Materials Transactions, Vol. 51, No. 2 (2010) pp. 310 to 316
Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis
#2010 The Japan Institute of Metals
Effect of Silver Addition on the 0 -Phase in Al-Mg-Si-Ag Alloy
J. Nakamura1 , K. Matsuda2; * , T. Kawabata2 , T. Sato3 , Y. Nakamura3 and S. Ikeno2
1
Graduate School of Science and Engineering for Education, University of Toyama, Toyama 930-8555, Japan
Graduate School of Science and Engineering for Research, University of Toyama, Toyama 930-8555, Japan
3
Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8522, Japan
2
The metastable 0 -phase that forms in an alloy with composition Al-1.0 mass% Mg2 Si-0.5 mass% Ag, over aged at 523 K, has been
investigated by transmission electron microscopy (TEM) in order to understand the effect of Ag-addition on the crystal structure of this phase.
According to the results of analyses of selected area diffraction pattern, high resolution TEM (HRTEM), energy dispersive X-ray spectroscopy
(EDS), the elemental maps by energy-filtered TEM (EFTEM) and high angle annular dark field scanning transmission electron microscopy
(HAADF-STEM), this precipitate consists of Mg, Si and Ag, and has a hexagonal unit cell which is similar to that of 0 phase in Al-Mg-Si alloys
without Ag. However, the unit cell a-axis of 0 with Ag was 0.69 nm, which is slightly smaller than the corresponding dimension in Ag-free 0
(0.71 nm). The bond overlap population (BOP) was also calculated for the bonding between atoms in the small cluster in the 0 -phase by the
discrete-variational (DV)-Xa method, suggesting the bonding in 0 with Ag was higher than in the Ag-free 0 .
[doi:10.2320/matertrans.MC200911]
(Received August 24, 2009; Accepted December 3, 2009; Published January 25, 2010)
Keywords: aluminum alloys, precipitation, transmission electron microscopy, silver-addition
1.
Introduction
It has been known that silver addition to the Al-Mg-Si
alloys improves strength, hardness, elongation, and accelerates the age-hardening response.1–4) However, studies of
the precipitation sequence are limited, especially about
precipitate crystal structures. It would be interesting, for
example, to compare the silver addition with reports about
the effect of copper addition to this alloy on precipitation.5–7)
In our recent study, an Al-1.0 mass%Mg2 Si-0.5 mass%Ag
alloy showed good ductility at peak aged condition as
compared with peak aged Al-1.0 mass%Mg2 Si alloy.3) On the
other hand, a new grain boundary precipitate has been found
in an Al-1.0 mass%Mg2 Si-0.5 mass%Ag alloy by HRTEM
method,8) designated as a quaternary Al-Mg-Si-Ag compound (the QAg -phase), similar to the Q0 -phase in Al-Mg-SiCu alloys.5–7) An addition of silver to this alloy also seems
to affect nucleation4) according to a differential scanning
calorimetric (DSC) analysis.
In this study, the 0 -phase formed in an Al-1.0
mass%Mg2 Si-0.5 mass%Ag alloy has been investigated by
TEM, the change of its lattice parameter has been shown,
and the possibility of a silver atomic column saturation has
been discussed.
2.
Experimental Procedure
Al-1.0 mass% Mg2 Si-0.5 mass% Ag (Al-0.68 at%Mg0.34 at% Si-0.14 at% Ag, (Ag-bearing)) and Al-1.0 mass%
Mg2 Si (Al-0.69 at%Mg-0.34 at% Si, (Ag-free)) alloys were
prepared using 99.99% purity Al, 99.9% purity Mg, Si and
Ag metals. The ingot obtained was hot- and cold-rolled to
0.2 mm thick sheets, heat treated at 848 K for 3.6 ks, and then
quenched into chilled water at 273 K. These sheets were
overaged at 523 K for 120 ks in a salt bath. Thin foils for
TEM observation were made by a conventional electrolytic
*Corresponding
author, E-mail: [email protected]
polishing technique using a solution of ethanol (9 parts) and
perchloric acid (1 part). HRTEM was performed using a
JEOL 4010T instrument operated at 400 kV, and chemical
analyses of precipitates were performed by energy-dispersive
X-ray spectroscopy (EDS). The elemental maps for Mg-K,
Si-K and Ag-L edges were also obtained by the three
windows method using the energy-filter (Gatan GIF200)
equipped with 400 kV-HRTEM. The elemental map for the
Ag-L edge (energy offset = 367 eV) was obtained using the
following condition; pre-edge 1 ðE1 Þ ¼ 312 eV, pre-edge 2
ðE2 Þ ¼ 337 eV and post-edge ðE3 Þ ¼ 500 eV. For the
Mg-K edge (energy offset = 1305 eV); E1 ¼ 1220 eV,
E2 ¼ 1280 eV and E3 ¼ 1370 eV. For the Si-K
edge (energy offset = 1839 eV); E1 ¼ 1744 eV, E2 ¼
1809 eV and E3 ¼ 1889 eV. The energy slit was 30 eV, an
exposure time for each window was 30 s. The total time to
record one elemental map therefore 90 s. A JEOL 2010F
TEM/STEM (200 kV, 0.19 nm point resolution) machine
was used for the acquisition of the ADF-STEM image. The
inner collector angle was 28 mrad. Small distortions may be
present in the ADF-STEM image due to the specimen drift
during the electron scan.
3.
Results
Figure 1 shows a bright field image from the aged sample
of the Ag-containing alloy. Many rod shaped precipitates can
be observed, which are aligned to [100] or [010] directions of
the matrix. The particle marked by white circle is the crosssection of a rod shaped precipitate which is parallel to [001]
direction of the matrix. The cross section of this precipitate is
shown in high resolution TEM in Fig. 2(a). Bright dots show
a hexagonal network having 0.69 nm spacing. This value
is slightly lower as compared to the previously reported 0
unit cell spacing of about 0.71 nm, found in a Ag-free
Al-1.0 mass% Mg2 Si alloy.9,10) The selected area electron
diffraction (SAED) pattern of the particle in Fig. 2(a) was
obtained and showed in Fig. 2(b). Figure 2(c) shows a
Effect of Silver Addition on the 0 -Phase in Al-Mg-Si-Ag Alloy
Fig. 1 Bright field image of the Ag-bearing alloy aged at 523 K for 120 ks.
The marked precipitate (circle) was observed by HRTEM in Fig. 2.
Fig. 2 (a) HRTEM image and (b) its SAED pattern obtained for the cross
section of a 0 -phase in the Ag-bearing alloy aged at 523 K for 120 ks.
(c) Schematic illustration of (b).
311
schematic illustration of this SAED pattern where large open
circles correspond to the matrix reflections, gray circles are
from the precipitate, and small open circles indicate very
weak or extinct ones also from the precipitate. The SAED
pattern of the precipitate shows a hexagonal arrangement
of diffracted spots, making an angle of 12 degrees with the
matrix. The third nearest spots marked by small open circles
were weak, and this is also observed in the 0 -phase in
the Ag-free alloy.9) Figure 3(a)–(c) show SAED patterns
obtained from rod-shaped 0 -phase in the Ag-free alloy,
perpendicular to their longitudinal directions and parallel to
h100i matrix.10) In comparison, Fig. 3(d)–(f) show corresponding patterns from 0 in the Ag-containing alloy. The
arrangements of diffraction spots in Fig. 3(a)–(c) are almost
the same as in Fig. 3(d)–(f) respectively, and indicate a zone
axis close to h112 0i0 for Fig. 3(a) and (d), and close to
h11 00i0 for Fig. 3(b) and (e). The zone axis for the 0
precipitates in Fig. 3(c) and (f) is between h112 0i0 and
h11 00i0 . Figure 4 shows schematic illustrations of the
SAED in Fig. 3. Figure 4(a)–(c) and (d)–(f) correspond to
Fig. 3(a)–(c) and (d)–(f) respectively. Large open circles
correspond to diffraction spots for the matrix, solid circles for
the precipitate, and small open circles for double diffracted
spots between the precipitate and the matrix. The arrangement of diffracted spots for the 0 precipitate in the Agaddition alloy is very similar to that in Ag-free alloy. The
spacings between the lines indicated as a, b, c, d and e are
compared with the spacing between 000 transmitted spot and
200 or 020 diffracted matrix spot, and their ratios are given in
Fig. 4: In Fig. 4(d), spacings for a, b and c were estimated as
0.53, 0.06 and 0.41 respectively, when the spacing between
000 and 020 matrix was fixed to 1. From Fig. 4(a), a and c
were estimated as 0.46, and b was 0.08. From Fig. 4(e),
spacings for a and b were estimated as 0.34, and for c was
0.32. However, spacings for a, b and c were equal to 0.33 in
the 0 of the Ag-free alloy, see Fig. 4(b). In the same way,
spacings for a, b and c in Fig. 4(f) were estimated as 0.14,
0.72 and 0.14 respectively. These data suggest the lattice
spacings of the 0 -phase in the Ag-bearing alloy are different
from those in the Ag-free alloy.
Fig. 3 SAED patterns obtained for the 0 -phase perpendicular to their longitudinal directions. (a)–(c) for the 0 -phase in the Ag-free alloy,
and (d)–(f) for the 0 -phase in the Ag-bearing alloy. The direction of incident electron beam is parallel to h100i matrix.
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J. Nakamura et al.
Fig. 4 Schematic illustrations of the SAED patterns in Fig. 3. Figure 4(a)–(c) and (d)–(f) correspond to Fig. 3(a)–(c) and (d)–(f),
respectively. The direction of incident electron beam is parallel to h100i matrix.
Fig. 5 SAED patterns from orientations perpendicular to the longitudinal directions of the 0 -phase, obtained with the matrix along
h130im directions. Figure 5(a) and (b) were obtained for the Ag-free alloy, and Fig. 5(c) and (d) were for the Ag-bearing alloy.
To confirm the difference of lattice spacings, SAED
patterns have been checked carefully. Figure 5 shows SAED
patterns obtained for the h130im direction which is also
perpendicular to the longitudinal direction of the 0 -phase.
Figure 5(a) and (b) were obtained from 0 in the Ag-free
alloy, and Fig. 5(c) and (d) were from 0 in the Ag-bearing
alloy. Zone axes for the 0 -phase are probably close to
h112 0i direction of 0 -phase for Fig. 5(a) and (c), and to
h11 00i direction of 0 -phase for Fig. 5(b) and (d). Figure 6
shows enlargements of the parts marked by rectangles in
Fig. 5. Figure 6(a) and (b) were obtained for the Ag-free
alloy, and Fig. 6(c) and (d) were for the Ag-bearing alloy.
Effect of Silver Addition on the 0 -Phase in Al-Mg-Si-Ag Alloy
313
Fig. 6 Enlarged pictures of the marked rectangles in Fig. 5. (a) and (b) were obtained for the Ag-free alloy, and (c) and (d) were for the
Ag-bearing alloy.
Spacings of d1 and d2 in Fig. 6(c) and (d) respectively are
wider than their equivalents in Fig. 6(a) and (b), although d3
was the same in each pattern, corresponding to the lattice
spacing of 0.203 nm of the Al-matrix. This means that the
a-axis of the 0 -phase in the Ag-bearing alloy is smaller than
that in the Ag-free alloy. According to this result, the lattice
constant of the 0 -phase which has a hexagonal crystal lattice
in the Ag-bearing alloy is estimated as a ¼ 0:69 nm and
c ¼ 0:405 nm.
4.
Discussion
A possible cause for the lattice constant of the 0 -phase in
the Ag-bearing alloy becoming smaller than in the Ag-free
alloy is suggested by the chemical analysis for the 0 -phase.
The chemical composition of the 0 -phase in the Ag-bearing
alloy was investigated by EDS analysis, and its result was
summarized by the Cliff-Lorimer plot in Fig. 7.11) The ratio
of Mg : Si : Ag was almost 3 : 2 : 1. It means that Ag enters
the composition of 0 -phase. Figure 8 also shows chemical
analysis by energy-filtered TEM method. Figure 8(a) is the
zero-loss image, and (b), (c) and (d) are elemental maps for
the Ag-L, Mg-K and Si-K edges. A brighter contrast in
Fig. 8(b)–(d) has been corresponded to the cross section of
the 0 -phase and it means that there are homogeneous
distributions of Mg, Si and also Ag inside the 0 -phase.
Figure 9 presents a HAADF-STEM image obtained from the
0 -phase in the Ag-bearing alloy, and it shows bright dots in
the cross section of the 0 -phase. It is well known that the
intensity in HAADF-STEM images depends on atomic
number,12) and this image means that the heaviest atom
among Mg, Al, Si and Ag, namely the Ag atom, is present in
the 0 -phase. The arrangement of the Ag-containing atomic
columns is hexagonal, with dimension close to 0.69 nm,
which is the same spacing as measured in the HRTEM image
Fig. 7 EDS analysis for the 0 -phase in the Ag-bearing alloy summarized
by the Cliff-Lorimer plot.
in Fig. 2(a). The image in Fig. 9 also shows there is one Agcontaining atomic column per unit cell. The particle structure
is complex, having domains that separate different Agcontaining areas. Some areas are Ag-free. There is a strong
Ag-enrichment at the particle interface with the Al matrix. It
is important to mention that the EDS, EFTEM and HAADFSTEM analyses do not exclude the possibility of Al entering
the composition of 0 , together with Mg, Si and Ag in the
present study.
The atom radii of Mg, Si and Ag are 0.160, 0.138 and
0.145 nm respectively.13) When there are 4 Mg atoms and 2
Si atoms inside the small hexagonal cluster defined by the
‘C’-atoms, with edge of 0.407 nm and depth of 0.405 nm at
the centre of the extended hexagonal crystal lattice9) in
Fig. 10(a), the volume of this cluster is 174:30 1030 m3 .
The cluster is shown in more detail in Fig. 11. When 1 Ag
atom substitutes 1 Mg atom in this unit, for example the
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J. Nakamura et al.
Fig. 8 Chemical analysis by energy-filtered TEM method. (a) the zero-loss image, (b), (c) and (d) are elemental maps for the Ag-L, Mg-K
and Si-K edges.
Fig. 9 HAADF-STEM image obtained from a 0 -phase in the Ag-bearing
alloy.
Fig. 10 (a) the extended hexagonal crystal lattice of the 0 -phase. (b)–(d)
are kinematical simulations of SAED patterns using Fig. 10(a). (b) Si, Ag
and Mg atoms occupy the C, A and B sites, (c) C site is Si and occupancy
of A and B sites is 25%Ag and 75% Mg, and (d) A and C sites are Mg and
Si, and the occupancy of B site was changed as 33%Ag and 67% Mg.
Effect of Silver Addition on the 0 -Phase in Al-Mg-Si-Ag Alloy
Fig. 11 The schematic illustration of a small hexagonal cluster surrounding C site in the extended hexagonal crystal lattice in Fig. 10(a). The A and
B sites are Mg atom on the same lattice plane of z ¼ 0 or 1, and the C site is
Si atom on the half plane of z ¼ 0:5 for the normal 0 -phase.
Table 1 The result of BOP values calculated using the small hexagonal
cluster in the 0 -phase as shown in Fig. 11. Mg in the A site in Fig. 11 is
substituted for the atom described as [A].
eV1
[A]-Mg
[A]-Si
Mg-Si
average
[A]=Mg
[A]=Ag
0.237
0.260
0.025
0.022
0.283
0.266
0.182
0.183
[A]=Si
0.232
0.015
0.277
0.175
Mg at A site, the Mg : Si : Ag ratio becomes 3 : 2 : 1 and
the a-axis becomes 0.696 nm, while the c-axis remains
the same 0.405 nm. This estimated value is smaller than
0.705 nm of the typical 0 -phase in the Ag-free alloy, and
it is in good agreement with data measured from SAED
patterns of Fig. 5. To understand the possibility of substitution of Ag for Mg in the 0 -phase, the bond overlap
population (BOP) between atoms was also calculated by the
discrete-variational (DV)-Xa method using the BONDODR
program.14) The small cluster presented in Fig. 11 has
similarities to the unit cell of the 0 -phase by Vissers
et al.,15) but has been simplified for the BOP calculations in
this work. A more accurate crystal structure for the Agcontaining 0 will be discussed in another publication. In the
Ag-free 0 -phase, the A and B sites are Mg atoms on the
same lattice plane of z ¼ 0 or 1, and the C site is Si atom on
the half plane of z ¼ 0:5. Here the spacing of A-A site is
0.705 nm, and C-C site is 0.407 nm. Therefore, in the Agfree 0 -phase the A site is occupied by an Mg atom, which is
described as [Mg] in Table 1. BOP between A and B, and A
and C sites, which are described as [Mg]-Mg and [Mg]-Si,
and are 0.237 eV1 and 0.025 eV1 , respectively. According
to the EDS result, it can be predicted that the Mg atoms at
the A sites would be substituted with Ag. In the case of Ag
at the A site, BOP for [Ag]-Mg was 0.260 eV1 , which is
higher than the [Mg]-Mg bonding. BOP for [Ag]-Si was
0.022 eV1 , which is similar to the [Mg]-Si bonding and
higher than [Si]-Si. All of calculated results were summa-
315
rized in Table 1. In conclusion, when the atom at the A site
is Ag instead of Mg, the [Ag]-Mg, [Ag]-Si and Mg-Si bonds
show higher values, and the average of the three bonds for
[Ag] is also higher. This means that small amounts of Ag
can make strong bonding, similar to a covalent bond in the
0 -phase.
Considering the results mentioned above, kinematical
SAED patterns were calculated using the unit cell in
Fig. 11. This is the hexagonal unit cell having lattice
parameters of a ¼ 0:69 nm and c ¼ 0:405 nm in the case
of Ag-containing 0 . In Fig. 10(b), Si, Ag and Mg atoms
occupy the C, A and B sites in this unit cell. The first and
third nearest neighbor diffracted spots indicated by arrows
showed weaker intensity slightly, but almost similar to
the other diffracted spots. The simulated patterns can be
compared to the experimental one in Fig. 2(b) that was
taken from a particle thin enough to assume kinematical
diffraction. In can be observed that every third nearest
neighbor diffracted spot in Fig. 2(b) was weaker than the
first nearest neighbor one, therefore the simulated SAED
pattern is not in good agreement with the experimental
one. When occupancies of A and B sites were changed
as 25%Ag and 75% Mg, intensities of the first and third
nearest neighbor diffracted spots disappeared, as shown in
Fig. 10(c). Finally, the occupancy of B site was changed
as 33%Ag and 67% Mg and simulated SAED pattern has
been obtained in Fig. 10(d). The intensity of third nearest
neighbor diffracted spot has been weakened in a similar
manner to the real SAED pattern in Fig. 2(b). These results
may indicate that Ag atoms do not have fixed positions in
the unit cell, because the intensity of SAED patterns for the
0 -phase would be strongly affected by the occupancy of Ag
in each site of the unit cell. The simulated SAED pattern in
Fig. 10(d) is still not in perfect match with the experiment,
because the intensity of first nearest neighbor diffracted
spot became weakened. More detailed analysis of SAED
patterns, that will include considerations of occupancy,
symmetry and inter-atomic distances in the unit cell will be
discussed elsewhere. However, it has been suggested that
small amounts of Ag could be included in the structure of
0 -phase in this alloy that will change its lattice parameter
and coherency with the matrix.
5.
Conclusions
To understand the effect of Ag-addition on the metastable
0 -phase, an Al-1.0 mass% Mg2 Si-0.5 mass% Ag alloy over
aged at 523 K has been investigated by TEM, HRTEM,
SAED, EDS and EFTEM methods. The following conclusions have been reached:
(1) Ag enters the composition of 0 -phase, shrinking its
hexagonal network from a ¼ 0:705 nm in the Ag-free phase
to a ¼ 0:69 nm, while the c direction remains the same
0.405 nm.
(2) 0 phase in the Ag-containing alloy has Mg, Si and Ag in
its composition, with ratios close to Mg : Si : Ag ¼ 3 : 2 : 1.
(3) The BOP values calculated with the DV-Xa method on a
simplified 0 crystal structure indicate stronger bonding
between atoms in the Ag-containing precipitate, as compared
to the Ag-free case.
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J. Nakamura et al.
Acknowledgement
The authors thank to Dr. C. D. Marioara and Dr. S. J.
Andersen in SINTEF, and Prof. R. Holmestad in NTNU,
Norway for their help of HAADF-STEM. A part of this study
was supported by Japan Society for the Promotion of Science
2006–2007 [Grant-in-Aid for Scientific Research (C)]
#18560675, and the 2007 research project in Venture
Business Laboratory, University of Toyama.
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