Journal of
MATERIALS RESEARCH
Welcome
Comments
Help
Precipitation of bcc nanocrystals in bulk Mg–Cu–Y
amorphous alloys
Wenshan Liu and William L. Johnson
W. M. Keck Laboratory of Engineering, 138-78, California Institute of Technology,
Pasadena, California 91125
(Received 17 August 1995; accepted 9 May 1996)
Coexistent amorphous and nanoscale bcc-Mg7 Li3 phases were found in slowly
quenched alloys of the Mg65–x Lix Cu25 Y10 system containing 3 to 15 at. % Li. The
crystallization behavior of these alloys has been studied. The grain size of the
nanocrystalline bcc phase which is formed ranges from 2 to 20 nm. The volume fraction
of nanocrystalline phases as well as the grain size of the nanocrystals increase as the Li
composition increases. Transmission electron microscopy studies suggest that the alloy
exhibits phase separation in the undercooled liquid state and that the nucleation and
growth of the bcc-nanocrystals is related to the phase separation. Some characteristic
thermal properties of the glassy phase are presented, and the composition dependence
of Tg and Tx is discussed. It is concluded that the addition of a small amount of Li is
essential for the production of a bcc nanocrystalline phase in the Mg –Li–Cu–Y system.
I. INTRODUCTION
In the past few years, both nanocrystalline and amorphous metallic materials have attracted increasing interest because they exhibit different physical properties than
conventional crystalline materials. Several systems have
been reported that exhibit coexistent nanocrystalline and
amorphous phases, such as fcc-Ni in Ni–Si –B,1 fccAl in Al–Ni –Ce,2 hcp-Mg in Mg –Zn–La,3 and hcpCo in Co–Zr–B,4 and have been observed to form
using rapid cooling techniques like splat-quenching or
melt-spinning. The thickness of these samples is below
50 mm, and the estimated cooling rate is about 106 Kys.
Such nanoscale amorphous structures have not been
generally found when lower cooling rates are used.
Recently, a new family of bulk metallic glasses in the
Mg –Cu–Y system having a wide supercooled liquid
region were found.5,6 Fully glassy rods with a diameter
of 7 mm were obtained by high-pressure die-casting. We
have observed that a nanocrystalline bcc intermetallic
phase forms in an amorphous matrix at relatively low
cooling rates when a few percent of Li is added to
the Mg–Cu–Y system. Here we define a bulk metallic
glass as having a minimum dimension of 1 mm which
is equivalent to a critical cooling rate of about 103 to
104 Kys.
II. EXPERIMENTAL PROCEDURE
Mg–Li–Cu–Y ingots were alloyed by induction
melting a mixture of the elements of purity ranging
from 99.9% to 99.999% on a water-cooled silver boat
under a Ti-gettered argon atmosphere. The ingots were
repeatedly turned over and remelted to ensure homo2388
J. Mater. Res., Vol. 11, No. 9, Sep 1996
geneity. Then they were further processed by casting
into copper molds under inert gas atmosphere to form
strip samples with 1 mm thickness. The structure of
the strips was examined by x-ray diffraction with an
Inel position sensitive detector using Co Ka radiation
(l ­ 1.790 Å). Thermal properties, including the glass
transition, crystallization, and melting behavior, were
measured by using a Perkin-Elmer differential scanning
calorimeter (DSC-4) interfaced to a personal computer
for data processing and analysis. The samples were
contained in molybdenum pans and scanned in a flowing
argon atmosphere to reduce oxidation. Microstructural
characterization was performed by a Philips TEM430
operating at 300 kV.
III. RESULTS AND DISCUSSION
Figure 1 shows the x-ray diffraction patterns of the
surfaces of as-cast Mg65 Cu25 Y10 and Mg65–x Lix Cu25 Y10
(x ­ 3, 6, 8, 10, and 15 at. %) strip samples with a fixed
thickness of 1 mm. It is clear that the alloys with Li
content higher than 6% are partially amorphous and
partially crystalline. The Mg50 Li15 Cu25 Y10 sample has a
complex structure containing several crystalline phases
while Mg57 Li8 Cu25 Y10 and Mg55 Li10 Cu25 Y10 contain
only one simple crystal phase identified as bcc-Mg7 Li3
with lattice constant a ­ 3.52 Å. The grain size of the
bcc phase is about 10 to 20 nm as estimated by fullwidth at half maximum of diffraction peaks. The x-ray
patterns taken from interior cross-sectioned surfaces of
the samples are the same as those taken from exterior surfaces. This implies that the bcc nanocrystalline
phase is nucleated not only on external surfaces but
also throughout the bulk samples (i.e., it is nucleated
 1996 Materials Research Society
W. Liu et al.: Precipitation of bcc nanocrystals in bulk Mg – Cu – Y amorphous alloys
FIG. 2. X-ray diffraction patterns of Mg59 Li6 Cu25 Y10 in the as-cast
state and in samples heated to 170 ±C and 230 ±C, respectively, in a
DSC at a heating rate of 20 Kymin and then subsequently cooled to
ambient temperature.
FIG. 1. X-ray diffraction patterns of as-cast strips of 1 mm thickness
for Mg65 Cu25 Y10 and Mg65–x Lix Cu25 Y10 (x ­ 3, 6, 8, 10, and 15).
uniformly throughout the sample). It is also observed that
the relative intensity of the diffraction peaks corresponding to bcc nanocrystalline Mg7 Li3 phase increases as the
Li content increases. This indicates that the amount of
bcc-Mg7 Li3 phase increases with increasing Li content.
The fact that the bcc-Mg7 Li3 phase is still found in
Mg50 Li15 Cu25 Y10 means this phase is relatively stable.
This can also be seen in the x-ray diffraction patterns of
Mg59 Li6 Cu25 Y10 samples which were heated to 170 ±C
or 230 ±C in the DSC at a heating rate of 20 Kymin
and then subsequently cooled to ambient temperature as
shown in Fig. 2. While the alloys with 3 or 6% Li look
fully amorphous by x-ray diffraction, from the selected
area diffraction patterns and dark-field TEM images of
these alloys shown in Fig. 3, one can see that there are
ultrafine nanocrystals distributed uniformly throughout
the amorphous matrix. The crystallite sizes range from
2 to 6 nm.
DSC traces at various compositions for Mg65–x Lix Cu25 Y10 (x ­ 3, 6, 8, 10, and 15) taken using a
heating rate of 10 Kymin are shown in Fig. 4. Two exothermic peaks are observed in the scan of Mg59 Li6 Cu25 Y10 beginning at 141 ±C and 208 ±C, respectively. It
was confirmed from the x-ray diffraction patterns for
annealed samples shown in Fig. 2 that the first peak was
due to precipitation and grain growth of the nanoscale
bcc-Mg7 Li3 while the second peak was due to the transformation of the remaining amorphous matrix to the intermetallic phase Mg2 Cu. The glass transition endotherm
and crystallization exotherm to bcc-Mg7 Li3 are overlapping in other scans. One also notes that the total heat release for crystallization of bcc-Mg7 Li3 phase is relatively
small, estimated to be about 90 calymol. This means the
free energy difference between the purely amorphous
phase and the partially crystallized nanocrystalline phase
is small, suggesting that the driving force for nanoscale
precipitation of the bcc-Mg7 Li3 phase is small.
Figure 5 shows a comparison of DSC scans for
Mg65–x Lix Cu25 Y10 (x ­ 6, 6, 8, and 10) in the as-cast
state (solid line) to the state previously annealed at
170 ±C to crystallize Mg7 Li3 (dash line). In the scans for
J. Mater. Res., Vol. 11, No. 9, Sep 1996
2389
W. Liu et al.: Precipitation of bcc nanocrystals in bulk Mg – Cu – Y amorphous alloys
FIG. 4. DSC scans for as-cast Mg65–x Lix Cu25 Y10 (x ­ 3, 6, 8, 10,
and 15) at a heating rate of 10 Kymin.
FIG. 3. Selected area diffraction patterns and dark-field TEM images of as-cast (a) Mg62 Li3 Cu25 Y10 and (b) Mg59 Li6 Cu25 Y10 . The
dark-field images were taken from the ring indicated by the arrow.
the annealed Mg –Li–Cu–Y, the Tg of the amorphous
matrix increases with both annealing and increasing Li
content. It is known7 that Tg tends to increase with an
increase of magnitude and number of the attractive bonds
among the constituent atoms. As the volume fraction
of the bcc Mg7 Li3 phase increases, one expects the Y
and Cu contents of the remaining amorphous matrix
to increase. In the amorphous Mg–Cu–Y system, both
increasing Cu and Y content will result in an increase
of Tg. Also, after annealing, the crystallization temperature, Tx, for Mg2 Cu decreases since the remaining
amorphous matrix has been enriched in copper due to
the precipitation of Mg7 Li3 .
The thermal behavior of Mg65–x Lix Cu25 Y10 alloys
near the melting temperature is shown in Fig. 6.
Mg62 Li3 Cu25 Y10 shows a relatively sharp melting endotherm while Mg50 Li15 Cu25 Y10 has a more complicated
melting structure exhibiting a series of exotherms. The
latter alloy starts to melt at a solidus temperature
2390
of 433 ±C followed by complete melting at liquidus
temperature 495 ±C. Table I summarizes the thermal
parameters including the glass transition temperature, Tg,
the crystallization temperature, Tx, for Mg2 Cu, the total
heat of crystallization, DHx, the melting temperature,
Tm, and the latent heat, Lm, where Tg, Tx, and Tm are
defined as the onset points of corresponding events. In
the case of melting, the solidus and liquidus temperatures
are given.
Figure 7 shows the high resolution electron micrograph for Mg62 Li3 Cu25 Y10 . There is an obvious contrast
in the TEM image which suggests two apparently different amorphous regions in the image. These amorphous
domains are separated by a relatively sharp boundary.
Based on these images, we suggest that the amorphous
alloy has undergone phase separation during solidification. The domains that exhibit lighter contrast appear to be fully amorphous. On the other hand, one
observes well-developed groups of crystalline lattice
fringes within the darker domains. These fringes can
be identified with the bcc nanocrystalline phase. The
overall domain sizes are about 10 nm while the grain
size of the nanocrystalline phase is even smaller. Since
J. Mater. Res., Vol. 11, No. 9, Sep 1996
W. Liu et al.: Precipitation of bcc nanocrystals in bulk Mg – Cu – Y amorphous alloys
FIG. 6. DSC scans for as-cast Mg65–x Lix Cu25 Y10 (x ­ 3, 6, 8, 10,
and 15) near melting point.
FIG. 5. DSC scans for Mg65–x Lix Cu25 Y10 (x ­ 6, 8, and 10) in
the as-cast state (solid-line) and previously heated up to 170 ±C to
crystallize Mg7 Li3 (dashed line).
the nanocrystalline phase is bcc-Mg7 Li3 , this suggests
that the darker regions are Mg –Li rich, while the lighter
regions are Mg –Li poor. It appears that phase separation
occurs in the liquid state prior to crystallization of the
nanophase during cooling from the melt. Nucleation
of the bcc nanophase then occurs preferentially in the
Mg –Li-rich amorphous phase. The subsequent growth of
these Mg–Li-rich nanocrystals would then be restricted
by the size of the phase-separated domains in which they
nucleated. Thus, the phase separation acts both to trigger
the nucleation and to limit the growth of the Mg7 Li3
crystals.
IV. SUMMARY
It is found that a nanoscale intermetallic phase bccMg7 Li3 with a grain size of 2 to 20 nm is formed
by nucleation in the bulk amorphous Mg65–x Lix Cu25 Y10
TABLE I. Thermal parameters for Mg65–x Lix Cu25 Y10 (x ­ 3, 6, 8, 10, and 15) alloys. Tg is the glass transition temperature of amorphous
liq
matrix. Tx is the crystallization temperature of Mg2 Cu. DHx is the heat of crystallization. Tmsol is the solidus temperature, Tm is the
liquidus temperature, and Lm is latent heat.
liq
Li (at. )
Tg (±C)
Tx (±C)
DHx (calymol)
Tmsol (±C)
Tm (±C)
Lm (calymol)
3
6
8
10
15
141
150
154
155
···
214
208
205
204
210
708
385
336
288
361
454
433
433
434
433
472
460
464
476
495
2051
1961
1768
1777
1537
J. Mater. Res., Vol. 11, No. 9, Sep 1996
2391
W. Liu et al.: Precipitation of bcc nanocrystals in bulk Mg – Cu – Y amorphous alloys
Adding Li changes the structure of Mg–Li–Cu–Y alloys
from bulk amorphous to amorphous plus nanocrystalline,
then finally to a mixture of several intermetallic phases.
From TEM, we suggest that Mg62 Li3 Cu25 Y10 undergoes
phase separation during cooling in the liquid state and
forms two phases that are, respectively, Mg–Li rich and
Mg –Li poor with a length scale of about 10 nm. The
primary crystallization occurs in the Mg–Li-rich regions,
and the crystallines are restricted to nanometer size
because the boundaries of these two regions constrain
the growth of the crystals. It has been found that the
presence of Li plays an important role in the formation
of nanoscale bcc structure.
ACKNOWLEDGMENT
The financial support from the Department of
Energy (Grant No. DEFG0386ER45242) is greatly
acknowledged.
REFERENCES
FIG. 7. The high resolution electron micrograph for as-cast
Mg62 Li3 Cu25 Y10 .
system. The nanocrystalline particles are homogeneously
distributed throughout the bulk sample. The ratio of
nanocrystal to amorphous phase and the grain size of
the nanocrystalline both increase as Li content increases.
2392
1. A. Inoue, T. Shibata, and T. Masumoto, Mater. Trans. JIM 33 (5),
491 –496 (1992).
2. A. Inoue, Y. H. Kim, and T. Masumoto, Mater. Trans. JIM 33 (5),
487 –490 (1992).
3. A. Inoue, N. Nishiyama, S. G. Kim, and T. Masumoto, Mater.
Trans. JIM 33 (4), 360 –365 (1992).
4. H. Kimura, A. Inoue, Y. Murakami, and T. Masumoto, SCI-RTOK-A 36 (2), 213 –223 (1992).
5. A. Inoue, A. Kato, T. Zhang, S. G. Kim, and T. Masumoto, Mater.
Trans. JIM 32 (7), 609 –616 (1991).
6. A. Inoue, T. Nakamura, N. Nishiyama, and T. Masumoto, Mater.
Trans. JIM 33 (10), 937 – 945 (1992).
7. H. S. Chen, Rep. Prog. Phys. 3, 353 (1980).
J. Mater. Res., Vol. 11, No. 9, Sep 1996