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Science in China Series E: Technological Sciences 2006 Vol.49 No.5 582—589
DOI: 10.1007/s11431-006-2012-3
Effects of Mg and Ag elements on the
aging precipitation of binary Al-Cu
alloy
SONG Min & XIAO Daihong
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Correspondence should be addressed to Song Min (email: [email protected])
Received March 13, 2006; accepted June 28, 2006
Abstract The effect of trace elements Mg and Ag on the aging precipitation has been
studied. It has been shown that both Mg and Ag have no obvious effect on the aging precipitates but influence the nucleation and growth velocities of the precipitates in binary
Al-Cu alloy if added separately. However, when Al-Cu alloy contains both elements Mg
and Ag, Mg and Ag atoms strongly attract each other and form atom clusters along the
matrix {111} planes. These atom clusters help the Cu atoms aggregate and nucleate heterogeneously along the matrix {111} planes, which makes Mg atom clusters the optimized
nucleation sites for Ω phase and inhibit the nucleation of θ′ phase.
Keywords: Al-Cu alloy, Ω phase, θ′ phase, atom clusters.
Al-Cu alloys have been widely used as structural materials in aerospace field due to
their excellent combined properties. However, the alloys are mainly used at room temperature. The properties obviously decrease due to the precipitates (θ′ phase) coarsening
when the temperature is above 100℃[1]. Polmear et al.[2,3] indicated that the alloy will
nucleate a new type of aging precipitates if trace elements Ag and Mg are added. This
phase has good coarsening resistance at high temperatures, and thus improves the mechanical properties of the alloy at elevated temperatures.
―
Previous studies[4 7] on Al-Cu-Mg-Ag alloy concentrated on the crystal structure of Ω
phase. Some studies[4,5] showed that the Ω phase has a face-centered orthorhombic structure (a = 0.496 nm, b = 0.895 nm, c = 0.848 nm). Chang and Howe[6] have redesignated
this phase on the basis that it has the same composition (Al2Cu) as the equilibrium θ
phase (a = b = 0.6066 nm, c = 0.4874 nm), and they suggested that the precipitates have a
tetragonal structure[7].
―
Recently, some researchers[8 11] studied the atom aggregation behavior and the nucleation mechanism of the precipitates in Al-Cu-Mg-Ag alloy using computer simulation.
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Effects of Mg and Ag elements on the aging precipitation of binary Al-Cu alloy
The studies indicated that when the Mg and Ag elements are added into the binary Al-Cu
alloy, Mg and Ag atoms strongly attract each other and form atom clusters, which causes
the Mg atoms to aggregate along the matrix {111} planes (Mg atom clusters). This interaction between Mg and Ag atoms enhances the aggregation of Cu atoms along the matrix
{111} planes, and inhibits the aggregation of Cu atoms along the matrix {100} planes to
decrease the induced lattice distortion energy to the matrix, and thus inhibits the nucleation of the θ′ phase. However, a systematic study about the effect of Mg and Ag elements
on the nucleation of the θ′ phase and Ω phase, and the mechanism of the Mg and Ag atoms on the aging precipitation are still proven illusive. This paper studies the effects of
Mg and Ag elements, and the interaction between Mg and Ag atoms on the nucleation of
θ′ phase and Ω phase, which focuses on the nucleation mechanism of the precipitates.
1
Experimental
Table 1 illustrates the chemical composition of the testing alloys. Alloy 1 is a binary
Al-Cu alloy, alloy 2 is a tertiary Al-Cu-Mg alloy, and alloy 3 is a tertiary Al-Cu-Ag alloy,
while alloy 4 is a quaternary Al-Cu-Mg-Ag alloy. The design of the alloy composition
was based on the Al-Cu-Mg and Al-Cu-Ag phase diagrams to avoid the nucleation of the
S phase and T phase, and thus provide an optimized condition to study the nucleation and
precipitation of the θ′ phase and Ω phase. The four types of alloys were prepared in an
induction furnace in argon atmosphere. The as-cast materials were homogenized at 500℃
for 10 h, followed by air cooling to room temperature. Then they were hot extruded with
a ratio of 18 at 450℃. The extruded bars were solution-treated for 1 h at 250℃, and then
water quenched. The bars were artificial aging-treated at 180℃. Vickers hardness measurement was performed on all aged samples. The precipitates in all specimens were studied by transmission electron microscopy (TEM). The TEM specimens were prepared by
twin jet electro-polishing in 30% nitric acid-70% methanol solution at 35℃ and examined in a JEM-100CXII microscopy operating at 100 kV.
Table 1 Chemical composition of the specimens (weight fraction, %)
Specimen
Alloy 1
Alloy 2
Alloy 3
Alloy 4
2
Cu
4
4
4
4
Mg
Ag
−
0.5
−
0.5
−
−
0.6
0.6
Al
Balance
Balance
Balance
Balance
Results
Fig. 1 gives the Vickers hardness (Hv) curves of the four types of alloys. All the alloys
show similar trend of the variation in Hv with aging time. That is, it increases as the aging
time increases. After reaching the highest value, it then starts to decrease, showing
over-aged stage. The Vickers hardness of the four types of alloys can be expressed as
Hv4>Hv2>Hv3≈Hv1, or the time required to reach the peak-aging stage can be expressed
as T4<T2<T1≈T3.
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Science in China Series E: Technological Sciences
Fig. 1. Vickers hardness of four types of alloys as a function of artificial aging time. Aging temperature is 180℃.
Fig. 2 is the microstructure and selected area diffraction pattern (SADP) of the corresponding precipitates in four types of the alloys after aging to peak-stage. It can be seen
that plate-like phase along matrix {100} planes can be found in binary Al-Cu alloy, tertiary Al-Cu-Mg and Al-Cu-Ag alloys from Fig. 2(a), (c) and (e). This type of phase is the
typical precipitates in Al-Cu alloy －θ ′ phase. The diffraction patterns of the precipitates
in these three alloys are same (brightness is different), which indicates that the separated
addition of Mg and Ag elements has no obvious effect on the aging precipitation in Al-Cu
alloy. Actually, Mg and Ag atoms exist as solute atoms if the concentration is below the
solution limits. However, Mg and Ag elements strongly affect the nucleation and growth
velocities of the θ ′ phase although they do not affect the type of the precipitates in Al-Cu
alloy. It can be seen from Fig. 2 that when Al-Cu alloy contains Mg, the density of the θ ′
phase is much higher and the size of the θ ′ phase is much smaller than those in Al-Cu
alloy without Mg. When Al-Cu alloy contains Ag, the length and density of the θ′ phase
is similar to those in Al-Cu alloy without Ag, but the thickness is much thinner.
It can be seen from Fig. 2(g) that plate-like precipitates can also be found in quaternary Al-Cu-Mg-Ag alloy. These plate-like phases can be divided into two different types:
one is θ ′ phase, while the other is Ω phase. Both two types of phases have semi-coherent
relationship with the matrix. Ω phase and θ′ phase have a specific directional relationship
in matrix when nucleate at the same place: one θ′ plate nucleates along matrix {100}
plane and <100> direction, while two Ω plates nucleate along matrix {111} plane and
<111> direction, respectively.
3
Effects of Mg and Ag elements on the aging precipitation
For binary Al-Cu alloy, Cu atoms can occupy octahedral interspace and tetrahedral interspace in Al matrix, and aggregate along the second close-packed {100} planes and the
close-packed {111} planes at the early stage of aging. Fig. 3 is the schematic of the Cu
atoms occupying octahedral interspace and tetrahedral interspace, respectively. The ag-
Effects of Mg and Ag elements on the aging precipitation of binary Al-Cu alloy
585
Fig. 2. Effects of Mg and Ag elements on the aging precipitates of Al-Cu alloy. (a) Al-Cu alloy; (c) Al-Cu-Mg alloy;
(e) Al-Cu-Ag alloy; (g) Al-Cu-Mg-Ag alloy. (b), (d), (f) and (h) are SADP of the corresponding precipitates.
gregation of Cu atoms along the second close-packed {100} planes and the close-packed
{111} planes forms GP zones along {100} planes and GPB zones along {111} planes
respectively, and thus provides the nucleation sites for θ ′ phase and Ω phase.
Fig. 4(a) is the schematic of the Cu atoms aggregating along the second close-packed
―
{100} planes. Previous studies[8,9,12 14] indicated that in binary Al-Cu alloy, Cu atoms
aggregate obviously along the {100} planes at the early stage of aging (Cu atom clusters).
The aggregation of Cu atom forms single-layer GP zone (multi-layer GP zone has also
been observed[15]). The GP zone does not aggregate along the close-packed {111} planes,
which is due to the lattice distortion energy. Since the radius of the Cu atoms and Al atoms are 0.128 and 0.144 nm respectively, Cu atoms will form plus-distortion zone. If the
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Science in China Series E: Technological Sciences
GPB zones nucleate along the {111} planes,
the lattice distortion energy will increase dramatically as the GPB zones grow, and thus increase the free energy dramatically. The increase in free energy will cause the GPB zones
to solute finally. Thus, based on the kinetics,
Cu atoms will aggregate and grow along the
second close-packed {100} planes to decrease
the free energy.
Fig. 3. Schematic of the Cu atoms occupying ocWhen binary Al-Cu alloy contains trace Mg,
tahedral interspace (a) and tetrahedral interspace (b).
Mg and Cu atoms will strongly attract each
other and form large numbers of Mg-Cu couple to decrease the induced lattice distortion
energy due to the large size of Mg atom (atom radius is 0.162 nm). The Mg-Cu couple
can improve the aggregation of Cu atoms along both matrix {100} planes and {111}
Fig. 4. Cu atoms aggregate along {100} planes in binary Al-Cu alloy (a), and Cu and Mg atoms aggregate along
{111} planes in tertiary Al-Cu-Mg alloy (b).
planes so as to decrease the induced lattice distortion energy. The aggregation of Cu atoms increases the aging hardening velocity of Al-Cu-Mg alloy, and thus the time required
for Al-Cu-Mg alloy to reach the peak-aging stage is shorter than binary Al-Cu alloy.
Since the lattice distortion energy caused by small size of the aggregation of Cu atom
along the matrix {111} planes can be accommodated, the formation of GPB zones along
the {111} planes only needs Mg instead of Ag according to the kinetics, and evolve to Ω
phase as the aging time increases. Fig. 4(b) is the schematic of the aggregation of Cu and
Mg atoms along the close-packed {111} planes. Actually, some studies[16,17] indicated
that Ω phase has also been found in tertiary Al-Cu-Mg alloy (only small amount), and the
main phase is still θ′ phase distributing along the matrix {100} planes. The reason is that
the resistance of nucleation and growth of Cu atoms along {111} planes is much higher
than those along {100} planes. This resistance makes the nucleation and growth of θ′
phase the optimized mechanism. Thus, Mg element is the kinetic condition instead of
dynamic condition for the precipitation of Ω phase. Fig. 5(a) is the microstructure of
Al-Cu-Mg alloy, and some Ω phase distributing along the matrix {111} planes can be
observed (see arrow). Fig. 5(b) is the corresponding SADP for the Ω phase in Fig. 5(a).
Effects of Mg and Ag elements on the aging precipitation of binary Al-Cu alloy
587
When binary Al-Cu alloy contains trace Ag element, Ag and Cu atoms will not attract
each other (Ag-Cu couple will increase the lattice distortion energy) due to the small size
of Ag atoms (atom radius is 0.126 nm). Ag atoms stay as solute state in the Al matrix.
This solute state of Ag atoms has no obvious effect on the aggregation of Cu atoms.
However, since both Ag and Cu atoms are smaller than Al atom, they repel each other to
decrease the lattice distortion energy. Thus, Ag atoms have effects on inhibiting the
growth of θ′ phase.
Fig. 5. Ω phase (see arrow) (a) and corresponding SADP (b) in Al-Cu-Mg alloy.
When binary Al-Cu alloy contains both elements Mg and Ag, Mg and Ag atoms
strongly attract each other to form atom clusters, which makes the Mg and Ag atoms aggregate along the matrix close-packed {111} planes. These clusters have been observed
by field-ion microscopy in previous study[18]. Li et al.[8,9] also demonstrated the aggregation behavior of Mg and Ag atoms at the early stage of aging using computer simulation.
The aggregation of Mg and Ag atoms along the {111} planes decreases the plus or minus
lattice distortion energy introduced by adding Ag and Mg elements separately. Fig. 6 is
the schematic of the aggregation of Cu, Mg and Ag atoms along the close-packed {111}
planes, and Fig. 6(a), (b) and (c) is the schematic of the aggregation of the atoms along
Fig. 6. Cu, Mg and Ag atoms aggregate along {111} planes in Al-Cu-Mg-Ag alloy.
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Science in China Series E: Technological Sciences
three neighboring {111} planes. It can be seen that the Mg and Ag atom clusters along
the {111} planes provide optimized condition for the aggregation of Cu atoms along the
{111} planes, which means that Cu atoms aggregate along the {111} planes to form GPB
zones so as to decrease the free energy. At the same time, since Mg atom is larger than Al
atom, Mg atom clusters will form minus lattice distortion zone. The existing of the minus
lattice distortion zone further helps the Cu atoms aggregate around the Mg atoms to decrease the introduced lattice distortion to the matrix. This phenomenon makes the Mg
clusters the optimized zone for the nucleation of Ω phase. The aggregation of Cu atoms
along the {111} planes inhibits the aggregation of θ′ phase along the {100} plane. Thus,
Ag element is the dynamic requirement for the precipitation of Ω phase. The induced
aggregation of Cu atoms along the {111} planes and heterogeneous nucleation by the Mg
and Ag atom clusters increase the aging hardening velocity and improves the hardness
and strength (see Fig. 1).
4
Conclusions
The effect of trace elements Mg and Ag on the aging precipitation has been studied. It
has been shown that both Mg and Ag have no obvious effect on the aging precipitates
and the precipitates are still θ′ phase if added separately. But Mg atoms improve the heterogeneous nucleation of the θ′ phase, and thus increase the density and decrease the size
of the θ′ phase. At the same time, Mg atoms improve the aggregation of Cu atom along
the {111} planes, and make the alloy nucleate some Ω phase. Ag atoms limit the increase
in thickness of θ′ phase by repelling Cu atoms. However, when Al-Cu alloy contains both
elements Mg and Ag, Mg and Ag atoms strongly interact with each other and form atom
clusters along the matrix {111} planes. Since Mg atom is larger than Al atom, Mg atom
clusters will generate plus lattice distortion energy, which helps the aggregation of Cu
atom along the matrix {111} planes to decrease lattice distortion. Thus, these atom clusters help the Cu atoms aggregate and nucleate heterogeneously along the matrix {111}
planes, which makes Mg atom clusters the optimized nucleation sites for Ω phase and
inhibits the nucleation of θ′ phase.
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