Journal of Alloys and Compounds 509 (2011) 7758 7763

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Journal of Alloys and Compounds

j our na l ho me p ag e: www.elsev ier .com

Microstructural and mechanical evaluation of Alfabrica

Z. Sadegha Department o y, Ahvb Department oc Institute for M rmany

a r t i c l

Article history:Received 17 JaReceived in reAccepted 27 AAvailable onlin

Keywords:Aluminum maMechanical allIn situ TiB2Spark plasma

ositewdercess

has bterinred saissio

owde resu

t graishowed a hardness value of 180 VHN and yield and tensile strength of 480 and 540 MPa, respectively,which are much higher than those reported for similar composites made by other processes.

2011 Elsevier B.V. All rights reserved.

1. Introdu

The phymatrix comand thermation in the aproperties sizes of cereter to nannanocompowhose hardenhanced wdue to the cposites, sysof these msynthesis ometallurgy to fabricatewhich the rrication by

Corresponulty of EngineeTel.: +98 611 3

E-mail add

0925-8388/$ doi:10.1016/j.ction

sical and mechanical properties exhibited by metalposites (MMCs), such as high specic modulus, strengthl stability, make them particularly attractive for applica-erospace and automotive industries [14]. Mechanicalof MMCs can be further enhanced by decreasing theamic particulates and/or matrix grains from microm-ometer level. Such materials are referred to as thesites [5]. Typical examples are Al-based composites,ness, strength, and Youngs modulus can be effectivelyhen the structure changes to nano scale [68]. So far,omplexities in the synthesis of metal matrix nanocom-tematic investigations on the mechanical propertiesaterials and a few reports have been focused on thef bulk aluminum matrix nanocomposites via powder(PM) routes [8]. In situ techniques have been developed

aluminum-based metal matrix composites (MMCs), ineinforcements are synthesized in the matrix during fab-chemical reaction of the constituents [9]. The in situ

ding author at: Department of Materials Science and Engineering, Fac-ring, Shahid Chamran University, Golestan Blvd., Ahvaz, Iran.332739; fax: +98 611 3336642.ress: zsadeghian@yahoo.com (Z. Sadeghian).

formed reinforcing particles are ner in size and their distributionis more uniform. Furthermore, they are thermodynamically stablein the matrix, leading to less degradation at high temperatures.

The structure and properties of the in situ ceramic phasehave been optimized over a variety of processing techniques[9]. These techniques include exothermic dispersion, reactive-gasinjection, reactive sintering, reactive milling and mechanical alloy-ing. Mechanical alloying (MA) as a solid state powder processingmethod has been considered for fabricating in situ MMCs. MA hasan advantage over other in situ fabrication routes as it is capableof producing nano structured composite powder with high unifor-mity.

Even though the in situ composites have signicant advantages,some synthesis routes may lead to composites with inhomoge-neous microstructure with various unstable and/or undesirablephases [1012]. These undesirable phases might drastically reducethe mechanical properties. For example, Tjong et al. [13] have inves-tigated the mechanical properties of in situ Al10 wt.% TiB2 andfound that the formation of Al3Ti phase has a negative impact onthe mechanical performance of in situ composites. They proposedthat the elimination of intermetallic Al3Ti phase is a primary taskfor developing in situ AlTiB2 composites with superior mechanicalperformance.

A challenge in processing of nanostructured materials is thatlong time exposure at high temperature sintering, often resultsin severe grain growth. The consolidation techniques best suited

see front matter 2011 Elsevier B.V. All rights reserved.jallcom.2011.04.145ted by mechanical alloying

iana,, B. Lota, M.H. Enayatib, P. Beissc

f Materials Science and Engineering, Faculty of Engineering, Shahid Chamran Universitf Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iranaterials Applications in Mechanical Engineering, RWTH Aachen University, Aachen, Ge

e i n f o

nuary 2011vised form 25 April 2011pril 2011e 10 May 2011

trix nanocompositeoying

sintering

a b s t r a c t

Production of bulk AlTiB2 nanocompTiB2 metal matrix nanocomposite poAl powder mixture. A double step proAl3Ti intermetallic compound, whichwas consolidated by spark plasma sinteristics of powder particles and sinteelectron microscopy (SEM) and transmconducted on the cross section of pextruded samples was evaluated. Theite has good thermal stability agains/ locate / ja l l com

TiB2 nanostructured composite

az, Iran

from mechanically alloyed powder was studied. Al20 wt.% was obtained by mechanical alloying (MA) of pure Ti, B andwas used to prevent the formation of undesirable phases likeeen described in our previous papers. The resultant powderg (SPS) followed up by hot extrusion. The structural charac-mples were studied by X-ray diffractometry (XRD), scanningn electron microscopy (TEM). Hardness measurements werer particles and sintered sample and the tensile behavior oflts showed that the prepared Al20 wt.% TiB2 nanocompos-n growth and particle coarsening. Extruded Al20 wt.% TiB2

Z. Sadeghian et al. / Journal of Alloys and Compounds 509 (2011) 7758 7763 7759

Fig. 1. XRD pa step Mrod.

to retain natric discharand spark discharges fully controis possible tples [14,15reported fowhich Al3Tmore, the rhas been renanocrystal

The objebulk nanoststeps includunwanted pcal properti

2. Materials

Elementaling materials. rotating speedof the chromiuwas evacuatedmechanical allballs and the agent. A doubAlTiB2 compo

A SEIFERT( = 0.15406 nmalloying. XRD step of 20 s. Thby a LEO scannstability of thescanning calorment at heatinmicroscopy obworking at 20(FIB) techniqu

Powders wThe MAed powbetween the gas the protect

ut at with

total tiSinteruitablture wrdnes

whilrdneseld H

ults

ay exoducin A

ain s2 (degree)

Inte

nsity

tterns of Al20 wt.% TiB2 nanocomposite (a) as-milled powder obtained from double

noscale grain size seem to be hot-pressing, hot elec-ge sintering such as plasma activated sintering (PAS),plasma sintering (SPS) that uses microscopic electricbetween the particles under pressure where, by care-lling the processing time, temperature and pressure, ito produce nearly fully dense nanostructured bulk sam-]. According to the literature two attempts have beenr synthesizing in situ AlTiB2 by mechanical alloying, ini intermetallic compound was also obtained. Further-esulting powders were not nano-scale and no studyported on the mechanical properties of consolidatedline AlTiB2 composites.ctive of this study was to synthesize Al20 wt.% TiB2ructured composite via several consecutive processinging MA, SPS and hot extrusion without the formation ofhases. Microstructure, thermal stability and mechani-

carried oto 550 Cand the 78 mm. obtain stemperaMicrohamachinemacrohaa Houns

3. Res

X-rder prof TiB2The gres of extruded samples were also investigated.

and methods

Al 63 m), Ti (4060 m) and B (2 m) powders were used as start-Powder mixtures were milled by a Fritsch planetary ball mill with a

of 360 rpm. The ball to powder weight ratio was 10 and the diameterm steel balls was 15 mm. The hardened chromium steel vial (400 ml)

and lled with argon to prevent oxidation of powders during theoying process. To avoid severe adhesion of aluminum powder to thevial, 1 wt.% zinc stearate was added to the mixture as a controllingle step MA process of elemental powders was used to obtain in situsite powder which is described elsewhere [16,17].

X-ray diffractometer employing monochromatic Cu K radiation) was used to investigate the structural changes during mechanical

scans were performed with a step size of 0.05 and a dwell time pere cross-sectional microstructure of the powder particles was studieding electron microscope (SEM). To evaluate the thermal behavior and

powder during subsequent processes, such as sintering, differentialimetry (DSC) experiments were conducted using Netzsch 402 equip-g rate of 15 C min1 under owing argon gas. Transmission electronservations were conducted using a FEI Tecnai G2 TEM instrument0 kV. The TEM samples were prepared by using focused ion beame.ere sintered by a FCT HP D 250 spark plasma sintering (SPS) machine.ders were placed inside the graphite die, and the die was placed

raphite electrodes of the SPS chamber. Argon (99.99% purity) was useding atmosphere during SPS experiments. The SPS experiments were

Williamsonhad similar

Accordingrains was the values SEM imagedistribution

The DSCMA processFurthermorrun remain(Fig. 1b), thnanocompodid not incraluminum fore the Althermal sta

The mecspark plasmSPSed samptheoretical

Fig. 1c scompared tA, (b) after annealing up to 550 C, (c) SPSed sample and (d) extruded

DC current of 6000 A and DC voltage of 10 V. Samples were heated up a constant pressure of 35 MPa, maintained during the heating periodme of the process was 600 s. The diameter of the sintered samples wased samples were then hot extruded by backward extrusion in order toe rods of AlTiB2 composites for mechanical testing. The preheatingas 400 C and the extrusion rate was 0.6 mm s1 with a ratio of 15.

s examination of AlTiB2 powders was conducted by a Leco testinge the hardness of consolidated samples was measured by a Zwicks machine. Tensile behavior at ambient temperatures was studied by50KS S machine at a strain rate of 0.001 mm s1.

and discussion

amination of the nal in situ Al20 wt.% TiB2 pow-ed by mechanical alloying showed only the presencel matrix and no other phases were observed (Fig. 1a).ize of TiB2 and Al in the powder was estimated by the

Hall (WH) equation [18]. It was found that both phases

grain size of about 20 nm.g to the dark-eld image (Fig. 2b) the average size of the

measured to be 20 nm which is in good agreement withobtained from the WH equation. By image analysis of

(Fig. 3) it was conrmed that TiB2 had a nearly uniform in the Al matrix with an average size of about 90 nm.

trace of powder particles obtained from double step (Fig. 4) represented no exothermic peak up to 550 C.e the X-ray diffraction pattern taken after the DSCed unchanged compared to that for as-milled powderat reects the good thermal stability of resulting AlTiB2site powder upon heating. The average grain size of TiB2ease during annealing in DSC, while the grain size of thematrix grew from about 20 nm to about 30 nm. There-TiB2 nanocomposite powder was expected to have goodbility during subsequent consolidation processes.hanically alloyed AlTiB2 powder was sintered usinga sintering (SPS), to cylindrical samples. The density ofle was measured to be 2.82 g cm3 which is 96% of thedensity of Al20 wt.% TiB2 (2.94 g cm3).hows the X-ray diffraction pattern of the SPSed sampleo the as MAed composite powder. No structural change

7760 Z. Sadeghian et al. / Journal of Alloys and Compounds 509 (2011) 7758 7763

Fig. 2. TEM images of Al20 wt.% TiB2 as-milled powder particle obtained from double step MA (a) bright eld image, (b) dark eld image.

occurred in the AlTiB2 nano-composite after consolidation by SPS.The mean g20 nm. The determinedas-milled pin Al duringprepared nawithstand ggraph of thsize of TiB2about 3 mthan 100 nmof about 3.8

Hot extrremaining pRadial crackof the extruhot extrusio

Fig. 3. A typicaparticle obtain

during direct hot extrusion attempts in this study, even at an extru-eed aratur

s1

the d no

wder obtaextruut 20th theasurhe TEb), w30 nmSEM shownanrain size of the TiB2 phase was measured to be aboutgrain size of the Al matrix for the sintered sample was

to be about 30 nm which is higher than that for theowder. Therefore, no signicant grain growth occurred

the sintering process. These results showed that thenocomposite powder has sufcient thermal stability torain growth during consolidation process. SEM micro-e sintered sample is presented in Fig. 5. The maximumparticles in the sintered sample was measured to be, with a large amount of particles remaining smaller. However, there was still a small amount of porosity

% in the SPSed sample.usion was applied on the SPSed billets to eliminate theorosity and produce rods suitable for tensile testing.s which progress from the surface towards the centredate, known as r tree defect, has been reported duringn of Al-based composites [19,20]. This defect occurred

sion sptempe0.6 mmshowsrevealethe poresultsin the be aboble wiThe ming of t(Fig. 6about

An rod is a few l cross sectional SEM micrograph of Al20 wt.% TiB2 as-milled powdered from double step MA.

50

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

Hea

t Flo

w [m

W m

g-1 ]

Fig. 4. DSC tradouble step Ms low as 1 mm s1. By indirect extrusion at a preheatinge of 400 C, extrusion ratio of 15 and extrusion speed ofthe billets showed no surface cracking or defects. Fig. 1ddiffraction pattern of the extruded sample. The results

structural change in the extruded sample compared to and the sintered sample. This is in agreement with DSCined from MAed powder. The sizes of Al and TiB2 grainsded sample were measured, using the WH method, to

and 35 nm, respectively. These results are compara-ose obtained for the powder and the sintered sample.ed grain size values were conrmed by image analyz-M dark eld image obtained from the extruded samplehich showed nano-sized grains with a mean value of.

micrograph obtained from the cross section of extrudedn in Fig. 7. The size of TiB2 particles ranged betweenometers to about 2 m. TiB2 particles showed noTemperature [C] 125 200 275 350 425 500

ce up to 550 C of the Al20 wt.% TiB2 as-milled powder obtained fromA.

Z. Sadeghian et al. / Journal of Alloys and Compounds 509 (2011) 7758 7763 7761

Fig. 5. Cross-sectional SEM micrographs of SPSed Al20 wt.% TiB2 sample.

Table 1Hardness values of AlTiB2 composites at different stages.

As milled powder(microhardness,VHN)

Sintered sample(hardness, VHN)

Extruded sample(hardness, VHN)

480 206 179

remarkablesintered samand once fodissolutionduring sinte

Fig. 8 shmicroscopysolved Ti orlimited to TTiB2 particl

Table 1 sent stages.

Fig. 7. Cross-sectional SEM micrographs of Al20 wt.% TiB2 extruded rod.

value had decreased by about 50% versus the powder. This canbe attributed to the elimination of cold work effects during expo-sure to high temperatures. The hardness of extruded sample wasabout 180 HV, which was much higher than those reported previ-ously [11,12]. For example for Al15 wt.% TiB2 composite fabricatedby mechanical alloying and powder extrusion the hardness was

ed to be 84 VHN [11]. Improvement of mechanical propertiess harduceion o1 Alpicashowmpotion wt.%

in Tmpo

er in growth compared to the as-milled powder and the as-ple. TiB2 is thermodynamically stable in the Al matrix

rmed does not decompose in the matrix [21]. Therefore, of the particles and diffusion of Ti and B cannot occurring and extrusion processes.ows the EFTEM (energy ltered transmission electron) of the extruded sample. There was no evidence of

B in the Al matrix and the distribution of Ti and B wasiB2 particles. These maps also conrmed that most ofes were smaller than 100 nm.hows the hardness values of Al20 wt.% TiB2 at differ-

reportsuch asize rereductof 606

A typle is nanocoelongaAl20 sentednanocoby othAfter consolidation of the powder by SPS the hardness extruded A

Fig. 6. TEM images of Al20 wt.% TiB2 extruded sample (a) bright dness has been stated by other investigators, when thes to nanoscale [22]. It has been shown by Han et al. thatf grain size to 200 nm improves the tensile properties10 wt.% Al2O3 as much as twice [23].l stressstrain curve obtained from the extruded sam-n in Fig. 9. It is obvious from the curve that AlTiB2site shows a brittle behavior in tensile testing. The totalof the sample was 1.4%. Tensile properties of extrudedTiB2 nanocomposite produced in this study are pre-able 2, and Fig. 10 shows the tensile properties of thissite in comparison with AlTiB2 composite reportedvestigators. Both yield strength and tensile strength oflTiB2 nanocomposite of this study are much highereld image, (b) dark eld image.

7762 Z. Sadeghian et al. / Journal of Alloys and Compounds 509 (2011) 7758 7763

Fig. 8. EFTEM elemental map of Al20 wt.% TiB2 extruded ro

Strain (%)

Stre

ss (M

Pa)

Fig. 9. A typical stressstrain curve obtained from the extruded Al20 wt.% TiB2composite.

Fig. 10. Tensile properties of some Al20 wt.% TiB2 composites prepared by differenttechniques.

Table 2Tensile proper

0.2% proof s

483

Fig. 11. Fractu

than those [12,13].

The imphere can bas a result omatrix, whibe noted thmode, withthe fracturemanner at m(Fig. 11). Zimfeatures forites, previo

4. Conclus

The douTiB2 particlThe TiB2 pafrom 10 nm100 nm. Hocantly. The to be 15 nmd for (a) Al, (b) Ti and (c) B.

ties of extruded AlTiB2 composite.

trength, MPa Tensile strength, MPa Elongation, %

543 1.4re surface morphology of extruded Al20 wt.% TiB2 composite.

reported in previous studies with similar compositions

roved yield strength of the nanocomposite presentede explained by an Orowan strengthening mechanismf presence of ne, closely spaced hard particles in thech hinder the movement of dislocations [24]. It shouldat the samples fractured with a macroscopically brittleout any evidence of necking. However, fractography ofd samples showed that the matrix behaves in a ductileicroscopic scale, resulting in the formation of dimplesmerman et al. and Tang et al. have also reported similar

the fracture surfaces of NiSiC and AlSiC nanocompos-usly [25,26].

ions

ble step MA process resulted in the in situ formation ofes in the Al matrix with a ne and uniform distribution.rticles size in as-milled Al20 wt.% TiB2 powder ranged

to 1 m although most of them were smaller thant extrusion did not increase the TiB2 particle size signi-average Al grains in the as-milled powder was measured

which increased to 35 nm after hot extrusion. The

Z. Sadeghian et al. / Journal of Alloys and Compounds 509 (2011) 7758 7763 7763

results showed that the prepared Al20 wt.% TiB2 nanocompositehas good thermal stability against grain growth and particle coars-ening. Extruded Al20 wt.% TiB2 nanocomposite showed a hardnessvalue of 180 VHN and yield and tensile strength of 480 and 540 MPa,respectively, which are much higher than those reported for similarcomposites made by other techniques.

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Microstructural and mechanical evaluation of AlTiB2 nanostructured composite fabricated by mechanical alloying1 Introduction2 Materials and methods3 Results and discussion4 ConclusionsReferences