Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.2, pp 97-104, 2008
jmmce.org Printed in the USA. All rights reserved
Porous Bulk Metallic Glass Fabricated by Powder Consolidation
Z.Y. Suo, S.W. Liu, L. Zhang, H. L. Gao, H. Y. Zhang and K. Q. Qiu*
School of Materials Science and Engineering, Shenyang University of Technology
110023, CHINA
*For Correspondence: Dr. Ke-Qiang Qiu, 58 South Xinghua Street, Shenyang University of
Technology, PIN-110023, Shenyang, CHINA, Email: [e-mail]
ABSTRACT
A synthesis method for the production of porous bulk metallic glass (BMG) is
introduced. This method utilizes the superplastic forming ability of amorphous powder in
the supercooled liquid (SCL) state and intenerating salt mixture as a placeholder to
produce BMG foam by using a hot die pressing method. Scanning electron microscope
(SEM), x-ray diffraction (XRD) and differential scanning calorimetry (DSC) were
employed to characterize the morphologies of foaming structure, the crystallization and
percentage of amorphous phase of the as-produced porous BMG. The results suggest that
the formation of porous structure by superplastic forming process is feasible. Good
bonding effect was observed between amorphous powder particles. None of crystalline
phases was formed during hot pressing, and less than 3.5% percent of residual salt was
enclosed in the foam. In order to remove any residual salt particles, salt preform with
three-dimensional network and good connec t ivity is necessary.
Keywords: porous BMG; powder consolidation; hot pressing; placeholders; superplastic
forming
1. INTRODUCTION
Since porous BMGs exhibited ductility and biologic compatibility[1], many
researchers were focused on the fabrication of such excellent materials. Schroers et al [2]
demonstrated the first porous BMG by entrapping gas in a liquid Pd43Ni10Cu27P20 alloy.
Wada and Inoue [3] reported foaming of the same alloy by casting around soluble NaCl
placeholders. Shortly afterwards, Brothers and Dunand [4] developed an alternative
method based on liquid filtration of beds of hollow carbon microspheres, appropriate for
use with reactive commercial BMG alloys, e.g. Zr-based alloys. In his another report, a
97

98 Z.Y. Suo, S.W. Liu, L. Zhang, H. L. Gao, H. Y. Zhang, K. Q. Qiu Vol.7, No.2
method based on a salt replication process for the processing of open celled foams from
the commercial BMG alloy Vit106 (Zr57Nb5Cu15.4Ni12.6Al10 ) was also described [5].
Recently, Ren et al [6] fabricated a porous BMG of 8 mm in diameter, which is the
largest porous BMG reported up to now.
Until now, Bulk amorphous foam generally fabricated by liquid route, which
arises many problems. Two important problems of these are the reduction of glass
forming ability (GFA) of based alloy and the contamination of liquid alloy during
processing. Reports offer evidence that amorphous metal powders can be consolidated in
the SCL region with little or no loss of strength by using hot pressing [7], warm rolling
[8], and conventional area reduction [9,10] or equalchannel angular [11] warm extrusion
techniques. It is easy to envision an incorporation of placeholders into these consolidated
compacts, providing a natural method for making low-density porous materials with low
processing temperatures. Recently, Qiu et al [12] fabricated bulk metallic glass foam by
powder hot pressing. Less than 6.5% percent of crystalline phases were formed after hot
pressing 15 to 18 minutes. In this paper, we introduce a SCL-state foaming route by
blending placeholders, usually salts with controllable intenerating temperature, with
amorphous metal powder to produce a porous BMG. But the powder consolidation by hot
pressing is limited in 10 minutes.
2. EXPERIMENTAL
A pre-alloying ingot with a nominal composition of Zr55Cu30Al10Ni5 was prepared
by induction melting the mixture of Zr, Cu, Al and Ni with a purity of 99.5% or better in
a BN crucible at high vacuum. The Zr55Cu30Al10Ni5 powders were produced from the
pre-alloying ingots by high pressure Ar gas atomization at a dynamic pressure of 3.5 MPa
after heating to 1623 K using an annular nozzle having a melt delivery inner diameter of
2.7 mm. Following atomization, powders were screened to size ranges of powder
particles having diameters not larger than 25μm. The particle size of the metal powder is
not critical but the metal particles must be considerably smaller than salt particles that are
chosen as the placeholder [13].
During SCL region processing, heating into the SCL region would enhance the kinetics
of crystallization. Consequently the time allowed for processing would be limited.
Therefore, the processing temperature for the hot pressing has to be carefully determined
by considering the time scale characterizing plastic forming is shorter than that
characterizing crystallization. To determine the appropriate hot press time available
before crystallization at a certain temperature, a time–temperature-transformation (TTT)
plot was constructed for the amorphous powder, as shown in Fig.1, by performing
isothermal DSC scans. This was accomplished by first heating the powder to selected

Vol.7, No.2 Porous bulk metallic glass fabricated by powder consolidation 99
annealing temperatures above Tg and then measuring the time until the sample starts to
crystallize, which is indicated by the onset of an exothermic reaction.
0510 15 20 2530
680
700
720
740
760
Crystalline
Supercooled liquid
Tx
Tg
Temperature /K
Time /
min
Figure 1. Time-Temperature-Transformation diagram for the onset of
crystallization of gas atomized powder isothermal heated to selected
temperature.
A mixture of NaCl and CoCl2 (NaCl:CoCl2=7:10 in weight) was first melted at
873K for 1h in low vacuum and then cracked and screened to average size of 300μm. The
placeholder was prepared from such a mixture with a relatively low melting temperature.
The measuring intenerating temperature of the mixing salt is 643K.The particle size of
the salt powder selecte d w i l l b e according to the i ntended cell size of the final foam.
The amorphous powder and salt powders were mixed with a volume fraction of
amorphous powder 0.3 of all solid materials excluding pores in the mixture. Ethanol was
used as the binder and a small amount, roughly 1vol.% of the amorphous powder/salt
mixture, was added during mixing. The amorphous powder/salt powder mixture was
poured into a copper can and both ends of the can were vacuum sealed after outgassed.
The dimension of the copper can is 30 mm deep by 20 mm inner diameter cavity and 23
mm outer diameter.
The experiment was performed using hot die pressing for consolidation. The can
was placed into a hot die which was isothermal heated at 713 K. Once the copper can was
placed into the hot die the hot pressing at a constant ram pressure of 800 MPa through a
die was applied. The hot pressing time was set as 5min and 10 min respectively. After
pressing, the copper can was first quenched into water with ambient temperature, and

100 Z.Y. Suo, S.W. Liu, L. Zhang, H. L. Gao, H. Y. Zhang, K. Q. Qiu Vol.7, No.2
then removed the copper coat. The salt was removed by flowing water, leaving the
resultant foam.
The structure of the as-produced foam samples was examined by an S-3400
scanning electron microscope (SEM). X-ray diffraction (XRD) analyses and weight
measurements were also carried out to determine the extent of amorphous structure and
salt removal.
3. RESULTS AND DISCUSSION
The typical cellular structure of the as-produced foams is shown in Fig.2. Fig.2(a)
shows a SEM macrograph of the foam. The foam exhibits uniformly distributed open
cells and a network of well-bonded metal particles. It has a porosity of 70% and cell size
in the range of 250–450µm. Fig.2(b) shows the cellular structure of the as-produced
foam. The powder particles are well connected and some tiny holes among the connected
particles are observed. Fig.2(c) shows the wall structure in a typical cell. The bonding
region between particles as indicated by arrows indicates that strong bonding has formed
between the particles. Many tiny cross lines were observed on the surface of the particles,
showing that the particles have suffered severe deformation during hot pressing.
(c)
(a) (b
FIG. 2 (a) Morphology of as-produced foam synthesized by hot pressing. (b) The
cellular structure of the as-produced foam. (c) Magnified view of the inner wall of the
foam, the arrows indicating the formation of bonding areas during hot pressing.

Vol.7, No.2 Porous bulk metallic glass fabricated by powder consolidation 101
XRD was adopted to analyze the amorphous structure of the as-produced foam, as
shown in Fig.3. It is obvious that the two as-produced foams are almost in amorphous
state with a broad diffraction. DSC scans were performed on the powder metallic glass
and BMG foam (produced by hot pressing for about 10 min) as shown in Fig.4. Both of
the amorphous powder and BMG foam show distinct glass transition temperature and
SCL region, and there is no obvious difference between those characteristic temperatures.
The glass transition temperature, Tg, crystallization temperature, Tx, the temperature
interval between Tg and Tx, i.e. SCL region ΔTx, and crystallization enthalpies ΔH are in
Tab1. It is that crystallization enthalpies between the two materials are almost same. So
the percentage of amorphous phase can be estimated from the relatively same enthalpies
in the BMG foam. Such estimation is coinci dent with the XRD results.
In most cases, the salt p articles in the hot pressing preforms could not be
dissolved completely. Some salt particles remained in the resultant foam. The residual
salt can be estimated according to the expression suggested by Zhao et al [13]. For each
specim
us foam
ith respect to the total salt in the initial hot pressing preform, ϕ, was determined by
[12]:
en, the weights of the initial amorphous powder, the salt powder, a sample cut
from the hot pressing preform, and the resultant amorphous foam were measured using a
balance to an accuracy of 0.001g. The fraction of the residua l salt in the amorpho
w
)1( amorp
amorpf
fW
fWW
−
−
= ϕ
Where Wf and Wp are the weights of the amorphous foam and the corresponding prediss-
olution hot pressing preform respectively, and famor is the amorphous powder weight
fraction in the initial mixture. The calculation result shows that the fraction of the
residual salt powder in the foam with respect to the total salt in the preform is less than
3.5%. The relative density of the foam is therefore expected to be a little higher than the
value estimated from the initial amorphous powder fraction. With a high volume fraction
of salt in the preform, most salt particles are in contact with each other and form a
continuous three-dimensional network. Because all the salt particles in the network can
e dissolved away by water, there is only small amount of residual salt particles in the
resultan ome salt
les
e
pen
bt foam. In contrast, with a low volume fraction of salt in the preform s
particles are enclosed completely by the amorphous matrix. Those isolated salt partic
cannot be dissolved away and remained in the foam. Therefore hot pressing the mixtur
of salt and amorphous powder is only suitable to those foams with high porosity and o
cell. In order to fabricate adjustable porosity, a salt perform with three-dimensional
network and a required porosity is necessary. This will be reported elsewhere.

102 Z.Y. Suo, S.W. Liu, L. Zhang, H. L. Gao, H. Y. Zhang, K. Q. Qiu Vol.7, No.2
igure 4. DSC curveter of 3 mm (a) and as-produced foam
).
Figure 3. XRD patterns from as-produced foams synthesized by hot pressing for (a) 5
min and (b) 10 min respectively
10 20 30 40 50 60 70 80 90
(b)
intensity
2θ/deg
(a)
700 800 900
Fes of as-cast BMG with a diam
(b
500 600
Heating rate: 20K/ min
Tx
Tg
(b)
(a)
Endotherm ic
Temperatur e/K

Vol.7, No.2 Porous bulk metallic glass fabricated by powder consolidation 103
The amount of tiny holes in the cell wall or struts is determined by the particles
orphology and size of the amorphous and salt powder as well as the compacting
ressure. It is from Fig.2 (b) and (c) that there exists some tiny holes, less than 3µm in
iameter. Under the current compacting conditions, the tiny holes might reduce the
rength and increase the porosity of BMG foam. Considering the enclosed residual salt
xisted in the foam that has reduced the porosity of the foam, the porosity of the foam is
mewhat unaffected.
The large resistance of Zr55Cu30Al10Ni5 BMG to crystallization results in a SCL
gion that is accessible on a convenient time scale for processing, as shown in Fig.1.
his allows for a superplastic foaming (SPF) process, where the amorphous powder
ehaves like a plastic. An ideal amorphous powder for SPF would be identified by a large
percooled liquid region, no embrittlement during heating treatment. An amorphous
owder with a low Tg is desirable due to it reduces heating temperature and contamin-
ed in some sort. Though the SCL region of BMG foam is small, the thermal
ability is increased because of its T is increased.
sealed in a copper can. The can is then isothermal heated and compacted into a preform.
During hot pressing, the amorphous powder is deformed plastically in the SCL region. A
BMG foam can be successfully synthesized by this method. The particles among
amorphous powder were well bonded. XRD and DSC results show that crystalline phases
g (K) Tx (K)
m
p
d
st
e
so
re
T
b
su
p
ation come from oxidation. The SCL region of BMG foam shows smaller than that of the
amorphous powder, as shown in Tab1. The glass forming ability of BMG foam is
reduc
st g
Table 1. Glass transition temperature Tg, crystallization temperature Tx, supercooled
liquid region ΔTx and crystallization enthalpies ΔH of the amorphous powder and
BMG foam at heating rate of 20K/min.
sample T
Δ
Tx (K) ΔH (J/g)
(a) powder 672 762 90 52.06
(b) fo
It should be noticed that when the designed mixture of salt powder is heated in the
SCL-state of amorphous powder, the mixing salt intenerates and will not be as obstacles
but as transfers of pressure during pressing, because the salt liquid is incompressible.
4. CONCLUSIONS
A new process, SPF, has been developed for manufacturing BMG foam. In SPF,
amorphous powder and salt powder are first mixed at specified ratio, outgassed and
am 682 764 82 51.70

104 Z.Y. Suo, S.W. Liu, L. Zhang, H. L. Gao, H. Y. Zhang, K. Q. Qiu Vol.7, No.2
w
phase. L
remove ith three-dimensional network and good
connectivity is necessary.
RS
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ere not formed during hot pressing, and BMG foam was absolutely made of amorphous
ess than 3.5% percent of residual salt was enclosed in the foam. In order to
any residual salt particles, salt preform w
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