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Hot deformation behavior of AA7085 aluminum alloy during isothermal compression at elevated temperature
Materials Science & Engineering A 596 (2014) 176–182
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Hot deformation behavior of AA7085 aluminum alloy during isothermal compression at elevated temperature Wenyi Liu, Huan Zhao, Dan Li, Zhiqing Zhang n, Guangjie Huang, Qing Liu College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2013 Received in revised form 2 December 2013 Accepted 3 December 2013 Available online 11 December 2013
The isothermal deformation compression tests of AA7085 aluminum alloy were performed on Gleeble1500 system in the temperature range from 250 1C to 450 1C and at strain rate range from 0.01 s 1 to 10 s 1. The microstructure of samples was observed using optical microscopy (OM) and transmission electron microscopy (TEM) techniques. The results show that the peak stress levels decreased with the increase of deformation temperatures or the decrease of strain rate, which can be represented by the Zener–Hollomon parameter in the exponent-type equation with the hot deformation activation energy of 249.11 KJ/mol. Dynamic recrystallization more obviously occurred in the sample with higher Z value than in the sample with lower Z value. Dynamic recrystallization is sensitively dependent on the deformation temperature. & 2013 Elsevier B.V. All rights reserved.
Keywords: AA7085 aluminum alloy Hot deformation Constitutive equation Softening mechanism Dynamic recrystallization
1. Introduction The 7000 (Al–Zn–Mg–Cu) series alloys have been widely used in various automobiles and aerospace industry because of their high strength to low densities and high fracture toughness, as well as resistance to stress corrosion cracking [1,2]. AA7085 alloy aluminum was first introduced by ALCOA in 2003 as the 7th generation of high strength thick plate alloy [3–6]. It is beginning to be used for aerospace applications, most recently for wing spar and rib structures on the new Airbus A380 aircraft [3]. It is well known that mechanical properties of aluminum alloy are closely related to its microstructure, which is greatly influenced by thermomechanical processing [7]. So it is very necessary to investigate the interactive effect between deformation behavior and thermomechanical processing on microstructure and relative mechanical property of aluminum alloys. Two changes will occur in the material during the thermomechanical processing. One is work hardening, and the other is dynamic softening. Dynamic softening includes dynamic recovery and dynamic recrystallization which are important processes responsible for the microstructure evolution during high temperature deformation of high strength aluminum alloys, and are part of the scientifically and industrially important subject of thermomechanical processing [8]. Generally, dynamic recrystallization is not prone to occur in aluminum because of high stacking fault energy. But it has been
n
Corresponding author. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.012
reported in the literatures that dynamic recrystallization occurs in the aluminum during the hot deformation [9–11]. This indicated that the stacking fault energy is not the critical factor of softening mechanism of aluminum alloy during isothermal deformation compression. Many reports have shown that dynamic recrystallization only occurs at low Zener–Hollomon parameters, which must be below or equal to a critical value [12,13]. Though the AA7085 aluminum alloy was successfully used in industries, limited works on hot deformation behavior and corresponding microstructure evolution of this alloy are found in open publications so far. The research of the relationship between Z values and the microstructure is very important for industrial production in order to provide evidence for controlling and predicting the structure and performance after hot deformation. In the present study, the deformation behavior and microstructure evolution of AA7085 aluminum alloy during isothermal compression were studied in detail by means of true stress–true strain curves building, constitutive equation calculation and microstructure characterization, which aimed to reveal the relationship among flow stress and deformation parameters, microstructure evolution and the softening mechanism at different conditions.
2. Experimental The experimental material in this isothermal deformation compression tests is AA7085 aluminum alloy with the chemical position of 7.0–8.0%Zn, 1.2–1.8%Mg, 1.3–2.0%Cu, 0.08–0.15%Zr, Fer 0.08%, Si r0.06%. AA7085 aluminum alloy was homogenized
W. Liu et al. / Materials Science & Engineering A 596 (2014) 176–182
at 400 1C for 12 h and 460 1C for 12 h before the isothermal compression tests. The microstructure of the experimental material is homogeneous, which consists of the equiaxed grains with an average size about 60 μm as shown in Fig. 1. In order to investigate the effect of processing parameters on the deformation behavior of AA7085 aluminum alloy, the isothermal deformation compression tests were carried out on a computer servo-controlled Gleeble-1500 simulator at the deformation temperatures ranging from 250 1C to 450 1C, the strain rates of 0.01 s 1, 0.1 s 1, 1 s 1, 10 s 1, and the true strain of 0.9. Cylindrical compression samples were machined with the size of 10 mm in diameter and 15 mm in height. The groove with 0.3 mm depth was machined on both ends of the samples in order to maintain the lubricant of graphite mixed with machine oil to reduce the friction with the beak iron during the isothermal deformation compression tests. Cylindrical compression specimens prior to isothermal deformation compression were heated to deformation temperature at a heating rate of 5 1C/s and held for 3 min by thermocoupled-feedback-controlled AC current so as to obtain a uniform deformation temperature. After compression, the samples
Fig. 1. Initial microstructure of AA7085 aluminum alloy.
177
were quenched to room temperature immediately to maintain deformation microstructure. The flow stress–strain curves were recorded by the computer automatically. The deformed samples were sectioned parallelly to the compression axis along the direction of centerline, electrolytic polished, and chemical etched in a Keller's agent for microstructural observation. The Axiovert 40 MAT microscope was used for microstructure observation. Thin foil TEM samples were prepared by cutting the longitudinal section of the deformed specimen using an electrical-discharge machining. The discs were ground to a thickness of about 50 μm followed by electro-polishing in a double-jet electro-polishing unit operating at 10 V and 35 1C using a 30% nitric acid and 70% methanol solution. TEM investigations were performed on a Zeiss Libra 200 FE transmission electron microscope, operated at 200 KV.
3. Results and discussion 3.1. Flow stress behavior of AA7085 aluminum alloy A series of true stress–true strain curves obtained during the isothermal compression of AA7085 aluminum alloy at strain rate from 0.01 s 1to 10 s 1 and deformation temperature from 250 1C to 450 1C are shown in Fig. 2. It can be seen from Fig. 2 that the true stress–true strain curves exhibit a peak stress at a certain strain followed by a rheological softening or a steady-state flow behavior. The certain strain corresponding to the peak stress increases with the increasing of strain-rate and the decreasing of temperature. The peak stress and the flow stress gradually increase with increasing strain-rate and decreasing deformation temperature. In the early stage of isothermal compressive deformation of AA7085 aluminum alloy at low strain rate such as 0.01 s 1 and high temperature such as 450 1C, the flow stress increased dramatically with strain increasing because of rapid dislocation
Fig. 2. The true stress–true strain curves during hot compression of AA7085 (a) ε_ ¼ 0.01 s 1, (b) ε_ ¼0.1 s 1, (c) ε_ ¼ 1 s 1, (d) ε_ ¼ 10 s 1.
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multiplication. So work hardening played a leading role during this period. In the same strain rate, the rheological curves are relatively smooth at low temperature deformation, while the flow curves showed obvious fluctuation at high temperature deformation, softening and hardening alternately, the peak stress is more obvious.
slopes of ln ε_ vs. ln(s) plots at high stress in Fig. 3b, so the value of α is 0.01601 MPa 1.
3.2. Constitutive analyses of AA7085 aluminum alloy
Taking the natural logarithm of both sides of Eq. (4) we can obtain as follows:
In the isothermal compression of aluminum alloy materials at high temperature, it is well accepted that the relationship between flow stress, strain rate and temperature can be expressed as [14–17] Z ¼ ε_ expð
Q Þ ¼ f 1 ð sÞ ¼ A 1 sn 1 RT
Z ¼ f 2 ðsÞ ¼ A2 expðβsÞ
ð1Þ ð2Þ
where A1, A2, n1 and β are constants; Z is the Zener–Hollomon parameter; Q is the activation energy for hot deformation; R is the gas constant. Eqs. (1) and (2) are suitable for high stress and low stress, respectively. In addition, the hyperbolic sine type equation, Eq.(3), proposed by Sellars and Mctegart, is more general in form suitable for stress over a wide range, especially for expressing the hot deformation behavior of aluminum alloy [2,7,12,18–21]. Z ¼ f ðsÞ ¼ Aðsinh αsÞn
ð3Þ
where A and n are constants; α is the stress multiplier and also the additional adjustable parameter and it is calculated as α ¼ β=n1 , where β is taken as the average values of the slopes of ln ε_ vs. s plots at low stress in Fig. 3a, n1 is taken as the average values of the
Fig. 3. Relationships between (a) s and (b) ln ε_ ln(s).
Hence the following constitutive equation Eq. (4) can be obtained from Eq. (1) to Eq. (3):
ε_ ¼ A3 ½ sinh ðαsÞn expð
ln ½ sinhðαsÞ ¼
Q Þ RT
ln ε_ Q ln A þ RT n n
ð4Þ
ð5Þ
Generally, most researchers are used to fixing temperatures and differentiating in Eq. (5) and the n value can be obtained as follows [22,23]: n¼
∂ ln ε_ ∂ ln ½ sinhðαsÞ
ð6Þ
As a result, the n value is equal to the average slope of the lines in Fig. 4a, which was derived from the measured stress–strain curves in Fig. 2. Fixing strain rates and differentiating in Eq. (5), the S value can be expressed as follows: S¼
∂ ln ½ sinhðαsÞ ∂ð1=TÞ
ð7Þ
Obviously, the S value is equal to the average slope of the lines in Fig. 4b, which was derived from the measured stress–strain
Fig. 4. Relationships between (a) ln ε_ ln(sinhαs) and (b) ln(sinhαs) 1000/T.
W. Liu et al. / Materials Science & Engineering A 596 (2014) 176–182
179
were homogenized at 450 1C for 24 h and then 470 1C for 30 h in reference [25]. The initial microstructure of the specimen is strongly dependent on their heat treatment condition. Higher activation energy Q in the present study may have connection to the difference in their initial microstructure. More work needs to be done to find out the effects of initial microstructure caused by different heat treatments on the hot compression behavior of this alloy. 3.3. Microstructure evolution of AA7085 aluminum alloy
Fig. 5. Relationships between ln Z and ln(sinhαs).
Table 1 Values of material constants of AA7085 aluminum alloy. A (s 1) 3.6 10
18
α (MPa 1)
n
Q (KJ/mol)
0.01601
5.4644
249.11
curves in Fig. 2 too. So the Q value can be expressed as follows: Q ¼ nR
∂ ln ½ sinhðαsÞ ∂ð1=TÞ
ð8Þ
We can determine the activation energy Q directly from Eq. (8). Taking the natural logarithm of both sides of Eq. (3) we can obtain as follows: ln Z ¼ ln A þ n ln ½ sinhðαsÞ
ð9Þ
So we can obtain the value of A from Fig. 5. By substitution of the values in Table 1 into Eqs. (1) and (3), we can get the isothermal compression equation of AA7085 aluminum alloy, shown as Eq. (10), with the deformation activation energy of 249.11 KJ/mol. 249110 ð10Þ ε_ ¼ 3:6 1018 ½ sinh ð0:01601sÞ5:4644 exp RT The hot deformation activation energy Q is an important physical parameter serving as an indicator of deformation difficulty degree in plasticity deformation [24]. In the present work, the Q value is calculated to be 249.11 KJ/mol, which is higher than that for self-diffusion in pure aluminum (165 KJ/mol), as well as homogenized AA7085 aluminum alloy (187.4 KJ/mol) [25]. So the deformation mechanism is likely to be cross-slip, which makes the AA7085 aluminum alloy soft, i.e., the softening mechanism is dynamic recovery [18,26]. The reason for high hot deformation activation energy may be due to the following aspects: (1) Smaller initial grain size. The grain size in the present study is about 60 μm, which is smaller than the average grain size ( 100 μm) in reference [25]. According to the Hall-Petch law, smaller grain size makes the higher flow stress, which makes the material prone to deformation difficult. So the activation energy needed for deformation is higher. (2) Lower heating rate during hot compression. The heating rate in the present study is 5 1C/s, which is lower than the 10 1C/s in reference [25]. Lower heating rate means longer diffusion time. The elements were more inclined to exist homogeneously, which caused higher flow stress. (3) Heat treatment condition. The specimens were homogenized at 400 1C for 12 h and then 460 1C for 12 h before hot compression in the present study, while the specimens
Optical microstructure of AA7085 aluminum alloy deformed at strain rate of 0.01 s 1 and temperature of 300 1C, 350 1C, 400 1C, 450 1C is shown in Fig. 6. The microstructure of AA7085 aluminum alloy after being deformed at 300 1C and 350 1C at strain rate 0.01 s 1 consists of elongated grains and banded structure, as shown in Fig. 6a and b, representing typical feature of dynamic recovery. Fine grains which are shown by white circles in Fig. 6c and d are found in the 400 1C, 450 1C compressed samples, indicating that dynamic recrystallization (DRX) occurred in evidence during hot compression deformation. More fine grain colonies co-existing near grain boundaries of elongated grains are found in the samples which hot deformed at 450 1C and different strain rates (Fig. 7). In the deformed structures appear the elongated grains with serrations developed in the grain boundaries; decreasing of Z value leads to more adequate proceeding of dynamic recrystallization and coarser recrystallized grains. The recrystallized grains in the 0.01 s 1, 450 1C sample are much bigger than those in the other conditioned samples, which is due to sufficient deformation time to allow recrystallized grains grow up during hot deformation. It is generally accepted that softening mechanism of alloys during high temperature deformation transforms from dynamic recovery to continuous dynamic recrystallization with decreasing Z value and the Z value is usually used as a criteria to determine dynamic flow softening mechanism [12,13]. Though the microstructure of samples with high and low Z value are always compared to determine dynamic soften mechanism at different deformation conditions [27], little research has been carried out on microstructure comparison and softening mechanism determination of samples with close Z value, especially when they are potentially dynamic recrystallized. The ln(Z) values of specimen deformed under various deformation conditions obtains from Eq. (3) are shown in Table 2. The transmission electron micrographs of specimens deformed at close ln(Z) values which ranged from 39.9 to 46.8 are shown in Fig. 8. Equiaxed grains can be found in all the specimens, indicating that recrystallization occurred during hot deformation in these samples. It should be noticed that clean equiaxed grains with straight grain boundaries are found in the samples which have been deformed at 400 1C, 10 s 1 and 450 1C, 10 s 1 (Fig. 8a and d), while the samples that have been deformed at 400 1C, 0.01 s 1and 400 1C, 1 s 1 consist of equiaxed grain and grains with dense dislocation tangles (Fig. 8b and c). The microstructure of specimens deformed at 400 1C, 1 s 1consists of three typical zones as shown by A, B, C in Fig. 8b: Dense dislocation tangles zone (A), recrystallized grain with straight grain boundaries zone (B), grain with straight cell wall zone (C). This clearly indicates that dynamic recovery and continuous dynamic recrystallization co-occurred in the specimen. The dislocation density weakened with decreasing strain rate; serrated grain boundaries become more clear in the specimen deformed at temperature of 400 1C and strain rate of 0.01 s 1 (Fig. 8c). Recrystallized grain and thick cell wall, as shown by white arrows in Fig. 8d, are found in the specimen deformed at temperature of 450 1C and strain rate of 10 s 1. This indicates that
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Fig. 6. Optical deformed microstructure (OM) of AA7085 aluminum alloy at strain rate of 0.01 s 1 and different temperatures: (a) 300 1C, (b) 350 1C, (c) 400 1C, (d) 450 1C.
Fig. 7. Optical deformed microstructure (OM) of AA7085 aluminum alloy at 450 1C and different strain rates: (a) 0.01 s 1, (b) 0.1 s 1, (c) 1 s 1, (d) 10 s 1. Table 2 ln(Z) values of the alloy deformed at different conditions. Deforming temperature (1C)
300 350 400 450
Strain rate 0.01 s 1
0.1 s 1
1 s1
10 s 1
47.7 43.5 39.9 36.8
50 45.8 42.2 39.1
52.3 48.1 44.5 41.4
54.6 50.4 46.8 43.7
dynamic recrystallization and dynamic recovery co-occurred in the specimen also, so they are assumed to responsible for dynamic flow softening during hot deformation. That dynamic recrystallization and dynamic recovery are co-responsible for the dynamic flow soften was also reported by other researchers during the hot compression of 7050 aluminum alloy at the temperature of 460 1C and strain rate of 0.1 s 1 [8]. It is well known that because of their high stacking fault energy, dynamic recovery can easily occur before dynamic recrystallization in aluminum alloys. Because low strain rate results in
W. Liu et al. / Materials Science & Engineering A 596 (2014) 176–182
181
A
C B
Fig. 8. TEM microstructure of AA7085 aluminum alloy hot deformed at various conditions. (a) 400 1C–10 s 1(46.8), (b) 400 1C–1 s 1(44.5), (c) 400 1C–0.01 s 1(39.9), (d) 450 1C–10 s 1(43.7).
sufficient time for dynamic recovery to consume a mass of dislocation and stored deformation energy, so dynamic recovery inhibits dynamic recrystallization during hot deformation. With decreasing Z value, that is increasing the deformation temperature or decreasing the strain rate, the subgrains size increased and the dislocation density decreased, the main softening mechanism transformed from dynamic recovery to dynamic recrystallization [27]. In the present study, by increasing the deformation temperature from 400 1C to 450 1C and the strain rate from 0.01 s 1to 10 s 1, the ln(Z) increased from 39.9 to 43.7. According to this theory dynamic recrystallization is more inclined to occur in the 400 1C and 0.01 s 1deformed sample, while the microstructure of the two samples in Fig. 8c and d show that dynamic recrystallization more obviously occurred in the 450 1C and 10 s 1 deformed sample. That dynamic recrystallization more obviously occurred in the sample with higher Z value than the one with lower Z value is conflict with the above theory. This can be explained by the following reasons: first, the Z value is calculated from the constitutive equation which takes deformation temperature and strain rate into account, thus it is a calculated value combined with deformation temperature and strain rate contributions. It is very difficult to separate their contributions clearly by this value, so the Z value is only a reference factor to indicate how possible dynamic recrystallization may occur and cannot be used to judge softening mechanism under this condition. Secondly, because the migration of atoms and the dislocation are temperature controlled, the
dynamic recrystallization is more sensitive to temperature than strain rate. When deformation temperature is elevated to 450 1C, dynamic recrystallization would be triggered more easily in the sample even it has higher Z value. Decreasing Z value only by decreasing the strain rate would be more inclined to inhibit recrystallization because of the enhanced recovery caused by long time deformation. Dynamic recrystallization occurred in the specimen deformed at high temperature (550 1C) and high strain rate (5 s 1) was also reported and explained by Zhang et al. [27].
4. Conclusions (1) The flow stress increased significantly with increasing strain rate and decreasing deforming temperature. It can be described by constitutive equation in hyperbolic sine function with the activation energy 249.11 KJ/mol, and also by a Zener– Hollomon parameter Z. (2) Dynamic recrystallization and dynamic recovery are coresponsible for the dynamic flow soften in the specimen deformed at temperature of 400 1C and strain rate of 1 s 1, temperature of 450 1C and strain rate of 10 s 1. (3) Dynamic recrystallization more obviously occurred in the specimen with higher Z value (450 1C, 10 s 1) than the one with lower Z value (400 1C, 0.01 s 1), Z value is a reference
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factor to indicate how possible dynamic recrystallization would occur. Acknowledgments This study is financially supported by the Chongqing Municipal Programs for Science and Technology (CSTC, 2010AA4047) and NSFC (No. 50901092). References [1] M. Rajamuthamilselvan, S. Ramanathan, J. Alloys Compd. 509 (2011) 948–952. [2] N.P. Jin, H. Zhang, Y. Han, W. Wu, J. Chen, Mater. Charact. 60 (2009) 530–536. [3] D.J. Chakrabarti, J. Liu, R.R. Sawtell, G.B. Venema, Mater. Forum 28 (2004) 969–974. [4] H. Luong, M.R. Hill, Mater. Sci. Eng. A 477 (2008) 208–216. [5] L.Z. He, X.H. Li, P. Zhu, Y.H. Cao, Y.P. Guo, J.Z. Cui, Mater. Charact. 71 (2012) 19–23. [6] S.Y. Chen, K.H. Chen, G.S. Peng, L. Jia, P.X. Dong, Mater. Des. 35 (2012) 93–98. [7] L. Zhen, H. Hu, X. Wang, B. Zhang, W. Shao, J. Mater. Process. Technol. 209 (2009) 754–761. [8] H.E. Hu, L. Zhen, B.Y. Zhang, L. Yang, J.Z. Chen, Mater. Charact. 59 (2008) 1185–1189.
[9] C.X. Wang, F.X. Yu, D.Z. Zhao, X. Zhao, Liang Zuo, Mater. Sci. Eng. A 577 (2013) 73–80. [10] R. Kaibyshev, O. Sitdikov, A. Goloborodko, T. Sakai, Mater. Sci. Eng. A 344 (2003) 348–356. [11] S. Gourdet, F. Montheillet, Mater. Sci. Eng. A 283 (2000) 274–288. [12] H.E. Hu, L. Zhen, L. Yang, W.Z. Shao, B.Y. Zhang, Mater. Sci. Eng. A 488 (2008) 64–71. [13] X.Y. Liu, Q.L. Pan, Y.B. He, W.B. Li, W.J. Liang, Z.M. Yin, Mater. Sci. Eng. A 500 (2012) 150–154. [14] E. Ceeri, E. Evangelista, A. Forcellese, Mater. Sci. Eng. A 197 (1995) 181–198. [15] H.J. McQueen, N.D. Ryan, Mater. Sci. Eng. A 322 (2002) 43–63. [16] T. Sheppard, A. Jackson, Mater. Sci. Technol. 13 (1997) 203–207. [17] C. Zener, J.H. Hollomon, J. Appl. Phys. 15 (1944) 22–27. [18] G. Meng, B.L. Li, H.M. Li, H. Huang, Z.R. Nie, Mater. Sci. Eng. A 517 (2009) 132–137. [19] S.F. Medina, C.A. Hernandez, Acta Mater. 44 (1996) 137–148. [20] S.M. Abbasi, A. Shokuhfar, Mater. Lett. 61 (2007) 2523–2526. [21] M. Zhou, M.P. Clode, Mech. Mater. 27 (1998) 63–76. [22] W. Li, H. Li, Z.X. Wang, Z.Q. Zheng, Mater. Sci. Eng. A 528 (2011) 4098–4103. [23] M.R. Rokni, A.Z. Hanzaki, A.A. Roostaei, H.R. Abedi, Mater. Des. 32 (2011) 4955–4960. [24] H. Zhang, N.P. Jin, J.H. Chen, Trans. Nonferrous Met. Soc. 21 (2011) 437–442. [25] S.Y. Chen, K.H. Chen, G.S. Peng, X.H. Chen, Q.H. Ceng, J. Alloys Compd. 537 (2012) 338–345. [26] B. Wu, M.Q. Li, D.W. Ma, Mater. Sci. Eng. A 542 (2012) 79–87. [27] H. Zhang, L.X. Li, D. Yuan, D.S. Peng, Mater. Charact. 58 (2007) 168–173.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Hot deformation behavior of AA7085 aluminum alloy during isothermal compression at elevated temperature Wenyi Liu, Huan Zhao, Dan Li, Zhiqing Zhang n, Guangjie Huang, Qing Liu College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2013 Received in revised form 2 December 2013 Accepted 3 December 2013 Available online 11 December 2013
The isothermal deformation compression tests of AA7085 aluminum alloy were performed on Gleeble1500 system in the temperature range from 250 1C to 450 1C and at strain rate range from 0.01 s 1 to 10 s 1. The microstructure of samples was observed using optical microscopy (OM) and transmission electron microscopy (TEM) techniques. The results show that the peak stress levels decreased with the increase of deformation temperatures or the decrease of strain rate, which can be represented by the Zener–Hollomon parameter in the exponent-type equation with the hot deformation activation energy of 249.11 KJ/mol. Dynamic recrystallization more obviously occurred in the sample with higher Z value than in the sample with lower Z value. Dynamic recrystallization is sensitively dependent on the deformation temperature. & 2013 Elsevier B.V. All rights reserved.
Keywords: AA7085 aluminum alloy Hot deformation Constitutive equation Softening mechanism Dynamic recrystallization
1. Introduction The 7000 (Al–Zn–Mg–Cu) series alloys have been widely used in various automobiles and aerospace industry because of their high strength to low densities and high fracture toughness, as well as resistance to stress corrosion cracking [1,2]. AA7085 alloy aluminum was first introduced by ALCOA in 2003 as the 7th generation of high strength thick plate alloy [3–6]. It is beginning to be used for aerospace applications, most recently for wing spar and rib structures on the new Airbus A380 aircraft [3]. It is well known that mechanical properties of aluminum alloy are closely related to its microstructure, which is greatly influenced by thermomechanical processing [7]. So it is very necessary to investigate the interactive effect between deformation behavior and thermomechanical processing on microstructure and relative mechanical property of aluminum alloys. Two changes will occur in the material during the thermomechanical processing. One is work hardening, and the other is dynamic softening. Dynamic softening includes dynamic recovery and dynamic recrystallization which are important processes responsible for the microstructure evolution during high temperature deformation of high strength aluminum alloys, and are part of the scientifically and industrially important subject of thermomechanical processing [8]. Generally, dynamic recrystallization is not prone to occur in aluminum because of high stacking fault energy. But it has been
n
Corresponding author. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.012
reported in the literatures that dynamic recrystallization occurs in the aluminum during the hot deformation [9–11]. This indicated that the stacking fault energy is not the critical factor of softening mechanism of aluminum alloy during isothermal deformation compression. Many reports have shown that dynamic recrystallization only occurs at low Zener–Hollomon parameters, which must be below or equal to a critical value [12,13]. Though the AA7085 aluminum alloy was successfully used in industries, limited works on hot deformation behavior and corresponding microstructure evolution of this alloy are found in open publications so far. The research of the relationship between Z values and the microstructure is very important for industrial production in order to provide evidence for controlling and predicting the structure and performance after hot deformation. In the present study, the deformation behavior and microstructure evolution of AA7085 aluminum alloy during isothermal compression were studied in detail by means of true stress–true strain curves building, constitutive equation calculation and microstructure characterization, which aimed to reveal the relationship among flow stress and deformation parameters, microstructure evolution and the softening mechanism at different conditions.
2. Experimental The experimental material in this isothermal deformation compression tests is AA7085 aluminum alloy with the chemical position of 7.0–8.0%Zn, 1.2–1.8%Mg, 1.3–2.0%Cu, 0.08–0.15%Zr, Fer 0.08%, Si r0.06%. AA7085 aluminum alloy was homogenized
W. Liu et al. / Materials Science & Engineering A 596 (2014) 176–182
at 400 1C for 12 h and 460 1C for 12 h before the isothermal compression tests. The microstructure of the experimental material is homogeneous, which consists of the equiaxed grains with an average size about 60 μm as shown in Fig. 1. In order to investigate the effect of processing parameters on the deformation behavior of AA7085 aluminum alloy, the isothermal deformation compression tests were carried out on a computer servo-controlled Gleeble-1500 simulator at the deformation temperatures ranging from 250 1C to 450 1C, the strain rates of 0.01 s 1, 0.1 s 1, 1 s 1, 10 s 1, and the true strain of 0.9. Cylindrical compression samples were machined with the size of 10 mm in diameter and 15 mm in height. The groove with 0.3 mm depth was machined on both ends of the samples in order to maintain the lubricant of graphite mixed with machine oil to reduce the friction with the beak iron during the isothermal deformation compression tests. Cylindrical compression specimens prior to isothermal deformation compression were heated to deformation temperature at a heating rate of 5 1C/s and held for 3 min by thermocoupled-feedback-controlled AC current so as to obtain a uniform deformation temperature. After compression, the samples
Fig. 1. Initial microstructure of AA7085 aluminum alloy.
177
were quenched to room temperature immediately to maintain deformation microstructure. The flow stress–strain curves were recorded by the computer automatically. The deformed samples were sectioned parallelly to the compression axis along the direction of centerline, electrolytic polished, and chemical etched in a Keller's agent for microstructural observation. The Axiovert 40 MAT microscope was used for microstructure observation. Thin foil TEM samples were prepared by cutting the longitudinal section of the deformed specimen using an electrical-discharge machining. The discs were ground to a thickness of about 50 μm followed by electro-polishing in a double-jet electro-polishing unit operating at 10 V and 35 1C using a 30% nitric acid and 70% methanol solution. TEM investigations were performed on a Zeiss Libra 200 FE transmission electron microscope, operated at 200 KV.
3. Results and discussion 3.1. Flow stress behavior of AA7085 aluminum alloy A series of true stress–true strain curves obtained during the isothermal compression of AA7085 aluminum alloy at strain rate from 0.01 s 1to 10 s 1 and deformation temperature from 250 1C to 450 1C are shown in Fig. 2. It can be seen from Fig. 2 that the true stress–true strain curves exhibit a peak stress at a certain strain followed by a rheological softening or a steady-state flow behavior. The certain strain corresponding to the peak stress increases with the increasing of strain-rate and the decreasing of temperature. The peak stress and the flow stress gradually increase with increasing strain-rate and decreasing deformation temperature. In the early stage of isothermal compressive deformation of AA7085 aluminum alloy at low strain rate such as 0.01 s 1 and high temperature such as 450 1C, the flow stress increased dramatically with strain increasing because of rapid dislocation
Fig. 2. The true stress–true strain curves during hot compression of AA7085 (a) ε_ ¼ 0.01 s 1, (b) ε_ ¼0.1 s 1, (c) ε_ ¼ 1 s 1, (d) ε_ ¼ 10 s 1.
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multiplication. So work hardening played a leading role during this period. In the same strain rate, the rheological curves are relatively smooth at low temperature deformation, while the flow curves showed obvious fluctuation at high temperature deformation, softening and hardening alternately, the peak stress is more obvious.
slopes of ln ε_ vs. ln(s) plots at high stress in Fig. 3b, so the value of α is 0.01601 MPa 1.
3.2. Constitutive analyses of AA7085 aluminum alloy
Taking the natural logarithm of both sides of Eq. (4) we can obtain as follows:
In the isothermal compression of aluminum alloy materials at high temperature, it is well accepted that the relationship between flow stress, strain rate and temperature can be expressed as [14–17] Z ¼ ε_ expð
Q Þ ¼ f 1 ð sÞ ¼ A 1 sn 1 RT
Z ¼ f 2 ðsÞ ¼ A2 expðβsÞ
ð1Þ ð2Þ
where A1, A2, n1 and β are constants; Z is the Zener–Hollomon parameter; Q is the activation energy for hot deformation; R is the gas constant. Eqs. (1) and (2) are suitable for high stress and low stress, respectively. In addition, the hyperbolic sine type equation, Eq.(3), proposed by Sellars and Mctegart, is more general in form suitable for stress over a wide range, especially for expressing the hot deformation behavior of aluminum alloy [2,7,12,18–21]. Z ¼ f ðsÞ ¼ Aðsinh αsÞn
ð3Þ
where A and n are constants; α is the stress multiplier and also the additional adjustable parameter and it is calculated as α ¼ β=n1 , where β is taken as the average values of the slopes of ln ε_ vs. s plots at low stress in Fig. 3a, n1 is taken as the average values of the
Fig. 3. Relationships between (a) s and (b) ln ε_ ln(s).
Hence the following constitutive equation Eq. (4) can be obtained from Eq. (1) to Eq. (3):
ε_ ¼ A3 ½ sinh ðαsÞn expð
ln ½ sinhðαsÞ ¼
Q Þ RT
ln ε_ Q ln A þ RT n n
ð4Þ
ð5Þ
Generally, most researchers are used to fixing temperatures and differentiating in Eq. (5) and the n value can be obtained as follows [22,23]: n¼
∂ ln ε_ ∂ ln ½ sinhðαsÞ
ð6Þ
As a result, the n value is equal to the average slope of the lines in Fig. 4a, which was derived from the measured stress–strain curves in Fig. 2. Fixing strain rates and differentiating in Eq. (5), the S value can be expressed as follows: S¼
∂ ln ½ sinhðαsÞ ∂ð1=TÞ
ð7Þ
Obviously, the S value is equal to the average slope of the lines in Fig. 4b, which was derived from the measured stress–strain
Fig. 4. Relationships between (a) ln ε_ ln(sinhαs) and (b) ln(sinhαs) 1000/T.
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were homogenized at 450 1C for 24 h and then 470 1C for 30 h in reference [25]. The initial microstructure of the specimen is strongly dependent on their heat treatment condition. Higher activation energy Q in the present study may have connection to the difference in their initial microstructure. More work needs to be done to find out the effects of initial microstructure caused by different heat treatments on the hot compression behavior of this alloy. 3.3. Microstructure evolution of AA7085 aluminum alloy
Fig. 5. Relationships between ln Z and ln(sinhαs).
Table 1 Values of material constants of AA7085 aluminum alloy. A (s 1) 3.6 10
18
α (MPa 1)
n
Q (KJ/mol)
0.01601
5.4644
249.11
curves in Fig. 2 too. So the Q value can be expressed as follows: Q ¼ nR
∂ ln ½ sinhðαsÞ ∂ð1=TÞ
ð8Þ
We can determine the activation energy Q directly from Eq. (8). Taking the natural logarithm of both sides of Eq. (3) we can obtain as follows: ln Z ¼ ln A þ n ln ½ sinhðαsÞ
ð9Þ
So we can obtain the value of A from Fig. 5. By substitution of the values in Table 1 into Eqs. (1) and (3), we can get the isothermal compression equation of AA7085 aluminum alloy, shown as Eq. (10), with the deformation activation energy of 249.11 KJ/mol. 249110 ð10Þ ε_ ¼ 3:6 1018 ½ sinh ð0:01601sÞ5:4644 exp RT The hot deformation activation energy Q is an important physical parameter serving as an indicator of deformation difficulty degree in plasticity deformation [24]. In the present work, the Q value is calculated to be 249.11 KJ/mol, which is higher than that for self-diffusion in pure aluminum (165 KJ/mol), as well as homogenized AA7085 aluminum alloy (187.4 KJ/mol) [25]. So the deformation mechanism is likely to be cross-slip, which makes the AA7085 aluminum alloy soft, i.e., the softening mechanism is dynamic recovery [18,26]. The reason for high hot deformation activation energy may be due to the following aspects: (1) Smaller initial grain size. The grain size in the present study is about 60 μm, which is smaller than the average grain size ( 100 μm) in reference [25]. According to the Hall-Petch law, smaller grain size makes the higher flow stress, which makes the material prone to deformation difficult. So the activation energy needed for deformation is higher. (2) Lower heating rate during hot compression. The heating rate in the present study is 5 1C/s, which is lower than the 10 1C/s in reference [25]. Lower heating rate means longer diffusion time. The elements were more inclined to exist homogeneously, which caused higher flow stress. (3) Heat treatment condition. The specimens were homogenized at 400 1C for 12 h and then 460 1C for 12 h before hot compression in the present study, while the specimens
Optical microstructure of AA7085 aluminum alloy deformed at strain rate of 0.01 s 1 and temperature of 300 1C, 350 1C, 400 1C, 450 1C is shown in Fig. 6. The microstructure of AA7085 aluminum alloy after being deformed at 300 1C and 350 1C at strain rate 0.01 s 1 consists of elongated grains and banded structure, as shown in Fig. 6a and b, representing typical feature of dynamic recovery. Fine grains which are shown by white circles in Fig. 6c and d are found in the 400 1C, 450 1C compressed samples, indicating that dynamic recrystallization (DRX) occurred in evidence during hot compression deformation. More fine grain colonies co-existing near grain boundaries of elongated grains are found in the samples which hot deformed at 450 1C and different strain rates (Fig. 7). In the deformed structures appear the elongated grains with serrations developed in the grain boundaries; decreasing of Z value leads to more adequate proceeding of dynamic recrystallization and coarser recrystallized grains. The recrystallized grains in the 0.01 s 1, 450 1C sample are much bigger than those in the other conditioned samples, which is due to sufficient deformation time to allow recrystallized grains grow up during hot deformation. It is generally accepted that softening mechanism of alloys during high temperature deformation transforms from dynamic recovery to continuous dynamic recrystallization with decreasing Z value and the Z value is usually used as a criteria to determine dynamic flow softening mechanism [12,13]. Though the microstructure of samples with high and low Z value are always compared to determine dynamic soften mechanism at different deformation conditions [27], little research has been carried out on microstructure comparison and softening mechanism determination of samples with close Z value, especially when they are potentially dynamic recrystallized. The ln(Z) values of specimen deformed under various deformation conditions obtains from Eq. (3) are shown in Table 2. The transmission electron micrographs of specimens deformed at close ln(Z) values which ranged from 39.9 to 46.8 are shown in Fig. 8. Equiaxed grains can be found in all the specimens, indicating that recrystallization occurred during hot deformation in these samples. It should be noticed that clean equiaxed grains with straight grain boundaries are found in the samples which have been deformed at 400 1C, 10 s 1 and 450 1C, 10 s 1 (Fig. 8a and d), while the samples that have been deformed at 400 1C, 0.01 s 1and 400 1C, 1 s 1 consist of equiaxed grain and grains with dense dislocation tangles (Fig. 8b and c). The microstructure of specimens deformed at 400 1C, 1 s 1consists of three typical zones as shown by A, B, C in Fig. 8b: Dense dislocation tangles zone (A), recrystallized grain with straight grain boundaries zone (B), grain with straight cell wall zone (C). This clearly indicates that dynamic recovery and continuous dynamic recrystallization co-occurred in the specimen. The dislocation density weakened with decreasing strain rate; serrated grain boundaries become more clear in the specimen deformed at temperature of 400 1C and strain rate of 0.01 s 1 (Fig. 8c). Recrystallized grain and thick cell wall, as shown by white arrows in Fig. 8d, are found in the specimen deformed at temperature of 450 1C and strain rate of 10 s 1. This indicates that
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Fig. 6. Optical deformed microstructure (OM) of AA7085 aluminum alloy at strain rate of 0.01 s 1 and different temperatures: (a) 300 1C, (b) 350 1C, (c) 400 1C, (d) 450 1C.
Fig. 7. Optical deformed microstructure (OM) of AA7085 aluminum alloy at 450 1C and different strain rates: (a) 0.01 s 1, (b) 0.1 s 1, (c) 1 s 1, (d) 10 s 1. Table 2 ln(Z) values of the alloy deformed at different conditions. Deforming temperature (1C)
300 350 400 450
Strain rate 0.01 s 1
0.1 s 1
1 s1
10 s 1
47.7 43.5 39.9 36.8
50 45.8 42.2 39.1
52.3 48.1 44.5 41.4
54.6 50.4 46.8 43.7
dynamic recrystallization and dynamic recovery co-occurred in the specimen also, so they are assumed to responsible for dynamic flow softening during hot deformation. That dynamic recrystallization and dynamic recovery are co-responsible for the dynamic flow soften was also reported by other researchers during the hot compression of 7050 aluminum alloy at the temperature of 460 1C and strain rate of 0.1 s 1 [8]. It is well known that because of their high stacking fault energy, dynamic recovery can easily occur before dynamic recrystallization in aluminum alloys. Because low strain rate results in
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A
C B
Fig. 8. TEM microstructure of AA7085 aluminum alloy hot deformed at various conditions. (a) 400 1C–10 s 1(46.8), (b) 400 1C–1 s 1(44.5), (c) 400 1C–0.01 s 1(39.9), (d) 450 1C–10 s 1(43.7).
sufficient time for dynamic recovery to consume a mass of dislocation and stored deformation energy, so dynamic recovery inhibits dynamic recrystallization during hot deformation. With decreasing Z value, that is increasing the deformation temperature or decreasing the strain rate, the subgrains size increased and the dislocation density decreased, the main softening mechanism transformed from dynamic recovery to dynamic recrystallization [27]. In the present study, by increasing the deformation temperature from 400 1C to 450 1C and the strain rate from 0.01 s 1to 10 s 1, the ln(Z) increased from 39.9 to 43.7. According to this theory dynamic recrystallization is more inclined to occur in the 400 1C and 0.01 s 1deformed sample, while the microstructure of the two samples in Fig. 8c and d show that dynamic recrystallization more obviously occurred in the 450 1C and 10 s 1 deformed sample. That dynamic recrystallization more obviously occurred in the sample with higher Z value than the one with lower Z value is conflict with the above theory. This can be explained by the following reasons: first, the Z value is calculated from the constitutive equation which takes deformation temperature and strain rate into account, thus it is a calculated value combined with deformation temperature and strain rate contributions. It is very difficult to separate their contributions clearly by this value, so the Z value is only a reference factor to indicate how possible dynamic recrystallization may occur and cannot be used to judge softening mechanism under this condition. Secondly, because the migration of atoms and the dislocation are temperature controlled, the
dynamic recrystallization is more sensitive to temperature than strain rate. When deformation temperature is elevated to 450 1C, dynamic recrystallization would be triggered more easily in the sample even it has higher Z value. Decreasing Z value only by decreasing the strain rate would be more inclined to inhibit recrystallization because of the enhanced recovery caused by long time deformation. Dynamic recrystallization occurred in the specimen deformed at high temperature (550 1C) and high strain rate (5 s 1) was also reported and explained by Zhang et al. [27].
4. Conclusions (1) The flow stress increased significantly with increasing strain rate and decreasing deforming temperature. It can be described by constitutive equation in hyperbolic sine function with the activation energy 249.11 KJ/mol, and also by a Zener– Hollomon parameter Z. (2) Dynamic recrystallization and dynamic recovery are coresponsible for the dynamic flow soften in the specimen deformed at temperature of 400 1C and strain rate of 1 s 1, temperature of 450 1C and strain rate of 10 s 1. (3) Dynamic recrystallization more obviously occurred in the specimen with higher Z value (450 1C, 10 s 1) than the one with lower Z value (400 1C, 0.01 s 1), Z value is a reference
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