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Hot deformation behavior of 7150 aluminum alloy during compression at elevated temperature
MA TE RI A L S CH A R A CT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0– 5 3 6
Hot deformation behavior of 7150 aluminum alloy during compression at elevated temperature Nengping Jina , Hui Zhanga,⁎, Yi Hanb , Wenxiang Wub , Jianghua Chena a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Suzhou Institute for Nonferrous Metal Processing Technology, Suzhou 215026, China
b
AR TIC LE D ATA
ABSTR ACT
Article history:
Hot compression tests of 7150 aluminum alloy were preformed on Gleeble-1500 system in
Received 30 October 2008
the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s− 1 to 10 s− 1,
Received in revised form
and the associated structural changes were studied by observations of metallographic
9 December 2008
and transmission electron microscope. The results show that the true stress–true strain
Accepted 12 December 2008
curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress
Keyword:
level decreases with increasing deformation temperature and decreasing strain rate,
7150 aluminum alloy
which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation
Hot compression deformation
with the hot deformation activation energy of 229.75 kJ/mol. In the deformed structures
Flow softening
appear the elongated grains with serrations developed in the grain boundaries, decreasing
Dynamic recrystallization
of Z value leads to more adequate proceeding of dynamic recrystallization and coarser
Dynamic precipitate coarsening
recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as increasing Z value. The dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value. © 2008 Elsevier Inc. All rights reserved.
1.
Introduction
The 7000 series alloys, based on the Al-Zn-Mg system, have a combination of high strength and fracture toughness, as well as resistance to stress corrosion cracking that renders them very useful in the aircraft and aerospace industry applications [1–3]. Increased strength of these alloys was anticipated by increasing Zn, Mg, and Cu concentration, as these are the principal basis of precipitation strengthening, but at the same time lowers the hot workability. Workability is usually defined as the amount of deformation that a material can undergo without cracking and reach desirable deformed microstructures at a given temperature and strain rate. Improving
workability means increasing the processing ability and improving properties of the materials, this probably can be achieved by optimum processing parameters. However, these difficulties usually arise due to the lack of knowledge of the hot deformation behavior of specific materials under the conditions encountered in the processing operation, such as extrusion, forging, rolling, etc. Previous studies have suggested generally that the flow stress during hot deformation of Al-Zn-Mg aluminum alloy diminishes with rise in deformation temperature and decrease in strain rate, and the flow curves are marked by a peak and softening which decline as deformation temperature rises [4–6]. The peak flow stress of various treated A1-Zn-Mg-Cu
⁎ Corresponding author. Tel.: +86 731 8664086; fax: +86 731 8821483. E-mail address: [email protected] (H. Zhang). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.12.012
531
M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0 –5 3 6
alloys (7012 and 7075) is related to the strain rate by the hyperbolic sine equation; the activation energy for precipitated alloys and over-aged materials is about 141–162 kJ/mol for 7012 and 143–156 kJ/mol for 7075, similar to that of the bulk selfdiffusion of pure aluminum, 142 kJ/mol. For the solutiontreated material, the values are 200–230 kJ/mol for 7012 and 300–400 kJ/mol for 7075. The flow behavior is the result not only of dynamic precipitation (DPN), but also of dynamic recrystallization (DRX) during hot deformation of 7012 and 7075 aluminum alloys [4]. The stress level of 7050 aluminum alloy can be presented by a Zener–Holloman parameter in a hyperbolic sine-type equation with the hot activation energy of 256.6 kJ/mol and the soften mechanism of the alloy during high temperature deformation transforms from dynamic recovery to continuous dynamic recrystallization with decreasing Z value [5]. Grain boundary sliding (GBS) can play a key role in the evolution of fine grains during hot compression of 7075 aluminum alloy [6], as well as the appearance of superplasticity during hot deformation of an unrecrystallized coarse grained 7075 aluminum alloy [7]. 7150 aluminum alloy, higher fracture toughness, SCC resistance and strength than that of 7050 alloy, has been developed [1,8]. The purpose of this is to gain a fundamental understanding of the hot deformation behavior of 7150 aluminum alloy, including the effects of thermomechanical
parameters on the flow stress and associated structural changes.
2.
Experimental Procedure
The experiments were carried out on 7150 aluminum alloy with main chemical compositions 6.29Zn, 2.33 Mg, 2.10Cu, 0.02Si, 0.05Fe, 0.09Zr, 0.04Ti and Al balance (mass, %). Cylindrical samples with 10 mm in diameter and 15 mm in height were machined from commercially semi-continuous chill cast billets with 180 mm in thickness and subsequently homogenized at 465 °C for 24 h, followed by water quenching. Convex depressions 0.2 mm deep were machined on both ends of the samples in order to maintain the lubricant of graphite mixed with machine oil during compression tests. Compression tests were carried out on a computer servo-controlled Gleeble 1500 system at a strain rate of 0.01–10 s− 1 and deformation temperature of 300–450 °C. The sample was resistance heated to deformation temperature at a heating rate of 10 °C/s and held at that temperature for 180 s by thermocoupled-feedback-controlled AC current before compression, the samples were deformed to half of their original height and water quenched immediately. The deformed structures were sectioned parallel to the compression axis along the direction of centerline and prepared by
.
Fig. 1 – True stress–true strain curves of 7150 aluminum alloy during hot compression deformation: (a) ε = 0.01 s− 1; . . . (b) ε = 0.1 s− 1; (c) ε = 1 s− 1; (d) ε = 10 s− 1.
532
MA TE RI A L S CH A R A CT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0– 5 3 6
Table 1 – Material constants of the alloy used in the hyperbolic sine equation. A/s− 1 4.161 × 1014
α/MPa− 1
n
Q/kJ·mol− 1
0.01956
5.14336
229.75
the conventional methods for the microstructural observations on MM-6 metallographic microscope (OM) and H-800 transmission electron microscope (TEM).
3.
Results and Discussion
3.1.
Flow Stress Behavior
A series of typical true stress–true strain curves obtained during hot compression of 7150 aluminum alloy at a strain rate of 0.01–10 s− 1 and deformation temperature 300–450 °C are shown in Fig. 1. It can been seen that the true stress–true strain curves exhibit a peak stress at certain strain, followed by dynamic flow softening until the end of compression. The peak stress and flow stress increase with increasing strain rate and
with decreasing deformation temperature. The strain corresponding to the peak stress increases with increase in strain rate and/or with decrease in deformation temperature due to the high hardening rate at initial deformation stages. The flow softening is probably subjected to the dynamic recovery and recrystallization, as well as dynamic coarsening of precipitates during hot deformation of aluminum alloys [4–6,9,10]. The actual softening mechanisms will be discussed in detail in the following section by comparison analysis with hot deformation activation energy calculations and microstructural observation. In hot deformation of metallic materials, it is commonly accepted that the relationship between the peak stress or steady state stress, strain rate and temperature can be expressed as [11–13]: Q : = f1 ðrÞ = A1 rn1 Z = eexp RT
ð1Þ
Z = f2 ðrÞ = A2 expðbrÞ
ð2Þ
where A1, A2, n1 and β are constants, Z is the Zener−Hollomon parameter, Q is an activation energy for hot deformation and R is the gas constant. The power law, Eq. (1), and the exponent-
Fig. 2 – Optical deformed microstructures of 7150 aluminum alloy under different conditions: (a) 450 °C, 0.01 s− 1; (b) 400 °C, 0.1 s− 1; (c) 350 °C, 1 s− 1; (d) 300 °C, 10 s− 1.
M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0 –5 3 6
Table 2 – LnZ values under different deformation conditions, s− 1.
300 350 400 450
°C °C °C °C
0.01 s− 1
0.1 s− 1
1 s− 1
10 s− 1
43.63 39.76 36.46 33.62
45.93 42.06 38.76 35.92
48.23 44.36 41.06 38.22
50.53 46.66 43.36 40.52
type equation, Eq. (2), break at a high stress and at a low stress, respectively. The hyperbolic sine type equation, Eq. (3), proposed by Sellars and McTegart [14] is more general in form suitable for stresses over a wide range. Z = f ðrÞ = Aðsinh arÞn
ð3Þ
where A and n are constants. α is the stress multiplier, also the additional adjustable parameter and a=
b b c n n1
ð4Þ
It is easier to obtain values of n1 or n and β by means of linear regression through Eqs. (1) and (2), respectively. Then a value of α was taken as a first approximation in Eq. (4) and the
Fig. 3 – High magnification optical deformed microstructures of 7150 aluminum alloy: (a) 450 °C, 0.01 s− 1; (b) 350 °C, 1 s− 1.
533
relationship of Eq. (3) was derived to obtain a new value of n, which was then iterated to obtain the optimum values of α, n, A and Q [13]. The material constants of the alloy obtained from the experimental data are shown in Table 1, which can be used for quantificationally describing the relationship of the flow stress and temperature and strain rate by a Zener– Hollomon parameter in the hyperbolic sine equation with the related coefficient of 0.9778. The hot deformation activation energy Q is an important physical parameter serving as indicator of deformation difficulty degree in plasticity deformation. The value of Q for 7150 aluminum alloy is 229.75 kJ/mol, which is higher than that of the aged 7150 alloy (158.8–161.4 kJ/mol) previously reported by Sheppard and Jackson [12], as well as the precipitated and over aged 7012 alloy (141–162 kJ/mol), similar to that of the bulk selfdiffusion of pure aluminum, 142 kJ/mol, but close to the values of the solution-treated 7012 alloy (200–230 kJ/mol) [4] and the asquenched 7050 aluminum alloy (256.6 kJ/mol) [5]. Generally, the higher deformation activation energy will be found in hot deformation of the solution treated aluminum alloys [4,5,9]. A stationary dislocation in a solid solution can be pinned by the solute atoms if they are able to diffuse to the dislocation and hence an additional increment of stress will be required to free the dislocation from its energetically favorable position. In addition, any dislocation moving through a solid solution will encounter friction drag thus raising the energy required for movement. Hence any increase in foreign atoms held in solution will increase the activation energy. In this case, it can be deduced that the higher Q of the alloy may be associated with an increase in the Zn, Mg and Cu held in solution by the homogenization sequence. During high strain rate deformation, recovery processes are also hindered by a higher solute atom vacancy binding energy which effectively reduces the number of vacancies available for dislocation climb whilst the misfit strain effectively raises the dislocation density; both processes contributing to an increase in activation energy for hot deformation.
Fig. 4 – Inhomogeneous strain distribution and frequent development of microshear bands deformed at 450 °C and 0.01 s− 1.
534 3.2.
MA TE RI A L S CH A R A CT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0– 5 3 6
Microstructural Evolution
Fig. 2 shows optical micrographs of 7150 aluminum alloy deformed under different conditions. The microstructure is composed of elongated newly refined grains separated by moderately high-angle boundaries as are observed, indicating that dynamic recrystallization (DRX) occurs in evidence during hot compression deformation and the DRX grain size is dependent sensitively on deformation temperature and/or strain rate, also on Zener–Hollomon parameter Z. Table 2 shows the lnZ values under different hot compression conditions, obtained from Eq. (1). Decreasing of Z value, that is increasing deformation temperature or decreasing strain rate, leads to more adequate proceeding of DRX and coarser recrystallized grains. The sufficient migration of atoms and dislocations cause the merging of some grains, and the low-angle grain boundaries
transformed into high-angle grain boundaries through absorbing dislocations. Subsequently, the recrystallized grains grew greatly through the migration of dislocations and high-angle grain boundaries. Correspondingly, the flow stress decreases dramatically with decreasing Z value (Fig. 1). From the high magnification optical deformed microstructures of 7150 aluminum alloy under different conditions, shown in Fig. 3, it can be seen that grain boundary sliding (GBS) takes place along the initial grain boundaries, resulting in inhomogeneous strain distribution and then frequent development of microshear bands in grain interiors (marked A). Subdivision of original grains into misoriented small domains leads to the average grain size evolved similar to the minimum size of regions fragmented by microshear bands, as shown in Fig. 4. The grain refinement occurs by a deformation-induced continuous reaction that is essentially similar to continuous dynamic
Fig. 5 – Transmission electron micrographs: (a) 450 °C, 0.01 s− 1; (b) 400 °C, 0.1 s− 1; (c) 350 °C, 1 s− 1; (d) 300 °C, 10 s− 1.
M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0 –5 3 6
recrystallization (CDRX) [7]. Continuous dynamic recrystallization occurs by gradual subgrain growth, leading to formation of high-angle boundary migration. Typically, in the mechanisms of continuous dynamic recrystallization nucleation it appears likely that some subgrains are coarsening through the merger of other adjacent subgrains to produce high-angle boundaries [10,15,16]. Subgrain boundary migration can occur when particle coarsening leaves some subgrain boundaries weakly restrained. Alternatively, elevated temperature straining may provide added driving pressure for boundary migration, accelerating continuous recrystallization [16]. Hence, recrystallization nucleation is also observed around the coarse precipitates (marked B), the number of coarsen particles increases with decreasing temperature and increasing strain rate. Indeed, it has been well known that aluminum alloys containing high densities of Al3Zr dispersoids or alloys containing high densities of several types of fine particles, which act as sufficient boundary drag pressures to prevent nucleation of discontinuous recrystallization and hence accelerating continuous recrystallization [10,16,17]. For these reasons, it can be concluded that the grain refinement during hot deformation of 7150 aluminum alloy occurs by the main type of the CDRX. Transmission electron micrographs show that the subgrains exhibited high-angle sub-boundaries with a certain amount of dislocations as was observed in the specimen deformed at temperature of 450 °C and strain rate of 0.01 s− 1, Fig. 5(a). The sub-boundary definitions become more indistinct with decreasing deformation temperature or increasing strain rate, owing to the dislocation tangles (Fig. 5(b–c)). Instead, large numbers of dynamic precipitates and coalesced particles exhibit in the subgrain interiors when deformed at temperature of 300 °C and strain rate of 10 s− 1, Fig. 5(d), indicating that the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening during hot compression of 7150 aluminum alloy at low deformation temperature and high strain rate. The soluble atoms and a uniform dispersion of fine particles interact with the dislocations reducing dynamic recovery and recrystallization, and hence the higher strain energy may speed up dynamic precipitation during hot deformation. The phenomenon is in correspondence also with higher hot deformation activation energy Q of the alloy by constitutive analyses.
4.
Conclusions
Hot compression tests of 7150 aluminum alloy were performed on Gleeble-1500 system in the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s− 1 to 10 s− 1, and the associated structural changes were studied by observations of metallographic and TEM. (1) The true stress–true strain curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation with the hot deformation activation energy of 229.75 kJ/mol.
535
(2) The deformed structures exhibit elongated grains with serrations developed in the grain boundaries. A decreasing of Z value leads to more extensive dynamic recrystallization (DRX) and coarser recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as Z value is increased. (3) Dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitation and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value.
Acknowledgments This work is supported by the National Basic Research (973) Program of China (No. 2008CB617608 and No. 2009CB623704), the National Natural Science Foundation of China (No. 50771043), and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.
REFERENCES [1] Immarigeon JP, Holt RT, Koul AK, Zhao L, Wallace W, Beddoes JC. Lightweight materials for aircraft applications. Mater Charact 1995;35:41–67. [2] Heinz A, Haszler A, Keidel C, Moldenhauer S, Benedictus R, Miller WS. Recent development in aluminium alloys for aerospace applications. Mater Sci Eng 2000;A280:102–7. [3] Williams James C, Starke Edgar Jr A. Progress in structural materials for aerospace systems. Acta Mater 2003;51:5775–99. [4] Cerri E, Evangelista E, Forcellese A, McQueen HJ. Comparative hot workability of 7012 and 7075 alloys after different pretreatments. Mater Sci Eng 1995;A197:181–98. [5] Hu HE, Zhen L, Yang L, Shao WZ, Zhang BY. Deformation behavior and microstructure evolution of 7050 aluminum alloy during high temperature deformation. Mater Sci Eng 2008;A 488:64–71. [6] Sakai T, Takahashi C. Flow softening of 7075 aluminum alloy under hot compression. Mater Trans JIM 1991;32:375–82. [7] Sitdikov O, Sakai T, Goloborodko A, Miura H. Grain fragmentation in a coarse-grained 7475 Al alloy during hot deformation. Scr Mater 2004;51:175–9. [8] Oliveira Jr AF, de Barros MC, Cardoso KR, Travessa DN. The effect of RRA on the strength and SCC resistance on AA7050 and AA7150 aluminium alloys. Mater Sci Eng 2004;A 379:321–6. [9] Zhang H, Li L, Yuan D, Peng D. Hot deformation behavior of the new Al-Mg-Si-Cu aluminum alloy during compression at elevated temperatures. Mater Charact 2007;58:168–73. [10] Doherty RD, Hughes DA, Humphreys FJ, Jonas JJ, Juul Jensen D, Kassner ME, et al. Current issues in recrystallization: a review. Mater Sci Eng 1997;A238:219–74. [11] McQueen HJ, Ryan ND. Constitutive analysis in hot working. Mater Sci Eng 2002;A322:43–63. [12] Sheppard T, Jackson A. Constitutive equations for use in prediction of flow stress during extrusion of aluminum alloys. Mater Sci Technol 1997;13:203–9. [13] Zener C, Hollomon JH. Effect of strain-rate upon the plastic flow of steel. J Appl Phys 1944;15:22–7.
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[14] Sellars CM, McTegart WJ. On the mechanism of hot deformation. Acta Metall 1966;14:1136–8. [15] Gourdet S, Montheillet F. An experimental study of the recrystallization mechanism during hot deformation of aluminium. Mater Sci Eng 2000;A283:274–88. [16] Wert JA. Thermomechanical processing of heat-treatable aluminum alloys for grain size control. In: Chia EH, Mcqueen
HJ, editors. Microstructural control in aluminum alloys: deformation, recovery and recrystallization. The Metallurgical Society, Inc.; 1985. p. 67–94. [17] Dougherty LM, Robertson IM, Vetrano JS. Direct observation of the behavior of grain boundaries during continuous dynamic recrystallization in an Al-4Mg-0.3Sc alloy. Acta Mater 2003;51:4367–78.
Hot deformation behavior of 7150 aluminum alloy during compression at elevated temperature Nengping Jina , Hui Zhanga,⁎, Yi Hanb , Wenxiang Wub , Jianghua Chena a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Suzhou Institute for Nonferrous Metal Processing Technology, Suzhou 215026, China
b
AR TIC LE D ATA
ABSTR ACT
Article history:
Hot compression tests of 7150 aluminum alloy were preformed on Gleeble-1500 system in
Received 30 October 2008
the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s− 1 to 10 s− 1,
Received in revised form
and the associated structural changes were studied by observations of metallographic
9 December 2008
and transmission electron microscope. The results show that the true stress–true strain
Accepted 12 December 2008
curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress
Keyword:
level decreases with increasing deformation temperature and decreasing strain rate,
7150 aluminum alloy
which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation
Hot compression deformation
with the hot deformation activation energy of 229.75 kJ/mol. In the deformed structures
Flow softening
appear the elongated grains with serrations developed in the grain boundaries, decreasing
Dynamic recrystallization
of Z value leads to more adequate proceeding of dynamic recrystallization and coarser
Dynamic precipitate coarsening
recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as increasing Z value. The dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value. © 2008 Elsevier Inc. All rights reserved.
1.
Introduction
The 7000 series alloys, based on the Al-Zn-Mg system, have a combination of high strength and fracture toughness, as well as resistance to stress corrosion cracking that renders them very useful in the aircraft and aerospace industry applications [1–3]. Increased strength of these alloys was anticipated by increasing Zn, Mg, and Cu concentration, as these are the principal basis of precipitation strengthening, but at the same time lowers the hot workability. Workability is usually defined as the amount of deformation that a material can undergo without cracking and reach desirable deformed microstructures at a given temperature and strain rate. Improving
workability means increasing the processing ability and improving properties of the materials, this probably can be achieved by optimum processing parameters. However, these difficulties usually arise due to the lack of knowledge of the hot deformation behavior of specific materials under the conditions encountered in the processing operation, such as extrusion, forging, rolling, etc. Previous studies have suggested generally that the flow stress during hot deformation of Al-Zn-Mg aluminum alloy diminishes with rise in deformation temperature and decrease in strain rate, and the flow curves are marked by a peak and softening which decline as deformation temperature rises [4–6]. The peak flow stress of various treated A1-Zn-Mg-Cu
⁎ Corresponding author. Tel.: +86 731 8664086; fax: +86 731 8821483. E-mail address: [email protected] (H. Zhang). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.12.012
531
M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0 –5 3 6
alloys (7012 and 7075) is related to the strain rate by the hyperbolic sine equation; the activation energy for precipitated alloys and over-aged materials is about 141–162 kJ/mol for 7012 and 143–156 kJ/mol for 7075, similar to that of the bulk selfdiffusion of pure aluminum, 142 kJ/mol. For the solutiontreated material, the values are 200–230 kJ/mol for 7012 and 300–400 kJ/mol for 7075. The flow behavior is the result not only of dynamic precipitation (DPN), but also of dynamic recrystallization (DRX) during hot deformation of 7012 and 7075 aluminum alloys [4]. The stress level of 7050 aluminum alloy can be presented by a Zener–Holloman parameter in a hyperbolic sine-type equation with the hot activation energy of 256.6 kJ/mol and the soften mechanism of the alloy during high temperature deformation transforms from dynamic recovery to continuous dynamic recrystallization with decreasing Z value [5]. Grain boundary sliding (GBS) can play a key role in the evolution of fine grains during hot compression of 7075 aluminum alloy [6], as well as the appearance of superplasticity during hot deformation of an unrecrystallized coarse grained 7075 aluminum alloy [7]. 7150 aluminum alloy, higher fracture toughness, SCC resistance and strength than that of 7050 alloy, has been developed [1,8]. The purpose of this is to gain a fundamental understanding of the hot deformation behavior of 7150 aluminum alloy, including the effects of thermomechanical
parameters on the flow stress and associated structural changes.
2.
Experimental Procedure
The experiments were carried out on 7150 aluminum alloy with main chemical compositions 6.29Zn, 2.33 Mg, 2.10Cu, 0.02Si, 0.05Fe, 0.09Zr, 0.04Ti and Al balance (mass, %). Cylindrical samples with 10 mm in diameter and 15 mm in height were machined from commercially semi-continuous chill cast billets with 180 mm in thickness and subsequently homogenized at 465 °C for 24 h, followed by water quenching. Convex depressions 0.2 mm deep were machined on both ends of the samples in order to maintain the lubricant of graphite mixed with machine oil during compression tests. Compression tests were carried out on a computer servo-controlled Gleeble 1500 system at a strain rate of 0.01–10 s− 1 and deformation temperature of 300–450 °C. The sample was resistance heated to deformation temperature at a heating rate of 10 °C/s and held at that temperature for 180 s by thermocoupled-feedback-controlled AC current before compression, the samples were deformed to half of their original height and water quenched immediately. The deformed structures were sectioned parallel to the compression axis along the direction of centerline and prepared by
.
Fig. 1 – True stress–true strain curves of 7150 aluminum alloy during hot compression deformation: (a) ε = 0.01 s− 1; . . . (b) ε = 0.1 s− 1; (c) ε = 1 s− 1; (d) ε = 10 s− 1.
532
MA TE RI A L S CH A R A CT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0– 5 3 6
Table 1 – Material constants of the alloy used in the hyperbolic sine equation. A/s− 1 4.161 × 1014
α/MPa− 1
n
Q/kJ·mol− 1
0.01956
5.14336
229.75
the conventional methods for the microstructural observations on MM-6 metallographic microscope (OM) and H-800 transmission electron microscope (TEM).
3.
Results and Discussion
3.1.
Flow Stress Behavior
A series of typical true stress–true strain curves obtained during hot compression of 7150 aluminum alloy at a strain rate of 0.01–10 s− 1 and deformation temperature 300–450 °C are shown in Fig. 1. It can been seen that the true stress–true strain curves exhibit a peak stress at certain strain, followed by dynamic flow softening until the end of compression. The peak stress and flow stress increase with increasing strain rate and
with decreasing deformation temperature. The strain corresponding to the peak stress increases with increase in strain rate and/or with decrease in deformation temperature due to the high hardening rate at initial deformation stages. The flow softening is probably subjected to the dynamic recovery and recrystallization, as well as dynamic coarsening of precipitates during hot deformation of aluminum alloys [4–6,9,10]. The actual softening mechanisms will be discussed in detail in the following section by comparison analysis with hot deformation activation energy calculations and microstructural observation. In hot deformation of metallic materials, it is commonly accepted that the relationship between the peak stress or steady state stress, strain rate and temperature can be expressed as [11–13]: Q : = f1 ðrÞ = A1 rn1 Z = eexp RT
ð1Þ
Z = f2 ðrÞ = A2 expðbrÞ
ð2Þ
where A1, A2, n1 and β are constants, Z is the Zener−Hollomon parameter, Q is an activation energy for hot deformation and R is the gas constant. The power law, Eq. (1), and the exponent-
Fig. 2 – Optical deformed microstructures of 7150 aluminum alloy under different conditions: (a) 450 °C, 0.01 s− 1; (b) 400 °C, 0.1 s− 1; (c) 350 °C, 1 s− 1; (d) 300 °C, 10 s− 1.
M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 5 3 0 –5 3 6
Table 2 – LnZ values under different deformation conditions, s− 1.
300 350 400 450
°C °C °C °C
0.01 s− 1
0.1 s− 1
1 s− 1
10 s− 1
43.63 39.76 36.46 33.62
45.93 42.06 38.76 35.92
48.23 44.36 41.06 38.22
50.53 46.66 43.36 40.52
type equation, Eq. (2), break at a high stress and at a low stress, respectively. The hyperbolic sine type equation, Eq. (3), proposed by Sellars and McTegart [14] is more general in form suitable for stresses over a wide range. Z = f ðrÞ = Aðsinh arÞn
ð3Þ
where A and n are constants. α is the stress multiplier, also the additional adjustable parameter and a=
b b c n n1
ð4Þ
It is easier to obtain values of n1 or n and β by means of linear regression through Eqs. (1) and (2), respectively. Then a value of α was taken as a first approximation in Eq. (4) and the
Fig. 3 – High magnification optical deformed microstructures of 7150 aluminum alloy: (a) 450 °C, 0.01 s− 1; (b) 350 °C, 1 s− 1.
533
relationship of Eq. (3) was derived to obtain a new value of n, which was then iterated to obtain the optimum values of α, n, A and Q [13]. The material constants of the alloy obtained from the experimental data are shown in Table 1, which can be used for quantificationally describing the relationship of the flow stress and temperature and strain rate by a Zener– Hollomon parameter in the hyperbolic sine equation with the related coefficient of 0.9778. The hot deformation activation energy Q is an important physical parameter serving as indicator of deformation difficulty degree in plasticity deformation. The value of Q for 7150 aluminum alloy is 229.75 kJ/mol, which is higher than that of the aged 7150 alloy (158.8–161.4 kJ/mol) previously reported by Sheppard and Jackson [12], as well as the precipitated and over aged 7012 alloy (141–162 kJ/mol), similar to that of the bulk selfdiffusion of pure aluminum, 142 kJ/mol, but close to the values of the solution-treated 7012 alloy (200–230 kJ/mol) [4] and the asquenched 7050 aluminum alloy (256.6 kJ/mol) [5]. Generally, the higher deformation activation energy will be found in hot deformation of the solution treated aluminum alloys [4,5,9]. A stationary dislocation in a solid solution can be pinned by the solute atoms if they are able to diffuse to the dislocation and hence an additional increment of stress will be required to free the dislocation from its energetically favorable position. In addition, any dislocation moving through a solid solution will encounter friction drag thus raising the energy required for movement. Hence any increase in foreign atoms held in solution will increase the activation energy. In this case, it can be deduced that the higher Q of the alloy may be associated with an increase in the Zn, Mg and Cu held in solution by the homogenization sequence. During high strain rate deformation, recovery processes are also hindered by a higher solute atom vacancy binding energy which effectively reduces the number of vacancies available for dislocation climb whilst the misfit strain effectively raises the dislocation density; both processes contributing to an increase in activation energy for hot deformation.
Fig. 4 – Inhomogeneous strain distribution and frequent development of microshear bands deformed at 450 °C and 0.01 s− 1.
534 3.2.
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Microstructural Evolution
Fig. 2 shows optical micrographs of 7150 aluminum alloy deformed under different conditions. The microstructure is composed of elongated newly refined grains separated by moderately high-angle boundaries as are observed, indicating that dynamic recrystallization (DRX) occurs in evidence during hot compression deformation and the DRX grain size is dependent sensitively on deformation temperature and/or strain rate, also on Zener–Hollomon parameter Z. Table 2 shows the lnZ values under different hot compression conditions, obtained from Eq. (1). Decreasing of Z value, that is increasing deformation temperature or decreasing strain rate, leads to more adequate proceeding of DRX and coarser recrystallized grains. The sufficient migration of atoms and dislocations cause the merging of some grains, and the low-angle grain boundaries
transformed into high-angle grain boundaries through absorbing dislocations. Subsequently, the recrystallized grains grew greatly through the migration of dislocations and high-angle grain boundaries. Correspondingly, the flow stress decreases dramatically with decreasing Z value (Fig. 1). From the high magnification optical deformed microstructures of 7150 aluminum alloy under different conditions, shown in Fig. 3, it can be seen that grain boundary sliding (GBS) takes place along the initial grain boundaries, resulting in inhomogeneous strain distribution and then frequent development of microshear bands in grain interiors (marked A). Subdivision of original grains into misoriented small domains leads to the average grain size evolved similar to the minimum size of regions fragmented by microshear bands, as shown in Fig. 4. The grain refinement occurs by a deformation-induced continuous reaction that is essentially similar to continuous dynamic
Fig. 5 – Transmission electron micrographs: (a) 450 °C, 0.01 s− 1; (b) 400 °C, 0.1 s− 1; (c) 350 °C, 1 s− 1; (d) 300 °C, 10 s− 1.
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recrystallization (CDRX) [7]. Continuous dynamic recrystallization occurs by gradual subgrain growth, leading to formation of high-angle boundary migration. Typically, in the mechanisms of continuous dynamic recrystallization nucleation it appears likely that some subgrains are coarsening through the merger of other adjacent subgrains to produce high-angle boundaries [10,15,16]. Subgrain boundary migration can occur when particle coarsening leaves some subgrain boundaries weakly restrained. Alternatively, elevated temperature straining may provide added driving pressure for boundary migration, accelerating continuous recrystallization [16]. Hence, recrystallization nucleation is also observed around the coarse precipitates (marked B), the number of coarsen particles increases with decreasing temperature and increasing strain rate. Indeed, it has been well known that aluminum alloys containing high densities of Al3Zr dispersoids or alloys containing high densities of several types of fine particles, which act as sufficient boundary drag pressures to prevent nucleation of discontinuous recrystallization and hence accelerating continuous recrystallization [10,16,17]. For these reasons, it can be concluded that the grain refinement during hot deformation of 7150 aluminum alloy occurs by the main type of the CDRX. Transmission electron micrographs show that the subgrains exhibited high-angle sub-boundaries with a certain amount of dislocations as was observed in the specimen deformed at temperature of 450 °C and strain rate of 0.01 s− 1, Fig. 5(a). The sub-boundary definitions become more indistinct with decreasing deformation temperature or increasing strain rate, owing to the dislocation tangles (Fig. 5(b–c)). Instead, large numbers of dynamic precipitates and coalesced particles exhibit in the subgrain interiors when deformed at temperature of 300 °C and strain rate of 10 s− 1, Fig. 5(d), indicating that the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening during hot compression of 7150 aluminum alloy at low deformation temperature and high strain rate. The soluble atoms and a uniform dispersion of fine particles interact with the dislocations reducing dynamic recovery and recrystallization, and hence the higher strain energy may speed up dynamic precipitation during hot deformation. The phenomenon is in correspondence also with higher hot deformation activation energy Q of the alloy by constitutive analyses.
4.
Conclusions
Hot compression tests of 7150 aluminum alloy were performed on Gleeble-1500 system in the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s− 1 to 10 s− 1, and the associated structural changes were studied by observations of metallographic and TEM. (1) The true stress–true strain curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation with the hot deformation activation energy of 229.75 kJ/mol.
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(2) The deformed structures exhibit elongated grains with serrations developed in the grain boundaries. A decreasing of Z value leads to more extensive dynamic recrystallization (DRX) and coarser recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as Z value is increased. (3) Dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitation and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value.
Acknowledgments This work is supported by the National Basic Research (973) Program of China (No. 2008CB617608 and No. 2009CB623704), the National Natural Science Foundation of China (No. 50771043), and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.
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