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Dry sliding tribological behavior and mechanical propertiesof Al20245 wt.%B 4C nanocomposite produced by mechanicalmilling and hot extrusion

Alireza Abdollahi, Ali Alizadeh , Hamid Reza BaharvandiFaculty of Materials & Manufacturing Processes, MUT University of Technology, Tehran, Iran

a r t i c l e i n f o

Article history:Received 21 April 2013Accepted 4 September 2013Available online 18 September 2013

Keywords:AluminumMechanical millingHot extrusionMechanical propertiesTribological behavior

a b s t r a c t

In this paper, tribological behavior and mechanical properties of nanostructured Al2024 alloy producedby mechanical milling and hot extrusion were investigated before and after adding B 4C particles.Mechanical milling was used to synthesize the nanostructuredAl2024 in attrition mill under argon atmo-sphere up to 50 h. A similar process was used to produce Al20245 wt.%B 4C composite powder. Themilled powders were formed by hot pressing and then were exposed to hot extrusion in 750 C withextrusion ratio of 10:1. To study the microstructure of milled powders and hot extruded samples, opticalmicroscopy, transmission electron microscopy and scanning electron microscopy (SEM) equipped withan energy dispersive X-ray spectrometer (EDS) were used. The mechanical properties of samples werealso compared together using tension, compression and hardness tests. The wear properties of sampleswere studied using pin-on-disk apparatus under a 20 N load. The results show that mechanical millingdecreases the size of aluminum matrix grains to less than 100 nm. The results of mechanical and weartests also indicate that mechanical milling and adding B 4C particles increase strength, hardness and wearresistance of Al2024 and decrease its ductility remarkably.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminum based composites are one of the most widely usedmetal based composites which have attracted much attention sothat a large number of studies in the eld of metal based compos-ites were belong to this area [1,2] . The reason is the very goodproperties of these composites such as low density, high strengthand stiffness, high specic modulus ( E /q ), very good wear resis-tance, low coefcient of thermal expansion, high damping capacityand excellent high temperature properties. These properties haveled to numerous applications of these composites in differentindustries such as automotive, aerospace, military and nuclearpower [35] . Wrought aluminum alloys have long being used asa matrix alloy in producing metal based composites. The maincause of this is the low density of aluminum [6,7] . Lillo [4] , as anexample, used 6061 alloy as matrix composite in his research.Al7075 alloy was also used in Sankar et al. [8] research. Li et al.[9] have used Al5083 alloy as matrix composite in their studies.Most wrought aluminum alloys are appropriate for extrusion and

most discontinuously reinforced aluminum composites are pro-duced this way. 2xxx and 7xxx series alloys, because of being heattreatable, are among the best aluminum alloys for producing alu-minum based composites [7] .

The reinforcement phase in aluminum based composites couldbe as particle, continuous ber, short ber or whisker; however,Particles reinforcement, due to easier manufacturing process andcreating isotropic properties in the composite is the most com-monly used form [10,11] . The most common ceramic particles usedin producing aluminum based composites are as follows: Al 2O3[12] , TiN [13] , B4C [14] , MgO [15] , MoSi 2 [16] , etc. These particlescan be used in nano and micron scales. Among ceramic particlesB4C, due to high melting point (2450 C), high modulus(445 GPa), good thermal stability, good hardness (B 4C is the thirdhardest material after diamond and cubic boron nitride (CBN)),high wear and impact resistance, high chemical resistance andlow density (2.51 g/cm 3 ), is an appropriate reinforcing materialfor producing aluminum based composites. Besides, because of high capacity for neutron absorption in isotope B 10 , AlB 4 C com-posites have special applications in nuclear industries [1721] .For example, Alizadeh et al. [22] compared the mechanical proper-ties of aluminum matrix composites reinforced with 1, 2 and4 wt.%B 4C nanoparticles fabricated via stir casting method. Theirresults show that with increasing amount of B 4C nanoparticles,

0261-3069/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.09.024

Corresponding author. Address: Faculty of Materials & ManufacturingProcesses, MUT University of Technology, P.O. Box: 15875-1774, Tehran, Iran.Tel.: +98 9125184189; fax: +98 77385712.

E-mail addresses: [email protected] (A. Abdollahi), [email protected] , [email protected] (A. Alizadeh), [email protected] (H.R. Baharvandi).

Materials and Design 55 (2014) 471481

Contents lists available at ScienceDirect

Materials and Design

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yield strength and tensile strength increased but elongation tofracture decreased.

AlB 4C composites are produced by three different methods:solid state methods (such as mechanical milling and powder met-allurgy), molten methods (such as stir casting) and semi-solidmethods [23,24] . Generally, solid state methods are usually usedto produce particle composites with high mechanical properties,because these methods provide a uniform distribution of second-ary phase particles in the matrix. Therefore, composites producedby these methods are isotropic. In addition, as the temperaturein these methods are much less than in molten methods, undesir-able interfacial interaction between the matrix and reinforcement,that usually leads to loss of mechanical properties, are avoided andthe segregation of reinforcement particles becomes minimized[13,18] . For example Zhang et al. [25] observed extensive debond-ing during mechanical testing of a bulk AlB 4 C composite fabri-cated with conventional powder metallurgy method attributableto a weak bonding between the reinforcement and matrix. Hanet al. [26] in their research on Al5083 alloy proved that mechanicalmilling enhances the strength of alloy up to 713 MPa but reducesits ductility to 0.3%.

Wear behavior of aluminum matrix composites depends onstrength of the interface between matrix and reinforcement parti-cle. If the interface between the matrix and reinforcement materialis not strong enough, reinforcement particles slow down and leadto considerable material losses [27] . In general, the effect of rein-forcing particles size on wear behavior of composite is related topowder preparation method. In composites produced by mechan-ical milling, reinforcement particles are distributed well and hencewith more ne particles, the wear resistance improves [28] . Ali-zadeh et al. [29] have studied wear behavior of nanostructuredAlCu alloy and AlCu/B 4 C nanocomposites produced by mechan-ical milling and hot extrusion. Their results revealed a lower fric-tion coefcient and a lower wear rate for the nanostructuredmatrix of Al in contrast to a commercial coarse grained Al matrix.The same pattern was also observed in the nanocomposite sampleswith respect to the base matrix.

Researchers have studied the effect of pure metals and differ-ent alloys grain size on wear properties. The results of theseinvestigations have shown that ner grains lead to increasedhardness and reduced wear rate [3035] . In a study on the wearproperties of nanostructured iron, Lv et al. [36] observed thatwear rate is higher comparing to coarse grained iron. Theyattributed this decrease in wear resistance to a sharp decreaseof exibility in a nanostructured sample. Studies on the wearbehavior and mechanical properties of the B 4C particulate-rein-forced MMCs with Al2024 matrices are, however, limited. Thiscould be due to the high strength and low compressibility of thisalloy [37] . Therefore, in the present research, Al20245 wt.%B 4Cnanocomposite was produced by mechanical milling and hot

extrusion. We studied the effect of B 4 C particles on microstruc-ture renement of Al2024 based composite in mechanical mill-ing process. The mechanical and wear properties of Al2024alloy, before and after incorporation of B 4C particles, were alsoexamined.

2. Experimental details

In this research, Al2024 powder, atomized by Argon gas, withaverage particle size of 60 l m, was used as the matrix. 5 wt%B 4Cpowder, with average particle size of 20 l m was also used asreinforcement.

An attrition mill equipped with water cooling system was used

for mechanical milling process and producing Al20245 wt.%B 4Ccomposite powder. Ball-to-powder weight ratio and the rotational

speed were dened 1:10 and 400 rpm respectively. To prevent oxi-dation and contamination of powders during mechanical milling,argon with 99/99% purity was used. To prevent cold welding dur-ing milling, 2 wt.% stearic acid was used as process control agent(PCA). In order to study the effect of boron carbide on microstruc-ture, wear resistance and mechanical properties, unreinforcedAl2024 aluminum alloy powder was milled for 50 h under samecondition. Microstructure of powder, after 50 h being milled wasstudied by Transmission electron microscopy (Philips-FEGC200)to dene the grain size and reinforcement particles distributionwithin the aluminum matrix. To study the effect of mechanicalmilling process on the grain size of matrix alloy and the amountof lattice strain, WilliamsonHall method was used [38,39] .

After mechanical milling process, hot extrusion process wasused for nal forming. To that end, powders were hot-pressed ina cylindrical mold. Then, the hot pressed powders were exposedto hot extrusion in 750 C with extrusion ratio of 10:1. It shouldbe noted that in order to study the effect of boron carbide particlesand mechanical milling on Al2024 alloy properties, a coarse-grainsample using unmilled Al2024 powder was produced through hot-pressing and then hot extrusion.

At the end of extrusion process, the microstructure of extrudedsamples was studied parallel and perpendicular to extrusion direc-tion by Optical microscopy (OM) and Scanning electron micros-copy (TESCAN XMU VEGA-II). To compare the mechanicalproperties of samples, tension, compression and hardness testswere applied. The samples of tension test were provided accordingto ASTM: B557 and the test was performed at roomtemperature ata speed of 1 mm/min. In order to determine the fracture mode of samples, the cross section fracture was examined by SEM after ten-sion test. The compression test was also performed at room tem-perature at displacement rate of 1 mm/min. The samples of compression test were produced with 1/4 length-to-diameter ra-tio. The hardness of samples was measured by Brinell hardness testwith ball diameter of 2.5 mm and 30 kg force.

The wear test in this research is performed by a pin-on-diskwear-testing apparatus. The pins used in this research were pro-duced of extruded samples as cylinder with diameter of 1 cmand height of 2 cm. The disk used in this research was also madeof steel with a hardness of 63 HRC. The wear test was performedin distance 3000 m, at speed of 0.6 m/s and with imposed load of 20 N. At the end of each stage, the surface of samples were cleanedand washed by alcohol, then weighed carefully and the weight lossis dened and recorded. In order to study the surface changes of worn samples and understand the mechanism of wear, the wornsurface was examined by scanning electron microscopy with anenergy dispersive X-ray spectrometer (EDS). At this stage, besidestaking microscopic image of worn surface and wear particle mor-phology, the chemical composition of tribological layer was alsodetermined by EDS.

3. Results and discussion

3.1. Average size of sub-grains

Fig. 1 shows a TEM image of Al2024 powder and Al2024B 4 Ccomposite powder after 50 h of mechanical milling and an EDSanalysis of aluminum based and boron carbide. As it is shown, fol-lowing the mechanical milling process, B 4C particles are dispersedquite uniformly in aluminumbased so that B 4 C particles have evenpenetrated into aluminum grains. Moreover, no clustering oragglomeration of particles is observed. The TEM image of Al2024powder ( Fig. 1 b) shows sub-grains of aluminum based with aver-

age size of 3550 nm which a little more than grain size calculatedby WilliamsonHall method for Al2024 powder.

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Fig. 2 shows the changes of matrix grain size (calculated by Wil-

liamsonHall method) and lattice strain of unreinforced Al2024powder and Al2024B 4C composite powder depending the milling

time. As expected, sever plastic deformation (SPD) of powder par-ticles during mechanical milling process reduced the size of alumi-

num based crystals to nanometer. Therefore, the size of matrixgrains decreased to less than 100 nm. Meanwhile, by increasingthe milling time, the size of matrix grains decreased because thecrystal size decrease in the beginning stages of milling happenedmore quickly.

Another point that should be noted in Fig. 2 is that the size of aluminum based grains in Al2024B 4C composite powder de-creased more than of those in unreinforced Al2024 powder whichshows the effect of hard B 4C particles on grain renement of alumi-num matrix during mechanical milling. This effect is also trueabout lattice strain so that the lattice strain in Al2024B 4C com-posite powder is more than in unreinforced Al2024 powder. Grainrenement of aluminummatrix could be attributed to the increaseof dislocation density which is a result of sever plastic deformation

in Al2024 powder particles and also to the difference in coefcientof thermal expansion of matrix and reinforcement [40] . Parvin

Fig. 1. TEM image of: (a) Al20245 wt.%B 4C nanocomposite powders and (b) nano-structured Al2024 powder after 50 h of mechanical milling (arrows indicate sub-grains);(c), (d) and (e) EDS analysis for B 4 C particles and Al matrix.

Fig. 2. Crystallite size and lattice strain of unreinforced Al2024 and Al2024B 4 Ccomposite powders as function of milling time.

A. Abdollahi et al. / Materials and Design 55 (2014) 471481 473


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et al. [41] reported similar results about Al6061 and Al6061SiCpowders.

3.2. Microstructure of the hot extruded samples

Figs. 3 and 4 show the microstructure of coarse-grained alumi-num (CG Al), nanostructured aluminum (NC Al) and AlB 4C nano-

composite (NC AlB 4 C), in parallel and perpendicular to extrusiondirection. The rst thing that should be noted is that many struc-tural changes occur during extrusion. These changes include grainsand particles orientation along extrusion direction, recrystalizationof matrix grains and porosities closure [42,43] . Here again B 4 C par-ticles orientation and extension of aluminum based grains alongextrusion direction are quite evident in microstructure of all threesamples. In addition, comparison of the microstructure of samplesmakes it clear that following the mechanical milling, grain size isdecreased remarkably so that in microstructure of NC Al and NCAlB 4C samples, grain boundaries are not easily visible.

As shown in SEM image of NC AlB 4 C sample, boron carbideparticles are distributed quite uniformly throughout aluminummatrix and no clustering or agglomeration of particles is observed.This uniform distribution of reinforcement particles will have agreat effect on increase of mechanical properties. Moreover, suc-cessive fracture and welding of aluminum particles, duringmechanical milling, leads to uniformity in B 4C particles distribu-tion. In fact, mechanical milling through cold welding-fracture-cold welding results in penetration of reinforcement particles intoaluminummatrix particles ( Fig. 1 ) and prevents their separation oragglomeration in Al matrix grain boundaries [44] . This is provedbyWang et al. [45] in the case of AlSiC composite.

3.3. Mechanical properties of the hot extruded samples

3.3.1. Tensile propertiesTable 1 shows the results of tensile, compression and hardness

tests. The engineering stressstrain curves of samples are alsocompared with each other in Fig. 5 . As it is shown, NC AlB 4C sam-ple has the highest strength and hardness but its elongation is theleast. In contrast, CG Al sample has the highest elongation and leaststrength and hardness.

Changes of yield strength and Ultimate Tensile Strength (UTS)in extruded samples could be explained with HallPetch and Oro-wan mechanisms. According to HallPetch mechanism, the rela-tion between yield stress and grain size is dened as follows:

r 0 r i KD 1 =2 1

where r 0 equals to yield stress, r i equals to stress opposing themovement of dislocations, D equals to grain size, and K is a constantnumber. According to the above equation, yield stress has an in-

verse relationship with grain size; therefore, with decrease of grainsize, yield stress increases [46,47] . As mentioned before, mechanicalmilling process leads to grain rening of structure to nanometer.Therefore, according to HallPetch relationship, yield of NC Al sam-ple would be more than CG Al sample. Physical analysis of this issue

Fig. 3. Optical micrograph of (a) CG Al and (b) NC Al samples in the extrusion direction.

Fig. 4. Microstructure of NC AlB 4C sample: (a) optical micrograph in the extrusiondirection and (b) SEM image normal to the extrusion direction.



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HallPetch relationship, is microstructure renement duringmechanical milling:

H H 0 KD 1 =2 4

where H 0 is the hardness of annealed coarse grained sample, D isgrain size and K is a constant number [40,51] . Given that the hard-ness of boron carbide is much more than of aluminum, increase of hardness as a result of boron carbide is not unexpected. This couldalso be analyzed simply based on mixture rule [51] . Therefore, it isclear that hardness of NC AlB 4 C sample is more than NC Al sample.Hardness increase as a result of reinforcement particles addition isattributed to dispersion strengthening. Because the increase of bor-on carbide to aluminum matrix increases the number of barriersacross dislocations movement (their movement delay) and as a re-sult hardness increases.

3.3.4. Compression test As can be seenin Table 1 and Fig. 5 , NC Al sample, comparing to

CG Al sample, enjoys more compressive strength. The reason ismicrostructure renement of aluminum matrix during mechanicalmilling. In other words, here again HallPetch relationship is trueand nanostructures sample show a higher compressive strength.It should be noted that just as the results of tensile test, in com-pression test NC Al sample has a lower ductility than CG Al sample

(its strain-to-failure is low). By adding 5 wt.%B 4C to NC Al sample,compressive strength increases but ductility decreases even fur-ther; because matrix grain size in NC AlB 4 C nanocomposite issmaller than NC Al sample. Moreover, because of boron carbidepresence in the structure, Orowan strengthening mechanism isalso activated and helps the increase of compressive strength.Meanwhile, B 4C particles delays crack initiation and growth partic-ularly in primary stages of barreling and this way, increases of compressive strength.

3.3.5. FractographyFig. 6 shows fracture surface of CG Al, NC Al and NC AlB 4C sam-

ples after tensile test. By comparing the fracture surfaces of milledsamples and unreinforced alloy (CG Al) it could be found that sam-ples fracture mode is totally different.

Ductile fracture could take many forms. The grains may slide onconsecutive planes so that nally the sample separate along planes

with 45 angle (planes with maximum shear stress) [46] . This kindof fracture, as shown in Fig. 7 a, has occurred in CG Al sample.In contrast with ductile fracture, brittle fracture occurs right in

the direction perpendicular to the tensile axis [46] . As shown inFig. 7 b, this kind of fracture has occurred in nanostructure samples(NC AlB 4 C and NC Al).

Fig. 6. Fracture surface of the: (a) CG Al, (b) NC Al and (c) NC AlB 4C samples (the arrows indicate the direction of crack growth).

Fig. 7. Fracture mode of the: (a) CG Al and (b) NC Al samples under tensile load.



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fracture has three main features: 1. Flat and smooth surface; 2.Slots on planes which indicate the crack grow path (arrows inFig. 6 b and c) 3. Change of cleavage plane which leads to stepsdevelopment in the place cleavage plane change [46] . Of course,the stressstrain curves also show that NC Al and NC AlB 4C sam-ples fractured without plastic deformation (or without necking) ina brittle manner, whereas CG Al sample fractured after bearing alot of plastic deformation and after passing the UTS point. It isfor this reason that the yield stress and ultimate tensile strength(UTS) in NC Al and NC AlB 4 C samples are equal. In general, innanostructures materials, there are a large amount of barriersacross dislocations movements (because the grains are very ne).Thus, dislocations move difculty and these materials have a lowpotential for work-hardening. In other words, these materials donot show a remarkable plastic deformation and after passing theyield point, without necking (plastic deformation) fracture in abrittle manner [56] .

One very important point about fracture surface of NC AlB 4Csample that must be noted is that in Fig. 6 c, no boron carbide is ob-served. In other words, debonding between particle and matrix inNC AlB 4 C sample has not occurred because of loading. This indi-cates that bonding between particle/matrix in composites pro-duced by mechanical milling is very strong does not fracturesimply. In other words, the interface between particle and matrixhas a good metallurgical quality (has no crack or inclusion) andload transformation from matrix to particle occurs simply and fast[20,57,58] .

3.4. Tribological behavior of the hot extruded samples

Fig. 8 shows the effect of grain renement and adding 5 wt.%boron carbide to wear rate of Al2024 alloy. As can be seen, the

wear rate of CGAl sample is higherthan NCAl and NC AlB 4C sam-ples. In this circumstances, it could be said that CG Al sample expe-riences a sever wear. Grain renement of aluminum alloy andadding boron carbide have a positive effect on wear behavior, sothat unlike coarse grained aluminum, under a load of 20 N, a milledwear occurs. The wear rate of NC Al sample under a load of 20 N is30% lower than CG Al sample. By adding 5 wt.% boron carbide toAl2024 alloy and producing Al2024B

4C nanocomposite, its wear

rate, comparing to coarse grained aluminum decreases 33%. The ef-fect of hardness on wear rate is shown according to Archardequation:

Q KH W

5

where Q equals to wear rate, W equals to force, H equals to hardnessof worn surface and K is wear coefcient. According to the aboverelation, the higher the hardness is in worn surface, the lower thewear is in material. As mentioned before, mechanical milling pro-cess decrease grain size and increase hardness. According to Arch-ard law, wear rate has reverse relationship with material hardness[59] , therefore, increase of aluminum alloy hardness because grain

renement, decreases wear rate.Studies conducted on wear behavior of metal based composites

show that presence of reinforcement particles increases hardnessand improve wear resistance. Moreover, reinforcement particlesreduce the tendency of adhesive bonding to the counterface [60] .

Fig. 9 shows the SEM micrographs of worn surface and wearparticles of CG Al sample after being worn under a load of 20 N.In this situation, one of distinguished features of CG Al worn sur-face is a series of craters and cracks in sliding direction. The resultsof EDS analysis of this samples surface in two different areas areshown in Fig. 9 c and d. Presence of a considerable amount of

Fig. 10. (a) SEM micrograph of worn surface of NC Al sample under a normal load of 20 N, (b) SEM micrograph of wear debris morphology, (c) EDS analysis of worn surface.



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(2) The results of mechanical tests show that mechanical mill-ing and presence of B 4C particles increases the strengthand hardness of Al2024 alloy but decrease its ductilityseverely. The reason is the increase of barriers across dislo-cations movement which limits their movement.

(3) The fracture surface of CG Al sample indicates the features of ductile fracture in comparison to NC Al and NC AlB 4 C sam-ples; in return, the fracture surface of NC Al and NC AlB

4C

samples are smoother and no dimples are observed in theirfracture surface. This shows that the fracture in these sam-ples occurred in a complete brittle manner.

(4) The results of wear rate of CG Al, NC Al and NC AlB 4C afterbeing worn for 3000 m distance in 0.6 m/s speed and under anormal load of 20 N shows that mechanical milling andincorporation of B 4C particles decreases the wear rate of Al2024 alloy severely.

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