Structure and phase transformation behavior of rapidly solidified alloys in the Ge-Al-La system

Article abstract of DOI:10.1016/S0025-5408(99)00200-7

The structure of rapidly solidified Ge-Al-La alloys, as investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), is discussed in comparison with that of Al-Ge-TM (TM = transition metals) alloys. Rapidly solidified Ge-Al-La alloy

Full text of DOI:10.1016/S0025-5408(99)00200-7

Materials Research Bulletin, Vol. 34, Nos. 12/13, pp. 1991–2001, 1999
Copyright © 2000 Elsevier Science Ltd
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PII S0025-5408(99)00200-7
THE STRUCTURE AND PHASE TRANSFORMATION BEHAVIOR OF
RAPIDLY SOLIDIFIED ALLOYS IN THE Ge–Al–La SYSTEM
Dmitri V. Louzguine* and Akihisa Inoue
Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-Ku,
Sendai 980-8577, Japan
(Refereed)
(Received November 30, 1998; Accepted February 15, 1999)
ABSTRACT
The structure of rapidly solidified Ge–Al–La alloys, as investigated by X-ray
diffraction (XRD) and transmission electron microscopy (TEM), is discussed
in comparison with that of Al–Ge–TM (TM ϭ transition metals) alloys.
Rapidly solidified Ge–Al–La alloys, being metastable materials, transform
into an equilibrium state upon heating. Phase transformations in the above-
mentioned alloys were studied by differential scanning and isothermal calo-
rimetry. © 2000 Elsevier Science Ltd
KEYWORDS: A. alloys, C. electron microscopy, D. microstructure, D. phase
transitions
INTRODUCTION
Ternary Al–Ge–TM (TM ϭ transition metals) systems have wide composition ranges for the
formation of an amorphous single phase in alloys produced by rapid solidification [1,2].
Amorphous alloys with high Ge content up to 50 at% were obtained in Al–Ge–Cr system
[1,2]. Attempts to improve the glass-forming ability of Ge–Al–Cr alloys and to produce an
amorphous single phase with higher Ge content, using a melt-spinning technique, have been
performed by adding several TM elements to the Ge–Al–Cr system [3], but also limiting the
amorphous single-phase formation region by 50 at% Ge. In the present study, we paid
attention to Ge–Al–RE (RE ϭ rare earth metals) and Al–Ge–RE alloys. The influence of RE
*To whom correspondence should be addressed.
1991

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D.V. LOUZGINE and A. INOUE
Vol. 34, Nos. 12/13
on the glass formation is reported in the present paper, using La as an example. Special
attention is given to the formation of the amorphous phase in Ge-rich alloys.
In accordance with the three empirical rules for achieving high glass-forming ability, i.e.,
(i) a multicomponent alloy system consisting of at least three components, (ii) large atomic
size ratios among the main constituent elements, and (iii) negative heats of mixing among the
constituent elements proposed in ref. 4, La was chosen as an alloying element to improve the
glass-forming ability of Al–Ge alloys, especially in Ge-rich areas. The atomic radii of La, Al,
and Ge are 0.188, 0.143, and 0.123 nm, respectively. The atomic radius of La is 1.31 and 1.53
times greater than that of Al and Ge, respectively. The large difference in atomic radii
between La on the one hand and Al or Cr on the other, along with the negative heat of mixing
[5] with Al and Ge, match the second and the third rules. The negative heat of mixing with
Al and Ge was estimated on the basis of binary [6,7] and ternary [8] phase diagrams, where
the formation of several stable binary and ternary compounds was observed.
EXPERIMENTAL
Ingots of Ge–Al–La alloys were prepared by arc-melting a mixture of pure Ge (99.9999
mass%), Al (99.99 mass%), and La (99.9 mass%) in an argon atmosphere. From these alloys,
ribbon samples about 0.015 mm in thickness and 0.9 mm in width were prepared by rapid
solidification of the melt on a single copper roller 200 mm in diameter, at a roller surface
velocity of 41.9 m/s. The estimated cooling rate was about 1 ϫ 106 K/s. The structure of
ribbon samples was examined by X-ray diffraction (XRD) with monochromatic Cu K␣
radiation. X-ray diffraction patterns of as-solidified samples were taken from the free side of
the ribbon samples (opposite the copper wheel), which was exposed to Ar atmosphere during
rapid solidification. Transmission electron microscopy (TEM) was carried out with a JEOL
JEM 2010 microscope operating at 200 kV. Samples for TEM were polished electrolytically
in a solution of 10 vol% perchloric acid and 90 vol% methanol at 208–213 K. The
transformation temperature and the heat of transformation were measured by isothermal
calorimetry and differential scanning calorimetry (DSC) at different heating rates.
RESULTS AND DISCUSSION
Formation Range of the Amorphous Phase and the Structure of Rapidly Solidified
Alloys. By analogy with the alloys of Al–Ge–TM systems [1,2] studied in the past, the La
content was varied from 10 to 20 at%. Compositions of Al60Ge30La10, Al55Ge30La15,
Ge50Al40La10, Ge45Al40La15, Ge50Al35La15, and Ge50Al30La20 alloys were chosen in order to
be close to the composition ranges of the fully amorphous Al–Ge–TM alloys obtained by the
same melt-spinning technique [2]. The compositions of the Ge-rich alloys were Ge60Al25La15
and Ge70La20Al10. The structure of all the aforementioned rapidly solidified alloys was found
to be a mixture of the amorphous and crystalline phase(s), except for the Al55Ge30La15 and
Ge70La20Al10 alloys, in which only crystalline phases were formed. The replacement of La
in the Ge50Al40La10 alloy by certain other rare-earth (RE) metals (e.g., Y, Ce, Nd, Sm, and
Gd) was not helpful for the production of an amorphous single phase. The amorphous phase
in Ge50Al40RE10 alloys, including Ge50Al40La10, was always found to form in coexistence
with crystalline phase(s). A relatively high-volume fraction of amorphous phase was formed
in rapidly solidified Ge50Al35La15 and Ge60Al25La15 alloys, as shown in X-ray diffraction
patterns of these alloys (Fig. 1). The strong broad peaks observed in these patterns correspond

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FIG. 1
X-ray diffraction patterns of the rapidly solidified Ge50Al35La15 and Ge60Al25La15 alloys.
to 32.43 (scattering vector k ϭ 22.77 nmϪ1) and 32.51 (k ϭ 22.83 nmϪ1) degrees of
double-theta angle values for Ge50Al35La15 and Ge60Al25La15 alloys, respectively. The shape
of the broad peaks is different from that of the amorphous Al–Ge–TM alloys, and no split is
seen in the first broad peak. Although it was impossible to obtain an amorphous single phase
in the Ge–Al–La alloys studied in the present work, the Ge-rich Ge50Al35La15 and
Ge60Al25La15 alloys were found to have a relatively high-volume fraction of the amorphous
phase. The crystalline peaks in the X-ray diffraction patterns of these alloys belong to some
unknown phases, the d-spacings and integration intensities of which are summarized in
Tables 1 and 2. It should be noted that the equilibrium phase diagram of a ternary Ge–Al–La
system has been previously studied [9], and several stable intermetallic compounds were
found to form. The crystalline structures of some compounds were determined, whereas the
structures of two other compounds in the Ge-rich area remained unknown.
The structures of Ge50Al35La15 and Ge60Al25La15 alloys, which are mixtures of amorphous
and crystalline phases, differ greatly from each other. As illustrated in Figure 2, an unknown
crystalline phase was homogeneously distributed in the amorphous matrix of the
Ge50Al35La15 alloy, whereas the structure of Ge60Al25La15 consisted of rough colonies of
crystalline phase that were crystallized separately from the amorphous matrix, as shown in
Figure 3(a,b). It is noted in Figure 3(c–e) that some nanosize crystalline particles were also
present in the amorphous matrix, but their volume fraction was low. The interface between
the amorphous matrix and a colony of crystalline phase is shown in Figure 3(f,g).
TABLE 1
d-Spacings and Integrated Intensities of Crystalline Peaks in the X-ray Diffraction Pattern
of the Rapidly Solidified Ge50Al35La15 Alloy
d-spacing, nm
integrated intensity, I/I0
0.3784
7
0.2649
100
0.1866
49
0.1422
55

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D.V. LOUZGINE and A. INOUE
TABLE 2
Vol. 34, Nos. 12/13
d-Spacings and Integrated Intensities of Crystalline Peaks in the X-ray Diffraction Pattern
of the Rapidly Solidified Ge60Al25La15 Alloy
d-spacing, nm
integrated intensity, I/I0
0.4409
9.3
0.4235
13
0.2787
5.4
0.2200
81
0.2171
100
0.2128
26
0.2109
35
Thus, the mixed amorphous ϩ crystalline structure of the Ge50Al35La15 alloy (see Fig. 2) was
considerably more homogeneous than that of the Ge60Al25La15 alloy. Crystalline particles of
about 50 nm in size were homogeneously distributed throughout the amorphous matrix of the
Ge50Al35La15 alloy (see Fig. 2). Enlarged bright-field, dark-field, and high-resolution TEM
FIG. 2
The structure of the rapidly solidified Ge50Al35La15 alloy: (a) bright-field TEM image, (b)
dark-field TEM image, and (c) selected-area electron diffraction pattern.

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FIG. 3
The structure of the rapidly solidified Ge60Al25La15 alloy. A colony of crystalline phase
crystallized separately from the amorphous matrix: (a) bright-field and (b) dark-field TEM
images. Amorphous matrix: (c) bright-field and (d) dark-field TEM images, and (e) selected-
area electron diffraction pattern. An interface between amorphous and crystalline phase: (f)
bright-field and (g) dark-field TEM images.

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Vol. 34, Nos. 12/13
FIG. 4
Enlarged micrographs of a single group of crystals in the structure of the rapidly solidified
Ge50Al35La15 alloy: (a) bright-field, (b) dark-field, and (c) high-resolution TEM s; and (d)
selected-area electron diffraction pattern.
images of a single crystalline particle embedded in the amorphous matrix are shown in Figure
4(a–c). Selected-area electron diffraction patterns taken from crystalline particles often
showed at least two slightly disoriented patterns, as seen in Figure 4(d), rather than one single
diffraction pattern. Such disoriented patterns are indicative of polygonization, i.e., the
formation of subgrains with small angle boundaries.
Phase Transformations in Ge50Al35La15 and Ge60Al25La15 Alloys Upon Heating. Rap-
idly solidified Ge50Al35La15 and Ge60Al25La15 alloys are metastable materials and transform
into an equilibrium state upon heating. DSC analysis of the Ge50Al35La15 alloy showed three
heat effects up to melting, as seen in Figure 5. The first heat effect shown in the DSC curve
of the Ge50Al35La15 alloy was related to the precipitation of the unknown phase, as shown
in Figure 6. In the second stage, a relatively small amount of pure Ge crystallized. During the
last reaction, the metastable phase transformed into a stable Al2Ge2La phase with a hexa-
gonal structure of a ϭ 0.428 nm and c ϭ 0.700 nm. In accordance with the equilibrium phase

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FIG. 5
DSC curves of the Ge50Al35La15 alloy taken at 0.33 K/s, and the Ge60Al25La15 alloy taken
at different heating rates.
diagram, the structure of the Ge50Al35La15 alloy after heating, using DSC up to 700 K,
consisted of the Fd3m Ge and Al2Ge2La phases, as shown in Figure 7.
The Ge60Al25La15 alloy exhibited a temperature-dependent transformation behavior. The
DSC curve for this alloy showed only a single exothermic peak up to melting, at a heating
rate of 0.33 K/s, whereas at lower heating rates of 0.17 and 0.08 K/s, for example, about three
heat effects (two relatively strong and an extremely weak intermediate one) were observed
(Fig. 5). At heating rates below 0.17 K/s, the DSC curve of the Ge60Al25La15 alloy shifted
to a lower temperature range but remained generally unchanged in shape. It is important to
note that the heat released during a single thermal effect, 85.7 J/g, observed at a heating rate
of 0.33 K/s, was almost equal to the total heat effects of 83.7 and 86.2 J/g observed at 0.17
and 0.08 K/s, respectively, calculated as a sum of all three effects (see Fig. 5). The single
peak observed at a heating rate of 0.33 K/s suggests that the three reactions that occurred
separately at a lower heating rate took place simultaneously at the higher rate. The reaction
related to the first DSC heat effect, scanned at heating rates of 0.17 K/s or less, was found
to be amorphous phase ϩ unknown crystalline phase 1 (see d-spacings in Table 1) 3
unknown phase 2. This reaction is similar to the peritectic reaction in binary systems with

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D.V. LOUZGINE and A. INOUE
Vol. 34, Nos. 12/13
FIG. 6
X-ray diffraction patterns of the Ge50Al35La15 and Ge60Al25La15 alloys after heating up to
615 K at 0.33 K/s and 608 K at 0.17 K/s, respectively.
amorphous instead of liquid phase. The unknown phase 2 generated a strong peak in the
X-ray diffraction pattern corresponding to d ϭ 0.2795 nmϪ1, as shown in Figure 6. The
corresponding d-spacings and integrated intensities of crystalline peaks are summarized in
Table 3. During the two subsequent reactions pure Fd3m Ge precipitated from an amorphous
matrix, and an unknown phase 2 transformed into an unknown phase 3. As has been
mentioned above, the unknown phase 3 precipitated in the Ge50Al35La15 alloy during the first
stage of transformation, upon heating (see Fig. 6). In the Ge50Al35La15 alloy, the unknown
phase 3 transformed into the Al2Ge2La phase through the high-temperature reaction, whereas
the unknown phase 3 remained in the structure of the Ge60Al25La15 alloy heated up to
melting temperature during DSC (see Fig. 7) or after homogenization annealing at 770 K for
100 h. Thus, one can say that this phase was stable in the Ge60Al25La15 alloy. In accordance
with the equilibrium Ge–Al–La phase diagram [8], it can be identified as the Ge2AlLa phase
of the unknown structure observed in ref. 9.
As has been pointed out, the difference in the shape of DSC curves notwithstanding, the
same reactions took place in the Ge60Al25La15 alloy during DSC carried out at all heating
rates studied, from 0.08 to 0.67 K/s, and the same phase composition was observed when all
phase transformations were completed, as shown in Figure 7. Moreover, the total heat effect,
as has been mentioned above, was equal. The difference was that, at a heating rate of 0.33
K/s and higher, the reaction of amorphous phase ϩ unknown crystalline phase 1 3 unknown
phase 2 was shown to be almost completed at the top of a single peak. The structure consisted
of the unknown phase 2 and the residual amorphous phase. The X-ray diffraction pattern of
the Ge60Al25La15 alloy heated at 0.33 K/s up to the top of the single peak, is the same as that
of the Ge60Al25La15 alloy shown in Figure 6. The latter pattern was taken from the samples
heat-treated up to 608 K at 0.17 K/s after the completion of the first heat effect (see Fig. 5).
During the second half of the single peak of the DSC curve taken at 0.33 K/s pure Fd3m
Ge precipitates from amorphous matrix and an unknown phase 2 transforms into unknown
phase 3.
In order to study the details of the first phase transformation in the Ge60Al25La15 alloy (i.e.,

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FIG. 7
X-ray diffraction patterns of the Ge50Al35La15 and Ge60Al25La15 alloys after heating up to
melting temperature. The unindexed peaks of the Ge60Al25La15 alloy belong to an unknown
phase.
amorphous phase ϩ unknown crystalline phase 1 3 unknown phase 2), an isothermal
differential calorimetry was carried out at 573 and 578 K. Fraction transformed vs. annealing
time dependence is shown in Figure 8. The volume fraction transformed at time t is assumed
to scale with the fraction of the total heat released. The transformation kinetics can be
described on the basis of the equation for volume fraction x transformed as a function of time
t [10]:
TABLE 3
d-Spacings and Integrated Intensities of Crystalline Peaks in the X-ray Diffraction Pattern
of the Rapidly Solidified Ge60Al25La15 Alloy Heated Up to 608 K at 0.33 K/s
d-spacing, nm
integrated intensity, I/I0
0.3273
3
0.2795
100
0.2470
15
0.2194
27
0.2107
21
0.1852
16
0.1676
15

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D.V. LOUZGINE and A. INOUE
Vol. 34, Nos. 12/13
FIG. 8
Volume fraction transformed versus annealing time dependence for the Ge60Al25La15 alloy.
x(t) ϭ 1 Ϫ exp[Ϫ(k(T)t)n]
(1)
(2)
Eq. 1 can be written as
ln[ln(1/(1 Ϫ x)] ϭ nln(k) ϩ nln(t)
where k is an effective rate constant and n is the Avrami exponent [10]. The Avrami plot of
ln[Ϫln(1 Ϫ x)] vs. ln(t) yields a straight line with slope n and intercept nln(k). From the data
shown in Figure 9, the average slope is calculated to be n ϭ 2.69. The slope value slightly
changes with temperature, going from 2.59 at 578 K to 2.79 at 573 K. Thus, the volume
fraction transformed as a function of time can be described on the basis of the eq. 1 with n ϭ
FIG. 9
Avrami plots for the Ge60Al25La15 alloy, constructed from the isothermal calorimetry
measurements.

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2.69. Such a value of the exponent n indicates an accelerated nucleation and three-dimen-
sional crystal growth by volume diffusion.
CONCLUSIONS
The structure of rapidly solidified alloys in the Ge–Al–La system (namely Al60Ge30La10,
Al55Ge30La15, Ge50Al40La10, Ge45Al40La15, Ge50Al35La15, Ge50Al30La20, Ge60Al25La15, and
Ge70La20Al10) was found to be a mixture of amorphous and crystalline phase(s), except for
the Al55Ge30La15 and Ge70La20Al10 alloys, in which only crystalline phases were formed.
The shapes of the broad peaks in the X-ray diffraction patterns produced by the amorphous
phase are different from those of the Al–Ge–TM alloys, and no split is seen in the first broad
peak. It can be said that, in the studied concentration range, La is a less effective alloying
element for improving the glass-forming ability of Al–Ge alloys than are the 3-d transition
metals (e.g, Cr).
Transformation behavior upon heating of the Ge50Al35La15 and Ge60Al25La15 alloys was
studied. The kinetics of the phase transformation amorphous phase ϩ unknown crystalline
phase 1 3 unknown phase 2 in the Ge60Al25La15 alloy can be described on the basis of the
Johnson–Mehl–Avrami relation, with an Avrami exponent n of 2.69.
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