Improvements in Microstructure and Mechanical Properties

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Improvements in microstructure and mechanical propertiesof AlSiCu alloyAl2O3 nanocomposite modified by ZrO2

Rahman Bajmalu Rostamia) and Mohammad Tajallyb)

Faculty of Materials Engineering and Metallurgy, Semnan University, Semnan, Iran 3513119111

(Received 5 May 2014; accepted 18 August 2014)

In the present research, the microstructures and mechanical properties of AlSiCu alloy matrixcomposites reinforced with nanosized alumina (Al2O3) and zirconia (ZrO2) were investigated.For this purpose, Al2O3 particulates were replaced with different wt% of ZrO2 to improvemicrostructure and mechanical properties. The T6 heat treatment was also performed to investigatemechanical properties in heat-treated condition. Tensile testing, hardness measurement, opticalmicroscopy, x-ray diffraction (XRD), energy dispersive spectroscopy, and scanning electronmicroscope examination were used to characterize the behavior of composite and matrix.The highest tensile strength was achieved in the specimen containing 1.25 wt% ZrO2 and 0.75 wt%Al2O3, which shows an increase to 36% in comparison with the nonreinforced base alloy.The hardness values indicated 11% increase following the heat treatment. Fracture surfaceexaminations revealed a transition from ductile fracture mode in as-cast aluminum alloy to ratherbrittle in AlSiCu alloy matrix hybrid composites.

I. INTRODUCTION

Aluminum matrix composites possess many advantagessuch as high specific stiffness, low density, good wearresistance, high specific strength, good thermal stability,and electromagnetic shielding capacities with thedevelopment of some noncontinuous reinforcementmaterials, fibers, whisker, or particles. In particular,particle-reinforced aluminum matrix composites notonly have good mechanical and wear properties butalso are economically viable.15

Usually, microceramic particles are used to improve theyield and ultimate strength. However, it was reported thatthe elongation, ultimate strength, and yield strength (YS)of the Al matrix composites reinforced with nanosizedceramic particles were enhanced more significantly thanthose reinforced with microsized particles.6,7Tahamtanet al.8 showed that decreasing alumina particle size frommicrometer (10 lm) to nanometer (100 nm) causedimprovement in tensile strength of composite. In anotherstudy, Ma et al.9 suggested that the tensile strength of1 vol% Si3N4 (10 nm)/Al composite was comparable tothat of the 15 vol% SiCp (3.5 lm)/Al composite, while theYS of the former is much higher than that of the latter.

Tian et al.10 posited that the tensile testing showed anincrease in the tensile strength and a considerable increaseof the elongation in the 2024Al matrix composites rein-forced by the ZrB2 nanoparticles, compared with the

unreinforced 2024Al alloy. Hemanth11 reported that thepresence of nano-ZrO2 particulates in Al matrix noticeablyimproved hardness, strength, and fracture toughness butwith slight reduction in ductility.Hybrid aluminum nanocomposites can be considered

as an outstanding material, where high strength andwear-resistant components were of major importance,particularly structural applications in the aerospace,automotive, and military industries.12 Thakur et al.13

showed that the addition of SiC nanoparticulates tohybrid (Al/Ti 1 SiC) composites assisted in increasingmicrohardness, macrohardness, AlTi interfacial hardness,0.2% YS, and ultimate tensile strength (UTS), whereasthe ductility was marginally affected. Ahamed andSenthilkumar14 fabricated Al alloy composite rein-forced by nanosized Al2O3 and Y2O3 particulates andreported that the 0.2% YS, UTS, and work of fracturewere improved over the unreinforced A16063 alloy, andfailure strain was decreased in reinforced nanocomposites.ANSI 332 aluminum alloy, with 9.5% Si, 3.0% Cu,

and 1.0% Mg, typically has applications in automotivepiston and parts requiring elevated temperature strength.15

In this study, combined addition of Al2O3 and ZrO2nanopowders on the mechanical properties of ANSI 332aluminum alloy was investigated. Nanocomposite wasproduced by stir casting in the present research. The mainadvantages of Al2O3 are its high hardness and wearresistance, whereas ZrO2 exhibits higher strength andfracture toughness, besides its lower Youngs modulus.16

Therefore, this study was conducted to investigate thepossibility of replacing ZrO2 with Al2O3 as suitablecandidates in the hybrid composites.

Address all correspondence to these authors.a)e-mail: [email protected])e-mail: [email protected]: 10.1557/jmr.2014.241

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II. EXPERIMENTAL PROCEDURE

Chemical composition of ANSI 332 alloy is given inTable I. In addition, physical and mechanical propertiesof this alloy are shown in Table II. The alloy was meltedin a graphite crucible in ambient atmosphere using anelectrical furnace. Nanoreinforcements were produced byball-mill processing of microalumina and microzirconiaparticulates. The average sizes of nanoalumina and nano-zirconia particulates were 30 and 35 nm, respectively.In the first stage of the study, the total amount of nano-powders including Al2O3 and ZrO2 was considered equalto 2 wt%. The names allocated to the samples and theamounts of any nanoceramics added to the alloy used inthis experiment are presented in Table III. In the nextstep, nanocomposites reinforced by 1 wt% Al2O3 anddifferent ZrO2 contents including 2 and 3 wt% ZrO2.After additions, the melt was stirred in 750 C for 5 minto acquire uniform distribution of powders within themelt. The melt was cast in a permanent mold at 725 C.

The as-cast samples were machined based on ASTM E8M04. A SANTAM Universal tensile/compression test-ing machine/STM-50 series (SANTAM-Iran, Tehran, Iran)equipped with a data acquisition system was used to carryout the tensile tests at room temperature at a strain rate of1 mm/min. To compare mechanical behavior of as-castand heat-treated samples, some samples were heat treatedthrough solution treatment at 500 C for 6 h followed byquenching in water at 60 C. Artificial aging was carriedout for samples at 205 C for 9 h. A programmableNabertherm furnace was used for heat treating. Thehardness of as-cast and heat-treated alloys and nano-composites was measured using an ERNST Brinell hard-ness tester at an applied load of 187.5 kgf and 2.5-mm

diameter ball. The samples were grounded, polished, andetched by Keller etch reagent for 15 s and then were studiedusing optical microscopy. A Bruker D8 ADVANCE x-raydiffractometer (Karlsruhe, Germany) and a TESCANVEGA II XMU scanning electron microscopy (coupledwith energy dispersive spectroscopy (EDS) to identifyelements) were used to study the intermetallic phasesof nanocomposites. Fracture surfaces of tensile test sampleswere examined by scanning electron microscope (SEM) todetermine the macroscopic fracture mode.

III. RESULTS AND DISCUSSION

A. Microstructural characterization

Optical micrographs of the as-cast and heat-treatedsamples are shown in Figs. 1 and 2, respectively.The microstructure of ANSI 332-cast alloy consists ofa-phase dendrites (light gray, 1), eutectic phase (mixture ofa-matrix, and rounded dark gray Si phases, 2) and someintermetallic phases. It is observed that large flakes ofeutectic silicon phases with sharp tips changed intospherical shape particles after the heat treatment. Thischange decreases stress concentration and susceptiblesites to crack nucleation.

A number of Fe-rich intermetallic phases, includinga (Al8Fe2Si or Al15(FeMn)3Si2), b (Al5FeSi),p (Al8Mg3FeSi6), and d (Al9FeSi3), have been iden-tified in AlSi cast alloys.1720 Intermetallic phaseAl15(FeMn)3Si2 (alpha- or a-phase) with cubic crystalstructure forms in skeleton like morphology or inform Chinese script. Presence of Mg with Si resultsin an alternative called p-phase form, Al5Si6Mg8Fe2,which has script-like morphology. Cu is also presentin AlSiCu cast alloys primarily as phases: Al2Cu,AlAl2CuSi, or Al5Cu2Mg8Si6. Al2Cu with tetragonalcrystal structure generally precipitates as fine spheroidalAlAl2CuSi ternary eutectic.

21 The compact skeleton-likemorphology that was seen in micrographs (Fig. 3) andXRD examination of experimental ANSI 332 alloy canbe an evident of existence of some of these phases suchas Al8Mg3FeSi6 and Al2Cu (Fig. 4).

Furthermore, investigation by SEM and EDSrevealed intermetallic compounds that are shown inFig. 5(a) by arrow marks. The Al, Si, and Zr elementswere identified by EDS analysis [Fig. 5(b)]. Zhuet al.22 fabricated AlZrO2C composite by in situ

TABLE I. Chemical composition of experimental ANSI 332 alloy.

Composition (wt%)

Si 9.7Cu 2.1Mg 1.0Mn 0.12Fe 0.45Zn 0.03Ni 0.07Cr 0.003Al Rem.

TABLE II. Physical and mechanical properties of ANSI 332 aluminum alloy based on ASTM SB-108.

AlloyMelting temperature

range (F)Tensile strength,min, ksi (MPa)

Typical Brinellhardness Temper Fluidity

Pressuretightness

Resistance tocorrosion Polishing AnodizingANSI UNS

332.0 A03320 9701080 31.0 (214) 105 T5 1a 2 3 4 5

a1 indicates best of the group; 5 indicates poorest of the group.

R.B. Rostami et al.: Improvements in microstructure and mechanical properties of AlSiCu alloyAl2O3 nanocomposite modified by ZrO2

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method and declared that Al3Zr phase could be formedthrough an exothermic reaction between Al and ZrO2at 1023 K (750 C) based on the reaction formula13Al 1 3ZrO2 5 2Al2O3 (a) 1 3Al3Zr. This coherentprecipitate, which is essentially inert in the matrix, acts ashard pinning points in the matrix inhibiting dislocationmotion leads to affect the strength of the material.23

The main advantage of the precipitate formed is itsstability at elevated temperatures because of the lowsolubility of this precipitate. However, Mohamed andSamuel24 highlight the fact that the intermetallic phase

of Al3Zr is the main feature in the microstructures andmechanical properties of AlSiCuMg alloys in thepresence of Zr, which also leads to the formation of Al3Zr.According to the EDS analysis result, the intermetallicphases observed in microstructure [Fig. 5(a)] can beattributed to Al3Zr. It is possible that greater precipitatingof this compound occurs throughout the matrix byincreasing in zirconia content.

Microstructure examination of nanocomposite surfacesshows that increase in alumina content results in porosity.However, this is not observed in samples with high contentof zirconia. The porosity is an inevitable consequence ofstir-casting method; however, the microscopic observa-tions revealed no porosity in nanocomposites reinforced by1.252 wt% ZrO2. The porosity happened in the samplereinforced by different amounts of reinforcements isshown in Fig. 6. Furthermore, microstructural analysisof composites revealed that agglomeration of aluminaparticulates occurred throughout the samples, Fig. 7(a).However, agglomeration of reinforcements was notobserved in composites reinforced by high content of ZrO2,even uniform distribution of nanoparticulates is noticeablein these samples [Fig. 7(b)].

B. Mechanical properties

1. Hardness

The hardness values of the as-cast and heat-treatedcomposites for different percent of reinforcements areshown in Fig. 8(a). It is observed that the average hardnessvalues improved nearly 10% with the addition of variousamounts of reinforcements in comparison with the basealloy. Furthermore, heat treatment resulted in hardnessimprovement. Heat treatment increases the averagehardness up to 11% compared with the as-cast samples,probably because of diffusion proceeding that leads tobetter bonding of the nanoparticulates to the matrix andprecipitation hardening. The most effect of additionswas observed in hardness value of samples 7 and 9 with13.5% growth in hardness, which indicated that hardnessenhanced with increasing Al2O3 amount. It canattribute to larger hardness of alumina than zirconiabecause the hardness of composites depends on thehardness of the reinforcement and the matrix25;(hardness: alumina 5 22 GPa and zirconia 5 14 GPa26).The comparison of hardness values of sample 1 andsample 9 reveals that the addition of 2 wt% ZrO2 and2 wt% Al2O3 increased hardness up to 5 and 13.5%,respectively, that is, higher hardness of alumina provideshigher hardness.

2. Tensile properties

The influence of weight percentage of reinforcementsand heat treatment on the YS of ANSI 332/Al2O3 1 ZrO2composites is shown in Fig. 8(b).The results indicate that

TABLE III. Designation of the nanocomposite samples.

Sample no. Al2O3 (wt%) ZrO2 (wt%)

1 0 2.02 0.25 1.753 0.5 1.54 0.75 1.255 1.0 1.06 1.25 0.757 1.5 0.58 1.75 0.259 2.0 010 (base alloy) 0 0

FIG. 1. Optical micrograph of as-cast ANSI 332 aluminum alloy.

FIG. 2. Optical micrograph of heat-treated ANSI 332 aluminum alloy.


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the average YS values increases by 8.6% after heattreatment. In the course of cooling, dislocations format the matrix/reinforcement interface owing to the thermalmismatch. Based on Besterci et al.,27 the YS of the Alalloys relates to the interaction between particulate anddislocations by means of the Orowan bowing mechanism.The tangled dislocations around the agglomeration ofthe particulates could contribute a reinforcement effectin the Al alloy matrix. Figure 8(b) shows that increase inthe amount of ZrO2 results in increase of YS values.Results present an increase of 23% YS in compositeover the as-cast condition base alloy and an increase of26% YS in the heat-treated composite over the heat-treatedbase alloy in the composite reinforced with 1.75 wt% ZrO2and 0.25 wt% Al2O3.

The general descending trend in YS values of sampleswith increasing Al2O3 indicates that increase in aluminapercentage has a negative effect on tensile behavior ofsamples. For composites reinforced with 1.75 wt% Al2O3,the tensile test results show that nanocomposites failed atsmallest yield values rather than the base alloy.

The UTS values of as-cast and heat-treated specimens isshown in Fig. 8(c). The UTS follows the same trend as theYS of ANSI 332/Al2O3 1 ZrO2 nanocomposites. There isan increase of 30% UTS in the composite reinforced with2 wt% ZrO2 over the as-cast alloy. There is also anincrease of 36% UTS in the heat-treated composite incomparison with the heat-treated ANSI 332 alloy whenthe metal matrix alloy is reinforced with 1.25 wt% ZrO2and 0.75 wt% Al2O3. Based on the obtained results, the

FIG. 3. Intermetallic phase: (a) Fe-rich phase. (b) EDS spectra of the phase (marked by arrow), which can be attributed to Al8Mg3FeSi6.

FIG. 4. XRD examination of experimental ANSI 332 alloy.





mean of UTS values increased by 12% using the T6 heattreatment for this Al alloy reinforced with alumina andzirconia. Furthermore, it is clearly observed that the T6heat treatment has a better effect on UTS values than theYS values of nanocomposites.

With increasing the amount of alumina, the values ofyield and ultimate strength decreases. The decrease in theyield and ultimate strengths can be due to the formation

of clusters of Al2O3 in the nanocomposites at high weightpercentage of Al2O3. The reason could be the inhomo-geneous distribution of reinforcements and degree ofclustering, which could reduce the effective amount ofpowders for strengthening. Furthermore, the porosityof samples increased when alumina content was higherthan zirconia content. Another reason for this behaviormay involve wettability of alumina nanoparticulates.According to the recent works, below 1000 C, the contactangle between aluminum and Al2O3 is greater than 90,resulting in poor wetting by the liquid metal.28,29 This poorwetting behavior favors clustering of the alumina particles,thereby limiting the degree to which Al2O3 can beincorporated in a liquid melt.

However, in the process of load transfer, the matrixtransfers the load to the nanoparticulates. So if theboundary is assumed to be strong, ceramic particlesprevent plastic deformation of the matrix and this leadsto the strengthening and higher work-hardening rate.30

In the case of Al2O3, weak bonding between the par-ticle and matrix declines the plastic deformation andUTS values.

The influence of the heat treatment on the ductility(measured in terms of tensile elongation) has illustrated inFig. 8(d). The results show that the average values ofelongation declined by around 5% applying the T6 heattreatment on nanocomposites. The general descendingtrend in ductility values of samples with increase in Al2O3weight percentage is shown in Fig. 8(d). It was verifiedfrom the work carried out by Hajizamani and Baharvandi31

that ductility of the composites reinforced with Al2O3decreased with increase in alumina content because ofporosity content. According to the YS and UTS values,it can be concluded that in simultaneous using of Al2O3and ZrO2 nanoparticulate reinforcements in ANSI 332

FIG. 5. (a) SEM micrograph of nanocomposite reinforced by 2 wt% ZrO2. (b) EDS spectra of the phase (marked by arrow), which can beattributed to (Al,Si)3Zr.

FIG. 6. SEM photographs of nanocomposites show porosity inreinforced samples (a) 2 wt% ZrO2, (b) 1.75 wt% Al2O3 and 0.25wt% ZrO2, and (c) 2 wt% Al2O3.





aluminum alloy, the mechanical behavior of nanocom-posites increased with increase in the weight percentageof zirconia.

According to the XRD and EDS analysis of micro-structures, the most intermetallic phases formed in nano-composites are Al2Cu, Al15(FeMn)3Si2, Al8Mg3FeSi6, andAl3Zr. The coherent precipitates such as Al3Zr areundissolved even in elevated temperatures.23

It is possible that volume fraction of the intermetalliccompound rises as the percentage of ZrO2 increases.The comparison in UTS values of composites shows

that the Zr and Al may interact between themselves toform new phases, which lead to increase in the strengthof nanocomposites.

For more study on the effect of ZrO2 on Al2O3rein-forced composite, the nanocomposites reinforced by1 wt% Al2O3 and different contents of ZrO2 including2 and 3 wt% ZrO2 have been investigated to confirm thebeneficial effect of zirconia particles. The influence ofadding zirconia content on tensile properties is shown inFig. 9. Although alumina content is constant, improve-ment in tensile properties can be attributed to increase inweight percentage of zirconia. It is observed from Fig. 9that the addition of 2 and 3 wt% ZrO2 changes UTSfrom 213 MPa to 227 and 248 MPa, respectively, and anincrease of 7 and 15% was observed over the compositereinforced by 1 wt% ZrO2. The YS values increased by3 and 7% showing that increase in zirconia content hada better effect on UTS than YS value.

Another possible mechanism that may play a role inthe improvement of properties with increase in zirconiacontent is the effect of zirconia as the barrier to crackpropagation during the tetragonal to monoclinic trans-formation. Rendtorff et al.32 suggested that tetragonal tomonoclinic phase transformation is a martensitic trans-formation and independent of temperature and atomsdiffusion. This transformation acts as a barrier to crackgrowth and increases resistance of the base alloy. Theexpansion occurred due to the tetragonal to monoclinic

FIG. 7. SEM photograph of nanocomposites (a) agglomeration ofalumina in reinforced 2 wt% Al2O3 composite, (b) uniform distributionof reinforcements in composite reinforced by 1.75 wt% ZrO2 and 0.25wt% Al2O3.

FIG. 8. Mechanical properties of as-cast and heat-treated samples. (a) Hardness values, (b) 0.1% proof stress, (c) UTS, and (d) elongation values.





transformation forms the impacted regions, which hardenscrack propagation. When this mechanism takes place incomposites, a part of energy is consumed by trans-formation during cracking and results in toughening ofthe material.33 Increase in zirconia content leads toincrease in monoclinic zirconia phases; therefore, theresidual stresses are raised by expansion because oftransformation that results in improvement in compositetoughness. The stressstrain diagrams for nanocompositesreinforced by 1 wt% Al2O3 and different contents of ZrO2are shown in Fig. 10. The fracture toughness (KIC) ofmaterials which is required energy for fracture and equalsto the area beneath the stress-strain diagram. Figure 10shows that the area beneath the stressstrain diagrams(the toughness fracture of nanocomposites) increases byincrease in the zirconia content.

3. Fracture behavior

The ductile to brittle fracture at macroscale fracturesurface examinations was observed. Tensile fracture sur-faces of heat-treated samples of numbers 2, 4, and 6 areshown in Figs. 11(a)11(c).

It is observed from SEM photographs that increasein alumina content results in porosity and brittlebehavior of fracture mode. Generally, porosities andvoids have an important role in brittle manner. Thefracture of metal matrix composites occurs in one orcombined condition of these mechanisms: reinforcementfracture,34 matrix/reinforcement interfacial decohesion,35

and failure in the matrix.36 Results show that fracturebehavior in this research followed by the third mecha-nism. Reinforcement nanoparticles are pushed toward bysolid/liquid interface during solidification and are localizedin interdendritic regions or the last solidified zones.

Figures 11(a) and 11(b) show the dendritic structuresin the fracture surface. These structures are observed invast areas of fracture surfaces. It shows that the mainmechanism of failure is interdendritic cracking. Thisfailure mode is identical for the unreinforced ANSI 332alloy that has been recently investigated.37 During solid-ification of the composite, the nanoparticles and alloyingelements (mainly Si) are rejected to the solidliquidinterface and segregate to the interdendritic regions.34

The microcracks propagate along interdendritic aluminumsilicon eutectic during the fracture and failure of thespecimens occurs. It indicates that fracture behavior iscontrolled by fracture of matrix because of aggregation ofnanoparticles in silicon eutectic.

Dimples that are observed in some areas of fracturesurfaces, shown in Fig. 11(c), may be a result of the voidnucleation at eutectic silicon particles and subsequent

FIG. 9. UTS and YS values of composites reinforced by 1 wt% Al2O3and different contents of ZrO2.

FIG. 10. Stressstrain diagrams for nanocomposites reinforced by1 wt% Al2O3 and different contents of ZrO2.

FIG. 11. SEM micrograph of fracture surface of heat-treated tensilesamples: (a) 0.25 wt% Al2O3, (b) 0.75 wt% Al2O3, (c) 1.25 wt% Al2O3.





coalescence by shear deformation and fracture process onthe shear plane.38 It is reported in the researches that allthe fracture surfaces of the composites consist mainly ofdimples in the matrix and fragmentation and decohesionof the particles from the matrix. The fracture and decohe-sion of the particles can be explained by work hardeningand the fragmentation of the ceramic phase caused by highstress concentration.37,39

Tensile fracture surfaces of nanocomposites reinforcedby 1 wt% Al2O3 and different contents of ZrO2 including1, 2, and 3 wt% are shown in Fig. 12(a)12(c), respec-tively. These figures clearly show that the brittle behaviorof fracture enhanced with increase in zirconia content.The glossy broken planes indicate cleavage fracture ofsamples. This type of fracture occurs by rapid crackpropagation and without appreciable macroscopic defor-mation. It is clearly observed that the dimple structurechanges to complete brittle manner by increasing thezirconia content.

It is reported by Zhang et al.40 that the fracture modeturns to the combination of transgranular failure andintergranular failure in aluminum composite with theaddition of ZrO2 because of stronger bonding of grains,which resulted in improvement in fracture toughness ofcomposite.

IV. CONCLUSION

The characteristics and mechanical behavior of ANSI332 matrix composite reinforced with nanosized alumina(Al2O3) and zirconia (ZrO2) were investigated. Thefollowing conclusions can be drawn:

(1) Replacing zirconia with alumina in constant totalwt% reinforcement leads to improvement in tensile

properties. It can be attributed to the formation of hardintermetallic phases such as (Al,Si)3Zr, enhancing inwettability Al2O3 particulates and reducing in porosity,which leads to enhancing UTS and YS.

(2) The highest tensile strength achieved in thespecimen containing 1.25 wt% ZrO2 and 0.75 wt%Al2O3 can be attributed to increase in zirconia content,which play a remarkable role in improving the micro-structure of composite.

(3) It has been found that the maximum value ofhardness is acquired at maximum values of Al2O3.

(4) The sensitivity of UTS to heat treatment is morethan YS.

(5) It is observed from SEM photographs that increasein alumina content converts the fracture behavior fromrather ductile, with fine ductile dimples to brittle, withcleavage facets.

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Improvements in Microstructure and Mechanical Properties