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Effect of backpressure on structure and properties of AA5083 alloy processed by ECAP
Journal of Alloys and Compounds 378 (2004) 233–236
Effect of backpressure on structure and properties of AA5083 alloy processed by ECAP V.V. Stolyarov a,∗ , R. Lapovok b a
Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450025, Russia b School of Physics and Materials Engineering, Monash University, Clayton, Vic. 3168, Australia Received 1 September 2003; accepted 14 October 2003
Abstract An ultrafine-grained microstructure was obtained in bulk billets of 20 mm × 20 mm × 80 mm from commercial AA5083 alloy by equal channel angular pressing (ECAP) at the ambient temperature with a different number of passes and the backpressure level. Backpressure enhances workability of alloys processed by ECAP. Strength and microhardness of AA5083 alloy were significantly enhanced. The room strength and microhardness (UTS = 427 MPa, 165 HV) of initially course-grained alloy, subjected to three passes of ECAP with backpressure 200 MPa, are much higher than its standard values after convention processing by hot pressing or cold rolling. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Nanofabrications; Transmission electron microscopy; Strain; High pressure
1. Introduction Al-alloys are very attractive as a base for development of cheap and high-strength constructional materials for application in the automotive and aircraft industries. However, enhancement of strength by conventional thermal treatment in some non-heat treatable alloys, such as AA5083, is impossible. To overcome this problem, the use of severe plastic deformation (SPD) as a processing method for nano- or ultrafine-grained metals is suggested. This method is now well known among researchers in materials science [1,2]. It allows to refine microstructure and improve significantly various solid state properties, especially strength and ductility. Equal channel angular pressing (ECAP) is the most attractive method among the SPD techniques because it can be used to produce not only laboratory samples but bulk nanostructured billets for subsequent metallurgical operations [3,4]. Recently, many original and review papers have been published on this technique, in connection with ultrafine-grained structure (UFG) and the improvement of mechanical properties during ECAP. The influence of the number of pressing passes, processing routes, strain rate, ∗
Corresponding author. Tel.: +7-3472-23-44-49; fax: +7-3472-23-34-22. E-mail address: [email protected] (V.V. Stolyarov). 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.10.084
temperature, and the design of ECAP tooling has been considered in detail [1]. However, the role of the backpressure which is an important processing parameter, has been reported only in a few papers [5–7]. Backpressure is especially important for ECAP alloys with low ductility, which otherwise fail after even one single pass. When backpressure is applied, the accumulation of damage in deformed samples decreases due to the fact that the shear strain takes place under compressive hydrostatic pressure [5,6]. Recently, the influence of the level of backpressure on the structure and mechanical properties of Al–5% Fe alloy processed by ECAP has been investigated [8]. The positive role of backpressure for structure refinement and mechanical properties has been shown. In the present paper, the use of this approach for production of a high strength nanostructured state in another nonheat treatable AA5083 alloy and a study of microstructure and mechanical properties is performed.
2. Experimental material and procedures The hot rolled AA5083 alloy subjected to solid solution treatment (SST) at 450 ◦ C for 5 h had a recrystallized structure with a mean grain size of 150 m. Before ECAP the billet was machined into the samples with dimensions of
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tudinal and transverse directions with gauge dimensions of 5 mm×3 mm×1 mm and of 20 mm total length. The elongation to failure was obtained by measurement of total length before and after testing using an optical microscope. 3. Experimental results Fig. 1. Equipment for ECAP with controlled back-pressure (die with the angle of 120◦ ).
20 mm×20 mm×85 mm. Detailed information on the ECAP apparatus used in this study can be found in [7,8]. ECAP was performed in the 90 and 120◦ dies (Fig. 1) at room temperature and a constant pressing speed of 2 mm s−1 , using route Bc. The equivalent strain in each pass was equal to 1.15 or 0.66 for 90 and 120◦ die, respectively. The values of backpressure were 2.5 (62.5), 5 (125), and 8 (200) tones (MPa). The microstructure of processed alloy was investigated by optical microscopy (OM) and transmission electron microscopy (TEM). The mechanical behavior was studied by measuring Vickers microhardness under a 1 kg load applied for 15 s and tensile tests performed at room temperature under a strain rate of 1.6 × 10−3 s−1 , using a hydraulic Instron testing machine. The tensile specimens were spark machined from samples processed by ECAP in the longi-
3.1. Microstructure The structure of as-received alloy in SST state was recrystallized and consisted of a course-grained Al matrix and relatively fine (about 10 m) Al6 (Mg, Fe)-type particles with a volume fraction less than 10% (Fig. 2a). ECAP (three passes, 200 MPa) in 90 and 120◦ dies leads to structure refinement of both matrix phase and aluminides in comparison with undeformed state (Fig. 2b). Typical bright field TEM images of the matrix microstructure and a selected area electron diffraction patterns (SAEDP) of the alloy after three passes of ECAP alloy with backpressure of 200 MPa are shown in Fig. 3a. There are some shear bands with elongated grains with a width 0.1 m and a length 1 m in the crosssection (Fig. 3b). After processing by ECAP, the majority of grain boundaries are not clearly visible and are spread. The complex diffraction contrast near the boundaries and within grains, as well as the observed blurring of reflections testify to a high level of internal stress in the crystal lattice. However, after an additional annealing at 200 ◦ C for 0.5 h
Fig. 2. Optical microstructure of as-received AA5083 alloy before (a) and after (b) ECAP. Table 1 Grain size, microhardness, and tensile properties of the AA5083 alloy in different states State
Test direction
ECAP, three passes
Transverse Longitudinal
H121
Longitudinal
SST
Transverse Longitudinal
a
Grain size (m) 0.25
150
HV
UTS (MPa)
YS (MPa)
165
427 418
375 353
ELa (%) 8.0 7.5
80
352
234
16
78
320 295
163 144
36.5 35.5
Elongation for ECAP and H121, SST states cannot be compared because of different gage length.
V.V. Stolyarov, R. Lapovok / Journal of Alloys and Compounds 378 (2004) 233–236
235
Fig. 3. TEM images with SAEDP of alloy processed by three passes of ECAP with the backpressure of 200 MPa. (a, c, d) Longitudinal-section; (b) cross-section; (c, d) after post deformation annealing at 200 ◦ C for 0.5 h (c) and 6 h (d).
more distinct contrast from high angle grain boundaries can be seen due to a stress relief during annealing (Fig. 3c). It allows to measure the mean grain size that is about 250 nm. This indicates that there is almost no grain growth. The density of lattice dislocations is not high. The view of SAEDP, taken from an area 2 m2 , with a large number of reflections arranged in a circle testifies that the structure formed is of a granular type with mainly high-angle grain boundaries. In contrast some recrystallized grains of 1 m in size are observed in the case of increasing annealing time to 6 h (Fig. 3d).
tion, 8%, was smaller compared to those after SST and H121 (ASTM B221M, cold rolling with strain degree about 75%), but the gain in strength is superior. Anisotropy of strength and ductility in ECAP processed billets is not significant.
3.2. Microhardness, workability, and mechanical properties Microhardness (HV) measurements and tensile tests have been performed on alloy after SST and ECAP, Table 1. In the absence of backpressure or at its low value of 62.5 MPa, the sample has cracked after first pass ECAP and has failed during the second pass. At backpressure of 200 MPa cracks in a sample were not visible after three passes of ECAP. The microhardness and tensile characteristics changed significantly after three passes of ECAP. Microhardness, US, and YS increased from 78 HV, 320 MPa, and 163 MPa to 165◦ , 427 MPa, and 375 MPa, respectively. The value of elonga-
Fig. 4. Stress–displacement curves in longitudinal direction for AA5083 alloy before (a) and after (b) ECAP processing.
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V.V. Stolyarov, R. Lapovok / Journal of Alloys and Compounds 378 (2004) 233–236
The strain rate hardening for the strain–stress curve is strongly different for non-deformed and ECAP states, Fig. 4.
# LX0211114). This research has also been, partially supported by the Russian Foundation for Basic Research (Grant # 01-03-32125).
4. Conclusions
References
1. For the first time, the hot-rolled AA5083 alloy withstand without failure a severe plastic deformation by ECAP up to three passes at room temperature due to application of a backpressure. 2. ECAP of the alloy led to formation of ultrafine-grained structure with a mean grain size of 250 nm. 3. Both strength and microhardness of the alloy processed by the ECAP technique were significantly enhanced with tolerable loss in ductility.
[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mater. Sci. 45 (2000) 103. [2] Y. Zhu, T. Langdon, R. Mishra, S. Semiatin, M. Saran, T. Lowe (Eds.), Ultrafine Grained Materials II, TMS, 2002. [3] V.V. Stolyarov, E.P. Soshnikova, I.G. Brodova, D.V. Bashlykov, A.R. Kilmametov, Phys. Met. Metall. 93 (6) (2002) 74–81. [4] V.V. Stolyarov, Y.T. Zhu, I.V. Alexandrov, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. 299 (1–2) (2001) 59–67. [5] R. Lapovok, R. Cottam, G. Stecher, R. Deam, E. Summerville, T.C. Lowe, R.Z. Valiev (Eds.), NATO Science Series, vol. 80, Kluwer Academic Publisher, 1999, pp. 303–312. [6] R. Lapovok, R. Cottam, Ultrafine Grained Materials II, in: Y. Zhu, T. Langdon, R. Mishra, S. Semiatin, M. Saran, T. Lowe (Eds.), TMS, 2002, pp. 547–556. [7] G. Raab, N. Krasilnikov, I. Alexandrov, R. Valiev, Phys. Tech. High Pressures 10 (4) (2000) 3–7. [8] V. Stolyarov, R. Lapovok, I. Brodova, P. Thomson, Mater. Sci. Eng. A 357 (2003) 159–167.
Acknowledgements We acknowledge the principal financial support from the Australian Research Council (Linkage International Grant
Effect of backpressure on structure and properties of AA5083 alloy processed by ECAP V.V. Stolyarov a,∗ , R. Lapovok b a
Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450025, Russia b School of Physics and Materials Engineering, Monash University, Clayton, Vic. 3168, Australia Received 1 September 2003; accepted 14 October 2003
Abstract An ultrafine-grained microstructure was obtained in bulk billets of 20 mm × 20 mm × 80 mm from commercial AA5083 alloy by equal channel angular pressing (ECAP) at the ambient temperature with a different number of passes and the backpressure level. Backpressure enhances workability of alloys processed by ECAP. Strength and microhardness of AA5083 alloy were significantly enhanced. The room strength and microhardness (UTS = 427 MPa, 165 HV) of initially course-grained alloy, subjected to three passes of ECAP with backpressure 200 MPa, are much higher than its standard values after convention processing by hot pressing or cold rolling. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Nanofabrications; Transmission electron microscopy; Strain; High pressure
1. Introduction Al-alloys are very attractive as a base for development of cheap and high-strength constructional materials for application in the automotive and aircraft industries. However, enhancement of strength by conventional thermal treatment in some non-heat treatable alloys, such as AA5083, is impossible. To overcome this problem, the use of severe plastic deformation (SPD) as a processing method for nano- or ultrafine-grained metals is suggested. This method is now well known among researchers in materials science [1,2]. It allows to refine microstructure and improve significantly various solid state properties, especially strength and ductility. Equal channel angular pressing (ECAP) is the most attractive method among the SPD techniques because it can be used to produce not only laboratory samples but bulk nanostructured billets for subsequent metallurgical operations [3,4]. Recently, many original and review papers have been published on this technique, in connection with ultrafine-grained structure (UFG) and the improvement of mechanical properties during ECAP. The influence of the number of pressing passes, processing routes, strain rate, ∗
Corresponding author. Tel.: +7-3472-23-44-49; fax: +7-3472-23-34-22. E-mail address: [email protected] (V.V. Stolyarov). 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.10.084
temperature, and the design of ECAP tooling has been considered in detail [1]. However, the role of the backpressure which is an important processing parameter, has been reported only in a few papers [5–7]. Backpressure is especially important for ECAP alloys with low ductility, which otherwise fail after even one single pass. When backpressure is applied, the accumulation of damage in deformed samples decreases due to the fact that the shear strain takes place under compressive hydrostatic pressure [5,6]. Recently, the influence of the level of backpressure on the structure and mechanical properties of Al–5% Fe alloy processed by ECAP has been investigated [8]. The positive role of backpressure for structure refinement and mechanical properties has been shown. In the present paper, the use of this approach for production of a high strength nanostructured state in another nonheat treatable AA5083 alloy and a study of microstructure and mechanical properties is performed.
2. Experimental material and procedures The hot rolled AA5083 alloy subjected to solid solution treatment (SST) at 450 ◦ C for 5 h had a recrystallized structure with a mean grain size of 150 m. Before ECAP the billet was machined into the samples with dimensions of
234
V.V. Stolyarov, R. Lapovok / Journal of Alloys and Compounds 378 (2004) 233–236
tudinal and transverse directions with gauge dimensions of 5 mm×3 mm×1 mm and of 20 mm total length. The elongation to failure was obtained by measurement of total length before and after testing using an optical microscope. 3. Experimental results Fig. 1. Equipment for ECAP with controlled back-pressure (die with the angle of 120◦ ).
20 mm×20 mm×85 mm. Detailed information on the ECAP apparatus used in this study can be found in [7,8]. ECAP was performed in the 90 and 120◦ dies (Fig. 1) at room temperature and a constant pressing speed of 2 mm s−1 , using route Bc. The equivalent strain in each pass was equal to 1.15 or 0.66 for 90 and 120◦ die, respectively. The values of backpressure were 2.5 (62.5), 5 (125), and 8 (200) tones (MPa). The microstructure of processed alloy was investigated by optical microscopy (OM) and transmission electron microscopy (TEM). The mechanical behavior was studied by measuring Vickers microhardness under a 1 kg load applied for 15 s and tensile tests performed at room temperature under a strain rate of 1.6 × 10−3 s−1 , using a hydraulic Instron testing machine. The tensile specimens were spark machined from samples processed by ECAP in the longi-
3.1. Microstructure The structure of as-received alloy in SST state was recrystallized and consisted of a course-grained Al matrix and relatively fine (about 10 m) Al6 (Mg, Fe)-type particles with a volume fraction less than 10% (Fig. 2a). ECAP (three passes, 200 MPa) in 90 and 120◦ dies leads to structure refinement of both matrix phase and aluminides in comparison with undeformed state (Fig. 2b). Typical bright field TEM images of the matrix microstructure and a selected area electron diffraction patterns (SAEDP) of the alloy after three passes of ECAP alloy with backpressure of 200 MPa are shown in Fig. 3a. There are some shear bands with elongated grains with a width 0.1 m and a length 1 m in the crosssection (Fig. 3b). After processing by ECAP, the majority of grain boundaries are not clearly visible and are spread. The complex diffraction contrast near the boundaries and within grains, as well as the observed blurring of reflections testify to a high level of internal stress in the crystal lattice. However, after an additional annealing at 200 ◦ C for 0.5 h
Fig. 2. Optical microstructure of as-received AA5083 alloy before (a) and after (b) ECAP. Table 1 Grain size, microhardness, and tensile properties of the AA5083 alloy in different states State
Test direction
ECAP, three passes
Transverse Longitudinal
H121
Longitudinal
SST
Transverse Longitudinal
a
Grain size (m) 0.25
150
HV
UTS (MPa)
YS (MPa)
165
427 418
375 353
ELa (%) 8.0 7.5
80
352
234
16
78
320 295
163 144
36.5 35.5
Elongation for ECAP and H121, SST states cannot be compared because of different gage length.
V.V. Stolyarov, R. Lapovok / Journal of Alloys and Compounds 378 (2004) 233–236
235
Fig. 3. TEM images with SAEDP of alloy processed by three passes of ECAP with the backpressure of 200 MPa. (a, c, d) Longitudinal-section; (b) cross-section; (c, d) after post deformation annealing at 200 ◦ C for 0.5 h (c) and 6 h (d).
more distinct contrast from high angle grain boundaries can be seen due to a stress relief during annealing (Fig. 3c). It allows to measure the mean grain size that is about 250 nm. This indicates that there is almost no grain growth. The density of lattice dislocations is not high. The view of SAEDP, taken from an area 2 m2 , with a large number of reflections arranged in a circle testifies that the structure formed is of a granular type with mainly high-angle grain boundaries. In contrast some recrystallized grains of 1 m in size are observed in the case of increasing annealing time to 6 h (Fig. 3d).
tion, 8%, was smaller compared to those after SST and H121 (ASTM B221M, cold rolling with strain degree about 75%), but the gain in strength is superior. Anisotropy of strength and ductility in ECAP processed billets is not significant.
3.2. Microhardness, workability, and mechanical properties Microhardness (HV) measurements and tensile tests have been performed on alloy after SST and ECAP, Table 1. In the absence of backpressure or at its low value of 62.5 MPa, the sample has cracked after first pass ECAP and has failed during the second pass. At backpressure of 200 MPa cracks in a sample were not visible after three passes of ECAP. The microhardness and tensile characteristics changed significantly after three passes of ECAP. Microhardness, US, and YS increased from 78 HV, 320 MPa, and 163 MPa to 165◦ , 427 MPa, and 375 MPa, respectively. The value of elonga-
Fig. 4. Stress–displacement curves in longitudinal direction for AA5083 alloy before (a) and after (b) ECAP processing.
236
V.V. Stolyarov, R. Lapovok / Journal of Alloys and Compounds 378 (2004) 233–236
The strain rate hardening for the strain–stress curve is strongly different for non-deformed and ECAP states, Fig. 4.
# LX0211114). This research has also been, partially supported by the Russian Foundation for Basic Research (Grant # 01-03-32125).
4. Conclusions
References
1. For the first time, the hot-rolled AA5083 alloy withstand without failure a severe plastic deformation by ECAP up to three passes at room temperature due to application of a backpressure. 2. ECAP of the alloy led to formation of ultrafine-grained structure with a mean grain size of 250 nm. 3. Both strength and microhardness of the alloy processed by the ECAP technique were significantly enhanced with tolerable loss in ductility.
[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mater. Sci. 45 (2000) 103. [2] Y. Zhu, T. Langdon, R. Mishra, S. Semiatin, M. Saran, T. Lowe (Eds.), Ultrafine Grained Materials II, TMS, 2002. [3] V.V. Stolyarov, E.P. Soshnikova, I.G. Brodova, D.V. Bashlykov, A.R. Kilmametov, Phys. Met. Metall. 93 (6) (2002) 74–81. [4] V.V. Stolyarov, Y.T. Zhu, I.V. Alexandrov, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. 299 (1–2) (2001) 59–67. [5] R. Lapovok, R. Cottam, G. Stecher, R. Deam, E. Summerville, T.C. Lowe, R.Z. Valiev (Eds.), NATO Science Series, vol. 80, Kluwer Academic Publisher, 1999, pp. 303–312. [6] R. Lapovok, R. Cottam, Ultrafine Grained Materials II, in: Y. Zhu, T. Langdon, R. Mishra, S. Semiatin, M. Saran, T. Lowe (Eds.), TMS, 2002, pp. 547–556. [7] G. Raab, N. Krasilnikov, I. Alexandrov, R. Valiev, Phys. Tech. High Pressures 10 (4) (2000) 3–7. [8] V. Stolyarov, R. Lapovok, I. Brodova, P. Thomson, Mater. Sci. Eng. A 357 (2003) 159–167.
Acknowledgements We acknowledge the principal financial support from the Australian Research Council (Linkage International Grant