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Effect of deformation condition on plastic anisotropy of as-rolled 7050 aluminum alloy plate
Materials Science and Engineering A 515 (2009) 1–9
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of deformation condition on plastic anisotropy of as-rolled 7050 aluminum alloy plate Wang Xin-yun a,∗ , H.E. Hu b , Xia Ju-chen a a b
State Key Lab of Material Processing and Die&Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, China Naval University of Engineering , Wuhan,430033,China
a r t i c l e
i n f o
Article history: Received 2 April 2008 Received in revised form 17 November 2008 Accepted 23 March 2009 Keywords: Aluminum alloy Plastic anisotropy Microstructure Texture
a b s t r a c t Plastic anisotropy of the as-rolled 7050 aluminum alloy plate was investigated by tensile tests conducted at different temperatures (340 and 460 ◦ C) and different strain rates of 1.0 × 10−4 , 1.0 × 10−2 , 0.1, and 1 s−1 . The results show that the plastic strain ratio of the as-rolled 7050 aluminum alloy plate deformed at 340 ◦ C is not sensitive to strain rate, which increases with increasing strain rate when the alloy was deformed at 460 ◦ C. Alloy elements of the alloy deformed at 460 ◦ C and different strain rates are in solution. The plastic strain ratio of the alloy deformed at 460 ◦ C was affected by texture components, especially the Brass orientation {0 1 1}2 1 1. Dynamic precipitation appears when the alloy was deformed at 340 ◦ C and different strain rates, and the alloy deformed is in over aged condition. Microstructure is the main influence of plastic anisotropy of the alloy deformed at 340 ◦ C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High-strength aluminum alloys with high strength-density ratio and excellent mechanical properties have been widely used for aeronautical applications [1–4]. Good formability during conventional forging and extrusion at elevated temperature is another important reason for extensive application of high-strength aluminum alloys. Thermomechanical processing is often used to acquire the final product form of high-strength aluminum alloys, which has been found to develop strong crystallographic textures and pronounced mechanical anisotropy [5–10]. The magnitude and cause of this anisotropy are legitimate concerns. Plastic anisotropy of aluminum alloys has been classified as inplane anisotropy in plate products, through-thickness anisotropy in thick plates or axisymmetric flow anisotropy in extrusions [11]. Causes of anisotropy have been studied in the past decades and significant arguments support texture rather than microstructure as the primary source of mechanical anisotropy [12]. Additional, modeling and simulating work by Carlos Tome et al. with their viscoplastic self-consistent code has shown that microstructure can have an effect but the effect is secondary compared to texture [13]. Generally, in-plane anisotropy in aluminum alloy sheets was explained on the basis of texture variations together with synergistic contributions from directional precipitates [14–17]. Modeling of in-plane anisotropy, with crystallographic texture, precipitate vol-
∗ Corresponding author. Tel.: +86 27 87543491; fax: +86 27 87543491. E-mail address: [email protected] (X.Y. Wang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.061
ume fractions, grain morphology and the associated grain aspect ratio as the major variables has been undertaken [18]. Crystallographic texture has both direct and indirect effects on the plastic anisotropy of aluminum alloys [19–21]. Direct effects are due to the orientation of the crystals and slip systems with respect to applied stresses and grain morphologies. Indirect effects, due to work hardening and precipitation arise during thermomechanical processing, and these include the orientation of precipitates with respect to slip systems, the distribution of dislocation densities among differently orientated slip systems and the corresponding distribution of precipitates. However, the anisotropy of the Zn–Al eutectic in deformation was attributed to slip in the Zn-rich phase, which caused the change in texture [22]. In the Sn–Pb eutectic, the anisotropy was mainly due to grain directionality, without a significant role for texture [23]. The former research results illustrate microstructure is an important effect on anisotropy of materials too. Usually, high-strength aluminum alloys provided are typically as-rolled plates or as-extruded rods with strong crystallographic textures. Meanwhile, microstructure changes during deformation of high-strength aluminum alloy at some special deformation conditions, such as dynamic precipitates appear. So it is necessary to research the effect of thermomechanical parameters on texture, microstructure and plastic anisotropy of high-strength aluminum alloys. Nevertheless, only a few reports [24,25] about relationship of thermomechanical parameter, texture, microstructure and plastic anisotropy of aluminum alloys have been published. Kashyap [24] reported that the plastic anisotropy ratio of Al–Li alloy increases
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Fig. 1. Optical micrograph showing microstructures of the as-quenched 7050 aluminum alloy. Fig. 2. Grain boundary map showing microstructures of the as-quenched 7050 aluminum alloy.
towards 1 with the increase in strain and strain rate, and there is only a marginal effect of test temperature on plastic anisotropy ratio. Crooks [25] concluded that the anisotropy of AA2195 aluminum alloy was a direct effect of texture, with no significant contribution from precipitate. To the best of our knowledge, there are only few researches reported on relationship of thermomechanical parameter, texture, microstructure and plastic anisotropy of high-strengthen aluminum alloys. In this paper, the effects of thermomechanical parameters on in-plane plastic anisotropy of the as-rolled 7050 aluminum alloy plate were studied in terms of the plastic anisotropy ratio (r = εw /εt , where εw is the width strain and εt is the thickness strain). The microstructures of the 7050 aluminum alloy deformed at different deformation conditions were analyzed by OM, EBSD technique and TEM to investigate the causes of plastic anisotropy of the as-rolled 7050 aluminum alloy plate.
2. Experimental Commercial as-rolled 7050 aluminum alloy plate with 80 mm in thickness and T7451 temper was used for the experiments. Tensile specimens with a 5 mm gauge width and thickness, respectively, and a 20 mm gauge length were made with the tensile axis parallel to the rolling direction and the thickness direction parallel to the normal direction of the as-rolled 7050 aluminum alloy plate. The tensile specimens were sectioned from the centerline layer of the as-rolled 7050 aluminum alloy plate to ensure the specimens with the same texture components before deformation. The specimens were solid solution treated at 477 ◦ C for 1 h and quenched into water. Then tensile tests were conducted to a predetermined elongation of 20% and carried out on an Instron-5500R universal testing machine at 340 and 460 ◦ C with the strain rates of 1.0 × 10−4 ,
Fig. 3. ODF figure showing texture components of the as-quenched 7050 aluminum alloy.
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1.0 × 10−2 , 0.1 and 1 s−1 , respectively. All the samples for analyses were sectioned from the gauge sections. The samples for OM and EBSD analysis were prepared in transverse plane and then mounted, polished and etched by Keller solution (1.5% HCl + 1% HF + 2.5% HNO3 + 95% distilled water, in vol.%) for optical microscope observation by a ZEISS HAL100 optical microscope. EBSD measurement samples were electro-polished using 10 vol.% HClO4 acids in alcohol followed by examined and analyzed using HKL Channel 5 software in a JEOL 733 electron probe at a sample tilt of 70◦ with an accelerating voltage of 20 kV. Step size of the maps was 1 m and the total number of data points of each scan was 50,000. In order to ensure the statistical reliability of the data, three map scans were performed over different areas of each sample. The demarcate ratios of EBSD in the present research are all higher than 94%. Samples for TEM were thinned to about 50 m followed by electro-polished in a double-jet polishing unit operated at 15 V and −20 ◦ C using a 30% nitric acid and 70% methanol solution, the disks were observed in a Philips Tecnai 20 microscope, operated at 200 kV. 3. Results 3.1. Microstructure and texture of as-quenched 7050 aluminum alloy The optical micrograph showing microstructures in transverse plane of the as-quenched 7050 aluminum alloy is shown in Fig. 1. The micrograph of the as-quenched 7050 aluminum alloy consists of elongated grains. Fig. 2 is grain boundary map of the as-quenched 7050 aluminum alloy. In the grain boundary map, coarse black lines
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Fig. 4. Plastic strain ratio of the 7050 aluminum alloy deformed at different deformation conditions.
represent high angle grain boundaries (>15◦ ) and fine red lines represent low angle grain boundaries (2–15◦ ). It is shown that the microstructures are characterized by elongated grains and some low angle boundaries in the elongated grains. The ODF figure of the as-quenched 7050 aluminum alloy shows that a well-developed ˇ fiber consisting of primarily the Brass orientation {0 1 1}2 1 1 (35◦ , 45◦ , 90◦ ), the S orientation {1 2 3}6 3 4 (57◦ , 37◦ , 63◦ ), and the Copper orientation {1 1 2}1 1 1 (90◦ , 35◦ , 45◦ ) is evident (see Fig. 3). The Brass orientation {0 1 1}2 1 1 is found to be the strongest orientation along the ˇ fiber and the maximal intensity of the Brass orientation {0 1 1}2 1 1 reach 20.1.
Fig. 5. Optical micrographs showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
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Fig. 6. Grain boundary maps showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
3.2. Plastic strain ratio Fig. 4 shows plastic strain ratios of the as-rolled 7050 aluminum alloy plate deformed at 340 and 460 ◦ C with different strain rates (1.0 × 10−4 , 1.0 × 10−2 , 0.1 and 1 s−1 ). It is shown that deformation parameters have great effect on plastic strain ratio of the 7050 aluminum alloy. The plastic strain ratio increases with increasing strain rate when the 7050 aluminum alloy was deformed at 460 ◦ C, and even reach 1 with the strain rate of 1 s−1 . However, the plastic strain ratio of the 7050 aluminum alloy deformed at 340 ◦ C is not sensitive to strain rate. 3.3. Microstructure and texture of the 7050 aluminum alloy deformed Optical microstructures of the 7050 aluminum alloy deformed at different deformation conditions are shown in Fig. 5. The microstructures of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates (10−4 and 1 s−1 ) consist of big size elongated grains (the average intercept length measured from random lines drawn parallel to the transverse direction > 50 m), and many small size grains (the average intercept length measured from random lines drawn parallel to the transverse direction < 20 m) are observed in some big size elongated grains (see Fig. 5(a) and (b)). However, when deformation temperature is 460 ◦ C, the microstruc-
tures of the 7050 aluminum alloy deformed at different strain rates (10−4 and 1 s−1 ) are characterized by only big size elongated grains (see Fig. 5(c) and (d)). Fig. 6 shows that grain boundary maps of the 7050 aluminum alloy deformed at different deformation conditions consist of big size elongated grains and some low angle boundaries in the big size elongated grains. In Fig. 6, few small size grains with high angle boundaries in big size elongated grains show that the small size grains in Fig. 5(a) and (b) are not real grains with high angle boundaries, but subgrains with low angle boundaries. On the other side, Fig. 6(a) and (b) shows that grain microstructures of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates are almost the same. However, the size of subgrains of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 is bigger than that at 460 ◦ C and 1 s−1 (see Fig. 6(c) and (d)). TEM micrographs show that dynamic precipitation appears when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates of 10−4 and 1 s−1 (see Fig. 7(a) and (b)) [26]. Grain boundary map results of Fig. 6(a) and (b) show that the grains in Fig. 7(a) and (b) are essentially subgrains. During high temperature deformation at 340 ◦ C, dynamic precipitates appear, especially in grain boundaries and subgrain boundaries (see Fig. 7(a) and (b)). So the subgrain boundaries are easy to be eroded in Keller solution as the grain boundaries, as shown in Fig. 5(a) and (b). Additionally, Fig. 7(a) and (b) shows that the sizes of subgrain of the 7050 alu-
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Fig. 7. TEM micrographs showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
minum alloy deformed at 340 ◦ C and different strain rates of 10−4 and 1 s−1 are comparative. The subgrain size results of Fig. 7(a) and (b) are accordant with that by OM and EBSD technique (see Fig. 5(a) and (b), Fig. 6(a) and (b)). However, no precipitate is observed in the 7050 aluminum alloy deformed at 460 ◦ C and different strain rates, as shown in Fig. 7(c) and (d). The size of subgrains of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 is bigger than that at 460 ◦ C and 1 s−1 . The subgrain size results are accordant with that of Fig. 6(c) and (d). ODF figures of the 7050 aluminum alloy deformed at different deformation conditions are shown in Fig. 8. Fig. 8(a) and (b)
shows that the main texture components are the Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates (10−4 and 0.1 s−1 ). The maximal intensities of the strongest texture component (the Brass orientation {0 1 1}2 1 1) are 18.4 and 20.5 when the strain rates are 10−4 and 0.1 s−1 , respectively. The intensity values of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates are comparative to that of the as-quenched 7050 aluminum alloy (see Fig. 3, Fig. 8(a) and (b)). When the 7050 aluminum alloy was deformed at 460 ◦ C and different strain rates
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(10−4 and 0.1 s−1 ), the main texture components are the Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 too. However, the intensities of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 460 ◦ C are higher than that of the as-quenched 7050 aluminum alloy. The intensity of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 even reaches 36.9, as shown in Fig. 8(c) and (d).
4. Discussion 4.1. Effect of texture on plastic anisotropy Fig. 7(c) and (d) shows that there is no precipitate in the 7050 aluminum alloy deformed at 460 ◦ C and the strain rates of 10−4 and 1 s−1 . Alloy elements of the deformed alloy are in solution. During deformation at 460 ◦ C and different strain rates, the 7050 aluminum
Fig. 8. ODF figures showing texture components of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/0.1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/0.1 s−1 .
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Fig. 8. (Continued ).
alloy can be looked as single phase solid solute. Texture is the main cause of plastic anisotropy of the 7050 aluminum alloy deformed at 460 ◦ C [12,13]. The Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 are the primary texture components in the as-quenched 7050 aluminum alloy, as shown in Fig. 3, and the Brass orientation {0 1 1}2 1 1 has the most large intensity value. On the other hand, the Schmid ¯ factors of slip systems (1 1 1)[0 1¯ 1] and (1 1 1)[1 0 1] along rolling direction of as-rolled plate with the Brass orientation {0 1 1}2 1 1
are the biggest in all slip systems and reach 0.408. So the Brass orientation {0 1 1}2 1 1 is the main texture component affecting plastic anisotropy of the 7050 aluminum alloy deformed at 460 ◦ C. The single crystal analysis will be carried on in the asrolled 7050 aluminum plate based on Schmid factor method as follows. It is assumed that the as-rolled 7050 aluminum alloy plate comprises only the Brass orientation {0 1 1}2 1 1 and is considered as single crystal. The spatial relationship between {1 1 1} plane
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Fig. 9. Spatial relationship between {1 1 1} plane and the Brass orientation {0 1 1}2 1 1.
of the as-rolled 7050 aluminum alloy plate and the Brass orientation {0 1 1}2 1 1 is shown in Fig. 9. It is shown that (1 1¯ 1) plane and (1¯ 1 1) plane are normal to the rolling plane of the as-rolled ¯ 7050 aluminum alloy plate, the angles between (1 1 1) plane, (1 1 1) plane and the rolling plane are all 35.3◦ . The black arrows rep¯ resent the two slip systems ((1 1 1)[0 1¯ 1] and (1 1 1)[1 0 1]) with the biggest Schmid factor of 0.408 during tensile deformation. The angle between slip system (1 1 1)[0 1¯ 1] and the normal direction [1¯ 1¯ 0] is 60◦ , which with the rolling direction [1 1¯ 2] and the tran¯ is 30◦ and 90◦ , respectively. Otherwise, secting direction [1 1¯ 1] ¯ the angle between slip system (1 1 1)[1 0 1] and the normal direction [1¯ 1¯ 0] is 60◦ , which with the rolling direction [1 1¯ 2] and the ¯ is 120◦ and 90◦ , respectively. The two transecting direction [1 1¯ 1] slip systems are the easiest to be operated during tensile deformation along the rolling direction. So the deformation results of the as-rolled 7050 aluminum alloy plate with the Brass orientation {0 1 1}2 1 1 single crystal are the decrease in thickness and elongation along the rolling direction and the size in width is kept constant. So the 7050 aluminum alloy with the microstructure in the study possesses of low plastic strain ratio. However, the above analysis is only adapted to the deformation condition with low strain rate. With the increase of strain rate, the flow stress increases. More slip systems operated with high strain rate result in the intensity of the Brass orientation {0 1 1}2 1 1 is lower than that with low strain rate, as shown in Fig. 8(c) and (d). So the plastic strain ratio of the 7050 aluminum alloy increases with increasing strain rate and even reach 1 with the strain rate of 1 s−1 .
precipitates in subgrain boundaries and property of texture (slip systems are nearly parallel to subgrain boundaries). So there is more plastic strain in PFZs than that interior subgrain. The primary stress in the as-rolled 7050 aluminum alloy plate during rolling is compressive stress in the thickness and the dominant deformation is thickness reducing and the elongation along the length, while the size variation along the width direction is smaller than that of the other two directions. Besides, the AL3 Zr particles generated from the Zr and Al elements in the alloy may impede the migration and combination of the subgrains and then restrain the recrystallization in the 7050 aluminum alloy during rolling. So that, most deformed subgrains are preserved to be elongated, as shown in Figs. 1 and 2. Since most plastic strain occurs in the PFZs, the strain in the thickness direction is lager than that in the width direction when the 7050 aluminum alloy was deformed at 340 ◦ C, namely the plastic strain ratio along the width and thickness direction is smaller than 1, which is 0.7 in the current study. Since the relative content of the precipitates depends merely on the deformation temperature and coarsening of the precipitates only occurs interior the subgrains during over aged with little change of PFZs width, the plastic strain ratio along the width and thickness direction does not vary obviously when the alloy is deformed at 340 ◦ C and different strain rates. The former discussion shows that the main cause of plastic anisotropy of the 7050 aluminum alloy deformed at 340 ◦ C is not texture, but microstructure. 5. Conclusions (1) All the plastic strain ratios of the as-rolled 7050 aluminum alloy plate deformed at 340 ◦ C and different strain rates are about 0.7, which increases with increasing strain rate when the 7050 aluminum alloy was deformed at 460 ◦ C. (2) The 7050 aluminum alloy deformed at 340 ◦ C and different strain rates consists of elongated grains and some low angle boundaries in the elongated grain. When the 7050 aluminum alloy was deformed at 460 ◦ C and different strain rates, the microstructures are characterized by big size elongated grains and fine grains along high angle boundaries. (3) Alloy elements of the 7050 aluminum alloy deformed at 460 ◦ C and different strain rates are in solution. The plastic strain ratio of the 7050 aluminum alloy deformed at 460 ◦ C was affected by texture components, especially the Brass orientation {0 1 1}2 1 1. (4) Dynamic precipitation appears when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates. The 7050 aluminum alloy deformed is in over aged condition. Microstructure is the main influence of plastic anisotropy of the 7050 aluminum alloy deformed at 340 ◦ C.
4.2. Effect of microstructure on plastic anisotropy Acknowledgements Fig. 7(a) and (b) shows that precipitates are observed in the 7050 aluminum alloy deformed at 340 ◦ C and the strain rates of 10−4 and 1 s−1 and the precipitates are distributed uniformly interior subgrains and in subgrain boundaries, respectively. PFZs (precipitate free zones) in Fig. 7(a) and (b) indicate that the 7050 aluminum alloy deformed at 340 ◦ C and the strain rates of 10−4 and 1 s−1 are all in over aged condition. Dislocations bow around, but not cut through, the precipitates during deformation of the over aged alloy. On the other hand, the precipitates in subgrain boundaries pin and inhibit the migration of subgrain boundaries. The 7050 aluminum alloy cannot be looked as single phase solid solute during deformation at 340 ◦ C. The friction of dislocations movement in PFZs is lower than that interior subgrains because there is only few precipitates in PFZs when the 7050 aluminum alloy is in over aged condition. Meanwhile, alternate slipping is easy to occur in PFZs because of few
This work was supported by NSFC(50705034), and State Key Laboratory of Materials Processing and Die & Mould Technology. References [1] A. Deschamps, Y. Brechet, Mater. Sci. Eng. A 251 (1998) 200–207. [2] K. Stiller, P.J. Warren, V. Hansen, J. Angenete, J. Gjønnes, Mater. Sci. Eng. A 270 (1999) 55. [3] J.C. Williams, E.A. Starke, Acta Mater. 51 (2003) 5775–5799. [4] G. Sha, A. Cerezo, Surf. Interface Anal. 36 (2004) 564. [5] H. Utsunomiya, T. Ueno, T. Sakai, Scr. Mater. 57 (2007) 1109–1113. [6] J.T. Liu, R.E. Dick, J.M. Fridy, T.N. Rouns, Mater. Sci. Eng. A 458 (2007) 73–87. [7] O. Engler, L. Löchte, J. Hirsch, Acta Mater. 55 (2007) 5449–5463. [8] H. Jin, D.J. Lloyd, Mater. Sci. Eng. A 399 (2005) 358–367. [9] J. Savoie, Y. Zhou, J.J. Jonas, S.R. Macewen, Acta Mater. 44 (1996) 587–605. [10] P.W. Kao, Mater. Sci. Eng. 74 (1985) 147–157. [11] K.V. Jata, A.K. Hopkin, R.J. Riojia, Mater. Sci. Forum 217–222 (1996) 647. [12] A.J. Beaudon, P.R. Dawson, K.K. Mathur, Int. J. Plast. 11 (1995) 501–521.
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of deformation condition on plastic anisotropy of as-rolled 7050 aluminum alloy plate Wang Xin-yun a,∗ , H.E. Hu b , Xia Ju-chen a a b
State Key Lab of Material Processing and Die&Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, China Naval University of Engineering , Wuhan,430033,China
a r t i c l e
i n f o
Article history: Received 2 April 2008 Received in revised form 17 November 2008 Accepted 23 March 2009 Keywords: Aluminum alloy Plastic anisotropy Microstructure Texture
a b s t r a c t Plastic anisotropy of the as-rolled 7050 aluminum alloy plate was investigated by tensile tests conducted at different temperatures (340 and 460 ◦ C) and different strain rates of 1.0 × 10−4 , 1.0 × 10−2 , 0.1, and 1 s−1 . The results show that the plastic strain ratio of the as-rolled 7050 aluminum alloy plate deformed at 340 ◦ C is not sensitive to strain rate, which increases with increasing strain rate when the alloy was deformed at 460 ◦ C. Alloy elements of the alloy deformed at 460 ◦ C and different strain rates are in solution. The plastic strain ratio of the alloy deformed at 460 ◦ C was affected by texture components, especially the Brass orientation {0 1 1}2 1 1. Dynamic precipitation appears when the alloy was deformed at 340 ◦ C and different strain rates, and the alloy deformed is in over aged condition. Microstructure is the main influence of plastic anisotropy of the alloy deformed at 340 ◦ C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High-strength aluminum alloys with high strength-density ratio and excellent mechanical properties have been widely used for aeronautical applications [1–4]. Good formability during conventional forging and extrusion at elevated temperature is another important reason for extensive application of high-strength aluminum alloys. Thermomechanical processing is often used to acquire the final product form of high-strength aluminum alloys, which has been found to develop strong crystallographic textures and pronounced mechanical anisotropy [5–10]. The magnitude and cause of this anisotropy are legitimate concerns. Plastic anisotropy of aluminum alloys has been classified as inplane anisotropy in plate products, through-thickness anisotropy in thick plates or axisymmetric flow anisotropy in extrusions [11]. Causes of anisotropy have been studied in the past decades and significant arguments support texture rather than microstructure as the primary source of mechanical anisotropy [12]. Additional, modeling and simulating work by Carlos Tome et al. with their viscoplastic self-consistent code has shown that microstructure can have an effect but the effect is secondary compared to texture [13]. Generally, in-plane anisotropy in aluminum alloy sheets was explained on the basis of texture variations together with synergistic contributions from directional precipitates [14–17]. Modeling of in-plane anisotropy, with crystallographic texture, precipitate vol-
∗ Corresponding author. Tel.: +86 27 87543491; fax: +86 27 87543491. E-mail address: [email protected] (X.Y. Wang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.061
ume fractions, grain morphology and the associated grain aspect ratio as the major variables has been undertaken [18]. Crystallographic texture has both direct and indirect effects on the plastic anisotropy of aluminum alloys [19–21]. Direct effects are due to the orientation of the crystals and slip systems with respect to applied stresses and grain morphologies. Indirect effects, due to work hardening and precipitation arise during thermomechanical processing, and these include the orientation of precipitates with respect to slip systems, the distribution of dislocation densities among differently orientated slip systems and the corresponding distribution of precipitates. However, the anisotropy of the Zn–Al eutectic in deformation was attributed to slip in the Zn-rich phase, which caused the change in texture [22]. In the Sn–Pb eutectic, the anisotropy was mainly due to grain directionality, without a significant role for texture [23]. The former research results illustrate microstructure is an important effect on anisotropy of materials too. Usually, high-strength aluminum alloys provided are typically as-rolled plates or as-extruded rods with strong crystallographic textures. Meanwhile, microstructure changes during deformation of high-strength aluminum alloy at some special deformation conditions, such as dynamic precipitates appear. So it is necessary to research the effect of thermomechanical parameters on texture, microstructure and plastic anisotropy of high-strength aluminum alloys. Nevertheless, only a few reports [24,25] about relationship of thermomechanical parameter, texture, microstructure and plastic anisotropy of aluminum alloys have been published. Kashyap [24] reported that the plastic anisotropy ratio of Al–Li alloy increases
2
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Fig. 1. Optical micrograph showing microstructures of the as-quenched 7050 aluminum alloy. Fig. 2. Grain boundary map showing microstructures of the as-quenched 7050 aluminum alloy.
towards 1 with the increase in strain and strain rate, and there is only a marginal effect of test temperature on plastic anisotropy ratio. Crooks [25] concluded that the anisotropy of AA2195 aluminum alloy was a direct effect of texture, with no significant contribution from precipitate. To the best of our knowledge, there are only few researches reported on relationship of thermomechanical parameter, texture, microstructure and plastic anisotropy of high-strengthen aluminum alloys. In this paper, the effects of thermomechanical parameters on in-plane plastic anisotropy of the as-rolled 7050 aluminum alloy plate were studied in terms of the plastic anisotropy ratio (r = εw /εt , where εw is the width strain and εt is the thickness strain). The microstructures of the 7050 aluminum alloy deformed at different deformation conditions were analyzed by OM, EBSD technique and TEM to investigate the causes of plastic anisotropy of the as-rolled 7050 aluminum alloy plate.
2. Experimental Commercial as-rolled 7050 aluminum alloy plate with 80 mm in thickness and T7451 temper was used for the experiments. Tensile specimens with a 5 mm gauge width and thickness, respectively, and a 20 mm gauge length were made with the tensile axis parallel to the rolling direction and the thickness direction parallel to the normal direction of the as-rolled 7050 aluminum alloy plate. The tensile specimens were sectioned from the centerline layer of the as-rolled 7050 aluminum alloy plate to ensure the specimens with the same texture components before deformation. The specimens were solid solution treated at 477 ◦ C for 1 h and quenched into water. Then tensile tests were conducted to a predetermined elongation of 20% and carried out on an Instron-5500R universal testing machine at 340 and 460 ◦ C with the strain rates of 1.0 × 10−4 ,
Fig. 3. ODF figure showing texture components of the as-quenched 7050 aluminum alloy.
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1.0 × 10−2 , 0.1 and 1 s−1 , respectively. All the samples for analyses were sectioned from the gauge sections. The samples for OM and EBSD analysis were prepared in transverse plane and then mounted, polished and etched by Keller solution (1.5% HCl + 1% HF + 2.5% HNO3 + 95% distilled water, in vol.%) for optical microscope observation by a ZEISS HAL100 optical microscope. EBSD measurement samples were electro-polished using 10 vol.% HClO4 acids in alcohol followed by examined and analyzed using HKL Channel 5 software in a JEOL 733 electron probe at a sample tilt of 70◦ with an accelerating voltage of 20 kV. Step size of the maps was 1 m and the total number of data points of each scan was 50,000. In order to ensure the statistical reliability of the data, three map scans were performed over different areas of each sample. The demarcate ratios of EBSD in the present research are all higher than 94%. Samples for TEM were thinned to about 50 m followed by electro-polished in a double-jet polishing unit operated at 15 V and −20 ◦ C using a 30% nitric acid and 70% methanol solution, the disks were observed in a Philips Tecnai 20 microscope, operated at 200 kV. 3. Results 3.1. Microstructure and texture of as-quenched 7050 aluminum alloy The optical micrograph showing microstructures in transverse plane of the as-quenched 7050 aluminum alloy is shown in Fig. 1. The micrograph of the as-quenched 7050 aluminum alloy consists of elongated grains. Fig. 2 is grain boundary map of the as-quenched 7050 aluminum alloy. In the grain boundary map, coarse black lines
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Fig. 4. Plastic strain ratio of the 7050 aluminum alloy deformed at different deformation conditions.
represent high angle grain boundaries (>15◦ ) and fine red lines represent low angle grain boundaries (2–15◦ ). It is shown that the microstructures are characterized by elongated grains and some low angle boundaries in the elongated grains. The ODF figure of the as-quenched 7050 aluminum alloy shows that a well-developed ˇ fiber consisting of primarily the Brass orientation {0 1 1}2 1 1 (35◦ , 45◦ , 90◦ ), the S orientation {1 2 3}6 3 4 (57◦ , 37◦ , 63◦ ), and the Copper orientation {1 1 2}1 1 1 (90◦ , 35◦ , 45◦ ) is evident (see Fig. 3). The Brass orientation {0 1 1}2 1 1 is found to be the strongest orientation along the ˇ fiber and the maximal intensity of the Brass orientation {0 1 1}2 1 1 reach 20.1.
Fig. 5. Optical micrographs showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
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Fig. 6. Grain boundary maps showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
3.2. Plastic strain ratio Fig. 4 shows plastic strain ratios of the as-rolled 7050 aluminum alloy plate deformed at 340 and 460 ◦ C with different strain rates (1.0 × 10−4 , 1.0 × 10−2 , 0.1 and 1 s−1 ). It is shown that deformation parameters have great effect on plastic strain ratio of the 7050 aluminum alloy. The plastic strain ratio increases with increasing strain rate when the 7050 aluminum alloy was deformed at 460 ◦ C, and even reach 1 with the strain rate of 1 s−1 . However, the plastic strain ratio of the 7050 aluminum alloy deformed at 340 ◦ C is not sensitive to strain rate. 3.3. Microstructure and texture of the 7050 aluminum alloy deformed Optical microstructures of the 7050 aluminum alloy deformed at different deformation conditions are shown in Fig. 5. The microstructures of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates (10−4 and 1 s−1 ) consist of big size elongated grains (the average intercept length measured from random lines drawn parallel to the transverse direction > 50 m), and many small size grains (the average intercept length measured from random lines drawn parallel to the transverse direction < 20 m) are observed in some big size elongated grains (see Fig. 5(a) and (b)). However, when deformation temperature is 460 ◦ C, the microstruc-
tures of the 7050 aluminum alloy deformed at different strain rates (10−4 and 1 s−1 ) are characterized by only big size elongated grains (see Fig. 5(c) and (d)). Fig. 6 shows that grain boundary maps of the 7050 aluminum alloy deformed at different deformation conditions consist of big size elongated grains and some low angle boundaries in the big size elongated grains. In Fig. 6, few small size grains with high angle boundaries in big size elongated grains show that the small size grains in Fig. 5(a) and (b) are not real grains with high angle boundaries, but subgrains with low angle boundaries. On the other side, Fig. 6(a) and (b) shows that grain microstructures of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates are almost the same. However, the size of subgrains of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 is bigger than that at 460 ◦ C and 1 s−1 (see Fig. 6(c) and (d)). TEM micrographs show that dynamic precipitation appears when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates of 10−4 and 1 s−1 (see Fig. 7(a) and (b)) [26]. Grain boundary map results of Fig. 6(a) and (b) show that the grains in Fig. 7(a) and (b) are essentially subgrains. During high temperature deformation at 340 ◦ C, dynamic precipitates appear, especially in grain boundaries and subgrain boundaries (see Fig. 7(a) and (b)). So the subgrain boundaries are easy to be eroded in Keller solution as the grain boundaries, as shown in Fig. 5(a) and (b). Additionally, Fig. 7(a) and (b) shows that the sizes of subgrain of the 7050 alu-
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Fig. 7. TEM micrographs showing microstructures of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/1 s−1 .
minum alloy deformed at 340 ◦ C and different strain rates of 10−4 and 1 s−1 are comparative. The subgrain size results of Fig. 7(a) and (b) are accordant with that by OM and EBSD technique (see Fig. 5(a) and (b), Fig. 6(a) and (b)). However, no precipitate is observed in the 7050 aluminum alloy deformed at 460 ◦ C and different strain rates, as shown in Fig. 7(c) and (d). The size of subgrains of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 is bigger than that at 460 ◦ C and 1 s−1 . The subgrain size results are accordant with that of Fig. 6(c) and (d). ODF figures of the 7050 aluminum alloy deformed at different deformation conditions are shown in Fig. 8. Fig. 8(a) and (b)
shows that the main texture components are the Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates (10−4 and 0.1 s−1 ). The maximal intensities of the strongest texture component (the Brass orientation {0 1 1}2 1 1) are 18.4 and 20.5 when the strain rates are 10−4 and 0.1 s−1 , respectively. The intensity values of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 340 ◦ C and different strain rates are comparative to that of the as-quenched 7050 aluminum alloy (see Fig. 3, Fig. 8(a) and (b)). When the 7050 aluminum alloy was deformed at 460 ◦ C and different strain rates
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(10−4 and 0.1 s−1 ), the main texture components are the Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 too. However, the intensities of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 460 ◦ C are higher than that of the as-quenched 7050 aluminum alloy. The intensity of the Brass orientation {0 1 1}2 1 1 of the 7050 aluminum alloy deformed at 460 ◦ C and 10−4 s−1 even reaches 36.9, as shown in Fig. 8(c) and (d).
4. Discussion 4.1. Effect of texture on plastic anisotropy Fig. 7(c) and (d) shows that there is no precipitate in the 7050 aluminum alloy deformed at 460 ◦ C and the strain rates of 10−4 and 1 s−1 . Alloy elements of the deformed alloy are in solution. During deformation at 460 ◦ C and different strain rates, the 7050 aluminum
Fig. 8. ODF figures showing texture components of the 7050 aluminum alloy deformed at different deformation conditions (a) 340 ◦ C/10−4 s−1 ; (b) 340 ◦ C/0.1 s−1 ; (c) 460 ◦ C/10−4 s−1 ; (d) 460 ◦ C/0.1 s−1 .
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Fig. 8. (Continued ).
alloy can be looked as single phase solid solute. Texture is the main cause of plastic anisotropy of the 7050 aluminum alloy deformed at 460 ◦ C [12,13]. The Brass orientation {0 1 1}2 1 1, the S orientation {1 2 3}6 3 4, and the Copper orientation {1 1 2}1 1 1 are the primary texture components in the as-quenched 7050 aluminum alloy, as shown in Fig. 3, and the Brass orientation {0 1 1}2 1 1 has the most large intensity value. On the other hand, the Schmid ¯ factors of slip systems (1 1 1)[0 1¯ 1] and (1 1 1)[1 0 1] along rolling direction of as-rolled plate with the Brass orientation {0 1 1}2 1 1
are the biggest in all slip systems and reach 0.408. So the Brass orientation {0 1 1}2 1 1 is the main texture component affecting plastic anisotropy of the 7050 aluminum alloy deformed at 460 ◦ C. The single crystal analysis will be carried on in the asrolled 7050 aluminum plate based on Schmid factor method as follows. It is assumed that the as-rolled 7050 aluminum alloy plate comprises only the Brass orientation {0 1 1}2 1 1 and is considered as single crystal. The spatial relationship between {1 1 1} plane
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Fig. 9. Spatial relationship between {1 1 1} plane and the Brass orientation {0 1 1}2 1 1.
of the as-rolled 7050 aluminum alloy plate and the Brass orientation {0 1 1}2 1 1 is shown in Fig. 9. It is shown that (1 1¯ 1) plane and (1¯ 1 1) plane are normal to the rolling plane of the as-rolled ¯ 7050 aluminum alloy plate, the angles between (1 1 1) plane, (1 1 1) plane and the rolling plane are all 35.3◦ . The black arrows rep¯ resent the two slip systems ((1 1 1)[0 1¯ 1] and (1 1 1)[1 0 1]) with the biggest Schmid factor of 0.408 during tensile deformation. The angle between slip system (1 1 1)[0 1¯ 1] and the normal direction [1¯ 1¯ 0] is 60◦ , which with the rolling direction [1 1¯ 2] and the tran¯ is 30◦ and 90◦ , respectively. Otherwise, secting direction [1 1¯ 1] ¯ the angle between slip system (1 1 1)[1 0 1] and the normal direction [1¯ 1¯ 0] is 60◦ , which with the rolling direction [1 1¯ 2] and the ¯ is 120◦ and 90◦ , respectively. The two transecting direction [1 1¯ 1] slip systems are the easiest to be operated during tensile deformation along the rolling direction. So the deformation results of the as-rolled 7050 aluminum alloy plate with the Brass orientation {0 1 1}2 1 1 single crystal are the decrease in thickness and elongation along the rolling direction and the size in width is kept constant. So the 7050 aluminum alloy with the microstructure in the study possesses of low plastic strain ratio. However, the above analysis is only adapted to the deformation condition with low strain rate. With the increase of strain rate, the flow stress increases. More slip systems operated with high strain rate result in the intensity of the Brass orientation {0 1 1}2 1 1 is lower than that with low strain rate, as shown in Fig. 8(c) and (d). So the plastic strain ratio of the 7050 aluminum alloy increases with increasing strain rate and even reach 1 with the strain rate of 1 s−1 .
precipitates in subgrain boundaries and property of texture (slip systems are nearly parallel to subgrain boundaries). So there is more plastic strain in PFZs than that interior subgrain. The primary stress in the as-rolled 7050 aluminum alloy plate during rolling is compressive stress in the thickness and the dominant deformation is thickness reducing and the elongation along the length, while the size variation along the width direction is smaller than that of the other two directions. Besides, the AL3 Zr particles generated from the Zr and Al elements in the alloy may impede the migration and combination of the subgrains and then restrain the recrystallization in the 7050 aluminum alloy during rolling. So that, most deformed subgrains are preserved to be elongated, as shown in Figs. 1 and 2. Since most plastic strain occurs in the PFZs, the strain in the thickness direction is lager than that in the width direction when the 7050 aluminum alloy was deformed at 340 ◦ C, namely the plastic strain ratio along the width and thickness direction is smaller than 1, which is 0.7 in the current study. Since the relative content of the precipitates depends merely on the deformation temperature and coarsening of the precipitates only occurs interior the subgrains during over aged with little change of PFZs width, the plastic strain ratio along the width and thickness direction does not vary obviously when the alloy is deformed at 340 ◦ C and different strain rates. The former discussion shows that the main cause of plastic anisotropy of the 7050 aluminum alloy deformed at 340 ◦ C is not texture, but microstructure. 5. Conclusions (1) All the plastic strain ratios of the as-rolled 7050 aluminum alloy plate deformed at 340 ◦ C and different strain rates are about 0.7, which increases with increasing strain rate when the 7050 aluminum alloy was deformed at 460 ◦ C. (2) The 7050 aluminum alloy deformed at 340 ◦ C and different strain rates consists of elongated grains and some low angle boundaries in the elongated grain. When the 7050 aluminum alloy was deformed at 460 ◦ C and different strain rates, the microstructures are characterized by big size elongated grains and fine grains along high angle boundaries. (3) Alloy elements of the 7050 aluminum alloy deformed at 460 ◦ C and different strain rates are in solution. The plastic strain ratio of the 7050 aluminum alloy deformed at 460 ◦ C was affected by texture components, especially the Brass orientation {0 1 1}2 1 1. (4) Dynamic precipitation appears when the 7050 aluminum alloy was deformed at 340 ◦ C and different strain rates. The 7050 aluminum alloy deformed is in over aged condition. Microstructure is the main influence of plastic anisotropy of the 7050 aluminum alloy deformed at 340 ◦ C.
4.2. Effect of microstructure on plastic anisotropy Acknowledgements Fig. 7(a) and (b) shows that precipitates are observed in the 7050 aluminum alloy deformed at 340 ◦ C and the strain rates of 10−4 and 1 s−1 and the precipitates are distributed uniformly interior subgrains and in subgrain boundaries, respectively. PFZs (precipitate free zones) in Fig. 7(a) and (b) indicate that the 7050 aluminum alloy deformed at 340 ◦ C and the strain rates of 10−4 and 1 s−1 are all in over aged condition. Dislocations bow around, but not cut through, the precipitates during deformation of the over aged alloy. On the other hand, the precipitates in subgrain boundaries pin and inhibit the migration of subgrain boundaries. The 7050 aluminum alloy cannot be looked as single phase solid solute during deformation at 340 ◦ C. The friction of dislocations movement in PFZs is lower than that interior subgrains because there is only few precipitates in PFZs when the 7050 aluminum alloy is in over aged condition. Meanwhile, alternate slipping is easy to occur in PFZs because of few
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