1

Development of high performance lightweight aluminum alloy/SiC hollow sphere syntactic foams and

compressive characterization at quasi-static and high strain rates

Dung D. Luong1, Oliver M. Strbik III

2, Vincent H. Hammond

3, Nikhil Gupta

1,*, and Kyu Cho

3

1Composite Materials and Mechanics Laboratory, Department of Mechanical and Aerospace Engineering,

Polytechnic Institute of New York University, Brooklyn, NY 11201 USA

2Deep Springs Technology, LLC, 4750 W. Bancroft St., Suite 5, Toledo, OH 43615 USA

3U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD

21005 USA

ABSTRACT

Aluminum alloy A356 filled with silicon carbide hollow spheres (SiCHS) is investigated for quasi-static

(10-3

s-1

) and high strain rate (up to 1520 s-1

) compressive properties. Such closed cell composite foams, called

syntactic foams, are of interest in weight sensitive structural applications. The present work is focused on

understanding the compressive failure mechanism and relating them with the material microstructure. The

compressive and plateau strengths of syntactic foams with SiCHS are found to be 163 and 110 MPa, respectively.

The measured properties are considerably higher than the existing fly ash cenosphere filled aluminum matrix

syntactic foams. Compressive failure mechanisms are studied for A356/SiCHS syntactic foams and direct evidence

of hollow sphere crushing at the end of the elastic regions is obtained. The predictions of compressive strength

obtained from an existing model are validated with the experimental results. Extensive analysis of data on open and

closed cell foams containing gas porosity and syntactic foams is presented. A clear advantage in terms of low

density and high yield strength is observed in A356/SiCHS syntactic foams compared to other foams. Yield strength

of aluminum foams may be different at high strain rate compression compared to quasi-static values but most of the

foams do not show strong evidence of strain rate sensitivity within the high strain rate regime.

Keywords: Metal-matrix composites (MMCs) (A); Mechanical properties (C); Porosity (C); Metallography

(D).

* Corresponding author. Ph: +1-718-260 3080, Fax: +1-718-260 3532, Email: ngupta@poly.edu

2

1 INTRODUCTION

Aluminum/SiC is one of the most widely studied aluminum matrix composite (AMC) systems. Aluminum

alloys and AMCs have been used to replace steel in automotive body panels, engine components, and brake rotors to

reduce structural weight, which has resulted in increased interest in these materials. Particles, whiskers, short fibers,

and continuous fibers of SiC have been used as reinforcements in AMCs [1-3]. Typically, the cost of reinforcement

increases with the aspect ratio. Therefore, interest in SiC particles (SiCp) has been very high for strengthening

aluminum alloys [4, 5].

Lightweight composites are attracting a great deal of attention due to the possibility of weight saving in

industrial applications [6]. Automobile weight reduction can directly translate into reduced fuel consumption.

Reduction in the weight of aircraft and marine vessels can lead to increased loading capacity. Motivated by these

applications, open and closed-cell metal foams of different densities, cell sizes, and materials have been studied [7,

8]. Open-cell foams comprise interconnected pores, whereas closed-cell foams have each pore separate. A class of

closed-cell foams synthesized by incorporating hollow particles in the matrix is called syntactic foams. The density

of syntactic foams is higher than that of open and closed-cell foams at the same level of porosity. However, they

also have higher modulus and strength. In structural applications where a certain level of modulus and strength are

required for load bearing capacity, syntactic foams present an option to replace higher density materials. Polymer

and metal matrix syntactic foams show a general trend in which the enhancement in strength or modulus is

associated with an undesired penalty in the foam density [9, 10]. If SiCHS lead to development of syntactic foams

with high mechanical properties at low density, then the weight savings in load bearing structural applications can

be realized.

Numerous reports on a variety of metal matrix syntactic foams can be found in existing literature. A review

article provides details on the synthesis methods and mechanical properties such as modulus, yield strength, and

peak compressive strength observed in syntactic foams [10]. This review shows that most of the existing studies

have used glass or fly ash particles as hollow fillers in aluminum, titanium, and magnesium alloy matrices. Selection

of constituent materials is an important part of designing syntactic foams, where strength, modulus as well as density

can be optimized. A clear advantage is observed in the compressive properties and density of aluminum and

magnesium alloy syntactic foams compared to the titanium matrix syntactic foams. Among available hollow

particles, fly ash cenospheres are widely used as fillers in syntactic foams because of their low cost and abundant

3

availability [11-14]. Cenospheres are not manufactured in an optimized synthesis environment and their structure

and properties vary over a wide range, which is also reflected in the properties of syntactic foams. Therefore, for

high performance applications, it is preferred to use high quality engineered hollow particles in synthesizing

syntactic foams. The benefits of using such particles would be justified in the improved mechanical properties of the

resulting syntactic foams. The present work is aimed at exploring the possibility of using SiC hollow spheres

(SiCHS) for development of high performance syntactic foams.

In this work SiC hollow sphere filled A356 matrix syntactic foams are developed and characterized for

quasi-static and dynamic compressive properties. High strain rate (HSR) properties of a variety of aluminum foams

have been studied by several groups but studies related to syntactic foams are scarce [15]. The mechanical properties

of A356/SiCHS syntactic foams are compared with properties of several existing syntactic foam systems to determine

the benefits of using SiC hollow spheres compared to other types of particles. Literature data are analyzed for a

variety of aluminum foams to understand the trends obtained for strain rate sensitivity.

2 MATERIALS AND METHODS

2.1 Materials

The SiCHS were manufactured by Deep Springs Technology (Toledo, OH). These particles have nominal

diameter and wall thickness of 1 mm and 70 µm, respectively. The SiCHS shape and size can be visualized in Fig. 1a.

A broken particle is shown in Fig. 1b, where the wall thickness of the hollow sphere can be observed. The wall

thickness may vary within a sphere. The densities of SiC and SiCHS are 3150 and 1160 kg/m3, respectively. A356

matrix syntactic foams filled with 60 vol.% SiCHS were synthesized using a pressure infiltration method [16]. The

specimens did not have any measurable porosity in the matrix, which was confirmed by extensive microscopy.

Density of the synthesized syntactic foams was measured to be 1819±14 kg/m3. Size and weight of compression test

specimens were measured and used to calculate the specimen density.

2.2 Quasi-static compression

The quasi-static compression testing was conducted using an Instron 4469 test system equipped with a 50

kN load cell. Bluehill 2.0 software was used for acquisition of load and displacement data. A constant crosshead

displacement rate was maintained during the test to obtain an initial strain rate of 10-3

s-1

. Cylindrical specimens with

4

nominal diameter and length of 10 and 5 mm, respectively, were used for compression testing. To minimize the

effect of friction, 111 Valve Lubricant & Sealant (Dow Corning, Midland, MI) was used at contact surfaces between

the specimen and compression platens.

2.3 High strain rate compression

The HSR compression tests were conducted using a Split-Hopkinson pressure bar (SHPB) setup [17]. In

SHPB testing, the specimen is sandwiched between the incident and transmitter bars. The striker bar is launched to

impact the incident bar, which generates a stress pulse that propagates along the incident bar length. A brass pulse

shaper was used during the testing to obtain a constant rise time and strain plateau in the incident pulse. The

selection of pulse shaper was based on calibration studies. At the incident bar-specimen interface, the incident pulse

splits into the transmitted and reflected parts based on the impedance mismatch between the bar and the specimen

material. Similar phenomenon happens at the specimen-transmitter bar interface. The incident, reflected, and

transmitted pulses were acquired by two strain gauges of type CEA-13-240UZ-120 (Vishay Precision Group,

Melvern, PA) and then recorded by an oscilloscope Tektronix TDS 2014B (Beaverton, OR). The data was processed

using an in-house developed MatLab code based on existing theories for SHPB testing [17].

In this work, incident, transmitted, and striker bars of Inconel alloy were used. The Young’s modulus and

density of Inconel used in the calculations were 195 GPa and 8190 kg/m3, respectively. The elastic wave velocity in

Inconel at room temperature was taken as 4879 m/s. The length and diameter of the incident and transmitter bars

were 1.27 m and 12.7 cm, respectively. In SHPB testing the strain rate is not directly controlled during the test but is

recovered from the test results. Therefore, instead of testing multiple specimens at each strain rate, the results for all

specimens tested under HSR compression are reported. Cylindrical specimens of nominal 10 mm diameter and 5

mm thickness were used for testing.

2.4 Failure analysis

The microstructural observations were taken using a Nikon EPIPHOT 200 microscope fitted with a Nikon

DS-Fi1 digital camera and the images were acquired on the computer by NIS Elements 3.0 software. Standard

metallography procedures were applied for preparing specimens for optical microscopy.

5

A Nikon D7000 digital SLR camera with AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED telephoto lens was

used for image acquisition during quasi-static compression. This macro lens is capable of taking high resolution

images of small objects with high depth of focus. In an optical microscope the depth of focus is small, which is

overcome in this digital camera arrangement at low magnifications.

Hitachi S-3400N scanning electron microscope (SEM) was used for surface imaging of specimens before

and after failure. The SEM is equipped with secondary electron (SE) and backscatter electron (BSE) detectors. The

SEM Specimens were sputter coated with gold before microscopy. An EDAX energy dispersive spectroscope (EDS)

is mounted on the microscope and the spectra are acquired and labeled using Gnensis software.

3 RESULTS

3.1 Microstructure

The microstructure of as-fabricated A356/SiCHS composite is shown in Fig. 2a and b at two different

magnifications. These optical micrographs show high survival rate of SiCHS in the matrix after the pressure

infiltration process. All the SiCHS visible in these figures are intact and are not infiltrated by the matrix. Fig. 2b

shows some closely spaced particles. A thin layer of matrix alloy can be clearly observed in the interparticle region

of closely spaced hollow spheres, which indicates good wetting at the matrix-sphere interface. Fig. 2c and d provide

further magnified views of the matrix-sphere interface. Intimate contact between matrix and particles at the interface

is observed in this figure, which can provide mechanical interlocking and result in high interfacial strength.

Previous studies have analyzed the interface in Al/SiCp composites [18, 19]. In these studies, a major

concern is the possible formation of Al4C3 at the interface. This brittle phase can lead to poor interfacial stress

transfer and is detrimental to the quality of the composite. Previous experimental results showed that in A356 alloy,

there is sufficient Si content to avoid formation of Al4C3 at the interface under appropriate processing conditions.

Although Al4C3 was not detected in the as-cast composites, heat treatment or prolonged exposure at high

temperature can result in the formation of Al4C3 [18]. Heat treatment conducted at 710C for 2 h led to Al4C3

formation [18]. Results of EDS analysis are shown in Fig. 3 for the as-cast A356/SiCHS specimens. The analysis on

the spheres showed dominant peaks for silicon and carbon and the analysis on the matrix prominently showed peaks

for aluminum and silicon. Oxygen was detected in both spectra. Carbon was not detected in the matrix, which

indicates that aluminum carbides have not formed in the matrix.

6

Fig. 2 shows that porosity is present inside the hollow spheres in the syntactic foams. The density of SiC

material (SiC ) and true particle density of hollow spheres (

HS ) can be used to calculate the porosity in syntactic

foams. The ratio of inner to outer radius of hollow spheres is defined as radius ratio (oi rr ). The radius ratio

relates to hollow sphere wall thickness as 1orw . The volume fraction of porosity in a hollow sphere can be

estimated as 3HS. Since the total hollow sphere volume fraction in the syntactic foam () is 0.6, the total

porosity in the syntactic foam slab is calculated as 36.0 v=0.38. The calculation shows that the syntactic

foams contain 38 vol.% porosity. This porosity will be available for compaction during compressive loading.

3.2 Compressive properties

3.2.1 Quasi-static compression

Quasi-static compressive stress-strain curves of A356/SiCHS syntactic foams are presented in Fig. 4. The

characteristics of these curves are similar to those previously observed for other metal and polymer matrix syntactic

foams [20-22]. The beneficial characteristic observed in the compressive behavior of syntactic foams is large

deformation strain, which is helpful in obtaining a high level of energy absorption in these materials. The quasi-

static compressive properties measured on five specimens of syntactic foams are presented in Table 1. In this table,

the compressive strength is defined as the peak stress at the end of the initial linear elastic region and plateau stress

is defined as the stress level after the initial drop. The stress plateau is not very stable in these specimens and an

increasing trend with small slope is seen. The reported value is calculated as the average from the start of the plateau

to the densification strain. The densification strain is defined as the strain in the plastic region where stress exceeds

the compressive strength value. The average values of compressive strength, plateau stress, and densification strain

are 163 MPa, 110 MPa and 0.46, respectively.

3.2.2 Dynamic compression

A representative set of incident and transmitter bar strain signals obtained from SHPB for the testing of

A356/SiCHS syntactic foams is shown in Fig. 5a. The reflected and transmitted strains are combined and compared

with incident pulse in Fig. 5b. A close matching between the two lines ensures that the losses are insignificant. This

strain response is converted to stress-strain and strain rate-strain graphs in Fig. 5c by

7

02 ltct rb

(1)

0AtAEt t

(2)

t

dt0

(3)

where A and E are the cross-section area and Young’s modulus of the bar materials, respectively, and cb is the sound

wave velocity in the bar. In addition, l0 and A0 are the initial length and cross-section area of the specimen,

respectively, r(t) and t(t) are the reflected and transmitted axial strain pulses with respect to time, respectively, and

is a time variable used for integration.

The majority of the stress-strain graph in Fig. 5c is obtained under an average high strain rate of 970 s-1

.

The general trend of dynamic stress-strain graphs is similar to quasi-static compression graphs, where a linear elastic

region is followed by drop in stress after the peak strength is reached. It should be noted that the strain in dynamic

testing is limited to the pulse width observed in Fig. 5c, which is dependent on the striker bar length. Therefore,

densification strain is not obtained in dynamic stress-strain graphs. The calculated dynamic mechanical properties

are presented in Table 2. The modulus data should be treated with caution in SHPB testing. Fig. 5c shows that the

strain rate rises sharply in the initial part of the stress-strain curve. This part is avoided during modulus calculation

and only the region that corresponds to constant strain rate is used for calculation. If the constant strain rate region is

not obtained until the peak stress is reached, the modulus cannot be calculated in this kind of testing.

A comparison of quasi-static and dynamic properties in Table 1 and Table 2, respectively, shows that the

compressive properties of A356/SiCHS syntactic foams are not strain rate sensitive. The peak strength value obtained

from the dynamic test is 123.5±3.8 MPa. These results are qualitatively similar to observations on cp-Al/cenosphere

syntactic foams, where no strain rate sensitivity was observed in the HSR range [23]. Several types of open and

closed cell aluminum foams are found to be strain rate insensitive [15, 24]. As the strain rate sensitivity is dependent

on strain rate [25], the present conclusions are limited to the strain rate range used in this work.

3.3 Failure analysis

The microstructure of as-fabricated specimens obtained from the SEM is presented in Fig. 6. Uniformity in

the SiCHS distribution can be seen in this figure. A closer observation shows that cracks exist in some of the SiCHS

8

particles (Fig. 7), which may have occurred during composite fabrication. These micrographs of as-fabricated

materials will be useful in understanding the failure mechanisms.

The failure pattern in syntactic foams was observed using a high resolution digital camera equipped with a

telephoto lens. Fig. 8 illustrates a quasi-static compressive stress-strain diagram of A356/SiCHS syntactic foams. Six

locations corresponding to various optical photographic observations captured in Fig. 9 are identified in this figure.

The specimen before starting the test is shown in Fig. 9a. During the elastic deformation, no SiCHS failure is

observed, as seen in Fig. 9b. As the stress peak is reached, SiCHS failure starts, as captured in the hollow sphere

marked by a circle in Fig. 9c. Some of the particles with pre-existing cracks, seen in the as-fabricated composite in

Fig. 7 may initiate failure. The initial SiCHS failure can be due to (a) the defects such as cracks present in the hollow

sphere walls, (b) relatively thin walled hollow spheres that may have lower strength than other hollow spheres, or

(c) localized stress concentrations resulting from factors such as a group of closely spaced spheres. Cracks in the

loading direction are observed in Fig. 9c as the SiCHS failure pattern. This pattern is similar to the failure of several

other SiCHS particles observed during compression. Fig. 9d shows that the cracks can extend to the matrix material

in the same direction, which is likely due to the strong interfacial bonding of the SiCHS with the matrix. Fig. 9e and

Fig. 9f correspond to plateau regions where progressive densification takes place. These figures show various

locations of SiCHS collapse, localized large matrix deformation, and densification of porosity due to compaction of

material. Shear cracks in the matrix are also observed during this stage, which lead to SiCHS fragmentation. The

hollow sphere crushing and densification in the shear band progressively increases and leads to complete

densification of the specimen.

Specimens that failed under quasi-static loading condition are further analyzed using the SEM. The

micrographs are shown in Fig. 10. The specimens are sectioned in the loading direction using an Isomat diamond

blade saw and the micrographs are taken on the as-cut surface. SiCHS particles are completely crushed in the

specimens at complete densification, as observed in these micrographs. BSE image in Fig. 10b and d clearly show

the hollow sphere and the matrix phases. Complete densification of porosity is observed in these figures due to

extensive plastic deformation of the matrix around debris.

A356/SiCHS specimens subjected to HSR compression are also analyzed for failure features. A lower

magnification image of failed specimen is shown in Fig. 11a. A shear band can be observed in this image, marked

by a rectangle. SiCHS particles along this band have crushed and their cavities are consumed by the compressing

9

material as observed in Fig. 11b. Large scale matrix deformation in this region has resulted in compaction of debris,

without the presence of matrix cracks in the vicinity. It can also be seen in Fig. 11a that spheres that are directly not

on the shear band, Fig. 11c, have developed cracks and fractured under compression as per the mechanism

demonstrated in Fig. 9c. These hollow spheres did not show signs of shearing at this stage. The formation of the first

shear band will depend on the presence of weak spheres along a plane but eventually additional shear bands generate

and all the hollow spheres fracture and densify as the compression reaches densification strain.

4 DISCUSSION

The compressive strength of syntactic foams can be obtained by a recent model [26] and the results can be

validated with the experimental values. The compressive strength of A356/SiCHS syntactic foam ( com ) is

expressed in terms of contributions of the yield strength of A356 matrix alloy (0) and the compressive strength of

hollow spheres (HS):

HScom C 23

0 1 (4)

where C is a constant. In some previous studies on similar materials, the value of C is estimated to be 0.3 [26, 27].

Since the SiCHS comprise a thin shell made of SiC and their inner part is hollow, their effective strength is different

from that of the SiC material. The compressive strength of SiCHS is expressed as [26]:

231 voidSiCHS VC (5)

where (SiC) is the compressive strength of the hollow sphere material and Vvoid is the void volume fraction of the

SiCHS, specified by

SiC

HSSiC

voidV

(6)

The densities of SiC and SiCHS are SiC =3150 kg/m3 and

HS =1160 kg/m3, respectively. The estimates

of SiC can be found in the literature varying from 3.9 to 4.6 GPa. Considering these values of SiC, the estimates for

SiCHS strength range from a minimum of 261.5 to a maximum of 308.4 MPa,. Using equation 4, the lowest and

highest values of com are calculated as 171.7 and 199.8 MPa. The experimental value of com is 163 MPa (Table 1),

which is close to the calculated lower limit. The theoretical estimates are 5.1 and 18.4% higher than the

experimental strength.

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Weight saving potential of syntactic foams is an attractive characteristic that enables their applications in

weight sensitive structures. In recent literature, syntactic foams comprising a number of matrix materials have been

studied, which include pure metals and alloys of magnesium, aluminum, titanium, and iron. The difference in

density of matrix metals ( 7.1Mg , 7.2Al , 5.4Ti g/cm3) is also reflected in the densities of syntactic

foams. Fig. 12a shows yield strength values of metal matrix syntactic foams extracted from various studies† [11, 16,

28-36]. From the references, where the data are represented in the form of figures, the data points are extracted to

the best possible accuracy. The yield strength of syntactic foam is normalized with the matrix yield strength in Fig.

12b. Most papers have not reported the yield strength values for the matrix alloys. Therefore, the base alloy

properties are taken from databases when necessary.

As a general trend, Fig. 12a shows that the yield strength of syntactic foams increases with density. Most of

the aluminum, titanium, and iron matrix syntactic foams confirm to this trend and fall inside a narrow ellipse. The

foams that defy this trend with better yield strength are mainly magnesium alloy and A356 aluminum alloy matrix

syntactic foams. The use of SiC hollow spheres has resulted in the development of aluminum alloy matrix syntactic

foams that match the density and yield strength of existing magnesium matrix syntactic foams in the literature,

which have used low volume fraction of fly ash cenospheres. The trends indicate that the use of engineered SiCHS in

magnesium alloys can result in syntactic foams with lower density and higher mechanical properties than most of

the existing syntactic foams.

The results of strain rate dependent compressive properties obtained in the present study are also analyzed

and compared with the limited number of previous studies on aluminum matrix syntactic foams [20, 23, 34, 37, 38].

High strain rate properties of cp-Al/cenosphere have previously been reported in [23]. Two compositions of

syntactic foams, containing cenospheres of 70 and 65 vol.% of 90 and 150 µm size, respectively, were studied. The

flow stress of foams containing 90 µm cenosphere was 21% higher than the matrix material. However, the foams

containing 150 µm cenospheres had 45% lower flow stress than the cp-Al. It is also noted that the compressive

strength values at high strain rate are higher than the quasi-static values. However, within the high strain rate range

(1400-5000 s-1

), the peak and plateau strengths did not show any variation. Similar trends were observed in another

study on A4032 matrix syntactic foams containing 5 wt.% fly ash cenospheres [34]. No measurable strain rate

† Attempt is made to obtain data from all widely cited studies. Missing any reference is inadvertent.

11

sensitivity was observed up to 2136 s-1

strain rate. The syntactic foam did not have strain rate sensitivity at high

strain rates but had higher strength compared to quasi-static values.

HSR compressive properties of cp-Al (commercially pure Al) and 7075 alloy filled with mullite-silica

microspheres have been reported in a previous work [20]. The cp-Al composites showed a peak strength of 109

MPa. The as-cast and T6 treated 7075 alloy composites showed a peak strength of 199 and 209 MPa, respectively.

At a strain rate of 2300 s-1

, the cp-Al, 7075, and 7075-T6 syntactic foams showed peak strengths of 140, 231, and

248 MPa, respectively. The results of this study corroborate with others that the peak strength values at high strain

rates were higher than those observed under quasi-static compression. However, the observation in other studies that

there was no measurable strain rate sensitivity within the high strain rate range cannot be confirmed for these foams

because this work presented results only at 2300 s-1

.

A series of papers have examined a composite foam containing fly ash cenospheres and closed-cell gas

porosity in A2014 alloy for strain rate sensitivity [37-39]. These composite foams show stress-strain curves similar

to syntactic foams, where a plateau region is observed and the densification strain is as high as 0.7. The compressive

testing is conducted in the strain rate range 10-2

-101 s

-1. Two of these papers present results from the same set of

experiments. The plateau stress increased with the foam density. However, no measurable effect of strain rate is

observed on the properties of the composite in the strain rate range explored in these papers.

In summary, the results from the published studies indicate that there is a difference in the quasi-static and

HSR compressive peak strength and plateau stress values. However, within the high strain rate range, these

properties did not show any difference. The applied theories for SHPB neglect several factors such as specimen

inertia, temperature rise (especially relevant at high compressive strains), and radial dispersion. These factors may

cause the HSR compressive strength to be different than the quasi-static values and explain why no difference is

observed as the strain rate is changed using SHPB technique.

Further comparison is drawn between the reported strength values with respect to compressive strain rate

for open and closed-cell aluminum foams containing gas porosity and syntactic foams in Fig. 13† [25, 37, 38, 40-

53]. It should be noted that the definition of strength may not be the same in all references. The peak strength

observed after the initial linear elastic region is termed as strength in most cases. However, sometimes yield strength

is termed as strength. Hence, there may be some differences in the values based on the definition used, which is not

† Attempt is made to obtain data from all widely cited studies. Missing any reference is inadvertent.

12

always clearly specified in the publications. The strength axis is represented on a log scale in Fig. 13. The strain rate

axis is plotted on normal and log scales in Fig. 13a and b, respectively, to clearly illustrate the trends obtained in the

data at high and quasi-static strain rates. In these figures, a dotted line separates the zones of syntactic foams and gas

porosity foams. The highest strength in gas porosity foams is observed as 31 MPa. It is important to note that most

aluminum matrix syntactic foams have a compressive strength over 100 MPa. These observations are limited to the

data reported in the literature but the numbers may be different for other compositions. The strength increases with

foam density for all types of foams. However, the density based trends are not included in this figure. Only some

foams show strain rate sensitivity as per the data compiled in this figure. A small positive or negative slope may not

conclusively mean strain rate sensitivity because standard deviation in the properties of foams can be considerably

large. It is observed in Fig. 13b that a large number of foams have strengths below 5 MPa. Use of such foams in

structural applications is difficult, unless they are present in the form of sandwich structures. These low strength

foams may be suitable for applications related to energy absorption under compression, vibration damping, and core

materials in sandwich structures.

The specimens used in the present work, and also in most published studies, are in the as-cast condition.

Heat treatments can improve the mechanical properties of these syntactic foams, as previously observed for 7075

alloys [54, 55]. The results of the present study show that the SiCHS can provide high yield strength to syntactic

foams and break the general trend observed between yield strength and density. Such benefits can be carried over to

other syntactic foams, possibly of magnesium alloys, to enhance the performance of those syntactic foams by using

high quality engineered hollow particles.

5 CONCLUSIONS

Aluminum alloy A356/SiC hollow sphere reinforced syntactic foams are studied under quasi-static and

dynamic compression. Mechanical properties and failure mechanisms are analyzed. Clear evidence of hollow sphere

failure at the peak stress is obtained. The failure initiates by the fracture of weak particles; some of the cracks can

propagate to the matrix as well. Shear band formation in the matrix and shearing of SiCHS lead to the major failure

activity. Debris of hollow spheres is compacted in their own cavity and densification is obtained. The compressive

strength and plateau strength are measured as 163 and 110 MPa for composites having 60 vol.% SiCHS. The

composites did not show strain rate sensitivity up to 1520 s-1

strain rate. Analysis of yield strength from previous

13

studies shows that most of the existing data fall along a narrow zone and shows a trend that the yield strength

increases with the composite density. In general, it is desired to have composites with high yield strength without

having to increase the density. A356/SiCHS syntactic foams show a clear advantage over other aluminum matrix

syntactic foams, especially those containing fly ash cenospheres.

The A356/SiCHS syntactic foams studied in the present work and most aluminum matrix syntactic foams

previously studied have not shown strain rate sensitivity in compressive properties (up to 5000 s-1

used in some

experiments). Differences are observed between quasi-static and HSR compressive yield strength values, which may

be related to the difference in the test techniques rather than the fundamental material behavior.

ACKNOWLEDGMENTS

Portions of the research reported in this paper were performed in connection with U.S. Army Research

Laboratory contract W911NF-10-2-0084 with DST and Cooperative Working Agreement W911NF-11-2-0096 with

NYU-Poly. The views and conclusions contained in this presentation are those of the authors and should not be

interpreted as presenting the official policies or position, either expressed or implied, of the ARL or the U.S.

Government unless so designated by other authorized documents. Citation of manufacturers' or trade names does not

constitute an official endorsement or approval of the use notwithstanding any copyright notation hereon.

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18

Figure Captions

Fig. 1. (a) Scanning electron micrograph of a sample of SiC hollow spheres and (b) wall thickness can be

visualized in a broken particle.

Fig. 2. Optical micrographs of A356/SiCHS composites showing (a,b) particle distribution in the composite.

The particles are wetted well with the alloy. Even the closely spaced particles have a layer of matrix between them;

(c,d) wetting of particle with the matrix alloy and mechanical interlocking at the particle-matrix interface.

Fig. 3. EDS analysis on (a) SiC hollow sphere and (b) matrix in the as-fabricated composite.

Fig. 4. Quasi-static compressive stress-strain graphs of A356/SiCHS syntactic foam.

Fig. 5. High strain rate test results from an A356/SiCHS syntactic foam tested at 970 s-1

strain rate (a) strain

pulses obtained from incident and transmitter bars, (b) validation of strain output, and (c) strain rate-strain and

stress-strain graphs.

Fig. 6. Scanning electron micrographs of an as-fabricated A356/SiCHS syntactic foam obtained using (a)

secondary electron and (b) back scattered detector.

Fig. 7. Cracks noticed in one of the hollow spheres.

Fig. 8. Locations on stress-strain diagram for observations presented in Fig. 9.

Fig. 9. Observations of various stages of material failure in A356/SiCHS syntactic foams (a)-(f) correspond

to successive stages of quasi-static compressive failure as represented in Fig. 8. Failure mechanisms marked with 1

and 2 are displayed in sketches included in c and e, respectively.

Fig. 10. Specimens failed and densified under quasi-static compression (a), (c) secondary electron and (b),

(d) back scattered electron SEM images. Particle crushing can be observed without interfacial separation.

Fig. 11. Scanning electron micrograph of A356/SiCHS syntactic foam tested at 1220 s-1

strain rate. (a) A

shear band with hollow sphere failure is observed, (b) close observation of a particle failed along shear band and (c)

failure of a particle away from the shear band; cracks in the loading direction initiate and cause failure of the

particle.

Fig. 12. Comparison of results obtained from the present study with literature data on aluminum,

magnesium, titanium, and iron matrix syntactic foams. The studies use different types of alloys and hollow particles

(only ceramic particle filled syntactic foams are added). The pressure listed for titanium matrix syntactic foams

refers to that used in composite synthesis.

19

Fig. 13. Comparison of strength values for open and closed-cell aluminum foams containing gas porosity

with those of aluminum matrix syntactic foams. The strain rate values are plotted on (a) normal axis and (b) log

axis. In both cases the strength values are plotted on log axis.

20

Table 1. Quasi-static compressive properties of A356/SiCHS syntactic foams.

Specimen

No.

Modulus

(GPa)

Compressive Strength

(MPa)

Plateau Stress

(MPa)

Densification Strain

(mm/mm)

1 2.08 161 107 0.46

2 1.85 152 111 0.48

3 1.71 161 102 0.42

4 2.02 162 105 0.48

5 2.10 181 126 0.45

Average 1.95 163 110 0.46

St. Dev. 0.17 11 9 0.03

21

Table 2. High strain rate properties of A356/SiCHS syntactic foams measured from SHPB testing.

Specimen

No.

Strain Rate

(/s)

Compressive Strength

(MPa)

Modulus

(GPa)

1 940 124.5 4.7

2 970 119.0 3.0

3 1160 125.5 5.2

4 1165 123.0 3.6

5 1220 120.7 5.7

6 1310 119.3 3.0

7 1425 130.1 3.5

8 1520 125.7 5.8

22

(a) (b)

Fig. 1. (a) Scanning electron micrograph of a sample of SiC hollow spheres and (b) wall thickness can be

visualized in a broken particle.

23

(a) (b)

(c) (d)

Fig. 2. Optical micrographs of A356/SiCHS composites showing (a,b) particle distribution in the composite.

The particles are wetted well with the alloy. Even the closely spaced particles have a layer of matrix between

them; (c,d) wetting of particle with the matrix alloy and mechanical interlocking at the particle-matrix

interface.

24

(a) (b)

Fig. 3. EDS analysis on (a) SiC hollow sphere and (b) matrix in the as-fabricated composite.

Energy (keV)1 2 3

Cou

nts

,

1000

1.3

2.5

3.8

6.3

5.0

Energy (keV)1 2 3

Cou

nts

,

1000

0.7

1.4

2.1

3.6

2.8

25

Fig. 4. Quasi-static compressive stress-strain graphs of A356/SiCHS syntactic foam.

0

50

100

150

200

250

300

-0.05 0.15 0.35 0.55

Str

ess

(M

Pa

)

Strain (mm/mm)

26

(a) (b)

(c)

Fig. 5. High strain rate test results from an A356/SiCHS syntactic foam tested at 970 s-1

strain rate (a) strain

pulses obtained from incident and transmitter bars, (b) validation of strain output, and (c) strain rate-strain

and stress-strain graphs.

-1.2

-0.8

-0.4

0

0.4

0.8

250 500 750 1000

Str

ain

(x

10

-3m

m/m

m)

Time (μs)

Incident pulse

Transmitted pulse

Reflected pulse

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250

Str

ain

(x

10

-3m

m/m

m)

Time (μs)

IncidentReflected+Transmitted

0

300

600

900

1200

0

30

60

90

120

0 0.05 0.1 0.15

Str

ain

ra

te (

s-1

)

Str

es

s (

MP

a)

Strain (mm/mm)

Stress Strain rate

27

(a) (b)

Fig. 6. Scanning electron micrographs of an as-fabricated A356/SiCHS syntactic foam obtained using (a)

secondary electron and (b) back scattered detector.

28

Fig. 7. Cracks noticed in one of the hollow spheres.

29

Fig. 8. Locations on stress-strain diagram for observations presented in Fig. 9.

0

40

80

120

160

200

0 0.1 0.2 0.3 0.4 0.5

Str

ess

(M

Pa

)

Strain (mm/mm)

a

b

c

d

e

f

30

(a) (b)

(c)

(d)

(e)

(f)

Fig. 9. Observations of various stages of material failure in A356/SiCHS syntactic foams (a)-(f) correspond to

successive stages of quasi-static compressive failure as represented in Fig. 8. Failure mechanisms marked

with 1 and 2 are displayed in sketches included in c and e, respectively.

Compression

direction

1 1

2 1 2

31

(a) (b)

(c) (d)

Fig. 10. Specimens failed and densified under quasi-static compression (a), (c) secondary electron and (b), (d)

back scattered electron SEM images. Particle crushing can be observed without interfacial separation.

32

(a) (b)

(c)

Fig. 11. Scanning electron micrograph of A356/SiCHS syntactic foam tested at 1220 s-1

strain rate. (a) A shear

band with hollow sphere failure is observed, (b) close observation of a particle failed along shear band and (c)

failure of a particle away from the shear band; cracks in the loading direction initiate and cause failure of the

particle.

33

(a)

(b)

Fig. 12. Comparison of results obtained from the present study with literature data on aluminum,

magnesium, titanium, and iron matrix syntactic foams. The studies use different types of alloys and hollow

particles (only ceramic particle filled syntactic foams are added). The pressure listed for titanium matrix

syntactic foams refers to that used in composite synthesis.

0

50

100

150

200

250

300

0 1 2 3 4 5 6

σys

(MP

a)

Density (g/cc)

Daoud (2009) Zn12Al foam

Daoud (2007) ZC63

Daoud (2009) 4032 foam

Tao (2009) 6082

Sudarshan(2008) A356

Luong(2011) A4032

Rohatgi (2006) A356

Rohatgi (2009) AZ91D

Luong(2011) AZ91D

Xue (2010) Ti (45 MPa)

Xue (2010) Ti (70 MPa)

Xue (2010) Ti (100 MPa)

Xue (2010) Ti (150 MPa)

Xue (2010) Ti (200 MPa)

Peroni (2012) Fe 99.7%

Present work

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6

σy

s/ σ

ym

Density (g/cc)

Daoud (2007) ZC63

Tao (2009) 6082

Sudarshan(2008) A356

Luong(2011) A4032

Rohatgi (2006) A356

Rohatgi (2009) AZ91D

Luong(2011) AZ91D

Xue (2010) Ti (45 MPa)

Xue (2010) Ti (70 MPa)

Xue (2010) Ti (100 MPa)

Xue (2010) Ti (150 MPa)

Xue (2010) Ti (200 MPa)

Peroni (2012) Fe 99.7%

Present work

34

(a)

(b)

Fig. 13. Comparison of strength values for open and closed-cell aluminum foams containing gas porosity with

those of aluminum matrix syntactic foams. The strain rate values are plotted on (a) normal axis and (b) log

axis. In both cases the strength values are plotted on log axis.

0.1

1

10

100

1000

0 1000 2000 3000 4000 5000 6000

σs

tre

ng

th(M

Pa

)

Strain Rate (s-1)

Deshpande (1999) Duocel Deshpande (1999) Alulight Mukai (1999) ALPORASDannemann (2000) ALPORAS Hall (2000) AlSi7 Kanahashi (2000) SG91 APaul (2000) ALPORAS Yi (2001) Al open-cell foam Miyochi(2002)ALPORASRuan (2002) CYMAT Balch (2005) cp-Al Balch (2005) A7075-OBalch (2005) A7075-T6 Han (2005) cp-Al Mukai(2005)ALPORASLee (2006) Duocel Wang (2006) Al foam Dou (2007) cp-AlKang (2007) Al Cady (2008) ALPORAS Edwin Raj (2009) AlShen (2010) ALPORAS Luong (2011) A4032 Wang (2011) Al foamsPresent work

Syntactic foams

Open and closed cell foams

0.1

1

10

100

1000

0.00001 0.001 0.1 10 1000 100000

σs

tre

ng

th(M

Pa

)

Strain Rate (s-1)

Deshpande (1999) Duocel Deshpande (1999) Alulight Mukai (1999) ALPORASDannemann (2000) ALPORAS Hall (2000) AlSi7 Kanahashi (2000) SG91 APaul (2000) ALPORAS Yi (2001) Al open-cell foam Ruan (2002) CYMATMiyochi(2002)ALPORAS Balch (2005) cp-Al Balch (2005) A7075-OBalch (2005) A7075-T6 Han (2005) cp-Al Mukai(2005)ALPORASLee (2006) Duocel Wang (2006) Al foam Dou (2007) cp-AlKang (2007) Al Cady (2008) ALPORAS Edwin Raj (2009) AlShen (2010) ALPORAS Luong (2011) A4032 Wang (2011) Al foamsPresent work

Syntactic foams

Open and closed cell foams