Materials Sciences and Applications, 2011, 2, 427-434
doi:10.4236/msa.2011.25056 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
Fine Scale Precipitates in Al-Mg-Zn Alloys after
Various Aging Temperatures
Nasser Afify*, Abdel-Fattah Gaber, Ghada Abbady
Physics Department, Faculty of Science, Assiut University, Assiut, Egypt.
Received March 5th, 2011; revised April 13th, 2011; accepted April 27th, 2011.
This article deals with investigation of fine-scale precipitation in Al-Mg-Zn alloys with compositions of Al - 2 at% Mg-
x at% Zn, (x = 1.8, 2 and 4.2). The precipitates morphology was examined by scanning electron microscope (SEM) and
correlated with the microhardness (HV) and differential scanning calorimetry (DSC) of the specimens. The precipitates
are characterized as
(MgZn2) and
(MgZn2) phases of hexagonal structure of the same composition with a slight
difference in lattice parameters. In addition, T-phase pf composition (Mg32 (Al, Zn)49) having a cubic crystal structure.
Owing to the determined activation energies of the precipitates, the kinetics associated with their nucleation and growth
can be characterized. The thermal energy acquired during aging leads to the agglomeration of precipitates to or larger
particle sizes.
Keywords: Al-Mg-Zn Alloys, Coherent Precipitates, Aging, Microhardness, Scanning Electron Microscopy,
Precipitation Kinetics
1. Introduction
Aluminum magnesium alloys are preferred, particularly
for automotive vehicle manufactures who aim to produce
lightweight vehicles. Reducing the weight and thus in-
creasing the fuel efficiency of vehicles is becoming an
important objective for the automotive industry as gov-
ernments introduce regulations for exhaust emission [1].
The addition of zinc to the Al-Mg alloy system reduc-
es the solid solubility of aluminum in magnesium, in-
creasing the amount of precipitate phase formed upon
aging and thus causing a moderate increase in strength
[1,2]. However, the phenomena of precipitation and the
mechanisms which govern them remain subject of inten-
sive research concerning the number and the nature of
the phases that appear in these alloys as well as the heat
and/or thermo mechanical treatment which are essential
for obtaining the necessary mechanical properties. The
phenomena of transformation of phases in these alloys
are most complex among all alloys based on aluminum
Aging processes in most aluminum alloys are complex
and the decomposition of saturated solid solutions ob-
tained by quenching takes place in several stages. Typi-
cally, coherent Guinier-Preston (G.P.) zones and semi-
coherent intermediate precipitates precede the formation
of the incoherent equilibrium precipitates [4,5]. The in-
termediate precipitates in the various alloys have gener-
ally been considered to have the same compositions as
the respective equilibrium precipitates, although the cry-
stal structures differ because the former maintain partial
coherency with the parent lattice.
The modification of precipitates involves the follow-
ing four transformation reactions [6]:
1) direct formation of coherent precipitates from the
solid solution; e.g. G.P. zones;
2) growth or transformation of coherent precipitates,
G.P. zones to other coherent forms;
' (MgZn2) and T'
Mg32 (Al, Zn)49 or (Al2Mg3Zn3);
3) dissolution of precipitates;
4) formation of incoherent precipitates
and T from
the solid solution at high temperature.
Other works reported that the precipitation sequence in
Al-Mg-Zn alloys is:
α-Supersaturated solid solution (α-sss) Guinier-Pre-
ston zones (GP zones) metastable phase
' (Hex)
equilibrium phase
(MgZn2) (Hex) [7,8].
2. Experimental Procedures
In order to study the decomposition and precipitation in
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
supersaturated Al-Mg-Zn alloys, three specimens of Al -
2 at% Mg - x at% Zn (x = 1.8, 2 and 4.2), were prepared.
The initial elements are Al (99.9% purity), Mg (99.9%
purity) supplied by Aluminum Company, Naga Ham-
mady, Egypt, and pure Zn (99.999% purity) from Aldrich
Chemical, Inc. USA. The prepared ingots were cylin-
drical in shape of 17 mm diameter.
Disc-shaped DSC samples of 5 mm diameter and 0.5
mm thickness were punched from the ingots. The speci-
mens were solution heat treated for 1 h at 803 K followed
by quenching into chilled water maintained at ~ 273 K. A
nearly similar weight of an annealed pure Al disc has
been used as a reference. Nonisothermal thermograms
were carried out for the as-quenched specimens using a
Shcimadzu DSC thermal analyser TA-50 at various
heating rates.
The dimensions of the disc-shaped specimens used for
microhardness measurements (Vickers hardness HV) are
about 15 mm diameter and 3 mm thickness. The speci-
mens were polished by abrasive papers of different grades.
The final polishing was achieved by using diamond paste
of particle size ~ 0.25 µm on cloth. Each HV value is ob-
tained from the average of at least ten readings distributed
over the whole surface of the specimen.
For SEM, the specimens surfaces were polished and
followed by etching using a solution of 1% HF + 2.5%
HNO3 (by volume) in H2O (Keller etcher). The SEM
examinations were performed after artificial aging for 30
min at various temperatures using JEOL-SEM 5400 LV-
Japan scanning electron microscope.
3. Results and Discussion
3.1. Precipitation Sequence
The studied specimens of Al- 2 at% Mg alloy which
contain 1.8, 2 and 4.2 at% Zn, were subjected to DSC
technique immediately after quenching from the solid
solution state (803 K). The specimens are non-isother-
mally scanned at a heating rate of 10 Kmin1 and printed
in Figure 1. Figure 2 shows the variation of Vickers
microhardeness, HV, of these quenched specimens as a
function of temperature.
From Figure 1 and Figure 2, five precipitation reac-
tions are detected and explained as follows:
Peak I; ascribed to the formation of Guinier-Preston
zones (G.P. zones). The abundant concentration of the
quenched-in vacancies have an effective role in forming
solute-vacancy complexes. The formed Zn-vacancy and
Mg-vacancy complexes may combine forming Zn-Mg-
vacancy clusters. These clusters act as preferable sites for
nucleation of G. P. zones, either vacancy rich G. P. zones
or solute rich G. P. zones.
Peak II; represents the precipitation of
' intermediate
Figure 1. DSC curves of Al - 2 at% Mg - x at% Zn, at a
heating rate of 10 K·min1.
phase, due to the dissociation of vacancies in the zones
and vacancy clusters and annealed out to grain bounda-
ries or free surface of the specimen leaving Mg-Zn clus-
' is a metastable phase (MgZn2), characterized by a
hexagonal structure with lattice parameter a = 0.50 nm, c
= 0.87 nm [9].
Peak III; can be ascribed to
stable phase (MgZn2).
' and
are hexagonal although the lattice parame-
ters are slightly different lattice parameter a = 0.5225 nm,
c = 0.8568 nm [9].
Shoulder IV; might be ascribed to the growth and
coarsening of
precipitates (MgZn2), noncoherent preci-
Peak V; can be ascribed to the nucleation and growth
T' precipitates (Mg32 (Al, Zn)49). The coarsening of
T'-precipitates to form T the crystal structure of this
phase is cubic structure of a = 1.422 nm noncoherent
(stable precipitates) is taking place [10].
As a representative, Al- 2 at% Mg- 2 at% Zn, Figure 3,
shows the variation of DSC thermograms as a function of
heating rate. The thermograms show that the peak tem-
peratures shift to higher temperatures as the heating rate
increases which indicate that the developed processes are
thermally activated.
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
Figure 2. Behaviour of the microhardness, HV, of the quen-
ched Al - x at% Zn - 2 at% Mg, as a function of aging tem-
The precipitation sequence can be concluded to be as
Supersaturated solid solution solute-vacancy clus-
ters + G.P. zones intermediate
' equilibrium
T'- phase T- phase
3.2. Isothermal Aging
The age hardening curves obtained during artificial aging
for Al- 2 at% Mg- 2 at% Zn at 393 K, 498 K, 533 K,
598K and 693K for representatives are shown in Figure
4. It can be observed that the age hardening behavior is
influenced by the aging temperature. Increasing the aging
temperature enhances the hardening behavior. In the
lowest aging temperature 393 K, the hardening precipita-
tion peak delayed to an aging time of higher than 1000
min, above which the precipitation hardening peak begin
to be reached, This result is in good agreement with L.
Hadjadj et al. [3].
On increasing the aging temperature to 498 K a slight
increase in the hardness appeared after 10 min of aging.
The peak hardness appeared after 150 min due to the
second hardness peak of the coherency precipitates
Aging at 533 K the first peak hardness did not appear and
Figure 3. DSC thermograms of Al - 2 at% Mg - 2 at% Zn
(balanced alloy), at different heating rates.
the seconded hardness peak starts at 20 min and reaches
its peak after ~ 80 min. By increasing aging temperature
to 598 K and 693 K the hardness peak shifts to the right
(lower time) which means that increasing the aging tem-
perature accelerates the nucleation of the precipitates.
The highest hardness of the specimen > 500 MP could
be achieved as a result of aging the specimen at 393 K
for a time above 1000 min.
The positive contribution to hardness depends on the
coherency of the precipitate with the matrix, size and
distribution of the precipitates and proximity of the par-
ticles [11,12].
3.3. Confirmation of Precipitation in Al- 2 at%
Mg- x at% Zn, (x = 1.8, 2 and 4.2) Alloys
To confirm the developed processes at the DSC reaction
peaks, the SEM is used for the aged specimens at each
reaction peak temperature. As the specimen is aged for
30 min at aging temperature 473 K, around the peak II in
DSC scans (
' phase) coherent phase. The SEM micro-
graph, Figures 5-7(a), shows the morphology of
' phase
MgZn2 which was identified as plate-shaped particles.
This result is in good agreement with T. Engdahl et al.
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
Figure 4. HV vs. aging time for Al - 2 at% Zn - 2 at% Mg.
As the aging temperature is increased to the corres-
ponding temperature of
-phase (peak III) for 30 min at
533 K, the SEM micrograph is shown in Figures 5-7(a).
-precipitates are blocky precipitates exhibit distinct
faceted platelets [4,12].
On raising the aging temperature to 673 K for 30 min;
the temperature of the peak V in the DSC scans, The
SEM micrograph revealed the existence of T phase
Mg32(Al,Zn)49 cigar-shaped, as shown in Figures 5-7(a).
The particle number density also decreased due to the
coarsening of T particles at the expense of the number
density [13] of the earlier precipitates developed at lower
In conclusion the surface density of precipitates
(number of precipitates per unite area) is decreased and
the particle size increased with increasing the aging tem-
In all studied alloys Al- 2 at% Mg- x at% Zn, (x = 1.8,
2 and 4.2), the particle size of grown precipitates in-
creased with increasing the aging temperature as shown
in Figure 8. The behaviour of particle size as a function
of aging temperature is found to fit exponential growth as:
exp T
Da y
 (1)
where a, b and y0 are constants of exponential formula
which are different for different alloys as shown in Table 1.
(a) (b) (c)
Figure 5. SEM micrograph of Al - 2 at% Mg - 1.8 at% Zn aged for 30 min at aging temperature. (a) 473 K; (b) 533 K; (c)
673 K.
(a) (b) (c)
Figure 6. SEM micrograph of Al - 2 at% Mg - 2 at% Zn aged for 30 min at aging temperature. (a) 473 K; (b) 533 K; (c) 673 K.
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
(a) (b) (c)
Figure 7. SEM micrograph of Al - 2 at% Mg - 4.2 at% Zn aged for 30 min at aging temperature. (a) 473 K; (b) 533 K; (c)
673 K.
Figure 8. Particle size of precipitates vs. aging time for Al -
2 at% Mg - x at% Zn.
Table 1. a, b and y0 constants for Al- 2 at% Mg- x at% Zn
a b y0
1.8 4.10589 118.15519 64.19479
2 0.002 47.52602 90.17734
4.2 0.00278 50.79372 172.37127
3.4. Dependence of the Volume Density on Aging
In the following, we tried to calculate the volume density
of the particles as a function of aging temperature in the
considered alloys. In this concern, calculating the linear
density of the particles NL m1 and the surface density
NA m2 are necessary. The volume density NV can be
calculated from [14]:
It can be easily noticed that NV decreases with in-
creasing aging temperature, because of its growth with
raising the aging temperature. The variations of NV as a
function of temperatures for all specimens Al-2 at% Mg-
x at% Zn, (x = 1.8, 2, and 4.2) alloys are represented in
Figure 9. The obtained values of NV ~ 1018 m
3 are in
good agreement with those obtained by Celotto [1] as 109
mm3 in the same range of temperature. The behaviour of
NV as a function of temperatures is presented in Figure 9.
3.5. Transformation Kinetics
Studies of the transformation kinetics for the precipitates
are always connected with the concept of the activation
energy. The study of precipitation processes is associated
with nucleation and growth processes, which dominate in
supersaturated alloys. In general, separate activation
energies must be identified with individual nucleation
and growth steps in a transformation, although they have
usually been combined with activation energy represent-
ative of the overall precipitation process [5,15]. In the
DSC non-isothermal method, the sample is heated at
various fixed rates
, and the heat evolved is recorded
as a function of temperature. For evaluating the activa-
tion energy of the processes, we have used Kissinger’s
method as it represents the most accurate method among
the other methods based on (Johnson-Mehl-Avrami)
(JMA) method.
Method of Kissenger [16]:
The Kissenger equation can be written as:
ln k
 (3)
is the heating rate, Tp is the reaction temperature
peak and E is the activation energy of the precipitation
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
Figure 9. Volume density vs. aging temperature for Al - 2
at% Mg - x at% Zn.
process, R is the universal gas constant and Ck is Kissin-
ger’s constant, which depends on the reaction stage and
on the kinetic model. According to Equation (3) plots of
versus 1
T exhibits straight lines. The slope
of the straight lines yields ER.
Figure 10(a)-(c) show
versus 1
T rela-
tionships. The values of E for the precipitates developed
in ternary alloy Al - 2 at% Mg- x at% Zn, (x = 1.8, 2 and
4.2) are determined and presented in Table 2.
It can be easily observed from the determined activa-
tion energies of G.P. zones and
' - phase formation; that
these values are slightly affected by the concentration of
Zn in Al - 2 at% Mg - x at% Zn. It increases slightly with
increasing Zn concentration in both precipitates. In con-
trast; the activation energies associated with T - precipi-
tates decreases with increasing Zn concentration in the
The determined activation energies of G.P. zones pre-
cipitation in the studied Al-Mg-Zn alloys is averaged as
54.11 3.3 kJ/mol. This value agreed with activations
energies of migration of Zn and Mg in Al of 53 and 59.9
KJ/mol respectively [17]. Our value is close to that ob-
tained by Ohta et al. [18] as 49.5 kJ/mol. Accordingly,
the precipitation kinetics of G. P. zones precipitation in
our alloy must have been controlled by migration of Zn
and Mg in Al - matrix.
The obtained average activation energy of
' - precipi-
tation is 82.23 8.7 kJ/mol is lower than the diffusion
energies of both Zn and Mg in Al of 116.2 - 129 kJ/mol
and 120 kJ/mol respectively [19]. In the same time the
activation energy is higher than the migration energies
obtained for Zn and Mg in Al. Therefore, the precipita-
tion kinetics of
' - precipitates could have been con-
trolled by a combination of both migration and diffusion
of Zn and Mg mechanisms in the studied alloys.
The activation energy associated with
- precipitation
Figure 10. ln[α/(Tp)2] versus 1000/Tp for different processes
in Al - 2 at% Mg- x at% Zn alloy. (a) 1.8 at% Zn; (b) 2 at%
Zn; (c) 4.2 at% Zn.
is 118.03 17.6 kJ/mol. One can see that the determined
activation energies are also close to each other and close
Fine Scale Precipitates in Al-Mg-Zn Alloys after Various Aging Temperatures
Copyright © 2011 SciRes. MSA
Table 2. Activation energies for Al - 2 at% Mg - x at% Zn associated with individual precipitates.
Activation energies of precipitates, kJ/mol
X Clusters and G.P. zones
1.8 50.36 0.13 73.47 0.13 113.49 0.07 279.79 0.19
2 55.13 0.34 82.32 0.07 137.51 0.12 270.71 0.58
4.2 56.86 0.08 90.89 0.09 103.10 0.03 234.89 0.32
to the activation energies of diffusion of Zn and Mg in Al
[19]. The activation energy of diffusion for Zn was ob-
tained by K. Asano et al. as 129 kJ/mol [19]. In addition
Kinzaku reported that the diffusion energy of Mg in Al
had the value of 125.2 kJ/mol [20]. This information
leads to the interpretation that
- precipitation process is
controlled by diffusion of Zn and Mg in Al – matrix.
The average activation energy of T' and/or T – phase
formation is obtained as 261.79 23.74 kJ/mol. This
value is significantly high which indicates that the driv-
ing force for T' and/or T – phase formation is high. This
information confirms that the precipitation of T' and/or T
– phase requires a high thermal energy; which interprets
T' and/or T – phase formed at relatively high temperature.
The kinetics controlling this process might not be con-
trolled by diffusion of Zn or Mg in the material. A more
reasonable explanation of the high activation energy as-
sociated with T' and/or T – phase formation is that this
process is not thermally activated but rather is tempera-
ture dependent.
4. Conclusions
1) The alloys can be described as having typically very
fine precipitates microstructure. The G.P. zones,
' and
T' phases are the most effective hardening phases. A high
number density of precipitates is responsible for high
2) The precipitation sequence of the supersaturated Al-
2 at% Mg - x at% Zn, (x = 1.8, 2 and 4.2) alloys, based
on the combined results of HV, and DSC, confirmed by
SEM and XRD can be written as follows:
Supersaturated solid solution (SSS) solute – vacan-
cy clusters + G.P. zones intermediate
' and T' phas-
es equilibrium
and T phases.
3) By increasing aging temperature the hardness peak
shifts to lower time which means that increasing the ag-
ing temperature enhances the precipitates nucleation
4) From SEM, the surface density of precipitates is
decreased and the particle size is increased with increas-
ing the aging temperature at the expense of the surface
density at lower aging temperature.
5) The volume density of the precipitated particles has
an average NV of ~1018 m
3, and the NV decreases with
aging temperature.
6) In the studied alloys, the average activation energy
for G.P. zones indicates that these zones could be con-
trolled by the migration of Zn and Mg in Al. Whereas the
activation energy of
' phase could be controlled by the
combination of both migration and diffusion Zn and Mg
in Al matrix and
- precipitation process is controlled by
diffusion of Zn and Mg in Al – matrix.
7) T – phase formation: this process is not fully ther-
mally activated but rather is temperature dependent.
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