Materials Science and Engineering A 380 (2004) 378–383

Study on bulk aluminum matrix nano-composite fabricated by ultrasonicdispersion of nano-sized SiC particles in molten aluminum alloy

Yong Yang, Jie Lan, Xiaochun Li∗

Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA

Received 4 March 2004; received in revised form 31 March 2004

Abstract

Lightweight metal matrix nano-composites (MMNCs) (metal matrix with nano-sized ceramic particles) can be of significance for automobile,aerospace and numerous other applications. It would be advantageous to produce low-cost as-cast bulk lightweight components of MMNCs.However, it is extremely difficult to disperse nano-sized ceramic particles uniformly in molten metal. This paper presents a new methodfor an inexpensive fabrication of bulk lightweight MMNCs with reproducible microstructures and superior properties by use of ultrasonicnonlinear effects, namely transient cavitation and acoustic streaming, to achieve uniform dispersion of nano-sized SiC particles in moltenaluminum alloy A356. Microstructural study was carried out with an optical microscope, SEM, EDS mapping, and XPS. It validates a gooddispersion of nano-sized SiC in metal matrix. It also indicates that partial oxidation of SiC nanopartilces results in the formation of SiO2 inthe matrix. Mechanical properties of the as-cast MMNCs have been improved significantly even with a low weight fraction of nano-sizedSiC. The ultrasonic fabrication methodology is promising to produce a wide range of other MMNCs.© 2004 Elsevier B.V. All rights reserved.

Keywords: Nano-composite; Ultrasonic dispersion; SiC; MMCs

1. Introduction

Metal matrix composites (MMCs) have been extensivelystudied in last two decades[1–15] and are significant fornumerous applications in the aerospace, automobile, andmilitary industries. MMC consists of a metallic base with areinforcing constituent, usually ceramic. The attractive phys-ical and mechanical properties that can be obtained withMMCs include high specific modulus, superior strength,long fatigue life, and improved thermal stability.

Normally, micro-ceramic particles are used to improvethe yield and ultimate strength of the metal. However, theductility of the MMCs deteriorates with high ceramic par-ticle concentration[14]. It is of interest to use nano-sizedceramic particles to strengthen the metal matrix, so-calledmetal matrix nano-composite (MMNC), while maintaininggood ductility [14,15]. With nanoparticles reinforcement,especially high temperature creep resistance and better fa-tigue life could be achieved. Currently, there are severalfabrication methods of MMNCs, including mechanical

∗ Corresponding author. Tel.:+1 608 262 6142; fax:+1 608 265 2316.E-mail address: xcli@engr.wisc.edu (X. Li).

alloying with high energy milling[3], ball milling [5],nano-sintering[7], vortex process[14], spray deposition,electrical plating, sol–gel synthesis, laser deposition, etc.For mechanical alloying, it normally involves mechanicalmixing (e.g. high energy ball milling) of metallic and ce-ramic powders[1] or different metallic powders[18] forthe fabrication of bulk MMNCs. The mixing of nano-sizedceramic particles is lengthy, expensive, and energy con-suming. With the other fabrication methods, generally thinfilms or plates are deposited.

Casting, as a liquid phase process, is capable of produc-ing products with complex shapes. It will be attractive toproduce as-cast lightweight bulk components of MMNCswith uniform reinforcement distribution and structural in-tegrity. However, nano-sized ceramic particles present dif-ficult problems. It is extremely difficult to obtain uniformdispersion of nano-sized ceramic particles in liquid metalsdue to high viscosity, poor wettability in the metal matrix,and a large surface-to-volume ratio. These problems induceagglomeration and clustering, as shown inFig. 1 (originalwork with mechanical mixing by the authors at Universityof Wisconsin–Madison).

High-intensity ultrasonic waves (e.g. above 25 W/cm2)could be especially useful due to their capability to generate

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2004.03.073

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Fig. 1. Nanoparticle cluster in aluminum alloy A356.

important non-linear effects in liquids, namely, transientcavitation and acoustic streaming[14–17], which are mostlyresponsible for refining microstructures, degassing of liq-uid metals for reduced porosity, and dispersive effects forhomogenizing. Acoustic cavitation involves the formation,growth, pulsating, and collapsing of micro-bubbles in liquidsunder cyclic high intensity ultrasonic waves (thousands ofmicro-bubbles will be formed, expanding during the negativepressure cycle and collapsing during the positive pressurecycle). By the end of one cavitation cycle (about the order of100 ms), the mciro-bubbles implosively collapse producingtransient (in the order of microseconds) micro “hot spots”that can reach very high temperatures, and pressures of about1000 atm, and heating and cooling rates above 1010 K/s[19]. Transient cavitations could produce an implosive im-pact strong enough to break up the clustered fine particlesand disperse them more uniformly in liquids. The strongimpact coupled with locally high temperatures in a veryshort time could also enhance the wettability between metalmelts and particles, thus making the preparation of as-castcomposites with micro-particles (e.g. 10–50�m) possible[21–23].

It is envisioned that strong ultrasonic nonlinear effectsmight efficiently disperse nanoparticles (less than 100 nm)into alloy melts and also enhance their wettability, thus mak-ing the production of as-cast high performance lightweightMMNCs feasible. It will be significant to carry out a funda-mental study to explore the feasibility.

2. Experiment setup and procedure

2.1. Materials for experiments

Aluminum alloy A356 was selected as matrix because itis readily castable and widely studied and used. The chem-ical composition of the A356 alloy is shown inTable 1. Itshould be noted that a high content of Si could constrain thereaction between the SiC and Al in the alloy. The nano-sizedceramic particles used in this study were�-SiC (spherical

Table 1Nominal chemical composition of matrix alloy A356 (in wt.%)

Si 6.5–7.5Fe 0.20Cu 0.20Mn 0.10Mg 0.25–0.45Zn 0.10Ti 0.20Al Balance

shape, average diameter≤ 30 nm, composition: SiC≥ 95%,[O]: 1–1.5%, [C]: 1–2%). SiC is selected because its den-sity is very close to Al, there will be no segregation duringprocessing, and the big difference of the thermal expansioncoefficients between the SiC and the Al.

2.2. Experiment setup

As shown inFig. 2, the experimental setup consists ofprocess and control parts. An electric resistance-heating unitwas used to melt the A356 in a small graphite crucible withsize 50 mm diameter by 50 mm height. A titanium waveg-uide, which was coupled with a 20 kHz, 600 W ultrasonicconverter (Misonix), was dipped into the melt for ultrasonicprocessing. Nano-sized SiC particles were added into meltsduring the process from the top of crucible. The aluminummelt pool was protected by Argon gas. One temperatureprobe was used to monitor the processing temperature.

2.3. Experiment procedure

The ultrasonic processing temperature was controlled to∼100◦C above the alloy melting point (610◦C). An ultra-sonic power of 80 W from the converter was found to gen-erate adequate processing function inside the crucible. Agraphite permanent mold with “U” shape and molybdenumfilter (∼1.0 mm2 mesh), which can filter out the large oxi-dation films, were used to cast bulk MMNCs rods that willbe machined into tensile test specimen. As observed dur-ing the processing, the viscosity of the melts significantlyincreased with nano-sized SiC particles in the melts. Thus,

Fig. 2. Schematic of experiment setup.

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after efficient ultrasonic processing, a higher casting tem-perature of 760◦C was used to ensure the flowability insidethe graphite mold.

Aluminum matrix nano-composites with various weightpercentages of nano-sized SiC were fabricated, including0.5, 1.0 and 2.0 wt.%. For comparison, samples of 0 wt.%SiC were prepared with and without ultrasonic processing,separately.

For microstructural study, samples of as-cast bulk MM-NCs were mounted, mechanically polished down to 0.1�m,and etched by Keller’s reagent (2 ml HF (48%), 3 ml HCl(conc.), 5 ml HNO3 (conc.) and 190 ml water). The opti-cal images were inspected with aNikon eclipse ME600.Scanning electron microscopy (SEM) images and energydispersion spectrum (EDS) mapping were obtained witha LEO1530. X-ray photoelectron spectroscopy (XPS) ma-chine,PHI 5400 ESCA/XPS, was used to analyze materialcomposition.

To measure the mechanical properties of the as-cast bulkMMNCs, specimens with a diameter of 6.35 mm (0.25 in.)and a gage length of 25.4 mm (1.0 in.) were prepared ac-cording to the standard of ASTM E8. Tensile testing wasperformed on aSintech 10/GL without extension meter.

3. Experiment results

During the experiment, when SiC particles were addedinto the molten alloys they tended to float on the surface ofthe melt, even though SiC has a slightly larger specific den-sity than that of the molten aluminum alloy. Possible causesmight be the high surface tension of the melt and the poorwetting between the particles and the melt. By applying highintensity ultrasonic waves the acoustic streaming trapped thenanoparticles into the melt efficiently.

3.1. Microstructural study

3.1.1. Optical microstructureFig. 3ashows the microstructures of samples of “pure”

alloy without ultrasonic processing. Dendritic grains are

Fig. 3. Microstructures of as-cast A356 alloy: (a) 0 wt.% SiC without ultrasonic; (b) 2.0 wt.% SiC with ultrasonic.

clearly revealed.Fig. 3b shows the microstructures of thecast aluminum alloy samples with 2.0 wt.% nano-sized SiCparticles with ultrasonic processing. The grain sizes fromthe samples with nanoparticles and ultrasonic processing aremuch smaller. Further study with high resolution SEM isneeded to verify the nanoparticle dispersion.

3.1.2. SEMCast samples after a 1.5 hours processing with an ul-

trasonic power of 80 W were examined with the highresolution (up to 10 nm) SEM (LEO1530), as shown inFig. 4. Nano-sized SiC particles (2.0 wt.%) were well dis-persed in the A356 matrix, although some small clustersremain in the microstructure, as shown inFig. 4a. Tinyscratches/cracks due to polishing are displayed. Singlenanopartilces (∼30 nm) were dispersed in the matrix, asshown inFig. 4b. It is believed that high intensity ultrasonicwaves generated strong cavitation and acoustic streamingeffects. Transient cavitations could have produced an implo-sive impact strong enough to break up the clustered particlesand disperse them more uniformly in the liquid. Moreover,acoustic streaming (a liquid melt flow due to an acousticpressure gradient) is very effective for stirring[20,21,23].Thus, a better understanding of these non-linear effects inmelts with nanoparticles will be essential to optimize theprocess parameters to further disintegrate the small clustersshown in the SEM pictures.

In comparison, conventional mechanical stirring couldalso mix small particles into the melt. However, when thestirring stopped, the particles tended to return to the sur-face and most of these particles still stick to each other asclusters[24], also shown previously inFig. 1. This resultcould be caused by the nanoparticles being surrounded bysurface gas layers. Obviously, the phenomenon of ultrasonicdispersion of nanoparticles is fundamentally different fromconventional mechanical stirring. The key difference lies inacoustic transient cavitation, which induces a violent col-lapse of micro gas bubbles around the nanoparticle clusters,thus breaking the clusters and dispersing the nanoparticlesinto the matrix. Moreover, the transient cavitation could re-move the gas layer from the surface of the nanoparticles,

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Fig. 4. SEM image of nano-composite: (a) nano-sized SiC dispersion in Al matrix; (b) higher magnification of nanoparticle dispersion.

improving the wettability between the nanoparticles and ma-trix significantly.

3.1.3. EDSIn order to verify the composition of the nano-composite,

EDS analysis was used. The typical result is shown inFig. 5.

Fig. 5. Spectrum of nano-composite EDS.

Fig. 6. Element distribution from EDS mapping.

It seems that the composite was protected well during fab-rication since the oxidation level is quite low. Since the av-erage size of the nanoparticles is less than 30 nm, it is verydifficult to use EDS spot analysis due to the limitation of thee-beam resolution in this instrument. Therefore, mappingscanning was employed.Fig. 6shows the distribution of the

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elements aluminum (Al), carbon (C), and silicon (Si), re-spectively. The results show that C is distributed uniformly,which probably indicates a good dispersion of SiC nanopar-ticles in matrix. From the mapping of Si element, there aresome concentrations from the eutectic Si of the alloy.

3.1.4. XPS analysisXPS analysis was conducted in our experiment to study

the oxidation of SiC nanoparticles. Surface analysis by XPSinvolves irradiating a solid in vacuum with monoenergeticsoft X-rays and analyzing the emitted electrons by energy.The spectrum is obtained as plot of the number of detectedelectrons per energy versus their binding energy. For eachelement in different compounds, XPS can show the differentpeaks corresponding to the element in different compounds.XPS conditions were: vacuum level (5× 10−9 Torr), sourceof X-ray (Al source), wattage (15 kV), and photon energy(1486.6 eV).

In order to eliminate the surface oxidation of samples dur-ing sample preparation, sputtering was conducted to deletethe surface layers of samples. The results of XPS are shownin Fig. 7. According to the results for Si element, only eu-tectic silicon was found in pure A356 while eutectic silicon,silica and SiC co-exist in the processed A356 with 2.0 wt.%SiC. It suggests that the SiC nanoparticles be partly oxi-dized during the process. The amount of oxidized particlesis less than 30% in terms of the peak heights and sensi-tive factors of SiC and SiO2. From the peaks for oxygenelement, it was found that only one wide peak existed inboth pure and MMNC samples. Due to the very small dif-ference of standard energies between pure oxygen and ox-ides (531 eV for pure oxygen, 529.8–531.7 eV for Al2O3,and 532.5–533.3 eV for SiO2), it is very possible that peakscorresponding to oxides and pure oxygen merge to form awider peak. Combined with the results for Si element, it isreasonable deduced that only Al2O3 and pure oxygen ex-ist in A356 while SiO2, Al2O3 and pure oxygen co-exist inA356 + 2% SiC. Because of the partly oxidation of SiC,more oxides exist in A356+ 2% SiC and the oxygen peakin A356 + 2% SiC is higher than that in pure A356.

3.2. Mechanical properties

The tensile testing results are shown inFig. 8. With only2.0 wt.% nano-sized SiC, the yield strength of as-cast alu-minum alloy A356 was improved approximately 50%, whichis significantly better than what aluminum alloy with thesame percentage of micro-particle reinforcement can offer.It should be noted that there is little change in the elonga-tion and ultimate tensile strength. It is expected that if theprocess physics is better understood and process parame-ters optimized, the dispersion and mechanical properties ofMMNCs will be further improved.

The strengthening mechanism for MMNCs has been stud-ied and one of the major attributes is the higher dislocationdensity in MMNCs[10,12]. The difference of the thermal

Fig. 7. XPS results for A356 and A356 with 2% nano-sized SiC: (a) Sipeak for A356; (b) Si peak for A356+ 2% SiC; (c) oxygen peak forA356; (d) oxygen peak for A356+ 2% SiC.

expansion coefficients between the matrix and the uniformlydispersed nano-SiC could induce high dislocation density,and these nano-SiC inclusions can work as the barriers fordislocations movement. It is believed that the propertiesof MMNCs would be enhanced considerably even with avery low volume fraction due to the high dislocation den-sity of matrix metal. For future study, a transmission elec-tron microscope (TEM) will be needed to investigate thehypothesized higher dislocation density around the nano-particles.

Y. Yang et al. / Materials Science and Engineering A 380 (2004) 378–383 383

Fig. 8. Tensile stress, yield stress and elongations vs. different wt.%.

4. Conclusion

Bulk Al-based nano-composites with nano-sized SiC werefabricated by an ultrasonic-assisted casting method. The mi-crostructure and mechanical properties were studied. Thenano-sized SiC particles are dispersed well in the matrix andthe yield strength of A356 alloy was improved more than50% with only 2.0 wt.% of nano-sized SC particles. Partialoxidation of SiC nanopartilces resulted in the formation ofSiO2 in the matrix. The study suggests that strong ultra-sonic nonlinear effects could efficiently disperse nanopar-ticles (less than 100 nm) into alloy melts while possiblyenhancing their wettability, thus making the production ofas-cast high performance lightweight MMNCs feasible. Fur-ther study will be needed to optimize the process and TEM

study will be used to improve the understanding of strength-ening mechanism.

Acknowledgements

The authors would like to thank Mr. Jae-Ik Chofrom Foundry Computer Laboratory at University ofWisconsin–Madison for his valuable discussion and techni-cal support.

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