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Microstructure and properties characteristic during Interrupted Muti-step Aging in Al-Cu-
Mg-Ag-Zr alloy
Jian-bo ZHANG, Yong-an ZHANG, Bao-hong ZHU, Feng WANG, Zhi-hui LI, Xi-wu LI, Bai-
qing XIONG
State Key Laboratory of Nonferrous Metals and Processes, General Research Institute for Non-
ferrous Metals, Beingjing, 100088, China
1010-07-26
zhang4318@163.com
ABSTRACT
The effects of Interrupted Muti-step Ageing on the microstructure and properties of
Al-Cu-Mg-Ag-Zr alloy was studied by tensile, hardness, electrical conductivity test and transmission
electron microscopy (TEM), respectively. Interrupted Muti-step Aging delayed the peak aging time
compared to that of one-step ageing, and kept the same level of hardness, electrical conductivity,
ultimate tensile strength (UTS), yield strength (YS) and elongation as these of T6 temper alloy while
increased the fracture toughness notably. Ω and little θ′ precipitated and grew simultaneously in the
process of one-step ageing at 160℃. During the second-step ageing at 65℃ of Interrupted Muti-step
Aging, no TEM characteristic of Ω precipitation could be found. During the third-step of Interrupted
Muti-step Aging, Ω began to dominant the microstructure like what happened in the process of one-
step ageing. The properties difference between the T6 temper and the Interrupted Muti-step Aged alloy
might be related to the different precipitation sequence in the process of the two heat treatment
technology.
KEY WORDS: Interrupted Multi-step Ageing; Al-Cu-Mg-Ag-Zr ；Precipitation; Fracture
toughness;
Introduction
The addition of small amount of Mg and Ag to Al-Cu alloys promote the precipitation of finely
dispersed plates of the orthorhombic Ω phase with the {111}α as its habit plane[1, 2], and the tetragonal
θˊ phase also precipitates on the {100}α plane as another strengthening phase. During the growth of Ω
phase Mg and Ag atoms segregate to the precipitate-matrix interface to minimize the misfit energy[3],
to retard coarsening and to facilitate the maintaining of coherency along the {111}α planes of Ω
phase[4, 5]. Because of these above, Ω retains its morphological stability and alloys based on the Al-
Cu-Mg-Ag system show increased strength and good high-temperature behavior compared to other
2xxx alloys[6, 7].
Similar with other age-hardening aluminum alloys, the properties of alloys based on Al-Cu-Mg-
Ag are influenced by the amounts, pattern, size, distribution, orientation and so on of the precipitates
controlled by the aging condition. Among the aging technology, Interrupted Muti-step Aging attracts
much attention which may enable simultaneous increase in both tensile and fracture toughness of a
wide range of aging-hardening aluminum alloys[8-10]. In view of the technical applications, controlling
the evolution of the structure of the alloy during Interrupted Muti-step Aging is a desirable objective,
which requires good knowledge of the microscopic mechanisms. However, many aspects of
Interrupted Muti-step Aging, including the very origin of the phenomenon, are yet unknown[11].
The present work contributes with information regarding the precipitation behavior during the
Interrupted Muti-step Aging process and relates this to the concomitant hardness, electric conductivity
and tear properties changing.
Experimental
The experimental material Al-Cu-Mg-Ag-Zr were prepared with pure Al, pure Mg, pure Ag and
Al-Cu, Al-Mn master alloys by ingot vacuum melting in a crucible furnace. Table 1 shows the
chemical compositions of the present alloy. After casting, the material were homogenized, scalped,
extruded to plates of 25 mm in thickness, solid solution treated at 520 , water quenched to room℃
temperature, and then treated in T6 temper or Interrupted Muti-step Ageing temper. Microstructural
characterization and mechanical property evaluation of the alloys were carried out under different heat
treated conditions. The details regarding the heat treatment conditions and the subsequent
characterization carried out are shown in Table 2.
Table 1. The composition of the alloy
Cu Mg Ag Zr Ti Fe Si Al
4.8-4.9 0.43-0.47 0.34-0.39 0.15 ≤0.1 ≤0.01 ≤0.01 Bal.
Table 2. Parameters of ageing condition
Solution treatment First step aging Second step aging Third step aging
520℃/2h, quench 160℃/1.5h, quench
65℃/67h 160℃/xh
65℃/116h 160℃/xh
65℃/240h 160℃/xh
T6 temper 160℃/12h
The tensile properties of the material were evaluated with a nominal tension rate of 1 mm/min at
room temperature in long transverse (LT) direction , using 25 mm gauge length specimens, and the tear
test was performed with a nominal tension rate of 1.3 mm/min using samples oriented in the
longitudinal-transverse (L-T) direction. Each datum is the average of three samples tested at room
temperature. Hardness tests were carried out on 430SVD digital Vickers hardness. Electric
conductivity was evaluated by a direct reading type conductivity meter. TEM observations were
performed using JEM-2000FX transmission electron microscopy. The specimens for TEM were
prepared by the standard twin-jet electro-polishing method using 75% methanol and 25% nitric acid
solution at about -30 .℃
Results and discussion
Vickers hardness and electrical conductivity
Fig. 1 (a), (b) shows the change of hardness of the alloy in the process of one-step ageing and
Interrupted Muti-step Ageing. The observation reveals that the hardness of the alloy increases with the
increasing time and decrease after reaching the maximum value for the two ageing conditions. The
time reaching the maximum hardness of the one-step ageing (160 /12h) is less than that of ℃
Interrupted Muti-step Ageing (160 /24h), and the maximum values for the two ageing conditions are ℃
almost the same. Apart from these, the second-step ageing time has little effect on the final hardness.
Fig. 2 (a), (b) gives the change of electrical conductivity of the alloy in the process of the two
ageing condition. Electrical conductivity of the alloy increases along the increasing time until reaches
peak value and remains unchanged, and the time reaching peak value of one-step ageing (20h) is less
than that of Interrupted Muti-step Ageing (40h), which suggests the process of Interrupted Muti-step
Ageing delays the hardening response of the alloy. Further observation reveals that the two value of
electrical conductivity are about at the same level and the second-step ageing time of Interrupted Muti-
step Ageing has little effect on the final electrical conductivity.
Fig.1. The effect of ageing condition on hardness of the alloy (a) one-step ageing; (b) the second-step
and third-step of Interrupted Muti-step Ageing
Fig.2. The effect of f ageing condition on electrical conductivity of the alloy (a) one-step ageing; (b)
the second-step and third-step of Interrupted Muti-step Ageing
Tensile test
The mechanical properties of alloy treated by Interrupted Multi-Step Ageing and T6 temper are
given in Table 3. It can be confirmed that the second-step ageing time of Interrupted Multi-Step Ageing
has little effect on the final properties of the alloy. With the increasing time of the third-step ageing,
UTS and YS increases increase with increasing ageing time, reaching a maximum value, before
increases with further increasing the ageing time. Comparing to T6 temper, the peak UTS, YS and
corresponding elongation of Interrupted Multi-Step Ageing keeps the same level.
Table 3. The effect of Interrupted Multi-Step Ageing and T6 temper on mechanical
properties
The second-step ageing The third-step ageing UTS ( MPa) YS ( MPa) A ( %)
65℃/67h
160℃/6h 500 455 14.0
160℃/24h 510 480 11.0
160℃/96h 495 450 10.5
65℃/116h
160℃/6h 505 455 13.0
160℃/24h 505 470 11.5
160℃/96h 490 450 12.0
65℃/240h 160℃/6h 505 460 16.0
160℃/24h 505 470 14.0
160℃/96h 490 445 13.0
T6 temper，160℃/10h 510 475 14.5
Tear test
Fig. 3 (a), (b) demonstrates the Initiation Energy (IE), Propagation Energy (PE) and Unit
Propagation Energy (UPE) of alloys corresponding to different stage of the two ageing condition,
respectively. It is obvious in Fig.3 (a) that the IE of one-step ageing alloy decreases first and then
increases with increasing ageing time, and the PE decreases along with the increasing time of one-step
ageing. The value of IE is always bigger than that of PE. The IE, PE of the second-step(65℃/10h) of
Interrupted Multi-Step Ageing is about the same with that of overaged alloy of one-step
ageing(160℃/10h). The IE, PE of the third-step is almost same with the value of underaged alloy of
one-step ageing.
It is obvious in Fig.3 (b) that UPE decreases with increasing ageing time of one-step ageing, and
the reduction between underaged and peak aged is much bigger than that of peak aged and overaged.
The UPE value of the second-step of Interrupted Multi-Step Ageing is between that of underaged and
peak aged. The UPE value of the third-step of Interrupted Multi-Step Ageing is much bigger than peak
aged of one-step ageing and keeps the same UPE level of under aged of one-step ageing.
Fig. 3. The effect of aging on Initiation Energy (IE), Propagation Energy (PE) and nit
Propagation Energy (UPE)
Considering the UPE value has significance as a relative index of fracture toughness[12] and the
results of hardness, electric conductivity and tensile test, it may be concluded that the Interrupted
Multi-Step Ageing increases the value of fracture toughness, and keeps the mechanical properties level
comparing to peak aged alloy of one-step ageing.
TEM
Fig. 4 gives the schematic diffraction patterns of Al-Cu-Mg-Ag alloy for zone axis <011>[13] and
TEM micrographs recorded closed to <011> of the present alloys treated under peak aged of one-step
ageing. It is apparent that both Ω and θ′precipitate simultaneously in peak aged alloys shown in Fig.
4(b) and (c): Plate shaped Ω distributed uniformly along [1 1 1] and [
—
111]，and littleθ′phases along [
—
100] are presented. Vietz and Polmear [5]reported that a small addition of Ag completely changed the
precipitation process that normally takes place in Al-Cu-Mg alloys and also simulated an enhanced age
hardening reaction. The predominant strengthening phases in Al-Cu-Mg areθ′and little S′ while that in
Al-Cu-Mg-Ag areΩand littleθ′, which are in good accordance with the results in this study.
Fig. 4. TEM micrographs of alloys of one-step ageing: (ａ) schematic diffraction patterns of Al-
Cu-Mg-Ag alloy for zone axis <011>[13]；(b),(c) peak aged of one-step ageing 160 /10h;℃
TEM microstructure of the second step ageing of Interrupted Multi-Step Ageing is given in Fig.
5. In this case no sharp contrast characteristic for Ω can be observed in bright field micrographs, and
only indistinct strings for Ω can be indentified in diffraction patterns which may be resulted from the
first-step of Interrupted Multi-Step Ageing at 160℃. θ′can be indentified unambiguously from
diffraction patterns and bright field micrographs. And a comparison of Fig. 5 (b) and (d) indicate that
the size of θ′ keep unchanged while the fraction increases with increasing ageing time in the second-
step of Interrupted Multi-Step Ageing.
Fig. 5. TEM micrographs after the second step of Interrupted Multi-Step Ageing : (ａ)、
(b): 160 /1.5h℃，quench，65 /67h ; (c)℃、 (d): 160 /1.5h℃，quench，65 /240h;℃
Fig. 6 demonstrates the TEM micrographs of alloy underwent the third step of Interrupted Multi-
Step Ageing. Similar with one-step ageing, both Ω and θ′can be indentified unambiguously from
diffraction and bright micrograph, and the phase of Ω dominant the microstructure with littleθ′phase .
The size and fraction of precipitates shown in Fig. 6 (b) is almost the same as that of peak aged of one-
step ageing shown in Fig. 4 (c).
Fig. 6. TEM bright field micrographs and corresponding selected area electron diffraction
patterns for the alloy after the third-step of Interrupted Multi-Step Ageing:
160 /1.5h℃，quench，65 /67h℃，160 /24h℃
Conclusions
In the present work, the properties and TEM micrographs of Al-Cu-Mg-Ag-Zr were
investigated, and the change of properties should be understood by the differences of precipitation
behavior between one-step ageing and Interrupted Multi-Step Ageing.
(1) The process of Interrupted Multi-Step Ageing constituted by three steps delays hardening
response comparing to that of one-step ageing process.
(2) The alloy underwent Interrupted Multi-Step Ageing increases the fracture toughness
evidently, and keeps the same level of hardness, electric conductivity, strength and
elongation as these of peak aged alloy underwent one-step ageing process.
(3) During the second-step of Interrupted Multi-Step Ageing, θ′precipitates continuously with
the increasing time while no TEM characteristic of the precipitation of Ω can be indentified.
In the period of the third-step of Interrupted Multi-Step Ageing, Ω and θ′ precipitate
simultaneously and Ω begin to dominant the microstructure just like what happened in the
one-step ageing process.
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