Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
Atom probe studies on the early stages of precipitation
in Al-Mg-Si alloys
M. Murayama1, K. Hono1, M. Saga2 and M. Kikuchi2
1
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305, Japan
2
Nippon Steel Corporation, Steel Research Laboratories, Futtsu 293, Japan
Abstract
Pre-precipitation stages of Al-0.70Mg-0.33Si and Al-0.65Mg-0.70Si (at.%) alloys have been
investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron
microscopy (HREM). Atom probe results show that clusters of Mg atoms are present in the asquenched state. After a prolonged aging at room temperature, clusters of Mg, Si and their co-clusters
are detected, although no contrast suggesting the presence of precipitates are observed in HREM
images. In the specimens aged at 175ºC for 30min., small equiaxed Mg-Si precipitates are observed
by TEM. APFIM results show that the ratio of Mg to Si atoms in the precipitates is close to 1, rather
than 2 which is expected from the equilibrium concentration of Mg2Si.
1. Introduction
shown. In Al-Si binary alloys, it is believed that
the initial stage of precipitation involves clustering
of Si [8], and it has been suggested that this
clustering may also occur in Al-Mg-Si alloys based
on a differential scanning calorimetry (DSC) study
[7]. Using atom probe field ion microscopy
(APFIM), Edwards et. al. [9, 10] reported recently
that the initial stage of precipitation at 70ºC starts
from separate clustering of Si and Mg, followed
by co-clustering of Si and Mg. Thus, they
proposed the following precipitation sequence:
Al-Mg-Si alloys are widely used as mediumstrength structural alloys. In the continuing drive
for automobile weight reduction, Al-Mg-Si alloys
are considered to be the most promising candidates
for heat treatable bodysheet materials. In the
automobile manufacturing process, these alloys are
subject to a room temperature aging during storage
and an artificial aging during an elevated
temperature paint-bake cycle. The temperature for
the paint-bake cycle is typically 175ºC, and the
duration is approximately 30min. Thus, the alloys
for automobile body sheet are required to show
strong age hardening by this heat treatment.
In the 6xxx series of alloys, Mg and Si are
added either in balanced amounts to form quasibinary Al-Mg2 Si alloys or with an excess of Si
above the quasi-binary composition. Several
studies [1-3] reported that alloys containing an
excess of Si showed pronounced age hardening
effects at 175ºC, while artificial age hardening
response after room temperature aging was
significantly suppressed. Since room temperature
aging can not be avoided in the manufacturing
process, understanding the mechanism of this
adverse age hardening effect due to room
temperature aging is strongly desired.
Previous works [4-7] reported that the
precipitation sequence in Al-Mg-Si alloy is:
separate Mg and Si clusters → co-clusters of Mg and Si →
small equiaxed precipitates → β” precipitates →
β’
precipitates → Mg 2Si (β).
As kinetics of artificial aging drastically change by
room temperature aging, it is necessary to study
solute clustering in the room temperature aged
condition (T4) in comparison with the as-quenched
condition. As Si content in the alloy also changes
the precipitation kinetics, difference in preprecipitation reactions in both balanced and Siexcess alloys should be clarified. Previous studies
examined only the artificial aging process [9,10].
Thus, this study attempted to study solute
clustering at the T4 and early stage precipitation
stages in two types of Al-Mg-Si alloys, one having
a balanced composition and the other having an
excess of Si, and to understand the roles of room
temperature aging and the excess amount of Si in
affecting the artificial age hardening responses.
Si clusters à GP 1 zone à GP 2 zoneà β’ à Mg 2Si (β).
However, substantial evidence for clustering in the
early stage of aging has not been convincingly
1
Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
2. Experimental
180
Chemical compositions of the alloys used in
this study are listed in Table 1. These alloys were
solution treated at 550ºC for 30min. and
subsequently ice-water quenched. The solution
treated samples were subjected to various heat
treatments including room temperature aging,
artificial aging at 175ºC and artificial aging after
room temperature aging (two-step aging). In this
work, specimens stored longer than 70days at
room temperature are called “room temperature
aged specimens”. For atom probe analyses, a
laboratory-made
reflectron-type
energy
compensated time-of-flight atom probe (APFIM)
was used. Field ion microscopy images were
observed at temperatures of 20-30K with He as an
imaging gas, and atom probe analyses were carried
out at about 30K with a pulse fraction (Vp/Vdc ) of
20% in UHV (~1x10-10 Torr). Microstructures of
the samples were examined with transmission
electron microscopes (TEM), Philips CM-200,
operated at 200kV, and selected specimens were
further examined by a high resolution transmission
electron microscope (HREM), JEOL JEM4000EX operated at 400kV. The electron beam
was diverged as much as possible for HREM
observation, so that the damage to the specimen
could be kept minimum, and was converged only
when taking photographs with minimum exposure
times.
170
160
150
13000
14000
15000
Mg
400
300
200
100
400
Si
300
200
100
10
20
40
30
3
Total Number of Detected Atoms / x10
Fig. 1
Integrated concentration depth profile of the asquenched Al-0.65Mg-0.76Si alloy. The number of detected
solute atoms are plotted as a function of total number of detected
atoms.
Table 1
Chemical composition of the alloys (at.%)
Alloy
Mg
Si
Fe
Ti
Al
Balance 0.699
0.336
0.024
0.006
bal.
Si excess 0.654
0.758
0.005
bal.
Figure 2 shows high resolution electron
microscopy (HREM) image of the Si-excess alloys
aged at room temperature. Only uniform contrast
from (002)Al matrix is observed, and no indication
of the presence of precipitates are recognized.
Figures 3 (a) - (c) show a part of sequence of
layer-by-layer FIM images of the (011)Al planes of
the same specimen. In this series of images,
(022)Al planes were evaporated layer-by-layer, and
their images were recorded from each layer. The
bright shots observed in the (110)Al planes are
presumably due to Si atoms, which are expected to
image brightly due to their higher evaporation
field. In Fig. 3 (a), three bright spots are observed
on the terrace; three spots are again observed
approximately in the same position in the next
layer in Fig. 3 (b). This kind of sequence
continues to several atomic planes, suggesting that
there are clusters of Si atoms in this specimen.
3. Results
Figure 1 shows integrated concentration depth
profiles, or ladder plots, obtained from an asquenched sample with an excess of Si. In these
diagrams, the number of detected solute atoms are
plotted as a function of the total number of detected
atoms. Thus, the slopes of the plots represent the
local concentration of the alloy, and the horizontal
axis corresponds to the depth. Steep changes in the
slope are recognized (indicated by arrowheads) in
the Mg ladder diagram. In these regions,
concentration of Mg is significantly higher than the
average concentration in the alloy, suggesting that
clusters of Mg atoms are present. It should be
noted that such clusters are not observed in the Si
ladder diagram.
2
Number of Detected Mg / Si Atoms
Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
0.20nm
Fig. 2 HREM image taken at [001] zone axis of Al0.65Mg-0.76Si aged at room temperature.
Si
160
140
Mg
120
100
80
60
40
20
2
4
6
8
10
Total Number of Detected Atoms /
12
x10
14
3
Fig. 4 Integrated concentration depth profiles of Si and Mg of an
Al-0.65Mg-0.76Si alloy aged at room temperature.
a significance level of 98% (α =0.03). This result
indicates there is a positive correlation between the
number of detected Si and Mg atoms, suggesting
that Mg and Si atoms form co- clusters. A similar
analysis of the as-quenched samples indicates there
is no significant evidence for co-clusters.
Figure 5 (a) and (b) show bright field images
and [001] zone SADP’s obtained from the Siexcess alloys which are subject to aging at 175ºC
for 30min and two-step aging (T4+175ºC
30min.), respectively. Both images show contrast
arising from extremely fine precipitates. However,
SAD patterns do not show any extra spots or
diffuse scattering indicating presence of
precipitates. This result suggests that the
precipitates are fully coherent and that the aspect
ratio of these particles are small, thus the
precipitates may be characterized as spherical
zones (equiaxed precipitates). Note that the density
of the precipitate is significantly smaller in the twostep aged specimen, suggesting that room
temperature aging delays the kinetics of
precipitation or decreases the number density of
nuclei.
Figure 6 (a) and (b) show integrated
concentration depth profiles obtained from the Siexcess alloys, which were subject to aging at
175ºC for 30min. and two-step aging (T4+175ºC
30min.), respectively. In both profiles, Si and Mg
atoms are concentrated at the same region,
indicating that the precipitates observed in Fig. 6
are composed of both Si and Mg atoms. The
results also show that the ratio of Mg atoms to Si
atoms in the precipitates is close to 1:1, rather than
2:1 which is expected from the equilibrium Mg2Si
phase.
Fig. 3 A sequence of layer-by-layer FIM image of the
(022) planes of an Al-0.65Mg-0.76Si alloy aged at room
temperature. Bright spots are believed to correspond to
Si atoms.
Figure 4 shows integrated concentration depth
profiles of the Si-excess alloy aged at room
temperature. In addition to separate clusters of Mg
and Si atoms (indicated by arrowheads), a cocluster of Si and Mg atoms is observed (shaded).
This result suggests that Mg and Si atoms tend to
aggregate each other to form co-clusters upon
aging, as previously reported by Edwards et al.
[9]. In order to test the tendency of co-clustering
more quantitatively, contingency tables for Mg and
Si were constructed. Using 50 atoms /group and a
4 x 4 table (equals 9 degree of freedom), the
calculated value for χ 2 is 19.79. When compared
with the chi-square distribution, the null
hypothesis (no correlation between the detected
numbers of Si and Mg atoms) can be rejected with
3
Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
Number of Detected Mg / Si Atoms Number of Detected Mg / Si Atoms
(a)
80
(a)
Mg
60
Si
40
20
30nm
(b)
0
0
500
1000
1500
2000
2500
3000
Total Number of Detected Atoms
80
(b)
Mg
60
40
Si
20
0
6000
7000
8000
9000
10000
11000
Total Number of Detected Atoms
Fig. 6 Integrated concentration depth profile of an Al-0.65Mg0.76Si alloy (a) aged at 175ºC for 30min., and (b) two-step aged
(T4+175ºC 30min.)
The average ratio of Mg to Si atoms is now 1.6:1.
This result indicates that the Mg content in the
precipitates gradually increases as the aging goes
on.
4. Discussion
In the room temperature aged specimens,
separate clusters of Si and Mg atoms were detected
as well as Mg-Si co-clusters. The ratio Mg:Si of
these clusters was approximately 1:1, which is
much lower than the 2:1 atomic ratio expected
from the equilibrium Mg2Si phase. The equiaxed
G.P. zones observed after aging at 175ºC for
30min. had the similar Mg:Si ratio. As this heat
treatment condition is close to the one used in the
paint-baking process, the microstructure and
mechanical properties of the commercial alloys are
believed to be controlled by such non-equilibrium
zones containing Mg and Si in atomic ratios close
to 1:1, as previously reported by Edwards et al.
[10].
The needle shape precipitates observed in Fig.
8 are believed to be β” based on the SADP. The
atomic ratio of Mg to Si in these precipitates is
1.6:1, which is closer to that of Mg2 Si. Although
we haven’t examined the chemical composition of
larger precipitates observed in much later stages, it
30nm
Fig. 5 Bright field TEM images and the [001] zone SAD
patterns of Al-0.65Mg-0.76Si alloys (a) aged at 175ºC
for 30min., and (b) two-step aged (T4+175ºC 30min.)
Figure 7(a) and (b) show bright field images
and the [001] zone SADP’s obtained from the Siexcess alloys with heat treatments at 175ºC for 3h
and two-step aging (T4+175ºC 3h), respectively.
Both bright field images show strain field contrast
from fine precipitates. In the case of directly aged
alloys, needle-shaped strain field contrast is
observed in the bright field image, and streaks in
the <100> directions are observed in the SAD
pattern. Therefore, the precipitates observed in this
stage are [001] coherent needles. The absence of
clear streaks on the SAD pattern of two-step aged
alloys suggests that the precipitation kinetics are
significantly delayed by T4 heat treatment. The
corresponding APFIM concentration depth profiles
of the directly aged alloy show that the precipitates
contain more Mg atoms than Si atoms (Fig. 8).
4
Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
Mg / at.%
30
25
20
15
10
5
0
25
20
Si / at.%
15
10
5
0
0
100
200
300
400
Number of Atoms (x50)
Fig. 8
Atom probe concentration depth profiles of an Al0.65Mg-0.76Si alloy aged at 175ºC for 3h.
analogous to that in the Al-Si alloy. Our finding of
Mg clusters from the as-quenched stage, and
subsequent formation of Si-clusters and Mg-Si coclusters strongly suggest that this criterion is not
valid. Our atom probe results have convincingly
shown that both Mg and Si are involved in the
precipitation reaction from the pre-precipitation
stage, and hence, it is necessary to consider
supersaturation from the Al-Mg2 Si pseudo-binary
equilibrium for understanding the precipitation
processes in Al-Mg-Si alloys.
One of the objectives of this work was to
clarify the role of excess Si on affecting the
precipitation reaction. It was reported that Siexcess alloy showed a more pronounced age
hardening response than the balanced alloy by
room temperature aging and by artificial aging
without T4 treatment. On the other hand, once the
Si-excess alloy is aged at room temperature, the
age hardening response at 175ºC is significantly
suppressed [3, 13]. As long as the atom probe
results are concerned, we did not find any
noticeable differences in the Si-excess and
balanced alloys in the types of clusters observed in
the T4 conditions. Both alloys contained clusters
of Si, Mg and their co-clusters after prolonged
aging at room temperature. Saga et al. [3] reported
that the number density of needle-like precipitate
decreases and the size of the precipitate increases
as the pre-aging temperature is lower. This
suggests that solute-clusters or co-clusters formed
at room temperature aging does not work as
Fig. 7 Bright field TEM images and their SAD patterns
taken at [001] zone axis of Al-0.65Mg-0.76Si alloys (a)
aged at 175ºC for 3h., and (b) two-step aged (T4+175ºC
3h.)
is believed that more stable precipitates such as β’
has the atomic ratio of 2:1 [12]. Thus, we believe
that the ratio of Mg to Si in the precipitate
gradually varies from 1:1 to 2:1 as aging proceeds.
Lynch et al. [11] studied the chemical
composition of metastable precipitates in the quasi
binary Al-Mg2 Si alloy using a scanning
transmission electron microscope (STEM)
equipped with a field emission gun, and concluded
that the ratio of Mg to Si atoms in the early stage
precipitate is 0.44:1, which is significantly smaller
than the present result. Based on these results,
they concluded that the precipitation reaction is
controlled by the supersaturation of the least
soluble component of the equilibrium precipitate,
i.e. the precipitation reaction in Al-Mg-Si alloy is
5
Preprint accepted for publication in the Proceedings of the 44th International Field Emission Society
(Mater. Sci. Eng. A., in press.)
nucleation sites for β” at the temperature for the
artificial aging. The clusters formed at room
temperature would be reverted instead of acting as
nucleation sites for the β” when the specimen is
heated at 175ºC. Therefore, we speculate that the
clusters formed by room temperature aging affect
only the kinetics of precipitation by trapping
quenched-in vacancies. The density of Si clusters
would be higher in the alloys containing excess
amount of Si, leading to pronounced age
hardening at room temperature by ‘clusterhardening’ [14]. However, the size of these
clusters are too small to act as nucleation sites for
the β” precipitates. Thus, upon heating at artificial
aging temperature, these clusters are completely
reverted and the vacancy concentration will reach
the thermal equilibrium concentration by being
released from the clusters. As these small clusters
may not provide nucleation sites for β”
precipitates, kinetics for artificial aging would be
delayed in comparison with directly aged
specimens with a lot of untrapped quenched-in
vacancies or the alloy pre-aged at higher
temperatures with sufficiently larger clusters which
may act as nucleation sites for β”. In addition,
chemical compositions of the equiaxed zone and
β” may be affected by the alloy concentration. In
order to clarify these points, further works with
AP and HREM are in progress.
have a slightly higher Mg:Si ratio of 1.6:1. This
result suggests that the atom ratio of Mg:Si
approaches the equilibrium value of 2:1 by
prolonged artificial aging. Based on this study,
solute clustering which occur at room temperature
must be playing a critical role in affecting the
kinetics of artificial aging after room temperature
aging.
5. Conclusions
Solute clustering behaviors in the preprecipitation stage in Al-0.65Mg-76Si alloy have
been characterized by APFIM. It has been shown
that Mg atoms form clusters in as-quenched stage
and eventually form co-clusters with Si. The
atomic ratio of Mg to Si atoms in the Mg-Si cocluster is close to 1:1. The equiaxed zones
observed by artificial aging for 3 hours at 175ºC
[11]
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