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Regulation Mechanism of Novel Thermomechanical Treatment for Microstructure and Properties of 2E12 Aluminum Alloy
Rare Metal Materials and Engineering Volume 44, Issue 10, October 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(10): 2341-2346.
ARTICLE
Regulation Mechanism of Novel Thermomechanical Treatment for Microstructure and Properties of 2E12 Aluminum Alloy Chen Zhiguo1,2,
Ren Jieke1,
Zhang Jishuai1,
Chen Jiqiang1,
Huang Yujin1,
Fang
Liang1 1
Central South University, Changsha 410083, China; 2 Hunan University of Humanities, Science and Technology, Loudi 417000, China
Abstract: The effects of a novel thermo-mechanical treatment (TMT) on the mechanical properties and the microstructure of 2E12 aluminum alloy were investigated by tensile test and fatigue crack propagation test, as well as scanning electron microscope and transmission electron microscope (TEM). Results show that a good combination of strength and plasticity can be achieved for 2E12 aluminum alloy by the new TMT, the yield strength increases by ~28% over that of T3, and the ultimate tensile strength also increases by 17%, while the high elongation of T3 condition (over 16%) is maintained. The fatigue crack growth rate of the TMT is lower than those of the T3 and T6 peak aged samples. A high density of tangled dislocation is contained in the TMT 2E12 aluminum alloy, and dislocation cell substructure is formed. The mechanism of the new TMT technique, by which the mechanical properties of the alloy is greatly increased, is the synergistic effect of composite structures, including dislocation cell substructures, the complex of Mg/Cu/vacancies, as well as GPB zones. Key words: 2E12 aluminum alloy; thermo-mechanical treatment; microstructure; mechanical property
High strength aluminum alloys have been used widely in aerospace industry, and 2E12 aluminum alloy is a new type of high-performance Al-Cu-Mg aero metal developed on the basis of AA 2524, which have potential for high mechanic strength, good heat resistance, excellent [1-3] formability and good resistance to damage . The thermo-mechanical treatment is a combination of plastic deformation and heat treatment, and it is used to impart desirable changes in the microstructure and the properties of the alloy while acquiring the required [4-7] shapes . However, most of the existing strengthening [7-10] TMT techniques result in decreased ductility . S. Singh [7,8] et al. indicated that by combining artificial aging with warm rolling (both at 160 oC), the strength and the fatigue property of AA 2014 could be improved; as a result, the ductility was decreased by 2%~5.5% compared to that of
T6. Presently, few TMT techniques could achieve both high strength and high ductility, and sophisticated processing [11] technique was involved. Kim et al. combined the equal channel angular pressing with artificial aging, to improve the yield strength of AA 2024 to 630 MPa while a 15% elongation was maintained. However, a large load is required in this technique, and the sizes of products are small. Thus developing of an effective and efficient TMT technique for 2E12 aluminum alloy is of both theoretical significance and practical value. The present study made an systematic exploration of a new TMT technique which improved the overall properties of 2E12 aluminum alloy, as well as a relatively thorough research on the mechanism of this TMT technique.
1
Experiment
Received date: October, 14, 2014 Foundation item: National Natural Science Foundation of China (50871123); Australia-China Special Fund (51011120052) Corresponding author: Chen Zhiguo, Ph. D., Professor, School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China, Tel: 0086-731-8325517, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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A cold rolled 4.45wt%Cu-1.50wt%Mg-0.54wt%Mn-Bal. Al alloy plate with a thickness of 2 mm was used in this investigation. The samples were solution treated at 500 oC for 1 h, and then quenched in water to room temperature. Such samples were subsequently aged at 80 oC or 100 oC for different periods of time, then cold rolled at room temperature to a total thickness reduction of 6%, 9% or 15%, and finally naturally aged for 15 d. The tensile tests were performed on a materials testing machine (MTS 858) with a gauge length of 20 mm at a constant strain rate of 8×10-4 s-1. The fatigue crack propagation test was performed on MTS 858 at room temperature. The test frequency was 10 Hz, a sinusoidal load waveform along the R-T direction was applied, and the stress ratio is 0.1. SEM observations were performed on Quanta 200 field emission scanning microscope, with an operating voltage of 25 kV. TEM observations were performed on TECNAI G220 transmission electron microscopy, with an operating voltage of 200 kV. The specimens for TEM observation were prepared by the standard twin jet electro-polishing method using a solution of methanol and nitric acid (3:1 in volume) below 25 oC.
2
Results and Discussion
2.1 Tensile properties The tensile properties of 2E12 aluminum alloy under the condition of TMT and T3 (6% strain, natural aging for 96 h), Table 1 Process
Pre-aging
T6 peak age (aged at 190 oC for 12 h), T8 peak age (6% strain, aged at 190 oC for 12 h) are shown in Table 1. As is shown, the strength of 2E12 aluminum alloy is significantly increased by TMT. In the case of TMT (100 o C/12 h pre-aging, 15% strain), the yield strength increases by over 150 MPa than that of T3. In the case of TMT (80 o C/100 h pre-aging, 9% strain), the tensile strength also increases by over 80 MPa. Generally, the strength of aluminium alloy increases at the expense of its plasticity. 2E12-T4 alloy has good plasticity as well as the lowest strength, while the 2E12-T8 alloy with a high strength has the lowest elongation. On the other hand, the TMT technique can increase the strength of 2E12 aluminum alloy while maintaining its plasticity. For TMT (80 oC/60 h pre-aging, 9% strain), the yield strength increases to 453.1 MPa, the ultimate strength 546.9 MPa, and the elongation is 16.5%, similar to that of T3 (16.8%). By comparing the mechanical properties under different aging conditions (Fig.1), it can be concluded that in TMT processing, the strength primarily depends on the extent of deformation, while the effect of aging conditions is unobvious; in the case of elongation, the mechanism is more complex, the over-all effect depends not only on aging time and aging temperature, but also on deformation. In the contrast test, as shown in Table 1, the samples without cold deformation, whether naturally aged or not, have a lower strength. We can conclude that the cold deformation is
Mechanical properties of 2E12 aluminum alloy under different conditions Strain/%
Final aging
YS/MPa
UTS/MPa
Elongation/%
80 C/12 h
6
Natural aging 15 d
324.2
490.3
16.8
80 oC/36 h 80 oC/60 h
6 6
Natural aging 15 d Natural aging 15 d
388.7 374.6
487.5 486.4
17.3 16
80 oC/84 h
6
Natural aging 15 d
398.9
483.2
14.4
100 C/12 h
6
Natural aging 15 d
464.7
485.2
13.7
100 oC/36 h 100 oC/60 h 80 oC/12 h
6 6 9
Natural aging 15 d Natural aging 15 d Natural aging 15 d
437.6 420.7 435.0
488.0 480.8 535.8
12.1 14.1 15.8
80 oC/36 h 80 oC/60 h 80 oC/100 h 100 oC/12 h 100 oC/36 h 100 oC/60 h
9 9 9 9 9 9
Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d
454.5 453.1 464.8 449.0 448.4 446.6
544.9 546.9 547.8 541.5 535.8 538.0
14.7 16.5 9.5 16.1 14.6 12.2
100 oC/12 h
15
Natural aging 15 d
506.7
539.4
10.5
T3 T4
－ －
6 －
Natural aging 96 h Natural aging 15 d
354.2 307.4
467.2 451.3
16.8 20.3
T6
－
－
190 oC/12 h
375.3
495.6
10.7
T8
－
6
190 oC/10 h
481.2
508.0
4.8
80 C/60 h
－
－
317.5
462.3
18.1
o
o
TMT
o
Contrast
o
80 C/60 h
9
－
405.3
527.4
14.7
80 oC/60 h
－
Natural aging 15 d
321.2
465.1
18.5
2342
o
80 C, 9%, strain o 100 C, 9%, strain o 80 C, 6%, strain o 100 C, 6%, strain
1E-4 1E-4
T3 T3 T6 T6 TMT TMT
1E-5 1E-5
55
12 24 36 48 60 72 84 96 108
18
b
16 Elongation/%
1E-3 1E-3
1E-6 1E-6
0
Fig.2
10 15 20 25 30 35 10 15 20 25 30 35 Δ K(MPA*m ) ΔK/MPa·m-1/2 -1/2
Fatigue crack propagation rate of 2E12 aluminum alloy under different conditions
14 12 o
80 C, 9%, strain o 100 C, 9%, strain o 80 C, 6%, strain o 100 C, 6%, strain
10 8
Fig.1
0.01 0.01
a
dA/dN(mm/cycle)
560 550 540 530 520 510 500 490 480
da/dN/mm·cycle-1
UTS/MPa
Chen Zhiguo et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2341-2346
0
12 24 36 48 60 72 84 96 108 Aging Time/h
Relation of mechanical properties and aging conditions of 2E12 alloy: (a) UTS vs time and (b) elongation vs time
a vital part of this processing. However, the pre-aged and cold deformed samples without natural aging also have a lower strength and an lower elongation than that of the TMT samples. These results show that the mechanism of the TMT is the synergistic effect of different processing techniques, rather than the simple stacking of precipitation strengthening and deformation strengthening. The conclusion can be drawn that the TMT technique can achieve both high strength and good plasticity, and this combination is unavailable to process such as T3 and T8. 2.2 Fatigue performance The relationship among fatigue crack propagation rates (da/dN) and stress intensity factor (ΔK) of TMT (80 oC/60 h pre-aging, 9% strain) and T3, T6 peak aged samples in an double logarithmic coordinate is shown in Fig.2. The three curves in the picture above show the typical feature of the three-region propagation [12-14]. Taking the curve of TMT sample for example: when the value of ΔK is lower than 6 MPa·m-1/2, the da/dN is lower than 10 -5 mm/cycle, which is the stable propagation region of micro-cracks. When the value of ΔK is between 6~20 MPa·m-1/2, the fatigue crack propagation rate and the stress intensity factor have a near linear relationship, which approximately satisfies the Paris law, and this is the stable propagation region of macro-cracks, also called the Paris region [15,16]. When the value of ΔK is over 20 MPa·m-1/2, the corresponding da/dN
increases dramatically, the sample breaks apart, and this is the rapid crack growth region. It can be observed that the corresponding da/dN of TMT sample is lower than that of the other two, indicating that the fatigue crack propagation rate of TMT sample is lower than those of T3 and T6 peak aging when the stress intensity factor is the same. It can be concluded that the fatigue crack propagation property of TMT sample is better than that of T3 and T6 peak aging. Paris region fracture morphologies of 2E12 aluminum alloy samples with fatigue crack propagation (80 oC/60 h pre-aging, 9% strain TMT, T3 and T6 peak age) are shown in Fig.3. The fracture of the two samples are covered with strips consisting of cracks, the crack propagation path is vertical to the propagation direction, and the path is remarkably rugged (Fig.3a and Fig.3b). In Fig.3c, the cleavage fracture is along the propagation direction in different parts of the alloy, and the fracture contains stairs of different heights. In TMT and T3 samples, many strips on the fracture will absorb a large amount of energy, magnifying the resistance to the fatigue crack propagation. It can be concluded that the driving force in these samples will be smaller when the stress intensity factor is identical. 2.3 Micrographic analysis Fracture morphologies of 2E12 aluminum alloy tensile test samples experienced by: TMT (80 oC/60 h pre-aging, 9% strain), T3, T6 peak age and T8 peak aging are shown in Fig.4. As shown in Fig.4a, the TMT sample fracture contains tough dimples with two different sizes, a large amount of smaller tough dimples are distributed around larger ones, and the larger ones are greater in depth, representing the properties of typical ductile fracture. The T3 sample (Fig.4b) has a similar fracture morphology to that of the TMT, sample. The larger tough dimples are scarcer in Fig.4c and
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a
Fig.3
c
b
Paris region fracture morphologies of samples with fatigue crack propagation: (a) TMT, (b) T3, and (c) T6 peak age
their depth is more shallow, shear platforms also appear in this figure, which implies that the plasticity of T6 samples is lower. The T8 peak age sample contains shear platforms in many locations, showing the properties of a partial shear fracture. It can be inferred that the TMT and T3 samples have a better plasticity than those of T6 and T8samples. Broken precipitate particles could be seen in the center of the tough dimples in Fig.4c and Fig.4d, which implies that the formation of cracks is related to coarse phases. It can be concluded that reducing the time and the temperature of aging will restrain the precipitation of coarse phases and retain the plasticity of the alloy. TEM images of 2E12 aluminum alloy under TMT (80 o C/60 h pre-aging, 9% strain), T3, T6 peak aging and T8 peak aging conditions are shown in Fig.5. It is shown in Fig.5a that the TMT sample contains a high density of dislocations, most of which are entangled together, no notable precipitates can be observed except for the dispersed phases containing Mn, and the corresponding diffraction patterns confirmed this. Fig.5b shows that the T3 sample contains a certain amount of dislocations, but no notable precipitates except for the coarse-phases containing Mn which can be observed, and it can be concluded that its strengthening effect mainly results from the work hardening of the strain. A certain amount of needle-like precipitates can be observed in Fig.5c, where the S' phases are formed during the artificial aging process, and few dislocations can be spotted, implying that the T6 peak age sample is mainly strengthened by precipitation- hardening. The values of strength of these two samples are relatively low, since there is only one strengthening mechanism, either work-hardening or precipitation- hardening. More needle-like S' phase is contained in the T8 peak aged sample (Fig.5d), more than those in T6 peak age sample, the precipitates are also finer and more evenly distributed, because the dislocations serve as the sites of heterogeneous nucleation for S' phase, implying a smaller probability of constricting. The dislocations are entangled around S'
Fig.4
a
b
c
d
Fracture morphologies of 2E12 aluminum alloy tensile strain test samples: (a) TMT (80 oC/60 h pre-aging, 9% strain), (b) T3, (c) T6 peak aging, and (d) T8 peak aging a
b
0.2 µm
0.2 µm d
c
200 nm
0.2 µm Fig.5
TEM
micrographs
of
2E12
aluminum
alloy
corresponding SAED patterns: (a) TMT (80
o
with
C/60 h
pre-aging, 9% strain), (b) T3, (c) T6 peak aging, and (d) T8 peak aging
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phase, leading to the stress concentration, resulting in that the strength of the alloy is dramatically increased at the expanse of its plasticity.
2.4 Discussion The Cu/Mg ratio of 2E12 aluminum alloy used in the present investigation is a medium one of 2.97. According to the phase diagram, this alloy is located in the phase zone of α+S(Al2CuMg), and its precipitation sequence is as follows: SSS→GPB1→GPB2/S''→S'→S. Though the TMT sample and T3 sample have some resemblance in their TEM images, by investigating the comparison of the properties of 2E12 aluminum alloy samples, it can be concluded that the mechanism of the TMT technique is not a simple work-hardening, and the TEM images and the corresponding diffraction patterns indicate that no notable precipitation-hardening is present. According to the research on the early stage of age hardening pattern of Al-Cu-Mg alloy, the hardening process can be divided into two stages: the first stage takes place swiftly, and then the hardness remains almost the same for a few hours, until the second stage begins. But either TEM or HRTEM could only spot the existence of GPB zones in the second stage. S. P. Ringer et al.[17,18] suggested that the first stage of hardening was due to the formation of Cu-Mg clusters. Clusters are different from the zones where the shape, size, composition, degree of order, orientation and structure of the clusters are less defined than that of a zone, and the actual hardening process of clusters can be regarded as a heightened form of solid solution strengthening. Moreover, Cu-Mg clusters generally contain a high amount of vacancies, because of the strong positive interactions between Cu-Mg clusters and vacancies. It is obvious that a composite of Cu-Mg clusters and vacancies is formed during the pre-aging stage of the TMT technique, and dense dislocations are formed during the cold-rolling followed by the pre-aging. The dislocations will usually become entangled through their interactions, and dislocation substructures will be formed, because the aluminum alloy has a relatively high stacking fault energy. In the final aging, i.e., natural aging, the alloy remains at room temperature for a certain period of time, some of the clusters will further segregate, and GPB zones will be formed at these sites. The composite of Cu-Mg clusters and vacancies will segregate around the dislocations via the diffusion progress at the same time, pinning the dislocations in the manner which is regarded as Cottrell Atmosphere. Artificial aging at a higher temperature is not applied; as a result, the formation of S' phase as well as the progress of the
recovery of the deformed alloy will be restrained. The former will cause a stress concentration when a lot of dislocations are entangled around the precipitates, leading to a decrease in plasticity; while the latter reduces the dislocation density, causing a diminishing of the strengthening effect.
3
Conclusions
1) The mechanical properties of 2E12 aluminum alloy are greatly improved by the new TMT. A good combination of strength and ductility is obtained, the yield strength increases by ~28% (T3 sample as a contrast), and the ultimate tensile strength also increases by 17%, while the high elongation of T3 condition (over 16%) is maintained. 2) The fatigue resistance of 2E12 aluminum alloy is greatly enhanced by the new TMT. In the Paris stage, the fatigue crack propagation rate of TMT samples is lower compared to that of T3 and T6 peak aged samples. 3) The great improvement of the comprehensive properties is likely to result from the synergistic effect of composite structures such as a high density of cell substructure of dislocations, the complex of Mg-Cu solute atoms clusters and vacancies, as well as GPB zones.
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Srivatsan T S, Kolar D, Maguuscn P. Materials & Design[J], 2002, 23: 129
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Manabu Nakai, Takehiko Eto. Materials Science and Engineering A[J], 2000, 285(1): 62
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Huang Yujin, Chen Zhiguo, Zheng Ziqiao. Scripta Materialia [J], 2011, 64: 382
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Huang Yujin, Chen Zhiguo, Zhou Xian et al. Materials Review[J], 2010, 24: 93 (in Chinese)
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Singh S, Goel D B. J Mater Sci[J], 1990, 25: 3894
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Singh S, Goel D B. Bull Mater Sci[J], 2005, 28(2): 91
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Chaturvedi M C, Chung D W, Doucette R A. Metal Sci[J], 1979, 13: 34
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Ning Ailin, Liu Zhiyi, Zeng Sumin. Transactions of Nonferrous Metals Society of China [J], 2006, 16(6): 1341
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Kim W J, Chung C S, Ma D S et al. Scripta Materialia[J], 2003, 49: 333
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Srivatsan T S, Kolar D, Magnusen P. Materials & Design[J], 2002, 23(2): 129
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Yang Sheng, Yi Danqing, Yang Shoujie et al. Journal of Material Engineering[J], 2007, 12: 26 (in Chinese)
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Liu Gang, Zheng Ziqiao, Yang Shoujie et al. Materials for
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Mechanical Engineering[J], 2007, 31: 65 (in Chinese) 15
Liu Liming, Duan Menglan, Liu Chuntu et al. Acta
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Zhao Tianwen, Zhang Jixi, Jiang Yanyao. International
Mechanica Sinica[J], 2003, 35(2): 171 (in Chinese)
17
Ringer S P, Hono K, Sakurai T et al. Scripta Materialia[J], 1997, 36: 517
18
Ringer S P, Sakurai T, Polmear I J. Acta Mater[J], 1997, 45: 3731
Journal of Fatigue[J], 2008, 30: 1169
2346
ARTICLE
Regulation Mechanism of Novel Thermomechanical Treatment for Microstructure and Properties of 2E12 Aluminum Alloy Chen Zhiguo1,2,
Ren Jieke1,
Zhang Jishuai1,
Chen Jiqiang1,
Huang Yujin1,
Fang
Liang1 1
Central South University, Changsha 410083, China; 2 Hunan University of Humanities, Science and Technology, Loudi 417000, China
Abstract: The effects of a novel thermo-mechanical treatment (TMT) on the mechanical properties and the microstructure of 2E12 aluminum alloy were investigated by tensile test and fatigue crack propagation test, as well as scanning electron microscope and transmission electron microscope (TEM). Results show that a good combination of strength and plasticity can be achieved for 2E12 aluminum alloy by the new TMT, the yield strength increases by ~28% over that of T3, and the ultimate tensile strength also increases by 17%, while the high elongation of T3 condition (over 16%) is maintained. The fatigue crack growth rate of the TMT is lower than those of the T3 and T6 peak aged samples. A high density of tangled dislocation is contained in the TMT 2E12 aluminum alloy, and dislocation cell substructure is formed. The mechanism of the new TMT technique, by which the mechanical properties of the alloy is greatly increased, is the synergistic effect of composite structures, including dislocation cell substructures, the complex of Mg/Cu/vacancies, as well as GPB zones. Key words: 2E12 aluminum alloy; thermo-mechanical treatment; microstructure; mechanical property
High strength aluminum alloys have been used widely in aerospace industry, and 2E12 aluminum alloy is a new type of high-performance Al-Cu-Mg aero metal developed on the basis of AA 2524, which have potential for high mechanic strength, good heat resistance, excellent [1-3] formability and good resistance to damage . The thermo-mechanical treatment is a combination of plastic deformation and heat treatment, and it is used to impart desirable changes in the microstructure and the properties of the alloy while acquiring the required [4-7] shapes . However, most of the existing strengthening [7-10] TMT techniques result in decreased ductility . S. Singh [7,8] et al. indicated that by combining artificial aging with warm rolling (both at 160 oC), the strength and the fatigue property of AA 2014 could be improved; as a result, the ductility was decreased by 2%~5.5% compared to that of
T6. Presently, few TMT techniques could achieve both high strength and high ductility, and sophisticated processing [11] technique was involved. Kim et al. combined the equal channel angular pressing with artificial aging, to improve the yield strength of AA 2024 to 630 MPa while a 15% elongation was maintained. However, a large load is required in this technique, and the sizes of products are small. Thus developing of an effective and efficient TMT technique for 2E12 aluminum alloy is of both theoretical significance and practical value. The present study made an systematic exploration of a new TMT technique which improved the overall properties of 2E12 aluminum alloy, as well as a relatively thorough research on the mechanism of this TMT technique.
1
Experiment
Received date: October, 14, 2014 Foundation item: National Natural Science Foundation of China (50871123); Australia-China Special Fund (51011120052) Corresponding author: Chen Zhiguo, Ph. D., Professor, School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China, Tel: 0086-731-8325517, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Chen Zhiguo et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2341-2346
A cold rolled 4.45wt%Cu-1.50wt%Mg-0.54wt%Mn-Bal. Al alloy plate with a thickness of 2 mm was used in this investigation. The samples were solution treated at 500 oC for 1 h, and then quenched in water to room temperature. Such samples were subsequently aged at 80 oC or 100 oC for different periods of time, then cold rolled at room temperature to a total thickness reduction of 6%, 9% or 15%, and finally naturally aged for 15 d. The tensile tests were performed on a materials testing machine (MTS 858) with a gauge length of 20 mm at a constant strain rate of 8×10-4 s-1. The fatigue crack propagation test was performed on MTS 858 at room temperature. The test frequency was 10 Hz, a sinusoidal load waveform along the R-T direction was applied, and the stress ratio is 0.1. SEM observations were performed on Quanta 200 field emission scanning microscope, with an operating voltage of 25 kV. TEM observations were performed on TECNAI G220 transmission electron microscopy, with an operating voltage of 200 kV. The specimens for TEM observation were prepared by the standard twin jet electro-polishing method using a solution of methanol and nitric acid (3:1 in volume) below 25 oC.
2
Results and Discussion
2.1 Tensile properties The tensile properties of 2E12 aluminum alloy under the condition of TMT and T3 (6% strain, natural aging for 96 h), Table 1 Process
Pre-aging
T6 peak age (aged at 190 oC for 12 h), T8 peak age (6% strain, aged at 190 oC for 12 h) are shown in Table 1. As is shown, the strength of 2E12 aluminum alloy is significantly increased by TMT. In the case of TMT (100 o C/12 h pre-aging, 15% strain), the yield strength increases by over 150 MPa than that of T3. In the case of TMT (80 o C/100 h pre-aging, 9% strain), the tensile strength also increases by over 80 MPa. Generally, the strength of aluminium alloy increases at the expense of its plasticity. 2E12-T4 alloy has good plasticity as well as the lowest strength, while the 2E12-T8 alloy with a high strength has the lowest elongation. On the other hand, the TMT technique can increase the strength of 2E12 aluminum alloy while maintaining its plasticity. For TMT (80 oC/60 h pre-aging, 9% strain), the yield strength increases to 453.1 MPa, the ultimate strength 546.9 MPa, and the elongation is 16.5%, similar to that of T3 (16.8%). By comparing the mechanical properties under different aging conditions (Fig.1), it can be concluded that in TMT processing, the strength primarily depends on the extent of deformation, while the effect of aging conditions is unobvious; in the case of elongation, the mechanism is more complex, the over-all effect depends not only on aging time and aging temperature, but also on deformation. In the contrast test, as shown in Table 1, the samples without cold deformation, whether naturally aged or not, have a lower strength. We can conclude that the cold deformation is
Mechanical properties of 2E12 aluminum alloy under different conditions Strain/%
Final aging
YS/MPa
UTS/MPa
Elongation/%
80 C/12 h
6
Natural aging 15 d
324.2
490.3
16.8
80 oC/36 h 80 oC/60 h
6 6
Natural aging 15 d Natural aging 15 d
388.7 374.6
487.5 486.4
17.3 16
80 oC/84 h
6
Natural aging 15 d
398.9
483.2
14.4
100 C/12 h
6
Natural aging 15 d
464.7
485.2
13.7
100 oC/36 h 100 oC/60 h 80 oC/12 h
6 6 9
Natural aging 15 d Natural aging 15 d Natural aging 15 d
437.6 420.7 435.0
488.0 480.8 535.8
12.1 14.1 15.8
80 oC/36 h 80 oC/60 h 80 oC/100 h 100 oC/12 h 100 oC/36 h 100 oC/60 h
9 9 9 9 9 9
Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d Natural aging 15 d
454.5 453.1 464.8 449.0 448.4 446.6
544.9 546.9 547.8 541.5 535.8 538.0
14.7 16.5 9.5 16.1 14.6 12.2
100 oC/12 h
15
Natural aging 15 d
506.7
539.4
10.5
T3 T4
－ －
6 －
Natural aging 96 h Natural aging 15 d
354.2 307.4
467.2 451.3
16.8 20.3
T6
－
－
190 oC/12 h
375.3
495.6
10.7
T8
－
6
190 oC/10 h
481.2
508.0
4.8
80 C/60 h
－
－
317.5
462.3
18.1
o
o
TMT
o
Contrast
o
80 C/60 h
9
－
405.3
527.4
14.7
80 oC/60 h
－
Natural aging 15 d
321.2
465.1
18.5
2342
o
80 C, 9%, strain o 100 C, 9%, strain o 80 C, 6%, strain o 100 C, 6%, strain
1E-4 1E-4
T3 T3 T6 T6 TMT TMT
1E-5 1E-5
55
12 24 36 48 60 72 84 96 108
18
b
16 Elongation/%
1E-3 1E-3
1E-6 1E-6
0
Fig.2
10 15 20 25 30 35 10 15 20 25 30 35 Δ K(MPA*m ) ΔK/MPa·m-1/2 -1/2
Fatigue crack propagation rate of 2E12 aluminum alloy under different conditions
14 12 o
80 C, 9%, strain o 100 C, 9%, strain o 80 C, 6%, strain o 100 C, 6%, strain
10 8
Fig.1
0.01 0.01
a
dA/dN(mm/cycle)
560 550 540 530 520 510 500 490 480
da/dN/mm·cycle-1
UTS/MPa
Chen Zhiguo et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2341-2346
0
12 24 36 48 60 72 84 96 108 Aging Time/h
Relation of mechanical properties and aging conditions of 2E12 alloy: (a) UTS vs time and (b) elongation vs time
a vital part of this processing. However, the pre-aged and cold deformed samples without natural aging also have a lower strength and an lower elongation than that of the TMT samples. These results show that the mechanism of the TMT is the synergistic effect of different processing techniques, rather than the simple stacking of precipitation strengthening and deformation strengthening. The conclusion can be drawn that the TMT technique can achieve both high strength and good plasticity, and this combination is unavailable to process such as T3 and T8. 2.2 Fatigue performance The relationship among fatigue crack propagation rates (da/dN) and stress intensity factor (ΔK) of TMT (80 oC/60 h pre-aging, 9% strain) and T3, T6 peak aged samples in an double logarithmic coordinate is shown in Fig.2. The three curves in the picture above show the typical feature of the three-region propagation [12-14]. Taking the curve of TMT sample for example: when the value of ΔK is lower than 6 MPa·m-1/2, the da/dN is lower than 10 -5 mm/cycle, which is the stable propagation region of micro-cracks. When the value of ΔK is between 6~20 MPa·m-1/2, the fatigue crack propagation rate and the stress intensity factor have a near linear relationship, which approximately satisfies the Paris law, and this is the stable propagation region of macro-cracks, also called the Paris region [15,16]. When the value of ΔK is over 20 MPa·m-1/2, the corresponding da/dN
increases dramatically, the sample breaks apart, and this is the rapid crack growth region. It can be observed that the corresponding da/dN of TMT sample is lower than that of the other two, indicating that the fatigue crack propagation rate of TMT sample is lower than those of T3 and T6 peak aging when the stress intensity factor is the same. It can be concluded that the fatigue crack propagation property of TMT sample is better than that of T3 and T6 peak aging. Paris region fracture morphologies of 2E12 aluminum alloy samples with fatigue crack propagation (80 oC/60 h pre-aging, 9% strain TMT, T3 and T6 peak age) are shown in Fig.3. The fracture of the two samples are covered with strips consisting of cracks, the crack propagation path is vertical to the propagation direction, and the path is remarkably rugged (Fig.3a and Fig.3b). In Fig.3c, the cleavage fracture is along the propagation direction in different parts of the alloy, and the fracture contains stairs of different heights. In TMT and T3 samples, many strips on the fracture will absorb a large amount of energy, magnifying the resistance to the fatigue crack propagation. It can be concluded that the driving force in these samples will be smaller when the stress intensity factor is identical. 2.3 Micrographic analysis Fracture morphologies of 2E12 aluminum alloy tensile test samples experienced by: TMT (80 oC/60 h pre-aging, 9% strain), T3, T6 peak age and T8 peak aging are shown in Fig.4. As shown in Fig.4a, the TMT sample fracture contains tough dimples with two different sizes, a large amount of smaller tough dimples are distributed around larger ones, and the larger ones are greater in depth, representing the properties of typical ductile fracture. The T3 sample (Fig.4b) has a similar fracture morphology to that of the TMT, sample. The larger tough dimples are scarcer in Fig.4c and
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a
Fig.3
c
b
Paris region fracture morphologies of samples with fatigue crack propagation: (a) TMT, (b) T3, and (c) T6 peak age
their depth is more shallow, shear platforms also appear in this figure, which implies that the plasticity of T6 samples is lower. The T8 peak age sample contains shear platforms in many locations, showing the properties of a partial shear fracture. It can be inferred that the TMT and T3 samples have a better plasticity than those of T6 and T8samples. Broken precipitate particles could be seen in the center of the tough dimples in Fig.4c and Fig.4d, which implies that the formation of cracks is related to coarse phases. It can be concluded that reducing the time and the temperature of aging will restrain the precipitation of coarse phases and retain the plasticity of the alloy. TEM images of 2E12 aluminum alloy under TMT (80 o C/60 h pre-aging, 9% strain), T3, T6 peak aging and T8 peak aging conditions are shown in Fig.5. It is shown in Fig.5a that the TMT sample contains a high density of dislocations, most of which are entangled together, no notable precipitates can be observed except for the dispersed phases containing Mn, and the corresponding diffraction patterns confirmed this. Fig.5b shows that the T3 sample contains a certain amount of dislocations, but no notable precipitates except for the coarse-phases containing Mn which can be observed, and it can be concluded that its strengthening effect mainly results from the work hardening of the strain. A certain amount of needle-like precipitates can be observed in Fig.5c, where the S' phases are formed during the artificial aging process, and few dislocations can be spotted, implying that the T6 peak age sample is mainly strengthened by precipitation- hardening. The values of strength of these two samples are relatively low, since there is only one strengthening mechanism, either work-hardening or precipitation- hardening. More needle-like S' phase is contained in the T8 peak aged sample (Fig.5d), more than those in T6 peak age sample, the precipitates are also finer and more evenly distributed, because the dislocations serve as the sites of heterogeneous nucleation for S' phase, implying a smaller probability of constricting. The dislocations are entangled around S'
Fig.4
a
b
c
d
Fracture morphologies of 2E12 aluminum alloy tensile strain test samples: (a) TMT (80 oC/60 h pre-aging, 9% strain), (b) T3, (c) T6 peak aging, and (d) T8 peak aging a
b
0.2 µm
0.2 µm d
c
200 nm
0.2 µm Fig.5
TEM
micrographs
of
2E12
aluminum
alloy
corresponding SAED patterns: (a) TMT (80
o
with
C/60 h
pre-aging, 9% strain), (b) T3, (c) T6 peak aging, and (d) T8 peak aging
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phase, leading to the stress concentration, resulting in that the strength of the alloy is dramatically increased at the expanse of its plasticity.
2.4 Discussion The Cu/Mg ratio of 2E12 aluminum alloy used in the present investigation is a medium one of 2.97. According to the phase diagram, this alloy is located in the phase zone of α+S(Al2CuMg), and its precipitation sequence is as follows: SSS→GPB1→GPB2/S''→S'→S. Though the TMT sample and T3 sample have some resemblance in their TEM images, by investigating the comparison of the properties of 2E12 aluminum alloy samples, it can be concluded that the mechanism of the TMT technique is not a simple work-hardening, and the TEM images and the corresponding diffraction patterns indicate that no notable precipitation-hardening is present. According to the research on the early stage of age hardening pattern of Al-Cu-Mg alloy, the hardening process can be divided into two stages: the first stage takes place swiftly, and then the hardness remains almost the same for a few hours, until the second stage begins. But either TEM or HRTEM could only spot the existence of GPB zones in the second stage. S. P. Ringer et al.[17,18] suggested that the first stage of hardening was due to the formation of Cu-Mg clusters. Clusters are different from the zones where the shape, size, composition, degree of order, orientation and structure of the clusters are less defined than that of a zone, and the actual hardening process of clusters can be regarded as a heightened form of solid solution strengthening. Moreover, Cu-Mg clusters generally contain a high amount of vacancies, because of the strong positive interactions between Cu-Mg clusters and vacancies. It is obvious that a composite of Cu-Mg clusters and vacancies is formed during the pre-aging stage of the TMT technique, and dense dislocations are formed during the cold-rolling followed by the pre-aging. The dislocations will usually become entangled through their interactions, and dislocation substructures will be formed, because the aluminum alloy has a relatively high stacking fault energy. In the final aging, i.e., natural aging, the alloy remains at room temperature for a certain period of time, some of the clusters will further segregate, and GPB zones will be formed at these sites. The composite of Cu-Mg clusters and vacancies will segregate around the dislocations via the diffusion progress at the same time, pinning the dislocations in the manner which is regarded as Cottrell Atmosphere. Artificial aging at a higher temperature is not applied; as a result, the formation of S' phase as well as the progress of the
recovery of the deformed alloy will be restrained. The former will cause a stress concentration when a lot of dislocations are entangled around the precipitates, leading to a decrease in plasticity; while the latter reduces the dislocation density, causing a diminishing of the strengthening effect.
3
Conclusions
1) The mechanical properties of 2E12 aluminum alloy are greatly improved by the new TMT. A good combination of strength and ductility is obtained, the yield strength increases by ~28% (T3 sample as a contrast), and the ultimate tensile strength also increases by 17%, while the high elongation of T3 condition (over 16%) is maintained. 2) The fatigue resistance of 2E12 aluminum alloy is greatly enhanced by the new TMT. In the Paris stage, the fatigue crack propagation rate of TMT samples is lower compared to that of T3 and T6 peak aged samples. 3) The great improvement of the comprehensive properties is likely to result from the synergistic effect of composite structures such as a high density of cell substructure of dislocations, the complex of Mg-Cu solute atoms clusters and vacancies, as well as GPB zones.
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