EckertJ.PhysRevB
2007;75:134201.
[13]HofmannDC,SuhJY,WiestA,DuanG,LindML,Demetrious
MD,etal.Nature2008;451:1085.
[14]HofmannDC,SuhJY,WiestA,LindML,DemetriouMD,Johnson
WL.ProcNatlAcadSciUSA2008;105:20136.
[23]SchryversD,FirstovGS,SeoJW,HumbeeckJV,KovalYN.Scripta
Mater1997;36:1119.
[24]SeoJW,SchryversD.ActaMater1998;46:1165.
[25]PaulyS,LiuG,WangG,KuhnU,MatternN,EckertJ.ActaMater
2009;57:5445.
[26]PaulyS,LiuG,GorantlaS,WangG,KuhnU,KimDH,etal.Acta
Mater2010;58:4883.
[27]ChengQ,WuHA,WangY,WangXX.ApplPhysLett
2009;95:021911.
[28]WuY,XiaoYH,ChenGL,LiuCT,LuZP.AdvMater2010;22:2770.
[29]DasJ,PaulyS,BostromM,DurstK,GokenM,EckertJ.JAlloy
Compd2009;483:97.
[30]CheungTL,ShekCH.JAlloyCompd2007;434:71.
[31]YuP,BaiHY,TangMB,WangWL.JNon-CrystSolids
2005;351:1328.
[32]LiuY,LiuCT,GeorgeEP,WangXZ.ApplPhysLett
2006;89:051919.
[33]LiuY,LiuCT,GeorgeEP,WangXZ.Intermetallics2007;15:557.
[34]WuWF,LiY,SchuhCA.PhilosMag2008;88:71.
[35]HanZ,WuWF,LiY,WeiYJ,GaoHJ.ActaMater2009;57:1367.
[36]ZhangZF,ZhangH,PanXF,DasJ,EckertJ.PhilosMagLett
2005;85:513.
[37]WuFF,ZhangZF,MaoSX,EckertJ.PhilosMagLett
2009;89:178.
[38]WuY,LiHX,LiuZY,ChenGL,LuZP.Intermetallics2010;18:157.
[39]WuY,LiHX,JiaoZB,GaoJE,LuZP.PhilosMagLett2010;90:403.
[40]BudrovicZ,SwygenhovenHV,DerletPM,PetegemSV,SchmittB.
Science2004;304:273.
[41]KlugHP,AlexanderLE.X-raydi?ractionproceduresforpolycrys-
tallineandamorphousmaterials.2nded.NewYork:Wiley;1974.
[42]KovalYN,FirstovGS,KotkoAV.ScriptaMetall1992;27:1611.
[43]LuZP,MaD,LiuCT,ChangYA.Intermetallics2007;15:253.
[44]Louzguine-LuzginDV,VinogradovA,XieGQ,LiS,LazarevA,
HashimotoS,etal.PhilosMag2009;89:2887.
[45]DasJ,KimKB,XuW,WeiBC,ZhangZF,WangWH,etal.Mater
Trans2006;47:2606.
[46]ChenG,BeiH,CaoY,GaliA,LiuCT,GeorgeEP.ApplPhysLett
2009;95:081908.
|
|
