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Materials Science Forum Vol. 560 (2007) pp 115-120
Online available since 2007/Nov/15 at www.scientific.net
© (2007) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.560.115
M.Montoya Dávila1,a,M.A.Pech Canul2,b,M.I.Pech Canul1,c∗
1
CinvestavSaltillo.Carr.Saltillo Mty.Km13,SaltilloCoah.México,25900
2
CinvestavMérida.Km.6AntiguacarreteraaProgresoApdo.Postal73,Cordemex,97310,
Mérida,Yuc.,México
a b
miguel.montoya@cinvestav.edu.mx, max@mda.cinvestav.mx,
cmartin.pech@cinvestav.edu.mx.
TheeffectofparticlesizedistributionandparticlesizeratioofSiC inSiC /SiO
p p 2
preformsonthemicrostructure,microhardnessofSiC reinforcements,modulusofrupture,
p
andsuperficialhardnessofAl/SiC compositesproducedbypressurelessinfiltrationhasbeen
p
investigated. SiC /SiO preforms in the form of plates (4cm x 3cm x 0.5cm) have been
p 2 o
pressurelessinfiltratedbythealloyAl'15.52Mg'13.62Si(wt.%)at1100 Cfor60min
underinertatmosphere.SiCpowderswithaverageparticlesizeof10,68and140mare
mixed with SiO powders and preforms of 40 % porosity with unimodal, bimodal and
2
trimodalsizedistributionsarepreparedbyuniaxialcompaction.Thebimodal(small:large)
andtrimodal(small:medium:large)preformsarepreparedwithdifferentparticlesizeratios
inthefollowinglevels:1:1,3:1,1:3,2:2:2,3:2:1,3:1:2.Resultsfromcharacterizationby
XRD,SEMandenergydispersiveX'rayspectrometryshowthatthetypicalmicrostructureof
the composites contains the MgAl O (spinel), AlN and MgO phases formed during
2 4
processingaswellaspartiallyreactedsilica,SiC,SiandAl.Itisfoundthatthedensity,
reinforcementmicrohardness,modulusofruptureandsuperficialhardnessofthecomposites
increaseallwithwiderparticlesizedistribution.However,whilstthemodulusofruptureis
mainlyaffectedongoingfromunimodalandbimodaltotrimodaldistribution,superficial
hardnessandmicrohardnessaremostlyinfluencedongoingfromunimodaltobimodaland
trimodaldistribution.
Aluminummatrixcompositeswithahighvolumefractionofaceramicreinforcementhave
beenthesubjectofintenseinvestigationsinthelastyearsduetoimprovedstrength,stiffness,
thermalconductivity,abrasionresistanceanddimensionalstability.TheAl/SiCsystemhas
attractedtheattentionofmanyresearchersparticularlyforthoseapplicationsdemandinga
lowcoefficientofthermalexpansion(CTE)andahighthermalconductivity[1'3].Itiswell
knownthatinordertoachievelargevolumefractionsoftheceramicinametal/ceramic
composite,itisnecessarytousereinforcementsofsubstantiallydifferentsizes[3'6].Fromthe
availableprocessingtechniquesfortheproductionofmetalmatrixcomposites,theinfiltration
ofceramicpreformsbyliquidmetalsisthemostconvenientrouteforthemanufactureof
compositeswithahighvolumefractionofthereinforcements.Bytheinfiltrationroute,itis
possibletoproducenear'netshapecompositeswithhighdimensionalstabilityandauniform
distributionofthereinforcements.
Mostoftheworkdoneonthefabricationofcompositeswithahighvolumefractionof
reinforcementshasbeenconnectedtothepressure'orvacuum'assistedinfiltrationtechniques
[3'6]. However, in order to abate processing costs, the development of alternative non'
assisted or pressureless infiltration routes is of paramount importance. The feasibility of
∗
Correspondingauthor
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 152.2.176.242, University of North Carolina at Chapel Hill, Chapel Hill, USA-30/11/14,22:20:03)
116 Advanced Structural Materials III
producing Al/SiC composites with thermal conductivities similar to those exhibited by
aluminumalloyshasbeenreportedbyArpónetal.[3]frominvestigationswithpressure'
assistedinfiltration,andCui[7]reportedthefabricationofAl/SiCcompositeswithaSiC
volumefractionof0.68bythepressurelessinfiltrationtechnique.However,aninconvenience
ofthemethodisthatlongprocessingtimesareusedforthepreparationofthepreforms(5hr)
andthecomposites(1'3hr)[7].
Inordertotakeadvantageofthethermalproperties(CTEandthermalconductivity)of
Al/SiC composites with high volume fractions of the reinforcements, it is also of vital
importancetofullycharacterizethemechanicalpropertiesofthecomposites.Theaimofthis
workwastoinvestigatetheeffectofparticlesizedistributionandparticlesizeratioofSiC in
p
SiC /SiO preformsonthemicrostructure,microhardnessofSiC reinforcements,modulusof
p 2 p
rupture,andsuperficialhardnessofAl/SiC compositesproducedbypressurelessinfiltration.
p
PreformswithaSiCpvolumefractionof0.60werepreparedwithsiliconcarbidepowdersof
threedifferentparticlesizes[10(small),68(medium)and140(large)m],threedifferent
particlesizedistributions(unimodal,bimodalandtrimodal)anddifferentparticlesizeratios
forbimodal(small:large)andtrimodal(small:medium:large)distributions:1:1,3:1,1:3,
2:2:2,3:2:1,3:1:2.Thedesignationforparticlesizeratioinvolvesboththetypeofpreform
(bimodalortrimodal)andtheproportionofeachparticlesize.Forinstance,3:1:2standsfora
trimodalpreformmadeofthreepartsofsmall,onepartofmediumandtwopartsoflargeSiC
particles.Unimodalpreformsaredesignatedwithonlythecorrespondingparticlesize.The
SiCpowdersweremixedwith10wt.%ofSiO powders(particlesize,346m)and8wt.%of
2
dextrinasabinder.Themixtureswereplacedintoasteelmoldanduniaxiallycompacted
usingapressureofabout3.5MPatoproducepreformswiththegeometryofplatesof3x4x
0.5cm.Withtheaimofpartiallyeliminatingthedextrin,thepreformswereheatedinanair
furnacefortwohoursat125ºCandthenfortwomorehoursat225ºC.AnAl'Mg'Sialloy
wasfabricatedinaninductionfurnacewithcommercialAl,MgandSimaterials.Table1
showsthechemicalcompositionofthefabricatedalloy.
Table1Chemicalcompositionofthealuminiumalloy[wt.%].
Al Mg Si Fe Mn Zn Others
Balance 15.5 13.6 0.99 0.15 0.13 2.61
Infiltrationtrialswereperformedinatubefurnacewithcontroloftheprocessatmosphere.
Thepreformandalloy were placed in a ceramic container and the whole assembly was
positionedinthefurnace.Thesystemwasheatedatarateof15ºC/minupto1100ºC,heldat
thistemperaturefor60minandthencooleddownatthesameratetoroomtemperature.In
ordertoenhancethewettingofthepreformbytheliquidalloyduringheatup,achangeinthe
processing atmosphere (Ar →N ) was made on reaching 1000 ºC. Once the system was
2
cooleddowntoroomtemperature,thecompositeswerepreparedforphysical,mechanicaland
microstructural characterization. The density was evaluated using Archimedes´ principle.
Specimens were characterized by X'ray diffraction (XRD), optical microscopy (OM),
scanning electron microscopy (SEM) and energy dispersive X'ray spectrometry (EDX).
Mechanicalcharacterizationwasconductedusingfour'pointbendingtests(ASTMC1161'94
standard)andRockwellsuperficialhardness.Vickersmicrohardnesstestswereperformedon
SiCparticlesof68(medium)and140(large)mincorporatedintothecomposites.
Materials Science Forum Vol. 560 117
ResultsfromXRDrevealthepresenceoftheMgAl O ,
2 4
AlN,MgOandMg Siphasesformedduringprocessing,inadditiontoAl,SiandSiC.These
2
phaseswereformedregardlessoftheparticlesizedistributionandparticlesizeratio.Atypical
XRDpatternfromacompositespecimenwithbimodalsizedistributionisshowninFig.1.It
isnoteworthythattheunwantedAl C phaseisnotdetectedinthecompositesmicrostructure.
4 3
Si
AlN
SiC
Al
Mg Si
400 2
MgAl O
2 4
MgO
y 300
t
i
s
n
e 200
t
n
I
100
0
10 20 30 40 50 60 70 80
2θ(degree)
Figure1.XRDpatternofacompositewithbimodalsizedistribution(1:1)(SiC,10and140
m).
Themagnesiumaluminate(spinel)isformedbythereactionofthesilicaaddedtothe
preformswithmagnesiumandaluminuminthealloy,accordingto:
2Al +2SiO +Mg =MgAl O +2Si (1)
(l) 2(s) (l) 2 4(s) (s)
AnalysisbySEMshowsthat,inatypicalmicrostructureofthecomposites,thespinelis
present at the periphery of partially reacted SiO particles. Figure 2 shows the typical
2
microstructureofthecompositesandFigure3aisaphotomicrographshowingapartially
reacted particle of SiO . Figure 3b is an EDX spectrum on the region corresponding to
2
MgAl O
2 4
Al SiC
SiC
Intermetallic
111100000000 ----mmmm
Figure2.SEMphotomicrographshowingthetypicalmicrostructureofthe
composites.
Figure4showstheeffectofparticlesize
distributiononthecompositesdensity.Inthisandthesubsequentfigures,thenumbers1,2
and3inthe'axisrefertounimodal,bimodalandtrimodaldistributions,respectively.The
118 Advanced Structural Materials III
3
measureddensities(intherangefrom2.77to2.91g/cm )aresimilartothosereportedby
Chenetal.forcompositespreparedbythepressure'assistedinfiltrationtechnique[6].The
SSii
MgAl O
2 4
AAll
3 SiO
2
MMgg
OO
555000---mmm
Figure3.(a)SEMphotomicrographshowingapartiallyreactedSiO particle,and(b)EDX
2
spectruminazonecorrespondingtoMgAl O .
2 4
changeinthedensityofthecompositeswithdifferentparticlesizedistributionsisexplained
intermsofthedegreeofpackingattainedwhenusingparticleswithdifferentsizesinthe
preforms.Inregardtotheinfluenceoftheparticlesizeratioinagivenpreformonthedensity
ofthecomposites,nosignificanteffectisobserved.
) 2,90
3m 2,88
c
/
r 2,86
g
(
y
t 2,84
i
s
n
e 2,82
D
2,80
1 2 3
Particlesizedistribution
Figure4.Averagedensitiesofthecompositesasafunctionofparticlesizedistribution.
.Resultsfromfour'pointbendingtestsrevealthattheMOR
doesnotsignificantlyincreasewhenthesizedistributionvariesfromunimodaltobimodal.
However,anincreaseofabout16%isobservedwhenachangeismadetotrimodalsize
distribution, as shown in Fig. 5. The average values of the moduli of rupture of the
compositesare117±8,119±17and145±13forunimodal,bimodalandtrimodalsize
distributions,respectively.TypicalvaluesofMORforsomespecimensareshowninTable2.
150
Table2.Modulusofruptureofcompositeswith 145
differentsizedistributionofreinforcements. 140
)
a 135
Particlesizeratioand MOR[MPa] P 130
M
( 125
distribution
R 120
Unimodal(68Jm) 117±8 O 115
M 110
1:1 136±17 105
1:3 102±18 100
3:1:2 129±12 1 2 3
3:2:1 113±14 Particlesizedistribution
2:2:2 194±10 Figure5.Modulusofruptureasafunctionof
particlesizedistribution.
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