Indian Journal of Engineering & Materials SciencesVol. 5, February 1998, pp. 1-8

Regarding the effect of aging heat treatment on the failure behaviour of aSiC reinforced Al based metal matrix composite

M Gupta', M K Surappa", S Qin' & L M Tharrr'

'Department of Mechanical and Production Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260

'Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India

Received 1 April 1997; accepted 21 January 1998

In the present study, 6061 Al metallic matrix was reinforced by 12.2 wt % of SiC particulates usingliquid metallurgy route. The composite material thus obtained was extruded and characterized in the as-solutionized and peak aged conditions in order to delineate the effect of aging associated precipitation ofsecondary phases on the tensile fracture behavior of the composite samples. The results ofmicrostructural characterization studies carried out using scanning electron microscope revealed theincreased presence of precipitated secondary phases in the metallic matrix and a more pronouncedinterfacial segregation of alloying elements in case of peak aged samples when compared to the as-solutionized samples. The results of the fractographic studies conducted on the as-solutionized samplesrevealed that the failure was dominated by the SiC particulates cracking while for the peak aged samplesthe fracture surface revealed a comparatively more pronounced SiC/6061 Al debonding and reducedSiC particulates cracking. This change in the failure behavior was rationalized in terms of embrittlementof the interfacial region brought about by the aging heat treatment and is correlated, in addition, with themechanical properties of the composite samples in as-solutionized and peak aged conditions.

Discontinuously reinforced metal matrixcomposites (MMCs) represent the unifiedcombination of the continuous metallic phase withthat of the ceramic phase. The discontinuousceramic reinforcements commonly includeparticulates, chopped fibers or whiskers I. Theselection of the metallic and ceramic phase isgoverned by the end properties desired for aspecific application I. 2. Discontinuously reinforcedmetal matrix composites derive their advantagesince they can be synthesized using conventionalprocessing techniques, exhibit near isotropicbehavior and can be plastically deformed andmachined using conventional techniques I. 2.

For the applications where high specificmechanical properties are desired aluminum alloymatrices based on 2XXX (AI-Cu), 6XXX (AI-Mg-Si) and 7XXX (AI-Zn-Mg-Cu) series areconsidered as a result of their low density andmoderate mechanical properties':'. Commonlyused reinforcements that are associated withstrength increment of the aluminum alloy matricesinclude silicon carbide (SiC) and aluminum oxide(AI203)3 •••

In order to qualify for the weight criticalstrength based applications, it is important that themetal matrix composites should demonstrate amoderate to high resistance to crack propagationso as to avoid catastrophic failures. Thus the studyof the failure mechanisms associated with theceramic reinforcing phase is important. In recentyears, investigators carried out numerous studies inorder to delineate the effect of particulate types,weight percentage of reinforcement", size ofreinforcement'", nature of metal-ceramicinterface", and the effect of under aging and overaging7•8 on the failure characteristics of thecomposite materials, however, no systematicstudies were carried out to investigate the failurecharacteristics of the SiC reinforced aluminummatrices in the presence and absence ofstrengthening precipitates.

Accordingly, the objective of the present studywas to provide preliminary information on thefailure characteristics of a 6061 AlISiC metalmatrix composite when tested under uniaxialconditions in the as-solutionized condition(absence of strengthening precipitates) and peak

2 INDIAN J. ENG. MATER. scr., FEBRUARY 1998

aged conditions (presence of strengtheningprecipitates). Particular emphasis was placed tocorrelate the microstructural variation of thesolutionized and peak aged samples with themechanical properties and the failure mechanismsexhibited by the composite samples fracturedunder tensile loading.

Experimental ProcedureMaterials-The nominal composition of the

matrix alloy (commercially designated asAA6061) used in the present study was (in wt %):0.6Si - 1.0Mg - 0.2Fe - 0.25Cu - 0.25Cr - Al (bal.).Silicon carbide (a-SiC) particulates with anaverage size of 54 f..lIT1 were selected as thereinforcement phase. The nominal composition ofSiC particulates was (in wt %): 0.75Si02 - O.4C -O.2A1203 - O.6Si - O.05Fe203 - SiC (bal.).

Processing-The synthesis of the metal matrixcomposites used in the present study was carriedout according to the following procedure-themetal ingots, prior to melting, were treated withwarm alkaline solution and washed with mixture ofacids in order to reduce the thickness of the oxidefilm and to eliminate other surface impurities. Thecleaned metal ingots were melted under a cover ofnitrogen gas in order to minimize the oxidation ofmolten metal. SiC particulates, preheated to900°C, were then added into the molten metalstirred using an impeller. The melt was alloyedwith small amounts of Mg and Zr (Mg+Zr <1wt%) in order to improve the wettability of SiCparticulates. The composite melt thus obtained waspoured into cylindrical cast iron moulds. In all thecases, residence time of SiC particulates in themelt was maintained between 10 and 15 min. Theas-cast composite bars were homogenized at540°C for 3 h and then extruded at 500°C in orderto close the residual porosity. The extrusion ratioobtained was 10:1.

Quantitative assessment of SiC particulates-Quantitative assessment of SiC particulates in theconventionally cast composite samples was carriedout using a chemical dissolution method. Thismethod involved: (i) measuring the mass ofcomposite samples, (ii) dissolving the samples inhydrochloric acid, followed by (iii) filtering toseparate the ceramic particulates. The particulates

were then dried and the weight fractiondetermined',

Density measurement-Density measurementswere carried out in order to ascertain the volumefraction of porosity in the composite samplesfollowing extrusion. Density measurements werecarried out using Archimedes' principle followingthe procedure as discussed elsewhere'.

Heat treatment studies-Heat treatment studieswere carried out in order to vary themicrostructural characteristics of the metallicmatrix. Two different types of heat treatmentcycles were investigated in this study. Heattreatment cycle #1 involved the solutionizing ofthe extruded composite samples at 530°C for lhfollowed by water quenching while the heattreatment cycle #2 involved solutionizing of theextruded composite samples, water quenchingfollowed by peak aging. In order to obtain the peakaging conditions specimens (10 mm diameter x6mm height) taken from extruded rods weresolutionized for one hour at 530°C, quenched incold water and aged isothermally at 177°C forvarious intervals of time. Superficial Rockwellhardness measurements were made on GNEHM160 Digital Hardness Tester using 1.58 mmdiameter steel ball indenter with 15 kg load.

Microstructural characterization-Micro-structural characterization studies were conductedon the as-solutionized and peak aged compositesamples in order to investigate the distribution ofSiC particulates, presence of secondary phases andthe constitutional and microstructural charac-teristics of the interfacial region between the AIalloy matrix and SiC particulates.

Microstructural characterization studies wereprimarily accomplished using a JEOL scanningelectron microscope (SEM) equipped with EDS[Energy Dispersive Spectroscopy]. The compositesamples were metallographically polished prior toexamination. Microstructural characterization ofthe samples were conducted in etched condition.Etching was accomplished using Keller's reagent[0.5 HF-I.5 HCI-2.5 HN03-95.5 H20].

Mechanical behavior-s- The smooth bar tensileproperties were determined on the as-solutionizedand peak aged composite specimens followingASTM standard E8-81. Tensile tests wereconducted using an automated servohydraulic

GUPTA et at.: SiC REINFORCED AI BASED METAL MATRIX COMPOSITE 3

Fig. I-Graphical representation of the aging studiesconducted on the 6061/SiC composite samples,

Fig. 2-Representative SEM micrograph taken from theextruded composite samples revealing the generalmicrostructural.feature.

Heat treatment studies-The results of agingstudies conducted on the extruded compositesamples are shown in Fig. 1. The results revealedthe presence of a well defined hardness peak at anaging time of 6 h. The results also exhibit that thehardness of the peak aged samples (76.1) issignificantly higher when compared to the as­solutionized samples (53.8).

Microstructure-The results of microstructuralcharacterization studies carried out on the as­solutionized and peak aged samples revealed--{a)completely recrystallized matrix, (b) oandeddistribution of SiC particulates in the direction ofextrusion, (c) limited presence of SiC clusters, (d)presence of porosity predominantly associatedwith SiC clusters, and e) partially debonded6061/SiC interface in case of some of the SiC

particulates (Fig. 2). In addition, the peak agedsamples also revealed the increased presence ofMg and Fe based intermetallics when compared tothe as-solutionized samples. The Mg basedintermetallics were found to be invariablysurrounded by a bright solute rich zone.

The results of EDS analyses conducted atvarious distances from the SiCAl matrix interface

in order to investigate the segregation behavior ofalloying elements are shown in Fig. 3. The resultsrevealed the presence of a relatively morepronounced segregation of silicon adjacent to SiCparticulates in case of peak aged .compositesamples. Moreover, the results also revealed thepresence of wider segregation zone in case of peakaged samples when compared to the as­solutionized samples (Fig. 3).

Mechanical behavior-The results of ambienttemperature testing on the as-solutionized and peakaged composite samples are summarized in Table1. The results in Table 1 reveal that peak agedsamples exhibit superior 0.2% yield stress (0.2%YS) and ultimate tensile strength (UTS) andinferior ductility when compared to as-solutionizedsamples. It may be noted that the UTS of the peakaged samples is approximately 1.4 times that of theas-solutionized samples.

Fracture behavior-Macroscopic investigationscarried out on the samples subjected· to heattreatment cycle #1 revealed a 45 degree chiselpoint shear fracture. Microscopic investigationsconducted on the fractured surface using SEM

10 12 14 16

Aging Time. h

4 6

5 wt nx~ m~~ e~! ~t "~ ~

o

Results

Quantitative assessment of SiC particulates­The results of acid dissolution experimentsconducted on the composite samples revealed theweight percentage of SiC particulates to beapproximately 12.2 %.

Density measurement-The results of densitymeasurements conducted on the extrudedcomposite specimens revealed a density value of2.63 g cm-3• The volume percent of the porositycomputed using the experimentally determineddensity value and the result of acid dissolutiontests was found out to be 4.37%.

Instron testing machine on 4 mm diameterspecimens using a crosshead speed of 0.254 mm·per minute.

Fracture behavior-Fractographic studies werecarried out on the fractured surface and subsurfaceof the tensile samples in order to provide insightinto the various fracture mechanisms· operativeduring tensile loading of the as-solutionized andpeak aged samples. Fractographic studies were·primarily accomplished using a JEOL scanningelectron microscope equipped with EDS [EnergyDispersive Spectroscopy].

4 INDIAN J. ENG. MATER. SCL, FEBRUARY 1998

Material

Table I-Results of room temperature mechanical properties

Condition 0.2% YS UTS(MPa) (MPa)

78.9 ± 0.5 183.5 ± 4.4171.8 + 7.9 254.5 ± 8.5

6061/SiC6061/SiC

As-solutionizedPeak aged

13.0 ± 4.87.8 ± 2.5

~ As-Solunomzcd--0--- Peak Aged2.5

"]<Ii

E~" 1.50...EOIl'i)

~0.5

0 10 15 20 25

Distance from Interface, urn

Fig. 3-Graphical representation of the segregation pattern ofSi in the interfacial region between SiC particulate andmetallic matrix obtained from as-solutionized and peak agedcomposite samples.

revealed: (a) presence of dimples, (b) minimalpresence of intermetallics at the core of dimples,(c) presence of SiC particulates exhibiting flatandsmooth surface, and (d) evidence of predominantlyobserved good 60611SiC interfacial bonding (Figs4a and 4b). Fractographic analysis conducted onthe subsurface region also exhibited a good.SiC/6061 interfacial integrity and in addition alsorevealed the presence of microcracks associatedwith SiC clusters (Fig. 4c).

Regarding the samples subjected to heattreatment cycle #2, the results of macroscopicinvestigations revealed a 45 degree chisel pointshear fracture similar to that observed in case ofas-solutionized samples, Microscopicinvestigations conducted on the fractured surfaceusing SEM exhibited: (a) presence of dimples, (b)presence of Mg and Fe rich intermetallics at thecore of dimples, (c) presence of broken SiCparticulates, and (d) frequently observed 60611SiCinterfacial debonding (Figs 5a&5b). The results offractographic analysis conducted on the subsurfaceregion also exhibited the increased tendency of theSiC particulates to debond from the 6061 Almetallic matrix (Fig. Sc) confirming theobservations made on the fractured surface of thepeak aged samples.

Ductility%

(a)

(b)

(c)

Fig. 4--SEM micrographs taken from the fractured compositesamples (as-solutionized condition) exhibiting: (a) theevidence of good 60611SiC interfacial bonding, (b) dimpledmatrix fracture, and (c) microcracks associated with SiCclusters.

GUPTA e/ al.: SiC REINFORCED AI BASED METAL MATRIX COMPOSITE 5

(a)

(b)

(c)

Fig. 5-SEM micrographs taken from the fractured peak agedcomposite samples revealing: (a) predominantly observedinterfacially debonded SiC particulates (marked A) and lessfrequently observed broken SiC particulates (marked B), andthe presence of dimples in the matrix, (b) presence of Mg richintermetallics (marked by arrows) in a dimple, and (c)evidence of SiC/6061 interfacial debonding in subsurfaceregion.

DiscussionHeat treatment studies-The results of the heat

treatment studies conducted on the 60611SiC alloyrevealed that the hardness of the compositesamples in the peak aged condition is - 1.4 timesthan that of the as-solutionized samples. This canbe attributed to the presence of strengtheningprecipitates (Mg2Si)9 in the matrix and theassociated increased resistance to the dislocationmotion under the application of localized load

during hardness testing. The results of the agingstudies also revealed a peak aging time of 6 h forthe 60611SiC samples. These results are consistentwith the results of other investigators" whoreported the similar aging kinetics for 6061/SiCcomposite samples heat treated under similarconditions.

Microstructure-The results of themicrostructural characterization studies revealedthat 6061 metallic matrix reinforced with 10.5volume percent of 54 urn SiC particulatescompletely recrystallized following the two typesof heat treatments used in this study. This is inaccordance with the recrystallization criterionestablished for 2-phase alloys and particulateMMCs II. The results further indicate that thethermal exposure during hot extrusion and thesubsequent heat treatments used in the presentstudy was sufficient to anneal out "deformationdislocations" generated during thermomechanicalprocessing".

Another important microstructural featureobserved in this study was the banded distributionpattern of SiC particulates in the extrusiondirection and the presence of ellipsoidal shapedand elongated SiC clusters in the transversedirection. This type of distribution originates as aresult of sequential deformation events duringextrusion of the ingot metallurgy processedMMCs. The detailed mechanisms describing themechanisms involved can be found elsewhere!'.

Microstructural characterization studiesconducted on extruded MMCs also revealed thepresence of finite amount (vol %=4.37) of porositypredominantly associated with the SiC clusters.This can be attributed to the inefficient packing ofSiC particulates in the clusters duringsolidification 14 and the inability of metallic matrixto plastically flow in the micrometer sized crevicesin the SiC clusters to close the porosity duringextrusion. The results thus suggest that a higherextrusion ratio should be employed in order tofurther reduce the extent of porosity and to obtainnear dense composite material followingconventional casting".

The results of microstructural characterizationstudies conducted on the SiC/6061 interfacialregion exhibit the presence of non-integralinterface in the case of some of SiC particulates

6 INDIAN 1. ENG. MATER. SCI., FEBRUARY 1998

following extrusion. This can be attributed to theresidual porosity associated with SiC particulatesthat could not completely close as a result ofinadequate extrusion ratio used in the presentstudy. It may be noted that the presence of porosityeither associated with SiC clusters or individualSiC particulates is also supported by the densitymeasurement results conducted on the bulkcomposite samples. The results of EDS analysisconducted in the near vicinity of SiC particulatesin case of as-solutionized and peak aged samplesrevealed the enrichment of the alloying elementssuch as silicon (Fig. 3). This phenomenon canprimarily be attributed to the presence of enhanceddislocation density in the interfacial region. Theenhanced dislocation density results due to thedifference in coefficient of thermal expansionbetween SiC particulates and the aluminummatrix 16 and promotes the dislocation-assisteddiffusion of the alloying elements from theadjacent dislocation lean areas of the matrix. Theresults also indicate a more pronouncedsegregation of silicon in case of peak aged sampleswhen compared to the as-solutionized samples.This may be attributed to the precipitation ofsilicon containing precipitates in the near vicinityof SiC particulates during aging and thesubsequent diffusion of silicon to the interfacialregion from the adjacent matrix region. This is in

Crack

~ --....... SiC

Particulate Cracking

(a)

Crack

~ --....... SiC

Crack

~SiC~

Particulate Cracking AI/SiC interfacial debonding

(b)

Fig. 6-Schematic illustrations showing the predominantfailure modes associated with SiC particulates in tensilefractured composite samples in: a) as-solutionized condition,and b) peak aged condition.

accordance with the results of other investigatorswho reported the presence of strengtheningprecipitates in the dislocation infested interfacialregion' adjacent to the ceramic particulates.However, further work is continuing in this area inorder to delineate the various factors that may beresponsible for this kind of segregation behavior.

Mechanical behavior-The results of themechanical properties characterization revealed asignificant improvement in the strength (0.2% YSand UTS) and reduction in ductility of the peakaged composite samples when compared to the as-solutionized samples. For example, the resultsshown in Table 1 indicate an increase in 0.2% YSand UTS of the peak aged samples by -2.2 and 1.4times when compared to the as-solutionizedsamples. The superior strength properties thusexhibited by the peak aged composite samples canprimarily be attributed to the presence ofstrengthening precipitates in the metallic matrixand the associated resistance towards motion ofdislocations during uniaxial tensile loading. Thestrength levels exhibited by the as-solutionized andpeak aged samples' should however be consideredas lower bound due to the presence of --4.37volume percent of porosity. The porosityassociated reduction in strength has beenpreviously established by other investigators forsteels, copper and aluminum based alloys":".Bocchini" and Payne et al}8, for example, assertedthat the presence of pores lead to weakening of amaterial by reducing the amount of stress bearingarea and therefore lower the amount of stress thematerial is able to withstand. It may veryinterestingly be noted that the increase in UTS ofthe peak aged samples by 1.4 times exhibit a one-to-one correlation with the increase in hardnessfrom the as-solutionized to peak aged condition forthe composite samples analyzed in the presentstudy. This is also in accordance with the results ofother investigators who have previously shownthat the tensile strength and hardness are linearlydependent for many aluminum alloys'Y".

Fracture behavior-The as-solutionized andpeak aged composite samples exhibited 45 degreechisel point fracture. This type of fracture ischaracteristic of the Al matrices reinforced withlow volume percent (10-15 vol %) of thereinforcing phases. The results are also consistent

GUPTA et al.: SiC REINFORCED Al BASED METAL MATRIX COMPOSITE

with the fractographic observations made on the6061 Al matrix reinforced with 10 vol % of SiCwhiskers by Me Danels", In addition, the resultsalso indicate the negligible influence of thestrengthening precipitates in the matrix on themacroscopic fracture mode of the 60611SiCcomposite samples.

SEM studies conducted on fractured surface ofas-solutionized samples exhibited the presence ofnon-equiaxed dimples indicative of the ductilemode of failure of the metallic matrix. Highmagnification SEM studies coupled with EDSpoint analysis revealed the core of the dimples tobe predominantly devoid of Mg or Fe basedintermetallics. This is also consistent with themicrostructural characterization studies conductedon the polished surface of as-solutionized samplesexhibiting minimal presence of Mg or Fe basedintermetallics. The results thus suggest thatintermetallics did not play a significant role inaffecting the deformation of metallic matrix in theas-solutionized condition. The failure modesassociated with the presence of SiC particulatesrevealed predominant SiC particulate breaking andminimal SiC/6061 interfacial debonding.Particulates breaking during tensile fractureprocess was indicated by the flat and smoothsurface of SiC particulates observed on thefractured surface of the composite samplesshowing no evidence of 6061 AI matrix. Thisobservation is consistent with the results offractographic studies carried out on 7091/SiCcomposite samples in the as-fabricated and peakaged composite samples by other investigators",The results also indicate that the stressaccumulation in the interfacial region at any stageduring tensile testing was not sufficient to debondthe interface in case of most of the SiC particulatesand that the solute enrichment in the interfacialregion during solutionizing was not sufficientenough to embrittle the interfacial region. Theobservations made on the fractured surface andsubsurface are also supported by the high ductility(13 %) exhibited by the as-solutionized compositesamples (Table 1). The predominant failure modeassociated with SiC particulates in the case of as-solutionized composite samples is alsoschematically represented in Fig. 6a.

7

The results of fractographic studies conductedon the peak aged composite samples also revealedthe ductile mode of failure of the 6061 metallicmatrix. The core of the dimples, however, werefrequently associated with the presence of Mg orFe based intermetallics (Fig. 5). This observationindicates that unlike as-solutionized samples thematrix failure in case of peak aged samples wassignificantly influenced by the presence ofintermetallics. Regarding the failure mechanismsassociated with SiC particulates the SEM studiesrevealed less frequently observed particulatebreaking and increased presence of decohered SiCparticulates when compared to the as-solutionizedfractured samples. The SiC particulate breakingduring tensile testing is inevitable as a result of arelatively coarser SiC particulate size (54 urn) usedin the present study. This is also consistent withthe work of other investigators who showed usingacoustic emission technique that the crackingtendency of the particulates greater than 12 urnincreases with an increase in the particulate size".The increased tendency of the decohesion of SiCparticulates exhibited by peak aged samples can beattributed to the interfacial embrittlement arisingas a result of enhanced segregation of alloyingelements (Fig. 3) and the pronounced precipitationof secondary phases (precipitateslintermetallics) inthe near vicinity of SiC particulates as a result ofaging heat treatment': 23. In related studies", forexample, investigators ascribed the presence ofprecipitates at the interfacial location to themaximization of the plastic incompatibilitystresses resulting due to the overlapping of theplastic fields of SiC reinforcement and precipitatedparticles. This, in addition, promotes earlymicrocracking and subsequent failure. This is alsoconsistent with the mechanical properties results(Table 1) which show that the ductility of the peakaged samples is almost 0.6 times when comparedto that of the as-solutionized samples. Fig. 6bshows a schematic representation of. the failuremodes associated with SiC particulates in case ofpeak aged samples.

The results of the fractographic analysesconducted oil the as-solutionized and peak agedsamples thus indicate a strong influence of agingstep of the conventional T6 heat treatment on thefailure behavior of the composite samples.

8 INDIAN 1. ENG. MATER. SCI., FEBRUARY 1998

ConclusionsThe primary conclusions that may be derived

from this work are, (i)The aging step of theconventional T6 heat treatment significantlyinfluence the constitutional and microstructuralcharacteristics of the interfacial region adjacent toSiC particulates, (ii)The change in failure modefrom predominantly observed particulate breaking(as-solutionized samples) to interfacial debonding(peak aged samples) can be attributed to theembrittlement of the interfacial region duringaging heat treatment, and (iii) The presence ofeither Mg or Fe based intermetallics at the core ofdimples indicates that the plastic deformation ofthe metallic matrix is strongly governed by thepresence of intermetallics in case of peak agedsamples when compared to the as-solutionizedsamples.

AcknowledgmentsAuthors would like to thank Mr. Thomas Tan,

Mr. Tung Siew Kong and Mr. Boon Heng(National University of Singapore, Singapore) fortheir valuable assistance and for many usefuldiscussions during the course of this investigation.

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