Gold-based bulk metallic glass alloys based on Au-Cu-Si are introduced. The alloys exhibit a gold content comparable to 18-karat gold. They show very low liquidus temperature, large supercooled liquid region, and good processibility. The maximum casting thickness exceeds 5 mm in the best glassformer. Au49Ag5.5Pd2.3Cu26.9Si16.3 has a liquidus temperature of 644 K, a glass transition temperature of 401 K, and a supercooled liquid region of 58 K. The Vickers hardness of the alloys in this system is similar to 350 Hv, twice that of conventional 18-karat crystalline gold alloys. This combination of properties makes the alloys attractive for many applications including electronic, medical, dental, surface coating, and jewelry

Johnson, William L.

Lohwongwatana, Boonrat

Peker, Atakan

Schroers, Jan

Caltech Authors - Main

Gold based bulk metallic glass
Jan Schroersa
Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena,
California 91125 and Liquidmetal Technologies, Lake Forest, California 92630
Boonrat Lohwongwatana and William L. Johnson
Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena,
California 91125
Atakan Peker
Liquidmetal Technologies, Lake Forest, California 92630
Received 7 February 2005; accepted 2 June 2005; published online 3 August 2005
Gold-based bulk metallic glass alloys based on Au–Cu–Si are introduced. The alloys exhibit a gold
content comparable to 18-karat gold. They show very low liquidus temperature, large supercooled
liquid region, and good processibility. The maximum casting thickness exceeds 5 mm in the best
glassformer. Au49Ag5.5Pd2.3Cu26.9Si16.3 has a liquidus temperature of 644 K, a glass transition
temperature of 401 K, and a supercooled liquid region of 58 K. The Vickers hardness of the alloys
in this system is 350 Hv, twice that of conventional 18-karat crystalline gold alloys. This
combination of properties makes the alloys attractive for many applications including electronic,
medical, dental, surface coating, and jewelry. © 2005 American Institute of Physics.
DOI: 10.1063/1.2008374
Gold became known to mankind thousands years ago,
and has been of inestimable value to civilization ever since.
It is the noblest of the noble metals, and has a good combi-
nation of high thermal conductivity, high electrical conduc-
tivity, and high corrosion resistance. These properties have
resulted in extensive use of gold and its alloys in electronic,
aerospace, astronomy, medical, and industrial applications.
For instance, gold can be found in many of today’s sophisti-
cated electronic devices. However, gold has always been
most valued as a jewelry material. Gold alloys are easy to
fashion, are nonallergenic, have a bright pleasing color, and
remain tarnish free indefinitely. Pure gold and higher karat
gold alloys, however, are rather soft and therefore vulnerable
to wear and scratching. This results in diminished aesthetic
appearance, and is a drawback of conventional crystalline
gold alloys.
In the last two decades, several alloys based on Pd,1–3
La,4 Zr,5,6 Fe,7–9 and Pt10 were found to form bulk amor-
phous phases. Fully amorphous samples are obtained when
the alloys are cast into copper molds of diameter up to cen-
timeters which indicates critical cooling rates for glass for-
mation of 100 K/s or less. These bulk metallic glasses
BMGs exhibit properties such as high strength, large elastic
strain limit, high hardness, and, in some cases, substantial
ductility.11 The compositions of these BMGs are typically
close to a deep eutectic composition. Consequently, their
melting temperatures are much lower than estimated from
interpolation of the alloy constituents’ melting temperatures.
The resulting low liquidus temperature is an attractive prop-
erty for casting alloys. The extraordinary stability of BMG
forming alloys against crystallization also results in a large
supercooled liquid region, T, T=Tx-Tg, Tx: crystalliza-
tion temperature, Tg: glass transition temperature, the tem-
perature region in which the amorphous phase first relaxes
into a highly viscous liquid before eventually crystallizing.
In this temperature region, BMG’s are amenable to super-
plastic processing using netshape processing methods similar
to those employed for thermoplastics.12
The binary gold silicon eutectic composition was the
first alloy found to exhibit metallic glass formation by Du-
wez and co-workers in 1960.13 The critical cooling rate for
glass formation of this alloy is of the order of 106 K/s, re-
sulting in a critical casting thickness, dc, below 50 microns.
In an effort to increase the glass forming ability of this alloy,
Si was partially replaced by Ge, which resulted in only mod-
erate increase of the glass forming ability and the width of
the supercooled liquid region.14,15 The poor glassforming
ability of these early metallic glasses and the low glass tran-
sition temperature make these alloys of marginal interest for
most applications and explains the limited interest in these
Au-based metallic glasses over the past several decades.
A suitable amorphous gold based alloy for applications
such as jewelry, electronics, or dental reguires a Tg of at least
370 K to be stable at ambient temperatures. Other desirable
properties include: a large gold content 18 karat or
higher, high hardness, good processibility, and a critical
casting thickness that permits fabrication of net shaped ar-
ticles such as jewelry. From a processing point of view, it is
desirable to possess a large supercooled liquid region which
gives access to a low forming viscosity, which in turn facili-
tates superplastic forming.
In this letter we report gold based bulk metallic glass
forming compositions of various complexities, with a gold
content comparable to 18-karat gold alloys. Samples were
prepared by alloying the elements Au: 99.95%, Cu: 99.9%,
Ag: 99.5%, Pd: 99.95%, Si: 99.95% purity in an arc-melter.
Copper mold quenching was performed to solidify the alloy
in its amorphous state, and to determine the critical casting
thickness. Thermal analysis was performed in a differential
scanning calorimeter DSC Netzsch DSC 404c. X-ray dif-
fraction XRD was carried out on an Inel XRG 3000 usingaElectronic mail: jan.schroers@liquidmetal.com
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Cu K radiation. Hardness tests were performed on a Leco
R-600.
Within this alloy development work a large number of
compositions was studied from various families of alloys.
The alloys that are explicitly introduced represents the com-
positions in each alloy family with the highest glass forming
ability. Figure 1 shows the DSC thermogram obtained by
heating at a rate of 20 K/min for four gold alloys. The ther-
mogram in Fig. 1a is that of Au55Cu25Si20 which was cast
in a 0.5 mm copper mold, Fig. 1b shows Au46Ag5Cu29Si20,
cast at 1 mm, Fig. 1c shows Au52Pd2.3Cu29.2Si16.5 cast at
2 mm, and Fig. 1d shows Au49Ag5.5Pd2.3Cu26.9Si16.3 cast at
5 mm. All compositions are in atomic percent. All alloys
show a glass transition and a crystallization peak suggesting
that at least some fraction of the material was amorphous. A
comparison of the heat of crystallization, H, and the heat of
fusion, Hf, which are summarized for the various alloys in
Table I gives approximately H /Hf =0.6, a value typical for
an entirely amorphous sample. When comparing with the Tg
of the early gold alloys of about 300 K Refs. 13–15 all
alloys have surprisingly high Tg. Their Tg is higher than
370 K with the exception of Au55Cu25Si20, thereby exceed-
ing requirements for most jewelry applications.
Figure 2 shows the x-ray diffractogram of the four al-
loys. The diffractogram in Fig. 2a is that of Au55Cu25Si20
cast in a 0.5 mm copper mold, Fig. 2b shows
Au56Ag5Cu29Si20 cast at 1 mm, Fig. 2c shows
Au52Pd2.3Cu29.2Si16.5 cast at 2 mm, and Fig. 2d shows
Au49Ag5.5Pd2.3Cu26.9Si16.3 cast at 5 mm. All spectra show
broad maxima typical for entirely amorphous material, sup-
porting the DSC results.
The results are summarized in Table I. All alloys
show liquidus temperatures of below 700 K.
Au49Ag5.5Pd2.3Cu26.9Si16.3 has a liquidus temperature as low
as 644 K. With the glass transition temperature of 401 K, the
alloy has the reduced glass transition temperature, Trg
=Tg /Tl, of 0.62, a value only seen among excellent bulk
metallic glassformers.16 The supercooled liquid region for
Au55Cu25Si20, Au56Ag5Cu29Si20, and Au52Pd2.3Cu29.2Si16.5 is
approximately 30 K. For Au49Ag5.5Pd2.3Cu26.9Si16.3, the su-
percooled liquid region reaches 58 K.
T is often used as a parameter that describes the super
plastic formability of a BMG.17,18 However, when comparing
the formability of different alloy systems, T should be nor-
malized to the width of the undercooled liquid region, Tl
−Tg Tl: liquidus temperature. The parameter, S=T / Tl
−Tg, shows better correlation with formability, even though
the assumption of identical fragility of the liquid and viscos-
ity at Tl is oversimplified.19 The S parameter for
Au49Ag5.5Pd2.3Cu26.9Si16.3 of 0.24 exceeds even the one for
Zr44Ti11Cu10Ni10Be25 of 0.2, an alloy frequently used and
known for its super plastic formability,20 but is below the
TABLE I. Summary of the properties of the various gold based alloys. The
maximum thickness the alloy could be cast amorphous, dc, was determined
for copper mold quenching.
Composition
at %
Tg
K
Tx
K
T
K
Tl
K
Trg
Tg /Tl
dc
mm
H
J/g
Hf
J/g
Au49Ag5.5Pd2.3Cu26.9Si16.3 401 459 58 644 0.62 5 35 46
Au52Pd2.3Cu29.2Si16.5 393 427 34 651 0.6 2 33 49
Au46Ag5Cu29Si20 395 420 25 664 0.59 1 28 46
Au55Cu25Si20 348 383 35 654 0.53 0.5 32 57
FIG. 1. DSC thermogram determined by heating with 20 K/min of gold
based alloys of various compositions that were cast in copper molds of
various sizes. a Au55Cu25Si20 cast in 0.5 mm copper mold; b
Au46Ag5Cu29Si20 cast in 1 mm copper mold; c Au52Pd2.3Cu29.2Si16.5 cast in
2 mm copper mold; d Au49Ag5.5Pd2.3Cu26.9Si16.3 cast in 5 mm copper
mold.
FIG. 2. X-ray diffraction thermogram of gold based alloys of various com-
positions that were cast in copper molds with various diameters. a
Au55Cu25Si20 cast in 0.5 mm copper mold; b Au46Ag5Cu29Si20 cast in
1 mm copper mold; c Au52Pd2.3Cu29.2Si16.5 cast in 2 mm copper mold; d
Au49Ag5.5Pd2.3Cu26.9Si16.3 cast in 5 mm copper mold.
FIG. 3. Au49Ag5.5Pd2.3Cu26.9Si16.3, superplastically formed from a rod shape
into this article. The forming was carried out in air at 423 K for 200 s under
a forming pressure of 100 MPa. The amorphous structure of the plastically
formed article was confirmed by DSC and x ray. Note that the as-formed
article is shown with no subsequent finishing. The flash and overflows have
not been removed.
061912-2 Schroers et al. Appl. Phys. Lett. 87, 061912 2005
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
parameter for Pt57.5Cu14.7Ni5.3P22.5 Ref. 10 of S=0.34, the
highest S value among all bulk metallic glass forming alloys.
The large S parameter for Au49Ag5.5Pd2.3Cu26.9Si16.3 suggests
good formability. Superplastic forming of
Au49Ag5.5Pd2.3Cu26.9Si16.3 was demonstrated by forming rod
shaped material into various geometries. Figure 3 shows an
article that was formed in air at 423 K for 200 s using an
applied pressure of 100 MPa. The amorphous structure of the
plastically formed articles was confirmed by DSC and x-ray.
The precise replication of the complex geometry confirms
the excellent formability as suggested already by the S pa-
rameter.
For the three bulk glassforming alloys,
Au56Ag5Cu29Si20, Au52Pd2.3Cu29.2Si16.5, and
Au49Ag5.5Pd2.3Cu26.9Si16.3, Vickers hardness was determined
to be approximately 360 Hv. This is an extraordinarily high
value when compared with conventional 18-karat gold alloys
which have Vickers hardness around 150–200 Hv.21 Ultra-
sonic measurements were carried out to determine the elastic
constants of amorphous Au49Ag5.5Pd2.3Cu26.9Si16.3. Youngs
modulus was determined to be 74.4 GPa, shear modulus to
be 26.45 GPa, and bulk modulus to be 132.31 GPa which
results in Poisson ratio of 0.406.
A larger composition range in the composition space
around the four alloys was studied and characterized to de-
termine the correlation of glass forming ability and T with
composition. As an example, during the development of the
Au49Ag5.5Pd2.3Cu26.9Si16.3 alloy, the Au content was varied
between 40% and 60%, Ag content was varied from 0 to
20%, Pd from 0 to 5%, Cu from 0 to 35%, and Si from 14%
to 20%. Among the constituents of these alloys, the Si and
Pd content have the strongest influence on the glass forming
ability and T. This is illustrated in Fig. 4 which shows the
dependence of the critical casting thickness and T on Si and
Pd content. The Si content was varied between 14% and 20%
resulting in a variation of dc from 1 mm to 5 mm, and a
variation of T from 35 K to 60 K. By varying the Pd con-
tent from 0 to 5%, T increases continuously with some
scatter whereas dc reaches a maximum at Pd=2.3%. In both
cases, a strong dependence of T and dc on the composition
variations can be observed. However, no obvious correlation
between T and dc, as previously reported to be positive22,23
or negative,20,24 was observed.
In conclusion, gold based bulk metallic glass forming
alloys were developed with a weight content of 18 karat
gold. The alloys show low liquidus temperature, large super-
cooled liquid region, large maximum casting thickness and
good processibility for both casting and thermoplastic
processing in a similar manner like thermo plastics. The
combination of these properties together with their high
hardness and esthetical appearance make them ideal for jew-
elry, dental, medical, and electronic applications.
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FIG. 4. Composition dependence of T and dc for
Au58.5Ag6.6Pd2.8Cu32.186-xSi14+x for x=0–6 top;
Au60.1Ag6.8Cu33.183.7-yPdySi16.3 for y=0–5% bottom. A strong depen-
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061912-3 Schroers et al. Appl. Phys. Lett. 87, 061912 2005
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Gold based bulk metallic glass

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Gold based bulk metallic glass

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