Advanced Powder Technology 25 (2014) 1693–1698

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Advanced Powder Technology

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Original Research Paper

Synthesis and characterization of Al (Al2O3–TiB2/Fe) nanocompositeby means of mechanical alloying and hot extrusion processes

http://dx.doi.org/10.1016/j.apt.2014.06.0160921-8831/� 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

⇑ Corresponding author.E-mail address: m.Tavoosi@ma.iut.ac.ir (M. Tavoosi).

Sh. Rizaneh, G.H. Borhani, M. Tavoosi ⇑Department of Material Science, Malek-Ashtar University of Technology, Shahin-Shahr, Isfahan, Iran

a r t i c l e i n f o

Article history:Received 21 April 2014Received in revised form 2 June 2014Accepted 19 June 2014Available online 12 July 2014

Keywords:Al-base nanocompositeComplex reinforcementMechanical alloyingHot extrusion

a b s t r a c t

Synthesis and characterization of Al–(Al2O3–TiB2/Fe) nanocomposite by means of mechanical alloyingand hot extrusion processes was the goal of this study. For this regards, mechanical alloying was donein two steps; formation of Al2O3–TiB2/Fe reinforcements and preparation of Al-base nanocomposite.Results showed that Al2O3–TiB2/Fe nanocomposite powders can synthesis by mechanical alloying andsubsequent heat treatment at 700 �C. Hot extrusion of powder samples lead to preparation of fully denseAl-base nanocomposite. With increasing the amount of complex reinforcements, the compressionstrength was increased and reached to 560 MPa. Consolidated samples show good ductility related tothe nature of Al2O3–TiB2/Fe reinforcements.� 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction Al based MMCs can be fabricated via various techniques such as

Aluminum-based metal matrix composites (MMCs) haveemerged as an important class of high performance material foruse in aerospace, automobile, chemical and transportation indus-tries due to their high specific modulus and strength, low density,superior wear and creep resistances [1,2]. Reinforcing the ductilealuminum matrix with stronger and stiffer second-phase rein-forcements provides a combination of properties of both the alu-minum matrix and the reinforcement components [3].

Different reinforcement particles such as aluminides (Al4Mo,Al3Ti, Al3Zr), carbides (SiC, TiC, ZrC), nitrides (AlN, Si3N4), oxides(Al2O3, SiO2) and borides (TiB2, ZrB2) have been incorporated inAl MMCs by various methods [1–6]. Among these reinforcements,TiB2 comes into prominence with its excellent properties such ashigh melting point (3225 �C), high hardness (25 GPa), superiorwear resistance, high electrical conductivity and considerablechemical stability [7–15]. Recently, new group of TiB2 base com-posite (multiple complex TiB2-ceramic-metallic component), suchas TiB2–TiC/Ni [16], TiB2–Al2O3/Al [17], TiB2–TiC/Fe [18,19], havebeen developed. This new group of composite could be a good can-didate for using as reinforcement for improving the mechanicalproperties of Al MMCs. It seems that, the ceramic components inthese reinforcements increase the strength and thermal stabilityand the metallic component increases the ductility of producedcomposite.

reactive squeeze casting and rapid solidification processing, exo-thermic dispersion, reactive hot pressing, self propagating hightemperature synthesis (SHS) and mechanical alloying (MA).Mechanical alloying is a solid-state powder processing techniqueinvolving repeated deformation, welding and fracturing of powderparticle. MA has been widely used to synthesize a variety of mate-rials, such as amorphous alloys, nanocrystalline materials, inter-metallic compounds, borides, carbides, nitrides and composites.In almost all cases, the final product has nanosized structure whichexhibits better properties and performance compared to the con-ventional coarse-grain materials [20–22].

In the present study, Al matrix composite reinforced with com-plex Al2O3–TiB2/Fe particles were fabricated via mechanical alloyingand hot extrusion processes. The effects of complex reinforcementon the microstructure and mechanical properties of produced com-posites were investigated.

2. Experimental procedures

The powders of aluminum (99.5% purity, average particle size45 lm,), ferrotitanium (0.8 wt%Ti–0.2 wt%Fe, average particle size60 mm), and B2O3 (99% purity, average particle size 45 lm) wereused as raw materials. The stoichiometric powders mixtureaccording to reaction (1) was milled in an attrition ball mill usinghard chromium steel vial and balls, under argon atmosphere. MAwas done with rotation speed of 365 rpm for predetermined hourswithout interruption and ball to powder ratio was 10:1. The vialtemperature was maintained at ambient temperature using of

Fig. 1. The XRD patterns of Al, ferrotitanium (Fe and Ti) and B2O3 powder mixtureafter different milling times.

Fig. 2. The DTA curve of Al, ferrotitanium (Fe and Ti) and B2O3 powder mixture after20 h of milling process at a constant heating rate of 20 �C/min.

Fig. 3. The XRD patterns of Al, ferrotitanium (Fe and Ti) and B2O3 powder mixtureafter 20 h of milling and annealing at 700 �C for 90 min.

Fig. 4. The SEM micrographs of Al, ferrotitanium (Fe and Ti) and B2O3 powdermixture after 20 h of milling and annealing at 700 �C for 90 min.

Fig. 5. The XRD patterns of Al-base composite with 1.25, 2.5 and 5 vol% of producedreinforcement (Al2O3–TiB2/Fe) after 10 h of milling.

Fig. 6. The morphological micrographs of Al-base composite with (a) 1.25, (b) 2.5and (c) 5 vol% of produced reinforcement (Al2O3–TiB2/Fe) after 10 h of milling.

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water circulating system. Stearic acid powder (1 wt%), supplied byMerck was used as the process control agent (PCA). In order to for-mation of final nanocomposites, Al powders were milled with 1.25,2.5 and 5 vol% of produced reinforcements (Al2O3–TiB2/Fe) in asame condition for 10 h. The produced composite powders withdifferent vol% of the reinforcements were consolidated with hotextrusion process (the extrusion ratio, rate and temperature wasabout 6:1, 5 mm/s and 500 �C, respectively).

2Alþ ð0:8Ti� 0:2FeÞ þ 0:8B2O3

! 0:8TiB2 þ 0:8Al2O3 þ 0:2Fe ð1Þ

Table 1Characteristics of Al (Al2O3–TiB2/Fe) nanocomposites after 10 h of milling.

Sample Al2O3–TiB2/Fe (vol%) Particle size (lm) Crystallite size (nm) Microhardness (Hv)

Al2O3 TiB2 Fe

1 1.25 43 40 1100.75 0.45 0.05

2 2.5 39 40 1501.5 0.9 0.1

3 5 41 40 1603 1.8 0.2

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X-ray diffractometry (Philips diffractometer (40 kV) with Cu Karadiation) was used to follow the structural changes of samplesduring milling and extrusion processes. The XRD patterns wererecorded in 2h range of 10–110� (step size: 0.05�; time per step1 s). The crystallite size of produced samples was estimated by ana-lyzing broadening of XRD peaks using Scherrer formula [23]. Micro-structural characterizations of synthesized samples were carriedout by scanning electron microscopy (SEM) (Seron AIS-2100, Korea)at an accelerating voltage of 20 kV. Differential thermal analysiswas done by using Rhemometric STA 1500 differential thermal ana-lyzer. The sample was placed in Al2O3 pans and heating in Ar atmo-sphere up to 1000 �C at heating rate 20 �C/min. The compressiontest was done according to ASTM E9. The Archimedes techniquewas also used to measure the density of consolidated samples.

3. Results and discussion

3.1. Production of Al2O3–TiB2/Fe nanocomposite

In fact, Al2O3–TiB2/Fe nanocomposite can be synthesized by adisplacement reaction of Al, ferrotitanium and boron oxide accord-ing to reaction (1). The Gibbs free energy and enthalpy changing ofthis reaction at ambient temperature were about �579 and�600 kJ/mol, respectively. These points illustrated that thisreaction is highly exothermic and can thermodynamically occursat room temperature [20–22].

The XRD patterns of powder mixture (according to reaction(1)) after different milling times are shown at Fig. 1. It can beseen that, in the early stage of milling, only broadening of theAl, ferrotitanium (Fe and Ti) and B2O3 peaks accompanied withremarkable decrease in their intensities occurred as a result ofrefinement of crystalline size and enhancement of lattice strain.Increasing milling time to 10 h, led to the disappearance of Feand Ti peaks. This can be due to the partial dissolution of Feand Ti in Al lattice as well as grain boundaries. Another possibilityis that these elements could disappear into the Al as very smallisolated particles which are virtually undetectable by XRD [21].The results shows that, by milling the powder mixture up to20 h, gradual grain refinement is the only considerable changethat occurs in the powder mixture, and no detectable reactiontakes place.

Fig. 2 has shown the DTA curve of Al, ferrotitanium (Fe and Ti)and B2O3 powder mixture after 20 h of milling process at aconstant heating rate of 20 �C/min. As seen in this figure, onlyone exothermic peak appears in the DTA curve at 650 �C. Toanalyze the reaction responsible for the exothermic peak, theproduced powder sample was annealed in an argon atmosphereat 700 �C for 90 min. The XRD pattern of annealed sample is pre-sented in Fig. 3. As seen, the annealed sample is composed of theAl2O3, TiB2 and Fe phases. Therefore, the exothermic peak inFig. 3 should be attributed to the formation of these phases fromthe milled powders. Crystallite sizes of Al2O3, TiB2 and Fe phaseswere about 60, 50 and 50 nm, respectively. The morphological

micrographs of milled samples after annealing are presented inFig. 4. As seen, the morphology of Al2O3–TiB2/Fe nanocompositepowder (complex reinforcement) is equiaxed and the average par-ticle size of that is about 5 lm.

3.2. Production of Al–(Al2O3–TiB2/Fe) nanocomposites

The XRD patterns of Al-base composites with 1.25, 2.5 and5 vol% of produced reinforcement (Al2O3–TiB2/Fe) after 10 h ofmilling are presented from Fig. 5. As seen, the diffraction peaksfrom XRD patterns are related to Al matrix and there are noany peak related to Al2O3, TiB2 and Fe phases. This result is attrib-uted to the low volume fraction of the Al2O3, TiB2 and Fe phasesand the fine size of these powders in the matrix. The morpholog-ical micrographs of produced nanocomposites powder are alsoshown in Fig. 6. As can be seen, the powders morphology of pro-duced composites in different volume fraction of complex rein-forcement were almost equiaxed and the average size of theseparticles were about 40 ± 7 lm. Several characteristics (volumefraction, particle size, Al crystallite size and microhardness) ofproduced nanocomposites are presented in Table 1. As shown,the hardness value of the samples increase with increasingamount of Al2O3–TiB2/Fe reinforcement from 1.25 to 2.5 and5 vol%.

3.3. Consolidation processes

The XRD patterns of consolidated samples with different per-centage of complex reinforcement (Al2O3–TiB2/Fe) are presentedin Fig. 7. Same to XRD results of powder samples, the XRD patternsof consolidated samples are only contain of Al peaks. The cross-sec-tional SEM micrographs of the consolidated samples with differentpercentage of reinforcements are shown in Fig. 8. According tothese micrographs, a fine and uniform distribution of Al2O3–TiB2/Fe reinforcement particles within the aluminum matrix is obtainedalthough most of the reinforcement particles appeared to be smallto be observed on SEM.

Table 2 shows the mechanical properties of produced nanocom-posite with different percentage of reinforcement. As seen, byincreasing the percentage of reinforcement, compression strength(and hardness) of samples increase strongly and rich to 560 MPa(150 Hv) in composite contain 5 vol% of Al2O3–TiB2/Fe particles.The strength of the present study is much higher than thosereported in previous studies [7–14]. The improved yield strengthof the nanocomposite presented here can be explained by an Oro-wan strengthening mechanism as a result of presence of fine clo-sely spaced hard particles in the matrix, which hinder themovement of dislocations [13,14].

Although, the ductility (fracture strain) of produced compositesdecreases with increasing the percentage of reinforcements, it isimportant to note that the ductility of these samples is higher thanother similar studies (composite with single ceramic reinforce-ments) [7–15]. In fact, this effect could be because of the nature

Fig. 7. The XRD patterns of consolidated samples (hot extruded at 500 �C) withdifferent percentage of complex reinforcement (Al2O3–TiB2/Fe).

Fig. 8. The SEM micrograph of consolidated samples with (a) 1.25, (b) 2.5 and (c)5 vol% of Al2O3–TiB2/Fe reinforcement.

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of complex Al2O3–TiB2/Fe reinforcements. Fig. 9 has shown thedistribution of reinforcement particle in Al matrix at high magnifi-cation. As seen, the reinforcements particles are contained of two

separated composed phases (light and gray phases). The EDSresults which taken from these phases (Fig. 9) showed that thelight and gray precipitates related to Fe rich and Al2O3/TiB2 phases,

Table 2Characteristics of consolidate Al (Al2O3–TiB2/Fe) nanocomposites with different volume percentage of complex reinforcement.

Sample Relative density (%) Al crystallite size (nm) Hardness (Hv) Compression strength (MPa) Fracture strain (%)

1 99.98 48 100 373 52 99.97 43 145 530 43 99.91 45 155 560 3.5

Fig. 9. The SEM micrograph and EDS analysis of separated phases in Al–(Al2O3–TiB2/Fe) nanocomposite.

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respectively. These results show that, during the production ofAl2O3–TiB2/Fe reinforcements, Fe atoms reject from ceramic matrixand form separated Fe rich phase. This phase stick in Al2O3–TiB2

ceramic components and can pin them to Al matrix. Moreover,the wettability and interaction of complex metal–ceramic rein-forcements with Al matrix is higher than ceramics only. Withregard to this point, it is expected that the produced Al–(Al2O3–TiB2/Fe) nanocomposite show higher ductility than other similarcomposites.

4. Conclusion

In the present paper, the fabrication of Al–(Al2O3–TiB2/Fe)nanocomposite by means of mechanical alloying and hot extrusionmethods has been investigated. The achieved results showed that:

1. Al2O3–TiB2/Fe nanocomposite powders can synthesis bymechanical alloying and subsequent heat treatment at 700 �C.The crystalline sizes of Al2O3, TiB2 and Fe in produced nanocom-posite were about 60, 50 and 50 nm, respectively.

2. Mechanical alloying of aluminum particles with produced rein-forcements for 10 h and then hot extrusion, lead to a more uni-form distribution of complex reinforcements and formation offully dense Al/(Al2O3–TiB2/Fe) nanocomposite.

3. The compression strength of consolidated samples increasestrongly with increasing the percentage of reinforcements andrich to 560 MPa.

4. Produced bulk samples show good ductility related to the nat-ure of complex Al2O3–TiB2/Fe reinforcements.

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