High-temperature centrifugation: A tool for finding eutectic compositions in multicomponent alloys

Article abstract of DOI:10.1063/1.1522820

The method for finding eutectic compositions in multicomponent alloys was discussed. The alloy was processed for 2 h in a centrifuge at a temperature of 530°C and them slowly cooled to room temperature during continuous centrifugation. It was found that high-temperature centrifugation is an effective tool to isolate and identify multiphase eutectic compositions and the sequence of crystallization in multicomponent alloys.

Full text of DOI:10.1063/1.1522820

High-temperature centrifugation: a tool for finding eutectic compositions in
multicomponent alloys
Jörg F. Löffler, Sven Bossuyt, Atakan Peker, and William L. Johnson
Citation: Applied Physics Letters 81, 4159 (2002); doi: 10.1063/1.1522820
View online: http://dx.doi.org/10.1063/1.1522820
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 22
25 NOVEMBER 2002
High-temperature centrifugation: a tool for finding eutectic compositions
in multicomponent alloys
J o¨ rg F. L o¨ fflera)
Department of Chemical Engineering and Materials Science, University of California,
Davis, California 95616
Sven Bossuyt
Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, United Kingdom
Atakan Peker
Liquidmetal Technologies, Lake Forest, California 92630
William L. Johnson
W. M. Keck Laboratory, California Institute of Technology, Pasadena, California 91125
͑
Received 5 June 2002; accepted 25 September 2002͒
A metallic melt of composition Al52.6Cu13.4Ge Si was processed for 2 h in a centrifuge at a
28
6
temperature of 530 °C and inertial acceleration of 60 000 g (gϭgravitational acceleration͒, and then
slowly cooled to room temperature during continuous centrifugation. Primary phases forming in the
melt during cooling were sequentially separated from the remaining liquid, changing the
composition of the liquid, until it solidified in a ternary eutectic microstructure of composition
Al Cu Ge . Presenting scanning electron microscope images, we demonstrate that this method of
6
4
3
33
high-temperature centrifugation is an efficient tool to isolate and identify multiphase eutectic
compositions and the sequence of crystallization in multicomponent alloys. The latter is particularly
useful for the discovery of new bulk metallic glasses. © 2002 American Institute of Physics.
͓
DOI: 10.1063/1.1522820͔
The main characteristic feature for good glass-forming
ability is a high ratio between the glass transition tempera-
ture T and the liquidus temperature T . When T /T ex-
Teutϭ524 °C ͑Al–Cu–Si phase diagram, 54 Phi, Ref. 3͒.
Furthermore, the eutectic in binary Cu–Ge has a composition
ratio of about 2 to 1 and the eutectic of Al–Ge is at about
g
l
g
l
4
ceeds a value of approximately two-thirds, nucleation is gen-
erally suppressed, which allows for the production of bulk
28% Ge. Based on these data, we prepared the composition
Al52.6Cu13.4Ge Si by induction melting the constituents on
28
6
1
,2
metallic glasses. Unfortunately, however, a T /T ratio ex-
a water-cooled copper boat in a Ti-gettered ultrahigh purity
Ar atmosphere. Subsequently, we remelted the resulting in-
g
l
ceeding two-thirds is generally only achieved in alloys con-
taining three or more elements. Whereas ternary phase dia-
grams can still be found for many alloy systems ͑see ten-
volume handbook of Ref. 3͒, only empirical data exist for
systems containing four or more elements, partly due to the
huge amount of data needed to be acquired and partly due to
the problems of visualization. Thus, for a systematic devel-
opment of bulk metallic glasses without laborious alloy de-
velopment, it is crucial to find a new method that can detect
eutectic compositions in multicomponent alloys.
Toward this end, we have developed a method of high-
temperature centrifugation, in which an alloy is heated above
its liquidus temperature and subsequently cooled to room
temperature during continuous centrifugation. The crystalline
phases that form and grow in the liquid on cooling are se-
quentially separated and stratified as primary crystals at the
sample ends, until finally eutectic microstructures solidify in
the central region of the sample. From the eutectic composi-
tion and the sequence of crystallization, useful information
towards the discovery of new bulk metallic glass composi-
tions can be extracted.
Ϫ5
got in a vacuum of 10 mbar and cast it into a copper mold
of 3 mm diameter and 4 cm length.
Using equations well known in separation science ͑see,
e.g., Ref. 5͒, one can estimate that inertial accelerations of
4
several 10 g at high temperatures are required to achieve
extensive sedimentation of small crystals ͑of 1 ␮m or less͒ in
relatively viscous metallic liquids. To meet such require-
ments, we collaborated with a company that routinely mea-
sures the room-temperature centrifugal strength of turboma-
chinery components6 and constructed an Inconel™ ͑Ni-
based superalloy͒ rotor of 25.4 cm diameter containing
internal cavities for the samples. Figure 1͑a͒ shows a picture
of the bottom part of the rotor. To suppress oxidation during
processing, the 4-cm-long sample was placed into a molyb-
denum crucible and hermetically sealed inside an Inconel™
,7
capsule by electron beam welding in a vacuum of
Ϫ4
1
0
mbar. The Inconel™ capsules fit precisely into the ro-
tor cavities, as can be seen in Fig. 1͑a͒. The rotor was spun
inside a circular furnace, which provided a uniform heating
across the sample with a temperature difference of less than
The Al–Cu–Si phase diagram shows a relatively deep
eutectic with composition Al80.6Cu13.4Si6 and temperature
1
K, as measured by four thermocouples connected to the
rotor via a rotating electrical feedthrough. The system is de-
signed to operate at temperatures of up to 875 °C at inertial
a͒Electronic mail: jfloeffler@ucdavis.edu
accelerations of up to 10 g. In the actual experiment, the
5
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0
1

4160
Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002
L o¨ ffler et al.
FIG. 3. Backscattered SEM images taken from regions at ͑a͒ 57%, ͑b͒ 65%,
͑
c͒ 74%, and ͑d͒ 98% sample height. ͑a͒ Al Cu ͑1͒, Ge ͑2͒, and some Al ͑3͒
2
crystals coexisting with eutectic microstructure. ͑b͒ Area near characteristic
crack. Above crack: Large homogeneous crystal of approximate composi-
tion Ge Si Al with a few Al–Cu crystals having a core-shell structure
77
21
2
FIG. 1. ͑a͒ Bottom part of Inconel™ rotor ͑diameter: 25.4 cm͒ with eight
symmetrically arranged internal cavities. Five cavities are filled with Inconel
capsules containing samples; the cavity at ‘‘three o’clock’’ contains a steel
rod to balance the weight of the sample opposite. ͑b͒ Optical micrograph of
Al52.6Cu13.4Ge Si embedded in a molybdenum crucible of 4.5 cm length
and inner diameter 3 mm. The sample end indicated as ‘‘Top’’ (hϭ0) is the
part closest the center of the centrifuge. The figure also indicates the regions
shown in the SEM pictures of Figs. 2 and 3, and marks the eutectic region.
͓AlCu ͑␩͒ core and Al2Cu ͑␪͒ shell, ͑4͔͒. Below crack: Al, some Al2Cu, and
Ge–Si crystals with core-shell structure ͓Ge–Si core, Ge shell, ͑5͔͒; see also
Al–Cu–Si phase diagram, 54 Phi ͓Ref. 3͔. ͑c͒ Similar microstructure as in
lower part of ͑b͒; volume fraction of Al2Cu has, however, increased. ͑d͒
Al–Cu crystals with core-shell structure ͑4͒ and Ge–Si crystals with core-
shell structure ͑5͒; only a few Al crystals ͑black͒ can be found.
28
6
For a more detailed data evaluation, we recorded a series
of 200 SEM images, each image covering an area of
sample was first ͑over-͒heated to a temperature of 560 °C at
low inertial acceleration. The angular frequency was then
increased to 2600 Hz during 10 min, which resulted in an
acceleration of 60 000 g at the center of the sample. After
reduction of the temperature to 530 °C, the sample was held
for 120 min at 60 000 g and subsequently cooled with a rate
of 4 K/min to room temperature at this inertial acceleration.
After removal from the Inconel™ capsule, the sample was
sectioned lengthwise with a diamond saw and prepared for
optical and scanning electron microscopy ͑SEM͒. The SEM
measurements were performed with an FEI high-resolution
2
00 ␮mϫ200 ␮m, from the ‘‘top’’ ͑closest to the center of
the centrifuge͒ to the ‘‘bottom’’ of the sample, and combined
these images to obtain a complete montage. Figure 2 shows
backscattered SEM images of characteristic microstructures
in the top half of the sample, the positions of which are
illustrated in Fig. 1͑b͒. The particles at the top of the sample
͓
black phase in Fig. 2͑a͔͒ are primary fcc-Al particles with
4
composition Al Ge ; Al has a solubility of 2% for Ge. At
9
8
2
the bottom of Fig. 2͑a͒ and the top of Fig. 2͑b͒, some pri-
4
mary Al Cu particles ͑␪ phase ͒ can be observed ͑gray
2
phase͒, with again ϳ2% Ge dissolved. As shown in Fig.
͑XL30sFEG͒ microscope equipped with an EDAX Phoenix
2
͑c͒, a ternary eutectic microstructure appears approximately
energy-dispersive spectrometry system to measure the el-
emental concentrations. All values are given in at. %.
Figure 1͑b͒ is an optical micrograph of the Al-based
sample, illustrating the stratification of the different crystal-
line phases. Primary phases can be observed at both sample
ends, whereas no clear microstructure is resolved in the
middle of the sample. The figure also labels the eutectic re-
gion and various positions in the sample that will be shown
in more detail in the SEM images of Figs. 2 and 3.
in the middle of the sample, containing primary Al ͑black͒,
Al2Cu ͑gray͒, and Ge ͑white͒. The approximate composition
of this ternary eutectic is Al Cu Ge .
6
4
3
33
Figure 3 illustrates characteristic microstructures from
the bottom half of the sample. Fig. 3͑a͒ shows a SEM image
from 57% sample height, where primary crystals coexist
with the eutectic microstructure. The gray phase in Fig. 3͑a͒
is again Al Cu with ϳ2% Ge ͑1͒, while the white phase
2
corresponds to primary Ge ͑2͒ with 1% Al and 1% Si. Some
primary Al particles ͑3͒ with 2% Ge are also observed.
Figure 3͑b͒ shows a conspicuous crack and its surround-
ings at 65% sample height. Above the crack, a large Ge–Si
crystal with approximate composition Ge Si Al exists to-
7
7
21
2
gether with some Al–Cu crystals, which have an AlCu core
4
͑
␩ phase ͒ and an Al Cu ͑␪͒ shell ͑4͒. Below the crack, the
2
primary Al phase and a few Al Cu crystals appear together
2
with Ge–Si crystals ͑5͒, the latter having a core-shell struc-
ture with a Ge–Si core and a Ge shell. With Si having the
higher melting temperature than Ge and being completely
4
miscible with Ge in the solid state, the liquid richest in Si
FIG. 2. Backscattered SEM images from ͑a͒ the top of the sample, ͑b͒ 9%,
and ͑c͒ 50% sample height. ͑a͒ and ͑b͒ Primary Al crystals ͑black͒ embed-
apparently nucleated first upon cooling and depleted the Si
content of the remaining liquid during growth, until finally
the Ge shell solidified. From the top down to a sample height
ded in eutectic microstructure, and some Al Cu crystals ͑gray͒. ͑c͒ Eutectic
2
microstructure with Al, Al Cu, and Ge ͑white͒ in equilibrium ͑see also
2
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Al–Cu–Ge phase diagram, 89 Efi, Ref. 3͒.
of 64% (ϳ400 ␮m above the crack͒, the average Si content
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Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002
L o¨ ffler et al.
4161
detected is р1%. Below the crack, however, it increases
abruptly to ϳ10% and is constant over the remaining part of
the sample.
nucleation of these three phases occurred independently over
a large area of the sample, the three phases nucleated at
roughly the same temperature. Otherwise, a more distinct
stratification would have been observed, as for example in
similarly processed Mg–Al based alloys.8
Moving further towards the bottom, the volume fraction
of primary Al Cu increases steadily at the expense of the
2
primary Al phase ͓compare Figs. 3͑c͒ and 3͑d͔͒. Finally, at
the far bottom of the sample ͓Fig. 3͑d͔͒, only a little primary
Al is observed, and the Al–Cu crystals appear again in a
Although the stratification is not complete, important in-
formation regarding the development of new bulk metallic
glass compositions can be gained. The fact that essentially all
Si is removed from the ternary eutectic and only found at the
bottom of the sample shows that the concurrent addition of
Si and Ge to the alloy is not useful in improving glass-
forming ability. Furthermore, Cu-rich crystals are found at
the bottom of the sample, and only 3% Cu is incorporated in
the ternary eutectic. This shows that Cu lowers the eutectic
temperature and thus appears to be useful in improving glass
formation up to 3%. Further Cu addition, however, results in
initial nucleation of the ␪ phase, or even the ␩ phase.
Since Si is removed during the early stages of solidifi-
cation, the last solidifying liquid is a three-phase eutectic
microstructure containing only the elements Al, Cu, and Ge.
The Al–Cu–Ge ternary phase diagram is not well explored
in literature, but a eutectic of composition Al59.5Cu Ge
core-shell structure ͑4͒ with an AlCu ͑␩͒ core and an Al Cu
2
͑
␪͒ shell, in addition to the Ge–Si/Ge crystals ͑5͒. Similar to
the latter, the growth of the peritectic AlCu ͑␩͒ phase having
the higher melting temperature depleted the remaining liquid
in Cu until finally the Al Cu ͑␪͒ shell solidified.
2
In the following, we discuss how the results of the high-
temperature centrifugation experiments can be used in the
search for new bulk metallic glass compositions. The ␩
phase and the Ge–Si phase ͑with ϳ20–30% Si͒ can only be
found at the bottom of the sample ͑or in a small area imme-
diately before the crack͒. This shows that the ␩ phase and the
Ge–Si phase nucleated first and were sedimented to the bot-
tom when most of the sample was still in the liquid state. As
illustrated in Fig. 3͑d͒, both phases solidified in a core-shell
6.5
34
melting below 420 °C ͑eutectic of binary Al–Ge͒ has been
reported ͑Al–Cu–Ge phase diagram, 89 Efi, Ref. 3͒, which
agrees well with the eutectic Al Cu Ge found in the
microstructure with Ge and Al Cu ͑␪͒ shells, respectively.
2
Furthermore, since both phases ͑4͒ and ͑5͒ are intercorre-
lated, it appears that the nucleation and growth of the Ge–Si
phase enriched the surrounding liquid in Cu and thus trig-
gered the nucleation of the ␩ phase. As one moves towards
the top, the overall Cu content decreases with the conse-
quence that the ␩ phase no longer formed. Apparently, the
Ge–Si phase then triggered the nucleation of the ␪ phase and
finally of the primary Al phase, which resulted in the net-
work structure seen in Fig. 3͑c͒. At ϳ64% sample height,
the Si content is reduced to such a low concentration ͑р1%͒
that the high-temperature Ge–Si phase also ceased to nucle-
ate. Thus, the area from the top down to 64% sample height
solidified much later than the remaining 36% towards the
bottom of the sample. During further processing, the core-
shell Ge–Si/Ge crystals near the liquid/solid phase boundary
apparently recrystallized to a large crystal of homogeneous
composition Ge Si Al with some smaller Cu-rich crystals
6
4
3
33
present study. Apparently, no quaternary eutectic exists be-
tween the ternary eutectics Al80.6Cu13.4Si6 (Teutϭ524 °C)
and Al59.5Cu Ge (TeutϽ420 °C). Thus, an alloy centrifu-
6.5
34
gally processed along a tie-line between these two ternary
eutectic compositions yields the deeper Al–Cu–Ge ternary
eutectic as the last solidifying liquid, as observed in the
present study. This shows that high-temperature centrifuga-
tion is able to find and physically isolate the deepest eutectic
composition in a multicomponent alloy.
In conclusion, high-temperature centrifugation is a
promising processing method for identifying deep eutectic
compositions in multicomponent alloys and for revealing the
sequence of crystallization. This method will be useful for
discovering new bulk metallic glasses without exploring
large numbers of alloy compositions.
7
7
21
2
embedded ͓Fig. 3͑b͔͒. The crack along the boundary of the
two different microstructures ͑homogeneous Ge Si Al
crystal versus inhomogeneous Ge–Si/Ge crystals͒ was either
caused by internal stresses during recrystallization, or intro-
duced after the experiment by the cutting of the sample.
This work was supported by DARPA ͑Grant No.
DAAD19-01-1-0525͒ and start-up funds from U.C. Davis.
The centrifugal experiments were performed with P. H.
Wawrzonek and B. Milatovic from Test Devices, Inc., Hud-
son, MA. One of the authors ͑J.L.͒ gratefully thanks M. Dun-
lap ͑U.C. Davis͒ for assistance with the SEM work.
7
7
21
2
In the area above the crack, only the phases Al, Al Cu,
2
and Ge are observed, which form more or less independent
layers of primary crystals ͓Figs. 2͑a͒, 2͑b͒, and 3͑a͔͒, as well
as a ternary eutectic microstructure of approximate compo-
1
T. Zhang, A. Inoue, and T. Masumoto, Mater. Trans., JIM 32, 1005 ͑1991͒.
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2
sition Al Cu Ge ͓Fig. 2͑c͔͒. The Ge and Al Cu crystals
3
4
5
6
4
3
33
2
P. Villars, A. Prince, and H. Okamoto, Handbook of Ternary Alloy Phase
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sedimented towards the bottom during the centrifugal experi-
3
ment, since their densities ͑5.35 and 4.82 g/cm , respec-
tively͒ are higher than the average density of the eutectic
3
3
composition (3.76 g/cm ). The Al Ge phase (2.75 g/cm )
9
8
2
floated to the top of the sample. Complete sedimentation,
however, was not always achieved. In some cases, the lighter
Al particles, for example, obstructed the sedimentation of the
551 ͑1998͒.
6
7
8
H. E. Sonnichsen, Global Gas Turbine News 37, 4 ͑1997͒.
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heavier Al Cu or Ge phase. This shows that although the
2
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