Scripta Materialia 50 (2004) 313–317
www.actamat-journals.com
Synthesis of stochastic open cell Ni-based foams
Douglas T. Queheillalt *, Yasushi Katsumura, Haydn N.G. Wadley
Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, 116 Engineers Way,
P.O. Box 400745, Charlottesville, VA 22904, USA
Received 12 August 2003; received in revised form 25 September 2003; accepted 16 October 2003
Abstract
Open cell, stochastic Ni-based foams have been synthesized on carbon foam templates by transient liquid phase bonding nickel
based superalloy (Ni–21Cr–9Mo–4Nb) and Ni–25Cr–10P powders. The mechanical properties of these Ni-based foams were similar
to those of other lower temperature metal foams of similar topology.
2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Metal foam; Porous materials; Nickel base superalloys; Mechanical properties
1. Introduction
Lightweight cellular solids with stochastic cells can be
manufactured from numerous metals and metal alloys
by a wide variety of vapor, liquid and solid-state processes [1]. The properties of these cellular materials depend upon the properties of the base metal alloy, the
relative density and the topology (i.e. open or closed cell,
cell size, etc.) [2]. Closed cell metal foams possess higher
moduli, strengths and impact energy absorbing characteristics than their open cell counterparts. While stochastic open cell foams are not as stiff or as strong as
their closed cell counterparts, they posses characteristics
that can be exploited for multifunctional applications
[3]. For example, when used as the cores of sandwich
panels, they can be used for both structural load support
and heat dissipation because of the ability to flow fluids
readily through the open structure [4–7], and they are
being investigated for high temperature supports for
catalytic applications [8–11].
Today, the majority of catalyst supports utilize ceramic honeycomb monoliths or foams, onto which the
catalytic metals or metal oxides are dispersed, wherein
various key factors in catalyst support design include:
catalyst size and shaping, catalytic performance,
strength and pressure drop across the system [12]. While,
ceramic foams are a relatively new catalyst support
*
Corresponding author. Tel.: +1-434-982-5678; fax: +1-434-9825677.
E-mail address: [email protected] (D.T. Queheillalt).
technology, they possess several advantages when compared with the honeycomb monoliths. These include a
wide range of available shapes and sizes due to the
method of manufacture. They also have a more tortuous
form of porosity, which improves reactant mixing, and
surface reactions [13]. Metal substrates for catalyst
supports are growing in importance due to their improved mechanical durability and superior heat transport [14,15]. The higher thermal conductivity of the
metal foam has been shown to promote a more uniform
temperature distribution throughout the structure,
thereby increasing the overall efficiency of the catalyst
device [16].
Precision investment casting has been used for the
manufacture of open cell, stochastic foams cast from Al,
Cu, Mg and Zn based alloys [17,18]. The process uses an
open cell polyurethane foam template fitted with appropriate vent and gating systems, which is invested by
conventional shell or flask techniques. The investment is
allowed to cure and the foam template burned out resulting in a negative mold that is filled with molten
metal. After solidification, the investment is removed by
water spray or other mechanical means leaving behind
the open cell, stochastic metal foam. This method is
limited to easily castable, low melting point alloys and
is quite difficult for high temperature alloys such
as stainless steels, nickel based alloys and other superalloys.
A more economical solid-state process has been used
for the manufacture of open cell stochastic foams made
from copper, stainless steel and FeCrAlY alloys. This
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.scriptamat.2003.10.016
314
D.T. Queheillalt et al. / Scripta Materialia 50 (2004) 313–317
process uses metal powders and binding agents that are
mixed together and coated onto the ligaments of polymer based open cell, stochastic foam templates. The
open cell polymer template is burnt-out and the foams
are sintered to increase the mechanical integrity of the
metal ligaments. This method proves difficult for oxidizing and reactive alloys and is generally limited to
easily sinterable alloys.
Here, we have extended the solid-state approach for
producing open cell, stochastic metal foams from a
nickel based Ni–21Cr–9Mo–4Nb superalloy, comparable to an Inconel alloy 625. This method can be simply
extended to other metal systems including copper and
iron based alloys. The approach is based on transient
liquid phase sintering of metal powders deposited on
open cell, stochastic carbon foam templates. The metal
foams have high porosities and high surface area to
volume ratios, which make them suitable for high temperature supports for a variety of thermal management
and catalytic applications.
50% in size. The excess furfuryl alcohol was removed by
compressing the infused foam between absorbent sheets
followed by spinning in a centrifuge for 10 min. This
critical drying step removes excess solution from the
ligament surfaces, which causes ligament cracking during subsequent high-temperature thermal treatments.
The infused polyurethane foam was thermoset at 250
C for 6 h in air. During thermosetting, the infused foam
contracts (roughly to size of the original polyurethane
foam) to form a rigid structure. The rigid foam structures were carbonized by heating at a rate of 2 C/min to
1100 C under a vacuum (104 Torr) and held for 30 min
followed by slow cooling. During carbonization, the
functional groups (i.e. H2 O, CH2 , CO2 , CO, H2 ) are
released at their respective temperatures resulting in an
open cell, stochastic carbonized foam where the carbon
is of the vitreous or glassy carbon form [19]. During
expulsion of the volatile functional groups, the foams
exhibited significant shrinkage. The cell size of the carbon foams were measured according to ASTM D357698 and the average cell diameter was determined to be
3.55 mm with a standard deviation of 0.324.
2. Experiments
2.2. Synthesis of metal foams
High temperature alloy foams made by solid state
routes such as sintering require the use of high temperature heat treatments to diffusively transport material at
interparticle necks. These high temperatures cannot
therefore easily be used to create stochastic foams using
a polymer template approach. Here we describe a process in which a reticulated polymer foam is converted to
a carbon foam and then used as a template for metal
particle coating. To reduce the time required for densification of the metal powder, a transient liquid phase
precursor is added to the powder.
2.1. Synthesis of vitreous carbon foams
Open cell, stochastic polyurethane foams are inexpensive and widely available from a number of commercial manufacturers with nominal cell sizes ranging
from 5 to 120 ppi. A polyurethane foam with a nominal
pore size of 10 pores per inch (nominal cell diameter of
2.54 mm) was used as the polymer template. The actual cell size of the as-received foams were determined
according to ASTM D3576-98 and determined to be
4.64 mm with a standard deviation of 0.287. These
polyurethane foams were modified to transform them
into open cell, stochastic carbon foams.
To produce the carbon foams, the polyurethane
foams were immersed in an unpolymerized solution of
furfuryl alcohol and oxalic acid (20:1 volume ratio) for
3 h. The oxalic acid was added as a catalyst for the
furfuryl alcohol. The mixture penetrates into the polyurethane foam ligaments to form a gel-like structure.
During this step, the infused foam swells approximately
30 · 30 · 15 mm3 carbon foam templates were dipped
in a binder solution (NICROBRAZ 520 cement) and
lightly shaken to remove excess binder. The bindercoated foams were then suspended in a fluidized airbed
containing a mixture of base alloy and braze powder.
The time spent in the fluidized bed was adjusted to vary
the amount of powder collected on the carbon ligaments
and thus the relative density of the resultant metal
foams. An Inconel alloy 625 nickel-based superalloy of
nominal
composition:
Ni–21.3Cr–8.8Mo–3.9Nb–
0.13Al– 0.19Ti, wt.% was chosen as the base alloy. It is a
non age-hardenable, solid solution strengthened alloy
with a face centered cubic (c-phase) crystal structure.
The base alloy powder was sieved to a 60 mesh size
which corresponds to a particle size of less than 250 lm.
A NICROBRAZ alloy 51 powder with a nominal
composition of Ni–25.0Cr–10.0P–0.03C, wt.% used as
the transient liquid phase sintering agent. The particle
size was less than 100 lm.
The powder coated carbon foams were dried and
placed in a vacuum furnace (104 Torr) and heated at a
rate of 10 C/min up to 550 C, holding for 1 h (to
volatilize the binder), then continued heating to 1050 C,
held for 2 h followed by furnace cooling at 25 C/min.
The transient liquid phase sintering agent has a brazing
temperature range of 980–1095 C. The resultant metal
foams consisted of an open cell, stochastic carbon core
uniformly coated by the variable density nickel alloy,
Fig. 1. It was found that as the volume fraction of
braze:base powder increased, the density of the metal
coating layer increased. Here, a braze:base powder ratio
C o m p r e ssi ve Str e ss ( M Pa )
D.T. Queheillalt et al. / Scripta Materialia 50 (2004) 313–317
315
12
polynomial fit
experimental data
9
densification
6
3
plateau region
yielding
0
0
20
40
60
80
100
S t ra i n ( %)
Fig. 2. A representative compressive stress–strain curve for the transient liquid phase sintered foams (foam relative density 7.39%).
Young's Modulus (E/Es)
1.0000
model prediction
nickel (TLPS)
aluminum (ERG)
0.1000
0.0100
*
 ρ *
E
------ =  -----
Es
 ρs
2
0.0010
op
en
ce
ll
0.0001
0.01
Fig. 1. Photomicrographs showing (a) the overall structure of the
nickel based open cell foam, (b) the mesoscopic and (c) the surface
structure of the open cell ligament structure.
of 1:9 was used. This resulted in a nearly fully dense
metal coating with a rough surface. The thickness of the
metal layer was 5–10 times the thickness of the carbon
core.
1.00
R e l a t i v e D e n s i t y (ρ / ρ s )
Fig. 3. Comparison of the relative Young’s modulus as a function of
relative density of the nickel based metal foams.
strength (defined as the yield strength of the foam, r,
divided by the yield strength of the parent solid, rys )
were measured from the stress–strain curves of samples
with a range of densities (Figs. 3 and 4) respectively.
1.0000
Compressive Strength ( σ/ σ ys )
3. Results and analysis
Uniaxial compressive test were performed on the
nickel-based foams at a strain rate of 0.024 min1 . Each
foam sample was initially loaded beyond the compressive yield point and cycled through several unload/reload stages at 2.5%, 5.0%, 7.5% and 10.0% strain. The
Young’s modulus measurements were made using the
unloading curve. The compressive stress–strain curves
for the transient liquid phase sintered foams exhibited
characteristics typical of open cell metal foams including, yielding (i.e. plastic collapse) followed by a plateau
region and then densification (Fig. 2).
The normalized Young’s modulus (defined as the
modulus of the foam, E divided by the modulus of the
parent solid, Es ) and the normalized compressive yield
0.10
model prediction
nickel (TLPS)
aluminum (ERG)
0.1000
*
 ρ *
σ
------ = 0.3 ⋅  -----
σs
 ρs
0.0100
0.0010
3⁄2
ell
nc
ope
0.0001
0.01
0.10
1.00
Relative Density ( ρ / ρ s )
Fig. 4. Comparison of the relative yield strength as a function of relative density of the nickel based metal foams.
316
D.T. Queheillalt et al. / Scripta Materialia 50 (2004) 313–317
Table 1
Mechanical properties of the transient liquid phase sintered Ni–21Cr–9Mo–4Nb open cell, stochastic foams (all values measured in compression)
Relative density q=qs (%)
Yield strength rys (MPa)
Young’s modulus E (GPa)
Densification strain eD
6.84
7.39
8.25
9.44
10.12
10.57
10.71
1.28
1.66
2.57
2.94
3.73
3.02
3.48
0.77
0.79
1.25
1.23
1.83
1.56
1.87
0.71
0.70
0.68
0.67
0.66
0.65
0.65
Nominal properties for the fully dense polycrystalline
alloy are: yield strength, rys ¼ 414 MPa, Young’s
modulus, Es ¼ 208 GPa and density, qs ¼ 8:44 g/cm3 .
Also shown are Gibson and Ashby theoretical relationships for the normalized Young’s modulus of an
open cell, stochastic foam
2
E
q
¼
ð1Þ
Es
qs
where q is the density of the foam, qs is the density of the
parent solid [2]. A similar relationship has been developed for the normalized yield strength
3=2
r
q
¼ 0:3
:
ð2Þ
rs
qs
There was good agreement with the experimental
data with both the Young’s modulus and the yield
strength. Table 1 summarizes the mechanical properties
of the transient liquid phase sintered Ni–21Cr–9Mo–
4Nb open cell, stochastic foams along with their respective densification strains, eD .
4. Discussion
A new process is described for the synthesis of Nibased foams. This process used to synthesize open cell
metal foams has many benefits to conventional pressureless sintering technology. During initial heating of
transient liquid phase sintering, the liquid initially
formed, due to its lower melting point, begins to spread
through capillary action. The formation of the liquid
film has the benefit of a surface tension force acting to
aid in densification, eliminate porosity (i.e. pinholes,
cracks, fissures) and reduce the overall interfacial energy
of the structure [20]. The capillary attraction due to the
wetting liquid gives rapid densification. Therefore, this
method may prove cost effective when compared with
solid state sintering. In addition, this process requires
lower processing temperatures and times when compared with conventional solid-state sintering. To prevent
carbon diffusion from the open cell foam template to the
metal alloy coating a diffusion barrier layer may be
applied to the foam template.
In addition, this process has the flexibility of being
used with other systems based on nickel, iron or copper
alloys, to manufacture the high temperature metal foam
supports. Nickel and iron based systems are well suited
for high temperature catalytic applications, where copper based systems are better suited for high temperature
thermal dissipation applications. This transient liquid
phase sintered foam provides a high geometric surface
area, low-pressure drop, and excellent high-temperature
and thermal–shock resistance, when compared with
their ceramic counterparts.
5. Summary
Open cell, stochastic nickel base alloy foams have
been synthesized by templated transient liquid phase
sintering and their microstructural and mechanical
properties evaluated. Vitreous or glassy carbon foams
were produced by infusing an open cell, stochastic
polyurethane foam with furfuryl alcohol, polymerizing
the infused foam then heat treating to carbonize the
foam. This resulted in ultra-lightweight vitreous carbon
foams, which provides a stable high temperature template for the synthesis of the metal foams. The transient
liquid phase sintering process used the carbon foam as a
template upon which a powder mixture of a nickel based
alloy (Ni–21Cr–9Mo–4Nb) and brazing alloy (Ni–
25Cr–10P) was coated onto. The powder coated carbon
foam was transient liquid phase sintered, densifying the
powder coating resulting in lightweight metal foams
consisting of an open cell, stochastic structure. The
compressive Young’s modulus and yield strength’s of
the metal foams were measured and found to be comparable to theoretical models for open cell, stochastic
foams.
Acknowledgements
This work has been performed as part of the research
of the Multidisciplinary University Research Initiative
(MURI) program on Ultralight Metal Structures (Harvard University, the Massachusetts Institute of Technology, Cambridge University, Princeton University
D.T. Queheillalt et al. / Scripta Materialia 50 (2004) 313–317
and the University of Virginia) supported by Dr. Steve
Fishman (ONR).
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