Materials and Design 54 (2014) 245–250

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Materials and Design

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Structural and mechanical characterization of Al-based compositereinforced with heat treated Al2O3 particles

0261-3069/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.08.036

⇑ Corresponding author. Tel.: +98 9159055046.E-mail addresses: M_hosseinzadeh@sun.semnan.ac.ir, Mohsen_mnp@yahoo.-

com (M. Hossein-Zadeh).

Mohsen Hossein-Zadeh a,⇑, Omid Mirzaee a, Peyman Saidi b

a Department of Materials Engineering, Semnan University, P.O. Box 35195-363, Semnan, Iranb Department of Science and Material Engineering, University of McMaster, Hamilton, ON, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 September 2012Accepted 8 August 2013Available online 19 August 2013

Keywords:Al–Al2O3 compositesHeat treatmentMicrostructureMechanical properties

In this study, the addition of 1.00 wt.% Al2O3 crystals to the metal matrix of the liquid aluminum wasstudied. In order to investigate the influence of heat treatment on activation of Al2O3 powders andmechanical properties of Al–Al2O3 composites, the Al2O3 particles were heated at 1000 �C. X-ray Diffrac-tion (XRD) analysis used to characterize the crystal lattice of Al2O3 and its variation during heat treat-ment. The size and morphology of the Al2O3 grains was evaluated by Scanning Electron Microscopy(SEM). The results showed a considerable change in morphology of Al2O3 grains during the heat treat-ment. Mechanical evaluation such as hardness, compression and wear tests showed enhancement inthe properties of Al–1.00 wt.% heat treated Al2O3 vs. Al–1.00 wt.% Al2O3 composite.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Metal–Matrix Composites (MMCs) because of their comprehen-sive properties such as low density, high stiffness, low coefficientof thermal expansion, high thermal conductivity, high strengthand high wear resistance, are attracting attention for applicationssuch as packaging, substrates and support structures for electronicdevices and a number of automobile components [1–4].

Many investigators have focused on the commercially impor-tant Al–Al2O3 system. There are several fabrication techniquesavailable to manufacture Al–Al2O3 composite. The fabricationmethods can be divided into three types. These are solid phase, li-quid phase and semi-solid fabrication processes [5].

Among the mentioned methods, liquid process due to its sim-plicity is more considered. Wettability of ceramic by molten metalin such techniques can be the most crucial factor in the productionof metal–ceramic composites. Al2O3 particles are not readily wet-ted by liquid Al that must be considered [6].

Various methods are applied to improve the wettability of par-ticles in the melt of Al such as: addition of some alloying elementsto the melt [7,8], making a wrapper on the surface of particles byCVD or PVD methods [9], applying force to the melt and controllingatmosphere [7]. Furthermore, primary heat treatment on Al2O3 im-proves wettability due to grooving at the grain boundary junctionsduring grain growth [10].

In the present research, the structural and mechanical charac-terization of Al–Al2O3 composite reinforced with heat treatedAl2O3particles has been investigated.

2. Experimental procedure

2.1. Materials

The raw materials used in this research are as follows: A356commercial alloy with composition mentioned in Table 1, Al2O3

particles with average particle size 30 lm.

2.2. Casting and sample preparation

The A356 ingot was heated in a crucible made of Al2O3, to pour-ing temperature 730 �C with heating rate of 20 �C/min to 500 �Cand 10 �C/min afterward.

In order to improve the wettability [6], Al2O3 particles heattreated in the controlled atmosphere tube furnace with heatingrate of 10 �C/min. Prior to entering the furnace, the gas passedthrough a silica gel column and a tube furnace containing copperchips at 300 �C to reduce the moisture and oxygen content. Theflow rate of argon was 5 l/min. The alumina particles were heldin the furnace at 1000 �C for 20 min [6].

0.3% Strontium was added to melt of A356 metal matrix formodification. Addition of 1.00 wt.% Al2O3 reinforcement to A356melt matrix was done. The Al2O3 particles had two different forms:

Table 1Chemical composition of used A356 ingot (wt.%).

Elements Al Si Mg Fe Mn Ti Ni

Weight percent Bal. 6.20 0.40 0.18 0.01 0.007 0.004

246 M. Hossein-Zadeh et al. / Materials and Design 54 (2014) 245–250

A. Alumina with the average grain size of 30 lm.B. Alumina with considerably smaller grain size (about

300 nm) achieved during heat treating.

The melt was cooled in the containers with circular crosssection.

2.3. Characterization

Characterization of the produced samples included metallo-graphic examination, SEM, XRD evaluation, hardness measure-ment, compression strength determination and wear resistance.Samples were extracted from the casted composites with circularcross section of 25 mm in diameter and 9 mm in height. In the cur-rent study, the specimens were prepared by grinding through 600,800, 1200, 1500 and 2000 grit papers, respectively and then werepolished with Al2O3 paste. XRD (Bruker D8-with the voltage andcurrent of 30 kV and 25 mA, respectively, and Cu Ka (k = 1.54 Å))analysis was employed to determine lattice before and after heattreatment.

The morphology of the particles before and after heat treatmentand composite were determined by SEM (Philips XL30). EnergyDispersive Spectrometer (EDS) was used for chemicalidentification.

Each 100 ml of the utilized etchant was consisted of 5 ml HCl,10 ml of H2SO4 and the rest distilled water [11].

Finally, in order to study the wear resistance of the cast sam-ples, Pin-on-Disk wear testing was performed according to ASTM:G99-95a. The cast sample was the disk and the pin was steel with ahardness of 62HR. The final load reached 20N with sliding speed of0.5 m/s for distances of 400, 800 and 1200 m at 18–20 �C and thehumidity of 37–41%. The machine was stopped at certain distanceand the weight losses of the samples were measured by an analyt-ical balance with the accuracy of ±10�4 g. The samples werewashed by acetone, vibrated by ultrasonic instrument and driedbefore being weighed.

Compression test was applied at the strain rate of 5 � 10�4 atroom temperature according to the ASTM: E9-89a standard weremade.

Hardness testing of the cast samples was done with Universalwear machine via Brinell method with a 2.5 mm ball and the forceof 10 kg.

Each test at least was repeated three times to obtain a preciseaverage value for each property.

3. Results and discussion

3.1. Composite synthesis

Fig. 1 shows the SEM micrographs of the alumina particles be-fore and after the heat treatment. The continuous structure of par-ticles before heat treatment is clearly observable in Fig. 1a.

Fig. 1b–d shows the different microstructure of Al2O3 particlesafter heat treatment.

As illustrated in Fig. 1a, the as-received alumina particles exhi-bit irregular shape with relatively broad size distribution. Afterheat treatment, the morphology of initial particles were modifiedand a change from irregular to spherical shape was noticed(Fig. 1b–d).

It is observed that the grain size decreased considerably to sub-micron and hence the active surface of the particles increased. Itcan improve the wettability of the second phase [10].

The mechanism of formation of the new structure after heattreatment can be described with respect to the grooving phenom-ena [10].

Grain-boundary grooves are important in grain growth becausethey tend to anchor the ends of the grain boundaries at surface.Fig. 2 (left-hand) represents a boundary attached to its groove,while the right-hand figure shows the same boundary moved tothe right and freed from its groove. Freeing this boundary fromits groove increases the total surface area and, therefore, the totalsurface energy.

On the other words, the heat treatment can provide sufficientenergy for grain boundary to leave their grooves and increase theeffective free surface [6,10]. The SEM photographs in Fig. 1 confirmthat the initial size of the alumina particles was reduced from30 lm to about 300 nm after heat treatment.

The XRD patterns in Fig. 3 clarified that the other allotropies ofalumina like (c-Al2O3) with the cubic structure transforms to thehexagonal structure during heat treatment.

The transformed volume will not fit perfectly into space origi-nally occupied by the matrix and this gives rise to a misfit strainenergy per unit volume of the matrix which intensifies the separa-tion of grains [12,13].

SEM micrograph of metal matrix of A356 without adding Al2O3

particles is shown in Fig. 4a. Fig. 4b shows the microstructure ofthe fabricated composite without applying heat treatment on alu-mina particles. Agglomeration of alumina particles in metal matrixis observed in Fig. 4b.

The SEM micrographs shown in Fig. 4c indicate distribution ofheat treated alumina particles in the matrix. Fig. 4c reveals gooddistribution of reinforcements and very low agglomeration in thecomposite.

The EDS analysis showed that darker phase is the Al matrix andthe white particles are alumina (Fig. 4d). The second phase doesnot seem to have any preferred site of accommodation. As it isclear in Fig. 4c, the amount of Al2O3 in the grain boundaries andthe bulk of the grains are the same.

The micrographs in Fig. 5 show that grain size of the reinforcedcomposite (Fig. 5b and c) is smaller than the A356 alloy withoutreinforcement (Fig. 5a) because particles act as nucleation sites[3,14,15]. Also, due to the modified structure of Al2O3 particlesafter heat treatment, the grain size was smaller than that of with-out heat treatment. As Fig. 5 reveals, during solidification of com-posite, reinforcement particles are pushed by Al dendrites into thelast freezing eutectic melt. Therefore, the strength particles aresurrounded by silicon eutectic [2,15].

3.2. Mechanical properties of the composite

Table 2 shows the results of hardness measurements and com-pression strength of samples. As it is expected, the hardness mea-surements verify that the hardness of A356 samples increases fromaverage value of 53.2 HBN to 70.6 and 63.4 HBN by addition of1.00 wt.% heat treated and not heat treated alumina respectively.It can be explained with respect to the distribution of the hard sec-ond phase particles in the matrix. The standard deviation of hard-ness was about 0.8 BHN.

Thus, results show that the hardness of the composite fabri-cated by heated alumina particles is more than the composite fab-ricated by alumina particles because of homogeneous distributionof Al2O3 heated in the metal matrix and also (Fig. 4c), partly due tothe decreasing grain size of Al–1.00 wt.% heat treated Al2O3 com-posite (Fig. 5b).The lower hardness in Al–1.00 wt.% nonheat treated

Fig. 1. SEM micrographs of the alumina particles. (a) Before heat treatment, (b–d) after heat treatment (at different magnifications).

Fig. 2. Moving a grain boundary away from its groove.

Fig. 3. XRD patterns of alumina particles. (a) Before heat treatment. (b) After heattreatment.

M. Hossein-Zadeh et al. / Materials and Design 54 (2014) 245–250 247

Al2O3 composite due to agglomeration and imperfect incorporationof particles (Fig. 4b).

The next mechanical property considered in the present re-search is compression strength. Fig. 6 illustrates the stress–straincurve in compression test for the Al–Al2O3 composites in compar-ison with the A356 alloy. The compression strength is reported inTable 2. The results showed that the compression strength in-creased from 213.56 to 374.81 and 264.41 MPa for the compositesreinforced with particles of heat treated and nonheat treated alu-mina respectively. It is due to the distribution of hard particles inthe matrix.

In the case, second phase particles trap the dislocations duringdeformation and cause higher work hardenability of the compositein comparison with the Al [16–19]. In addition, the Al2O3 particlesdistort the matrix and therefore increase the compression strengthof the composite to about 264.41 MPa.

Improvement of mechanical properties can be attributed to twophenomena:

(1) Strengthening from grain boundaries: A general relationshipbetween yield stress (and other mechanical properties) and grainsize proposed by Hall–Petch according to r0 = ri + K. D�1/2 equa-tion (r0, ri, K and D are yield stress, friction stress, locking param-eter and grain diameter, respectively).

In this system, by adding Al2O3 to the molten aluminum itsgrains decreased and thereby its mechanical properties excessivelyrose (Fig. 5).

(2) Strengthening from fine particles: Small second phase parti-cles distributed in a ductile matrix are a common source of alloyStrengthening. In this system, Al2O3 as fine particles throughoutthe matrix can act as barriers to dislocations and thus mechanicalproperties were elevated [3,16,19].

Fig. 4. SEM micrographs of (a) aluminum, (b) composite reinforced with 1.00 wt.% nonheat treated Al2O3 particles, (c) 1.00 wt.% heat treated Al2O3 particles, and (d) EDSmicrograph from white particles in (c).

Fig. 5. Optical micrographs of (a) A356 alloy, (b) composite reinforced with 1.00 wt.% heat treated alumina particles, and (c) composite reinforced with 1.00 wt.% aluminaparticles.

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Fig. 7 shows the weight losses of samples in wear test. Theweight loss at three different distances determined for A356 alloyand the composites.

Although the variation of the weight loss as a function of thedistance shows a linear trend for Al and the composites, the secondphase particles affect the wear resistance after a critical distance.

Table 2Mechanical properties of the A356 alloy and the Al–Al2O3 composites.

Sample A356 Composite

1.00 wt.% heattreatedalumina

1.00 wt.% aluminawithout heattreatment

Compression strength(MPa)

213.56 374.81 264.41

Hardness (HBN) 53.2 70.6 63.4

Fig. 6. Comparison of stress–strain curve for A356 alloy and Al–Al2O3 composites.

Fig. 7. The weight losses of the samples vs. distance.

M. Hossein-Zadeh et al. / Materials and Design 54 (2014) 245–250 249

On the other words, it is observed that for distances shorter than400 m, the weight losses of composites are almost the same. How-ever, by increasing the wear distance, the weight losses meet sig-nificant differences. It is generally believed that incorporation ofAl2O3 particles to alloys contributes in improving the wear resis-tance of aluminum to a great extent [20]. The results suggest thatthe wear resistance of Al–Al2O3 composite increases by the addi-tion of 1.00 wt.% heat treated alumina particles in all distances,as shown in Fig. 7

During dry sliding, it seems that the Al2O3 particles do not easilycome out in debris, which might verify good bonding betweenAl2O3 particles and the matrix [21,22]. Also according to Archardequation, due to higher hardness of composite sample, the wearresistance of Al–Al2O3 composites are better than A356 alloy [23].

4. Conclusions

In this research, the structural and mechanical properties ofA356 based composite reinforced with 1.00 wt.% heat treated and

nonheat treated Al2O3 were investigated. The findings can be sum-marized as followings:

1. It is observed that the active surface of the Al2O3 particles isincreased after heat treating at 1000 �C for 20 min. Groovingphenomena is supposed to be responsible for formation of thenew structure after heat treatment.

2. The microstructure of composite reinforced with heat treatedAl2O3 particles showed good distribution of particles and verylow agglomeration of alumina in contrast with Al-nonheat trea-ted Al2O3 composite.

3. Heat treatment of Al2O3 powder resulted in the significantimprovement in hardness Al–Al2O3 composite.

4. According to the compression test, compression strength of theAl–1.00 wt.% Al2O3 increases while Al2O3 heat treated particlesadded to A356 metal matrix.

5. The degree of improvement of wear resistant of composite isstrongly depended on heat treated particles of reinforcement,specially in long distances (up to 400).

Acknowledgements

The authors would like to thank the research board of SemnanUniversity and gifted student at Semnan University for the supportof this research.

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