Proceedings of the 9th International Conference on Aluminium Alloys (2004)
Edited by J.F. Nie, A.J. Morton and B.C. Muddle
 Institute of Materials Engineering Australasia Ltd956

Nano-cluster Controlled Precipitation in Al-Cu and Al-Mg-Si Alloys
Containing Microalloying Elements
T. Sato1, K. Hirose2, S. Hirosawa1
1 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
2 Graduate student, Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1,O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Keywords: Al-Cu alloy, Al-Mg-Si alloy, microalloying, cluster, GP zone, precipitation, AP-FIM,

Abstract
The formation behaviour of nano-clusters in age-hardenable Al-Cu and Al-Mg-Si alloys
containing microalloying elements was investigated using TEM and 3D-AP techniques.
Hardness and electrical resistivity measurements and a computer simulation method werealso performed. In the Al-Cu alloys containing Mg and Ag, Cu/Mg and Cu/Mg/Ag nanoclusters were found to exist by the 3D-AP technique and confirmed to be quite effective tocontrol precipitate microstructures of Al-Cu alloys. A new type GP111 zone was formed onthe {111} matrix planes by the assistance of nano-clusters. In Al-Mg-Si alloys containing
Cu and Ag, the age-hardening behaviour was influenced by the microalloying elements, i.e.
the initial stage was suppressed whereas the subsequent stage was accelerated by themicroalloying of Cu and Ag. The behaviour was also well understood by taking into
account the formation of nano-clusters such as Mg/Ag.
1. Introduction
It has been realized that the nanoscale clusters (i.e., nano-clusters) are generally formedin the initial stage of phase decomposition and are extremely important to control
precipitate microstructures of age-hardenable aluminium alloys. Microalloying elements
are expected to affect both the formation kinetics and structures of nano-clusters. In ourprevious papers[1-3], the formation behaviour of nano-clusters was examined using
electrical resistivity change, calorimetry, TEM and a computer simulation. In this work we
performed the 3D-AP technique together with TEM to detect nano-clusters directly in the
typical age-hardenable Al-Cu and Al-Mg-Si alloys containing several microalloying
elements such as Mg, Ag, Cu and Zn. Several types of nano-clusters containing soluteand microalloying atoms were found. It is also assumed that these nano-clusters stronglyinteract with quenched-in excess vacancies. The nano-clusters are expected to act aseffective heterogenous nucleation sites for the subsequent precipitates and affectmicrostructures and resultant alloy properties. In this work the role of microalloying
elements on the nano-cluster formation and heterogeneous nucleation behaviour arediscussed.

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2. Experimental Procedures

3. Results and Discussion
3.1 Al-Cu Alloys3.1.1 Effects of Microalloying Elements

Micro Vickers hardness

Micro Vickers hardness

Al-3.7%Cu, Al-3.7%Cu-0.29%Mg and Al-4.1%Cu-0.28%Mg-1.03%Ag alloys (in mass%)
were prepared for the Al-Cu alloy system. Al-0.83%Mg-0.51%Si, Al-0.83%Mg-0.48%Si0.27%Cu and Al-0.81%Mg-0.51%Si-0.38%Ag
140(in mass%) were also prepared for the Al-Mg-Si(a)
Al-Cu binary
130
alloy system. The alloy ingots wereMg-added120
homogenized and hot- and cold-rolled to sheets.
(Mg+Ag)-added
110
All the specimens were solution treated and100
then quenched into iced-water before several
90
aging treatments. TEM observation was80
performed using a JEM3010 microscope. For70
3D-AP observation sharp needle-shaped
specimens were carefully prepared and60
10-1 1A.Q.
10 102 103 104
examined using a new 3D-AP machine
Aging time , t/ks(Polaron CVT Ltd/Oxford nanoScience Ltd) at
150
20K. The atom distribution were analysed byAl-Cu binary(b)140
the “PoSAP” software.
Mg-added
130120
110
100
9080
7060

(Mg+Ag)-added

A.Q.

10-1110102103104

The age-hardening curves for Al-Cu alloys
Aging time , t/ks
containing microalloying elements of Mg andAg are shown in Figure 1. The characteristic
increase in hardness is found for Al-Cu (a)binary and Mg or (Mg+Ag)-added alloys inFigure 1(a). In the Mg-added alloy the agehardening is suppressed in the initial stage ofaging at RT and then accelerated in thesubsequent stage compared with the Al-Cualloy. At the aging temperature of 373K
(Figure 1(b)) the age-hardening of the Mg- (b)added alloy is greatly accelerated from thebeginning of aging. On the other hand, theage-hardening of the (Mg+Ag)-added alloy ismuch suppressed in the initial stage andsubsequently greatly accelerated compared
100nm
with other two alloys. The tendency is more
pronounced at 373K. The Ag-added alloy
Figure 2: TEM micrographs and corresponding(single addition) exhibited almost similar agediffraction patterns for (a) Al-Cu and (b) Mg-added
alloys aged at RT for 18ks.
hardening curves to those of the Al-Cu alloy,
suggesting that the single addition of Ag Figure 1: Age-hardening curves for Al-Cu binary,causes almost no influence on age- Mg-added and (Mg+Ag)-added alloys aged at (a)hardening kinetics. TEM micrographs and RT (room temperature) and (b) 373K.
corresponding diffraction patterns for the AlCu and Mg-added alloys aged at RT for 18ks are shown in Figure 2. An imaging plate wasused to record the weak intensity of diffraction patterns precisely for both alloys. The

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diffraction patterns of both alloys exhibit streaks running towards the <100> matrix
directions, showing that GP zones are formed at this aging stage. The quantitative
analysis of the streaks of both alloys revealed that the streaks of the Mg-added alloy was
more broad than those of the binary alloy, demonstrating that the GP zones of the Mgadded alloy are much finer than those of the binary alloy. Mg has the effect to produce
finer GP zones and possibly to affect nucleation of GP zones.
In order to know the reason for the greatly increased hardness of the (Mg+Ag)-added alloy
aged at 373K, TEM observation was performed. The observed microstructure exhibitedfine platelets co-existing with the conventional GP(1) zones aligned to the <100> directions.
The fine platelets seem to be aligned on the {111} matrix planes. To confirm this point
HREM images were taken from the alloy aged at 373K for 604.8ks and an example isshown in Figure 3. Small platelets with the single layer contrast were observed aligned onthe {111} matrix planes. The precipitates on the {111} planes are fully coherent with thematrix and exhibit strain field contrast. These platelets are named as the GP111 zones inthis work, which are different from the conventional GP(1) zones. From the above
observation the highest hardness is attributed to the characteristic microstructures of theGP111 zones on the {111} matrix planes, the slip planes of the fcc structure. The combined
addition of (Mg+Ag) was found to have the effect to produce fine GP111 zones. It is alsosuggested that the combined addition of Mg and Ag affects the nucleation of the fine {111}platelets.
3.1.2 Nano-Clusters[111]

As is discussed in our previous papers [1-3]small clusters with the nano-scale size,
nano-clusters, are generally formed in theinitial stage of phase decomposition of agehardenable aluminium alloys. The nanoclusters are expected to affect the formationof GP zones and metastable phases. In thiswork we have applied the 3D-AP (3dimensional Atom Probe) technique to detect
directly nano-clusters formed in the alloys[110]
containing microalloying elements. Figure 41][20shows 3D atom maps of Al-Cu and Mgadded alloys aged at RT. The crystal
5nm
orientations are indicated in the figure. To
avoid confusion Cu and Mg atom are only Figure 3: High resolution TEM image showing adisplayed in the maps. In the binary alloy Cu new type of GP111 zones co-existing with theatoms aggregate to form small clusters at RT. conventional GP zones in the (Mg+Ag)-added
alloy aged at 373K for 604.8ks.
The statistic analysis of atom positions
revealed that the Cu atoms are no longer
distributed homogeneously even in the very initial stage of aging. In the Mg-added alloy Cuand Mg atoms aggregate to form small clusters which are closely correlated. Therefore,Cu and Mg atoms are incorporated inside the nano-clusters. It is also reported thatquenched-in excess vacancies are incorporated inside nano-clusters. The nano-clusters
containing solute atoms (Cu), microalloying atoms (Mg) and vacancies are named ascomplex clusters. Based on our computer simulation the complex clusters are found to act
as effective nucleation sites for GP zones.

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Figure 5 shows 3D atom maps of the binary and (Mg+Ag)-added alloys aged at 373K. In
Figure 5(b) some GP zones are observed to be aligned to the <100> matrix directions. On
the contrary, a number of small clusters containing Cu, Mg and Ag atoms are found in
Figure (c), (d). In Figure 5(d), some of the clusters are elongated on the {111} matrix
planes. This indicates that the clusters have the tendency to aggregate on the {111}planes and resultantly to accelerate the formation of GP111 zones.
<220>

(a)

(a)

<220>

<220>

(b)

<111>

(c)
(b)

<220>

<111>

(d)
Cu

Mg

5nm

Figure 4: 3D-AP atom maps showing nanoclusters in Al-Cu binary and Mg-added alloys. (a)RT, 21.6 ks, (b)RT, 12.6 ks.

Cu

Mg

Ag5nmFigure 5: 3D-AP atom maps showing nanoclusters in Al-Cu binary and (Mg+Ag)-added
alloys aged at 373K. (a) Al-Cu, 373K for 0.6ks,(b)Al-Cu, 373K for 86.4ks, (c) (Mg+Ag)-added
alloy, 373K for 0.6ks, (d)(Mg+Ag)-added alloy,
373K for 86.4ks.

3.1.3 Heterogeneous Nucleation
The nano-clusters containing solute atoms, microalloying atoms and vacancies areexpected to act as nucleation sites for the subsequent precipitates. Therefore, to identify
the characteristic features of nano-clusters is extremely important. However, to identify
nano-clusters experimentally is not easy. Then, we performed a computer simulation toanalyse the very early stage of phase decomposition. Figure 6 shows one of the typicalexamples demonstrating the formation sequence of the complex clusters and nanoclusters. A Mg/vacancy cluster is initially formed (Figure 6(b)), indicating that vacancies
are trapped by Mg atoms. In the next step Cu atoms aggregate on the Mg/vacancy cluster

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to form a Cu/Mg/vacancy complex cluster (Figure 6(c),(d)). This is an important sequenceto represent the heterogeneous nucleation behaviour of GP zones, that is, the nanocluster assist processing (NCAP) for the GP zone formation. Mg atoms trap vacancies toretard phase decomposition (vacancy trapping) and subsequently aggregate to foam
Cu/Mg/vacancy complex clusters which accelerate the GP(1) zone formation. Similarly, the(Mg+Ag) combined addition also acts as strong vacancy trap sites in the form of
Mg/Ag/vacancy clusters, then contributes to form nano-clusters of Cu/Mg/Ag/vacancy,
which effectively act as the nucleation sites for GP111 zones formed on the {111} matrix
planes (Figure 7). This is again the heterogeneous nucleation of GP zones on nanoclusters.
(a)

(b)

Mg/Ag/vacancy
Cluster

Strongvacancy
trap

Suppressed
Cu clusters

Cu/Mg/Ag/vacancy
Cluster(c)(d)

Heterogeneous
nucleation

Cu

Mg

Increased hardness

GP111 zonevacancy

Figure 6: Formation sequence of complex
clusters and nano-clusters obtained in thecomputer simulation for Mg-added alloy. (a)start,(b) 5×105, (c) 5×106 and (d) 5×108 MCS.

Figure 7: Schematic illustration showing
heterogeneous nucleation of GP111
zones on nano-clusters.

3.2 Al-Mg-Si Alloys
3.2.1 Effects of Microalloying Elements
80
Micro Vickers Hardness (HV)

Age-hardening curves of Al-Mg-Si alloys
containing Cu and Ag at RT are shown in Figure
8. With increasing aging time hardness increases
rapidly for Al-Mg-Si, Cu-added and Ag-added
alloys. Compared with the Al-Mg-Si alloy, Cuadded and Ag-added alloys show the suppressedage-hardening in the initial stage thenaccelerated in the subsequent stage. Figure 9shows age-hardening curves at 453K for singleaged and two-step aged alloys. Two-step aging
was performed after pre-aging at RT for 86.4ks.
It is clear that Cu and Ag additions enhance thepeak hardness in the single aging condition. TheTEM micrograph in Figure 10 also demonstrates
that the Ag addition produces finer precipitates
which are effective to increase hardness.

7060
Base alloy
Cu-addedAg-added

50

40A.Q.

101102

103

104

Aging Time, t / ks

Figure 8: Age-hardening curves for Al-MgSi, Cu-added and Ag-added alloys at RT.

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The EDX analysis revealed
that Ag atoms are muchincorporated
insideprecipitates as shown inFigure 11. It is assumed that
Mg/Ag/vacancy clusters are
preferentially formed at thebeginning of aging as issuggested in Figure 5. TheAg added
Base alloy
Cu added
formation of Mg/Ag/vacancy
clusters retards the initial ageAging time at 453K, t / ks
hardening at RT (Figure 1). In
9: Age-hardening curves for Al-Mg-Si, Cu-added andthe next stage or at higher Figure
Ag-added alloys at 453K. Open symbol: single aging at 453K,aging
temperatures, Solid symbol: two-step aging after pre-aged at RT fro 86.4ks.
Mg/Ag/vacancy nano-clusters
accelerate the formation of GP zones and intermediate phases and increase the peakhardness. In the two-step aging, however, Mg/Ag/vacancy clusters will cause theincreased amount of Si-rich clusters at RT and resultantly enhanced negative effect of twostep age-hardening of the alloy (Figure 9).

100nm

Figure 10: TEM micrograph of Ag-added AlMg-Si alloy aged at 453K for 86.4ks.

Figure 11: EDX analysis results showing
incorporation of Ag atoms inside precipitates
shown in Figure 10.

4. Conclusions(1)

The microalloying elements of Mg and Ag in Al-Cu alloys suppress the initial agehardening at RT and accelerate the subsequent age-hardening, especially at 373K.
The 3D-AP clearly revealed that Cu/Mg and Cu/Mg/Ag nano-clusters are formed. The
nano-clusters are effective to accelerate the heterogeneous nucleation of GP zones
and a new type of GP111 zones.

(2)

The microalloying elements of Cu and Ag in Al-Mg-Si alloys also suppress the initial
age-hardening at RT and accelerate the age-hardening at the subsequent stage andat high temperatures. Mg/Ag nano-clusters are assumed to be initially formed andaffect the single aging and two-step aging behaviour.

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Acknowledgements
The present study was conducted as a part of “Nanotechnology Metal Project” supported
by New Energy and Industrial Technology Development Organization (NEDO) and The
Japan Research and Development Center for Metals (JRCM).
References
[1]
[2]
[3]

T. Sato, Materials Science Forum, 331-337, 85-96, 2000.
K. Hirose, S. Hirosawa and T. Sato, Materials Science Forum, 396-402, 795-800, 2002.
T. Sato, S. Hirosawa, K. Hirose and T. Maeguchi, Metall. Mater. Trans.,34A, 2745-2755, 2003.