Materials Science and Engineering A323 (2002) 138– 147
www.elsevier.com/locate/msea
Ultrasonic characterization of cellular metal structures
Douglas T. Queheillalt *, David J. Sypeck, Haydn N.G. Wadley
Department of Materials Science and Engineering, School of Engineering and Applied Science, Uni6ersity of Virginia, Charlottes6ille,
VA 22904 -4745 USA
Received 13 February 2001; received in revised form 8 March 2001
Abstract
A new class of cellular materials synthesized by partially consolidating hollow metal powders has been gaining interest for
multi-functional applications (where load support combined with other functionalities such as acoustic damping, thermal
insulation or energy storage is required). These functionalities depend upon the volume fraction of porosity, type (open/closed)
and pore size. The independent controllable pore volume fraction, pore size and fraction of open and closed porosity of these
structures offers the possibility of tailoring a structures properties to specific applications. Here, the elastic stiffness and acoustic
attenuation of highly porous structures made from hollow gas atomized superalloy spheres have been measured. The Young’s and
elastic shear moduli were deduced from ultrasonic wave velocities while the acoustic attenuation was evaluated from the temporal
decay of laser induced standing acoustic waves. The elastic stiffness of these structures varied with the pore volume fraction and
the acoustic attenuation scaled with the pore volume fraction and size. The unique attributes of these hollow powder cellular
structures offer the potential to tailor their properties for individual applications thereby exploiting the multi-functional nature of
these structures. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ultrasonic; Elastic shear modulus; Frequency
1. Introduction
Numerous methods have been developed for the
synthesis of cellular metal foams [1,2]. In open cell
metal foams the material is contained within cell ligaments and plateau borders, whereas in closed cell structures each cell is sealed from its neighbor with the metal
distributed in cell faces, the intersections of cell faces
and plateau borders. The properties of cellular structures depend on those of the metal, the structures
relative density, and the spatial distribution of metal
within the cellular solid [3 – 5]. Closed cell metal foams
possess high modulus and strength characteristics, high
impact energy absorbing characteristics [6,7], very low
thermal conductivities [8] and excellent acoustic damping characteristics [9,10], when compared with the metal
from which they are made. Open cell foams are not as
stiff or as strong, but they possess characteristics which
can be exploited in multifunctional load supporting and
* Corresponding author.
E-mail address: [email protected] (D.T. Queheillalt).
heat dissipation applications because of the ability to
flow fluids readily through the heated structure [11,12].
Some also have a high surface area to volume ratio and
can be used as high temperature supports for catalysts
and electrodes in electrochemical cells [13 –15]. Cellular
metal structures made by compacting hollow metal
powders contain both types of pores [16 –21]. Schematic illustrations are shown in Fig. 1.
In hollow powder cellular solids, the closed cell fraction can be controllably varied by selection of the
particle relative density. The open cell fraction is established by interstitial voids, which is controlled by the
degree of consolidation. In principle, the volume fraction of open celled porosity can approach the maximum interstitial void space for a random packing of
uniform sized spheres. The volume fraction of closed
cell porosity is dependent on the ratio of the shell wall
thickness to the particle radius. These structures have
significant potential for multifunctional applications
where a combination of impact energy absorption,
acoustic attenuation, thermal insulation or heat dissipation is required in addition to structural load support.
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 5 7 - 0
D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
This arises from the fact that closed cell enhance thermal insulation characteristics whereas, open cell promote enhanced heat dissipation. The ability to vary the
sphere sizes and relative densities throughout a structure leads to functionally graded materials.
Several processes have been developed to manufacture hollow metal spheres and powders [22]. Hollow
spheres made from plastic, metal and glass materials
have been produced by flowing an annular fluid jet
through a coaxial nozzle with a pressurized gas in the
inner tube. As the molten tube is extruded from the
nozzle, the jet may break up either because of the
Raleigh instability [23– 26] or induced acoustic vibrations [27,28]. These molten, hollow droplets are allowed
to spherodize and solidify. In an alternative method, a
metal, ceramic or glass slurry powder with a polymer
additive can be blown through a coaxial nozzle and a
gas pressurized inner nozzle can be used to blow bubbles within this plasticized annular jet [29– 31]. The
hollow spheres are hardened during free fall. A subsequent thermal decomposition treatment burns out the
polymer binder and sinters the solid spheres.
139
A recent approach utilizes small styrofoam spheres,
which can be spray coated with a fine metallic powder
and polymer binder, allowed to dry and subsequently
pyrolized resulting in hollow metallic shells [32]. This
process has been used to produce a wide variety of
hollow metallic shells [33,34]. Although the production
of Fe and Ti based hollow spheres has been demonstrated by this method, it shows little promise for
production of lower melting point alloys such as Al and
Mg alloys into hollow metal spheres.
Hollow powders are a common by-product of inert
gas atomization of metal alloys for powder metallurgy
processes. It has been suggested that during gas atomization, the dynamics may lead to the stable formation
of a hollow bag like droplet and eventual hollow sphere
formation of the molten metal droplet entrapping inert
gas inside the powder [35]. Generally, these hollow
powders are undesirable for powder metallurgy applications and often discarded or remelted. However, a
recently developed technique has been used to separate
gas atomized powders by size and particle density.
Here, cylindrical structures have been synthesized by
hot isostatic pressing, thus partially consolidating them
into hollow powder cellular solids and a laser ultrasonic
technique (pulsed laser generation and interferometric
detection) used to evaluate their elastic and acoustic
attenuation properties.
2. Sample preparation
Fig. 1. Schematic illustration of an (a) open cell reticulated foam, (b)
closed cell foam and (c) a hollow powder structure.
An Inconel® alloy 625 (Ni–21.3Cr–8.8Mo–3.9Nb–
0.13A1– 0.l9Ti, wt.%) was chosen for consolidation of
the hollow powder structures. The gas atomized powder was produced by Crucible Materials (Pittsburg,
PA). Inconel® alloy 625 is a solid solution, matrix
stiffened alloy with a face centered cubic (g-phase)
crystal structure which is microstructurally stable up to
650°C. This alloy is typically used for high temperature
applications where corrosion and pitting resistance is
required and was chosen because of its high resistance
to temperature dependent softening of its elastic properties. Nominal properties for the fully dense isotropic
polycrystalline alloy at room temperature are: Young’s
modulus, Es = 208 GPa, shear modulus, Gs =81 GPa,
Poisson’s ratio, w= 0.28 and a density of zs =8.44 g
cm − 3.
The as-received size distribution of the argon atomized powder was − 10/ + 45 mesh, which corresponds
to a diameter range of 355 mm to 2 mm. The as received
powder contained both solid and porous powder along
with some flake and often contained small satellite
particles attached to them. Predominantly spherical
powders were separated by agitating an inclined surface, which allowed the spherical powders to roll and
be collected. These separated spherical powders were
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D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
lar structures. Additional information on the consolidation process is detailed elsewhere [36].
Fig. 3a–c shows cross sectional scanning electron
micrographs of the as HIPed structures for the 850–
1000 mm powder. These correspond to structures with
relative densities of 0.40, 0.54 and 0.65. It can be
clearly seen from Fig. 3a–c, the transition of thin
walled, thick walled and solid powders. However, it
can been seen that the as-HIPed structures contain
various defects. These defects are classified as inherent
processing defects incurred during gas atomization
such as uneven shell wall thickness, non-spherical
powders, powders with satellites and/or broken and
fractured powders or consolidation induced defects
such as incomplete bonding, flattening of uneven shell
walls, crushing and buckling of uneven cell walls and
rupturing of thin walls.
Fig. 2. Waveforms showing the (a) temperaturization and (b) pressurization schedules used for partial consolidation of the hollow powder
structures.
sieved by size and separated by particle relative density using an elutriation device [21].
Two powder size distributions (850– 1000 and 355–
425 mm in diameter) were chosen for consolidation
into cylindrical structures by hot isostatic pressing.
Prior to consolidation the powders were degreased
and ultrasonically cleaned and packed in a 304 stainless steel hot isostatic press (HIP) canister with dimensions 0.6875 in. OD×0.6175 in. ID× 1.25 in. L.
In addition, 0.04 in. thick Inconel® alloy 625 face
sheets were placed at the top and bottom of the HIP
cans. A thin layer of boron nitride followed by a 25
mm thick molybdenum foil included on the inner wall
and end caps was added to help facilitate separation
of the consolidated structure from the canister after
HIPing. The HIP cans were evacuated to 5× 10 − 4
torr for 30 min and electron beam welded at KTI
(East Windsor, CT) prior to consolidation. An Asea
Brown Boveri HIP equipped with a two-zone molybdenum furnace was used for consolidation. The temperature was increased at a rate of 10 °C min − 1 up
to 1050 °C and held for 25 min before furnace cooling. The HIP chamber was initially pressurized to 5
MPa (the minimum pressure attainable for this HIP)
and released concomitantly with the temperature. The
temperature and pressure schedules used for the consolidation cycle are shown in Fig. 2a and b, respectively. The HIP cycle was chosen through trial and
error, to provide an acceptable amount of cell deformation and interparticle bond strength. Table 1
shows the particle relative densities of the separated
powders and the densities of the hollow powder cellu-
Fig. 3. Cross sectional micrographs of the as HIPed powders for the
850 – 1000 mm diameter powders with relative densities of (a) 0.40, (b)
0.54 and (c) 0.65.
Powder size = −18/+20 mesh
Powder size =−40/+45 mesh
Separation velocity
(mm ms−1)
Particle density
(g cm−3)
As-HIPed density
(g cm−3)
Relative density
(z*/zs)
Separation velocity
(mm ms−1)
Particle density
(g cm−3)
As-HIPed density
(g cm−3)
Relative density
(z*/zs)
15B6B20
20B6B25
40B6B45
6.1
7.6
8.44
3.38
4.56
5.49
0.40
0.54
0.65
6B11
13B6B15
19B6B21
7.5
8.0
8.3
5.15
5.32
5.57
0.61
0.63
0.66
D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
Table 1
Densities of the Ni–21.3Cr–8.8Mo–3.9Nb–0.13A1–0.19Ti wt.% powders for the separated and consolidated states
141
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D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
Fig. 4. Schematic illustration of the laser ultrasonic sensing facility used for the evaluation of the acoustical and elastic properties of the hollow
powder structures.
3. Ultrasonic principles
The propagation of high frequency elastic waves
(ultrasound) in isotropic polycrystalline bodies is directly related to the dynamic elastic moduli of the body.
The linear elastic behavior of isotropic bodies can be
fully described by two independent elastic stiffness constants [37]. In the long wavelength limit, the longitudinal (wl) and shear (ws) wave velocities can be expressed
in terms of the low frequency limit static elastic constants and the density. Therefore, by monitoring the
time of flight’s (the ultrasonic velocities can be found
by dividing the propagation distance by the respective
time of flight) of both the longitudinal and shear waves
enables the elastic properties to be readily determined.
In addition, by simultaneously monitoring the temporal decay of the laser induced resonant frequency
standing wave one can obtain valuable information
about the amplitude attenuation coefficient using the
reverberation technique [38]. Classic ultrasonic attenuation losses can be classified into two main categories;
elastic losses which conserve the mechanical energy
within the samples volume and inelastic losses in which
the mechanical energy is dissipated primarily as heat
[39,40]. One important elastic loss encountered in polycrystalline materials occurs when ultrasound is scattered by grain boundaries, which is caused by the
change in acoustic impedance due to local anisotropy.
In porous samples additional elastic losses are incurred
by pore scattering.
A recently developed method using laser generated
and detected ultrasound has been developed for the
direct measurement of ultrasonic absorption by monitoring the reverberation field [41–43]. It is assumed that
the incident ultrasonic wave loses its coherence by
scattering and after a finite period of time the elastic
energy of the wave becomes distributed over the whole
volume of the sample [44]. It is further assumed that the
temporal decay of this ‘homogeneous diffuse field’ is
determined by absorption mechanisms. It has been
earlier shown that shear modes dominate this diffuse
field with the partitioning of the energy between the
various modes of propagation, i.e. longitudinal, shear,
surface waves etc., depending on the respective longitudinal and shear wave velocities. The amplitude attenuation coefficient h can be calculated from the decay of
this reverberation field and can be represented by
h=
1
y
20 log
2L
Q
(1)
where L is the sample length in meters and Q is the
quality factor (Q= yfo/p) where fo is the fundamental
frequency of the reverberation field and p is the displacement amplitude decay assuming the reverberation
decay follows the form exp(−pt) where t is the time.
The ultrasonic time-of-flight (TOF) between defined
source and receiver points was measured using a laser
ultrasonic system Fig. 4. A 10 ns duration Qswitched Nd:YAG laser pulse of 1.064 mm wavelength
was used as the ultrasonic source. The energy per pulse
was 30 mJ and the roughly Gaussian beam of the
multimode pulse was focused to an approximate circular spot of 1 mm diameter. Thus, the source power
density was 375 MW cm − 2. The shortest wavelength
D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
of the detected ultrasound (u =6/f, where u is the
wavelength, 6 is the ultrasonic velocity, and f is the
frequency of ultrasound) was larger than the diameter
of either the open or closed celled pores.
The ultrasonic receiver was a Mach– Zehnder heterodyne laser interferometer, which responded to the outof-plane surface displacement, associated with the
wavefront arrivals. It was powered by a 5 mW HeNe
laser, which produced a continuous Gaussian beam of
632.8 nm wavelength focused to a circular spot 100
mm in diameter. The interferometer had a displacement
sensitivity of 0.4 A, mV − 1 and exhibited linear output
for displacements up to about 300 A, . For the experiments reported here, maximum surface displacements
were on the order of 75 A, . The signal from the interferometer (35 MHz bandwidth) was recorded with a precision digital oscilloscope at a 10 ns sampling interval for
the TOF measurements and 10 ms sampling interval for
the resonant frequency measurements using 8-bit
analog-to-digital conversion. To improve the signal-tonoise ratio, each waveform was an average of 25
pulses. A fast photodiode identified the origination time
for the ultrasonic signals.
Fig. 5. Normalized elastic properties versus relative density (z*/zs)
for (a) Young’s modulus (E*/Es) and (b) the shear modulus (G*/Es)
of the hollow powder structures.
143
4. Results and analysis
4.1. Elastic properties
The longitudinal and shear wave velocities were evaluated and a temporal correction factor of 0.388 and
0.702 ms (for wave propagation in the two face sheets)
was incorporated for the longitudinal and shear wave
time of flight’s, respectively. Therefore, the elastic properties reported here are for the porous core only. Once
the two ultrasonic velocities were determined, the
Young’s and the shear moduli were evaluated from the
respective density of each sample (Table 1). Fig. 5a and
b shows the normalized Young’s modulus (E*/Es) and
shear modulus (G*/Gs) versus relative density (z*/zs)
for the hollow powder cellular structures.
Two approaches can be taken when modeling the
elastic properties of these porous materials. These models may be classified as micromechanics based models
or ones, which are based on pertinent cross-sectional
geometries (minimum solid area models). Micromechanics based models such as the composite sphere
method (CSM) assume each unit cell is a sphere of
matrix material with a spherical pore concentrically
placed within it. It is assumed that the unit cells are
distributed in a completely void filling manner, i.e.
infinite range of sizes. The infinite size range is necessary such that smaller particles fill all the interstices
between larger particles, i.e. there is no porosity in the
body other than that from the pores within each spherical particle. This type of model was originally introduced by Hashin [45,46].
The CSM model was again looked at by Nielsen [47]
and Ramakrishnan and Arunachalam [48,49]. Nielsen
set forth to predict the elastic moduli for two phase
materials of any geometry and points out that the CSM
model is a markedly hypothetical material, in that the
very precise placing of particles is not practical and that
it is inconceivable that real particles can be gradiated in
size such that they can provide a completely dense
phase. Ramakrishnan and Arunachalam noted that the
residual stress at the surface of a composite sphere in
an assemblage is not zero, in contrast to that in an
isolated composite sphere assumed by Hashin. They
assumed each composite sphere to experience an effective pressure higher that the pressure on the assemblage
to account for the apartment intensification of pressure
in the composite sphere. Table 2 summarizes the earlier
mentioned analytical models.
Since these hollow powder cellular structures contain
two distinct differences in pore morphology, each will
exhibit a different elastic response and accordingly each
must be treated independently. The first pore morphology (open) utilizes the space between packed particles
or spheres. The amount and morphology of open
porosity is related to both the size and packing arrange-
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D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
Table 2
Composite sphere models for porous materials.
Functional form
Operative
mechanism
Hashin [46,47]
M/MO = (1−P)
/1+CP
Nielsen [48]
M/MO = (1−P)2
/1+CP
Ramakrishanan
Arunachalan [49,50]
M/MO = (1−P)2
/1+CP
CE = (1+6)
(13−156)/2(7−56)
CG = 2(4−56)
/7−56
CE = (1−56)
/7−56
CG = 2(4(13)−56)
/7−56
2−36
CG = 11−196
/4(1+6)
Fig. 6. Schematic illustration of a simple cubic unit cell stacking of
hollow spheres.
ment of the particles as well as their degree of bonding
(i.e. consolidation). The second pore morphology
(closed) utilizes pores within the particles or spheres.
The amount of closed porosity is related to the radius
of the spherical pore and the radius of the solid spherical particle. Therefore, the elastic properties of the
hollow powder cellular structures will be dependent
upon the amounts of each type of porosity (open and
closed).
Fig. 6 shows an idealized 3×3 × 3 unit cell with
simple cubic packing The upper limit of the porosity
for the fundamental stacking of uniform spherical particles with simple cubic stacking is 47.6% [50– 52]. This
upper limit represent the percolation limit. Any higher
porosities for simple cubic stacking of uniform spherical particles would result in particles that no longer
touch and hence no longer form a solid body. Conversely, the stacking of uniform spherical pores (simple
cubic array) results in a percolation limit of 52.4%. As
the porosity increases beyond the percolation limit
(which is dependent on the coordination number of the
stacking) the porosity becomes open due to pore overlap, i.e. the structure tends toward a reticulated foam
structure.
Many models for the porosity dependence of physical
properties of materials have been derived using idealized structures to calculate solid cross-sectional areas.
Minimum solid areas for solid particles are the bond
areas between them. For spherical pores the minimal
solid area is the minimum web cross-sectional areas
between the pores. Through the use of these idealized
models, i.e. only the minimum solid area and pore
shape encompass all of the characteristics needed to
fully characterize the elastic response [50–52]. Although
the hollow powder cellular structures synthesized here
are randomly packed, it has been suggested that cubic
and random packing of particles yield similar porosity
because the latter must involve some lower density (e.g.
particle bridging) and higher density (e.g. orthorhombic
or rhombic) packing to give the same porosity as simple
cubic packing [53–55].
In addition to the experimental data, Fig. 5 also
shows the composite sphere model and the minimum
solid area model predictions (simple cubic packing) of
uniform spherical particles and pores. The normalized
Young’s modulus shows reasonable agreement with the
model predictions, whereas the shear moduli falls below. The decrease in the elastic properties of these
hollow powder cellular structures is partially affected
by the presence of defects in the consolidated structure.
It has been earlier shown that defects in both open and
closed metal foams such as cell wall wiggles and uneven
material distribution in the cell walls and plateau borders significantly decrease the elastic properties [56–58].
Therefore, it is reasonable to assume that a decrease in
the elastic response is attributed to a combination of
inherent processing defects incurred during gas atomization such as uneven shell wall thickness, non-spherical powders, powders with satellites and/or broken and
fractured powders in addition to consolidation induced
defects such as incomplete bonding, flattening of nonuniform shell walls, crushing and buckling of uneven
shell walls and rupturing of thin walls. Various defects
of both inherent and processing induced were observed
in Fig. 3.
Although the micromechanics based and minimum
solid area models reasonable represent the structure,
further work is needed in the area of modeling the
elastic stiffness of hollow powder structures which contain both open and closed porosity. Models, which
incorporate factors such as total amount of porosity as
well as have the capability to vary the amount of open
and closed porosity are needed to more accurately
capture the elastic stiffness response of these hollow
powder structures.
D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
4.2. Acoustic attenuation
Fig. 7a–c shows the time-dependent damping spectrum of the reverberation field for the 850– 1000 mm
diameter powder samples with relative densities of 0.40,
0.54 and 0.65, respectively. It is observed that the
reverberation fields exhibit a strong first order resonant
145
frequency. This decay is qualitatively observed to decrease in intensity and time as the relative density
decreases. Fig. 8a–c shows the temporal decay of the
laser induced reverberation field for the three samples
consolidated from 850 to 1000 mm diameter powders. It
can be seen clearly that as the relative density of the
structures increased from 0.40 to 0.65, the rate of decay
Fig. 7. Time dependent frequency damping characteristics of the laser induced reverberation field for the 850 – 1000 mm diameter powder structures
with relative densities of (a) 0.40, (b) 0.54 and (c) 0.65.
Fig. 8. Temporal decay of the laser induced ultrasonic reverberation field for the 850 – 1000 mm diameter powder structures with relative densities
of (a) 0.40, (b) 0.54 and (c) 0.65. Also shown in (d) are the resonant frequency spectrums for each reverberation field.
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D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
to be attributed to pore scattering from both the open
and closed pores. Therefore, the attenuation should
increase with a decrease in relative density for each
powder size. In addition the smaller powder is roughly
half the diameter of the larger powder, there are
roughly twice the number of interstitial voids in the
structure available for pore scattering. Therefore, the
rate of increase in attenuation characteristics should
increase with a decrease in powder size.
5. Summary
Fig. 9. Amplitude attenuation coefficient of the laser induced ultrasonic reverberation field for the 850 – 1000 mm diameter and the
355 – 450 mm diameter powder structures.
of the reverberation field decreased. To examine the
frequency components of the reverberation field, the
fast Fourier transform was calculated using the matrix
software MATLAB™. Fig. 8d shows the frequency
response obtained from the fast Fourier transform of
the reverberation fields of Fig. 8a– c. It can be clearly
seen that both the magnitude and frequency response of
the reverberation field vary according to the relative
density. The ultrasonic velocity of the reverberation
field can be calculated from 6 =2Lfo where L is the
sample length and fo is the characteristics frequency of
the reverberation field, Fig. 8d.
To evaluate the attenuation characteristics of the
reverberation field for these structures, the temporal
decay of the reverberation fields were band pass filtered
about their fundamental resonant frequency (9 2 kHz
about fo ). The peak values of this band pass filtered
signal were determined using a local forward-backward
looking peak finding algorithm and plotted as a function of time. A subsequent non-linear least squares
routine was used to fit these extracted peak values for
the displacement amplitude decay assuming that the
reverberation field followed the form exp(− pt), where
p is the decay exponent and t is the time in seconds.
Once p of the peaks values of the reverberation field
were determined for each structure, the amplitude attenuation coefficient was evaluated from Eq. (1). Fig. 9
shows the amplitude attenuation coefficient for the
hollow powder structures consolidated from both the
850−1000 and 355− 450 mm diameter powders. It can
be seen that for both sets of structures that the amplitude attenuation coefficient increased as the relative
density decreased. It is also interesting to note that the
rate of increase of the amplitude attenuation coefficient
as a function of relative density was greater for the
structures consolidated from the smaller diameter powders. We believe the dominant attenuation mechanism
During gas atomization of metal powders a small
fraction of hollow powders (generally regarded as byproducts) are routinely produced which are normally
remelted or discarded. Here, a recently developed technique for separating powders by their size and particle
relative density has been used to obtain a distribution
of powders with controlled particle densities. From
these powders, cylindrical samples were consolidated
via hot isostatic pressing from three different particle
density distributions corresponding to solid, thick
walled and thin walled powders of two nominal diameters (850–1000 and 355–450 mm).
The elastic moduli of these hollow powder cellular
structures were evaluated by measuring the time-offlight of pure longitudinal and shear waves using a laser
ultrasonic technique. It was determined that the elastic
moduli of these hollow powder structures scaled with
relative density and are reasonably approximated by
current analytical models. The amplitude attenuation
coefficient for these hollow powder cellular structures
was determined from the temporal decay of the resonant frequency standing acoustic wave using the reverberation technique. It was found that the amplitude
attenuation coefficient varied with relative density and
particle size.
6. Conclusions
This unique class of porous materials constructed
from hollow metal powders has sparked interest in their
use for multi functional military, aerospace and commercial applications. The ability to independently control the relative volume fractions of both closed and
open porosity levels offer the potential to tailor the
elastic stiffness, acoustic damping, thermal insulation
and heat dissipation properties for individual applications. Therefore, due to their bimodal nature, structures
consolidated from hollow powders offer advantages
characteristic of both closed celled and open cell reticulated foams.
D.T. Queheillalt et al. / Materials Science and Engineering A323 (2002) 138–147
Acknowledgements
We would like to thank Dr Kumar P. Dharmasena
for his help with the HIP consolidation of the hollow
powder structures. This work has been performed as
part of the research of the Multidisciplinary University
Research Initiative (MURI) program on Ultralight
Metal Structures. We are grateful for the many helpful
discussions with our colleagues in these organizations.
The consortium’s work has been supported by
DARPA/DSO under contract N00014-96-I-1028 monitored by Dr Steve Wax (DARPA) and Dr Steve Fishman (ONR).
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