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Influence of strain rate on ultimate tensile stress of coarse-grained and ultrafine-grained copper
Materials Letters 64 (2010) 2344–2346
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of strain rate on ultimate tensile stress of coarse-grained and ultrafine-grained copper Tibor Kvačkaj a, Andrea Kováčová a, Michal Kvačkaj a, Imrich Pokorný a, Róbert Kočiško a, Tibor Donič b,⁎ a b
Department of Metal Forming, Faculty of Metallurgy, Technical University of Košice, Vysokoškolská 4, 042 00 Košice, Slovakia Department of Applied Mechanics, Faculty of Mechanical Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia
a r t i c l e
i n f o
Article history: Received 14 May 2010 Accepted 15 July 2010 Available online 22 July 2010 Keywords: Nanomaterials Deformation and fracture Strain rate Mechanical properties ECAP
a b s t r a c t In the present paper OFHC (oxygen free high conductivity) copper was tested by static and dynamic tensile tests at room temperature owing to strain rate investigation. Because of coarse-grained (CG) and ultrafinegrained (UFG) microstructure observation the copper was subjected to drawing and ECAP processes. The investigation of strain rate and microstructure was focused on the ultimate tensile stress (UTS) after the tensile tests. Following this study, it was found that strain rate is an important characteristic influencing the mechanical properties of copper. The ultimate tensile stress grew with strain rate increasing and this effect is more visible at high strain rates (ε˙ ~ 102 s−1). Moreover, it was revealed that strain rate hasn't got any influence on the failure mechanism of the copper on the other hand it has an influence on the values of dimple size. While strain rate increases the dimple size decreases. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The UFG materials (db 1 μm) have been studied over the last years due to their unique properties. They have high strength and good ductility at the same time in comparison to CG materials which have been produced by classical deformation technologies as rolling, drawing and pressing [1]. The equal-channel angular pressing (ECAP) is one of the methods which have been used on UFG metal formation [2–8]. It is well known that strain rate has a great influence on the mechanical properties of materials. Static tensile properties have been intensively studied in recent years [1]. Nowadays, Kolsky bar equipment has been used to study material dynamic properties in the strain rate range ε˙ N 1000 s−1 [9]. For the purpose of strain rate investigation in the range ε˙ ~ 100–1000 s−1 very limited studies exist in literature. The influence of these strain rates can be studied by a rotating flywheel machine [10,11]. The main advantage of a rotating flywheel is the kinetic energy which is created during the process. The kinetic energy is greater than the work done on the sample deformation this means that the constant velocity of deformation can be assumed [10]. 2. Material and methods The starting material for this study was OFHC copper in rod shape after drawing and ECAP processes at room temperature. The first series of samples was drawn from diameter 15 mm to 10 mm. The
⁎ Corresponding author. Tel./fax: +421 55 6024198. E-mail address: [email protected] (T. Donič).
second one (diameter 10 mm, length 100 mm) was processed by ECAP die (φ = 90°) with 5 passes and rotated by route C. The samples with CG and UFG microstructure were investigated at room temperature in consideration of their mechanical tensile properties and fracture morphology. The classical tensile testing machine LabTest 5.20 ST and rotating flywheel machine offer the possibility of obtaining a wide range of strain rates (from 10−3 to 10−1 s−1 for static conditions and~102 s−1 for dynamic ones). The geometrical parameters of the samples for static and dynamic tensile tests were L0 = 28 mm, d0 = 4 mm and L0 = 8, d0 = 4 mm, respectively. The achieved values from measurements were applied for mathematical analysis by the software product “R”. The fractured surface morphology of samples after tensile tests was investigated using scanning electron microscopy JEOL 7000F. The dimple sizes of ECAP samples after dynamic tensile tests were measured from a statistical file of 400 features. 3. Results and discussion 3.1. Strength properties The ultimate tensile stress (UTS) dependence on strain rate is illustrated in Fig. 1. Because of strain hardening the increase of UTS on strain rate is visible in both cases (drawing, ECAP). In the dynamic regime, the increase of UTS is steep what implies higher strain rate sensitivity in comparison with the static regime, what was confirmed by authors [9], too. These results can be explained by faster strain rate hardening elevation in the dynamic regime than in the static regime. Based on the literary source [12] as well as our experiment, the reached results could be explained by different deformation
0167-577X/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.047
T. Kvačkaj et al. / Materials Letters 64 (2010) 2344–2346
Fig. 1. The ultimate tensile stress progress in dependence on strain rate.
mechanism performance during plastic deformation. The higher strain rates caused a great amount of dislocation pile-up generation and their higher speed of movement resulting in greater strain hardening. Fig. 1 also shows little influence of UFG and CG microstructure on UTS. As noted previously, strain rate research on the tensile mechanical properties of UFG copper has not been described in the literature enough. The authors [9] investigated strain rate influence on flow stress progress however in experimental conditions based on dynamic compression tests. Comparing results from our experiment and results presented by authors [9] different data were achieved as is illustrated on Fig. 1. Using compression tests, lower dynamic strength properties in compare with tensile tests were obtained owing to different stress states. The results shown by authors [9] have only informative status due to only three point existence in every regime (without ECAP, 2 and 8 ECAP passes). Our results (5 ECAP passes) show a slow increase of strength properties until ε˙ ~ 0, 4 s−1 was reached. On the other hand, a steep increase of strength properties was shown in the strain rate range ε˙ = 〈313−563〉 [s−1], Fig. 1 Our presented results have confirmed the author's [9] idea about linear strength property dependence on strain rate. At the same time, the results defined UTS dependence on strain rate more precisely in compare with authors [9]. The obtained experimental results which are described above were processed by the mathematical method of nonlinear regression (method of least squares). The relation between ultimate tensile ˙ is described by Eqs. (1) and (2) for the stress (UTS) and strain rate (ε)
Fig. 2. The measured and calculated ultimate tensile stress data of the drawing state.
2345
Fig. 3. The measured and calculated ultimate tensile stress data of the ECAP state.
drawing and ECAP states, respectively. UTSDrawing = 408 + 32:53 × 1:45 ˙
UTSECAP = 436 + 0:78 × 2:72
lnε
ε˙
ln
ð1Þ ð2Þ
The calculated data using Eqs. (1) and (2) and measured data were plotted to Figs. 2 and 3. From graphical dependences, a close conformity between date was found what it also implied by the correlation index (I). The correlation index is 0.998 for Eq. (1) and 0.997 for Eq. (2).
Fig. 4. SEM images of a) drawn b) ECAP samples after static tensile tests (~10−3 s−1).
2346
T. Kvačkaj et al. / Materials Letters 64 (2010) 2344–2346
fracture mode with typical dimple morphology in both strain rate regimes independent from applied experimental methods (drawing, ECAP) was found. The fractured surface of ECAP samples after dynamic tensile tests was investigated in detail. The decrease of the dimple size with strain rate increasing from 2.3 μm to 1.2 μm (200–400 s−1) was found. 4. Conclusion The ultimate tensile stress of OFHC Cu depends on strain rate. This dependence is stronger at higher strain rates (~102 s−1) than at lower ones (from 10−3 to 10−1 s−1) and is described by the following equations ε˙ ε˙ UTSDrawing = 408 + 32:53 × 1:45ln and UTSECAP = 436 + 0:78 × 2:72ln . The results revealed negligible differences in UTS progress in the whole strain rate range for the drawing and ECAP states. The fracture investigation revealed no strain rate and microstructure influence on the failure mechanism. However, the decrease of the dimple size from 2.3 μm to 1.2 μm with strain rate increasing was observed. Acknowledgements This work is supported by the Slovak Agency APVV-20-027205 and the Bilateral Project SK-PL-0011-09. References [1] [2] [3] [4] [5] [6] Fig. 5. SEM images of a) drawn b) ECAP samples after dynamic tensile tests (~102 s−1).
3.2. Fractography
[7] [8] [9]
After the static and dynamic tensile tests, the fractured surfaces of samples were investigated. The typical micrographs are shown in Figs. 4 and 5. Based on the fractured surface investigation, a ductile
[10] [11] [12]
Zhu YT, Langdon TG. JOM 2004;56:58–63. Valiev RZ, Langdon TG. Prog Mater Sci 2006;51:881–981. Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103–89. Valiev RZ, Korznikov AV, Mulyukov RR. Mater Sci Eng 1993;A168:141–8. Bidulsky R, Bidulska J, Actis Grande M. High Temp Mater Process 2009;28:337–42. Kvackaj T, Bidulska J, Fujda M, Kocisko R, Pokorny I, Milkovic O. Mater Sci Forum 2010;633–634:273–302. Bidulska J, Kocisko R, Bidulsky R, Actis Grande M, Donic T, Martikan M. Acta Metall Slovaca 2010;16:4–11. Kocisko R, Kvackaj T, Bidulska J, Molnarova M. Acta Metall Slovaca 2009;15(4): 228–33. Mishra A, Martin M, Thadhani NN, Kad BK, Kenik EA, Meyers MA. Acta Mater 2008;56:2770–83. Niechajowicz A, Tobota A. ACME 2008;8(2):129–37. Pawlicki J. Rudy i Metale Niezelazne 2009;11:798–802. Wang M, Shan A. J Alloys Compd 2008;455(1–2):L10–4.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of strain rate on ultimate tensile stress of coarse-grained and ultrafine-grained copper Tibor Kvačkaj a, Andrea Kováčová a, Michal Kvačkaj a, Imrich Pokorný a, Róbert Kočiško a, Tibor Donič b,⁎ a b
Department of Metal Forming, Faculty of Metallurgy, Technical University of Košice, Vysokoškolská 4, 042 00 Košice, Slovakia Department of Applied Mechanics, Faculty of Mechanical Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia
a r t i c l e
i n f o
Article history: Received 14 May 2010 Accepted 15 July 2010 Available online 22 July 2010 Keywords: Nanomaterials Deformation and fracture Strain rate Mechanical properties ECAP
a b s t r a c t In the present paper OFHC (oxygen free high conductivity) copper was tested by static and dynamic tensile tests at room temperature owing to strain rate investigation. Because of coarse-grained (CG) and ultrafinegrained (UFG) microstructure observation the copper was subjected to drawing and ECAP processes. The investigation of strain rate and microstructure was focused on the ultimate tensile stress (UTS) after the tensile tests. Following this study, it was found that strain rate is an important characteristic influencing the mechanical properties of copper. The ultimate tensile stress grew with strain rate increasing and this effect is more visible at high strain rates (ε˙ ~ 102 s−1). Moreover, it was revealed that strain rate hasn't got any influence on the failure mechanism of the copper on the other hand it has an influence on the values of dimple size. While strain rate increases the dimple size decreases. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The UFG materials (db 1 μm) have been studied over the last years due to their unique properties. They have high strength and good ductility at the same time in comparison to CG materials which have been produced by classical deformation technologies as rolling, drawing and pressing [1]. The equal-channel angular pressing (ECAP) is one of the methods which have been used on UFG metal formation [2–8]. It is well known that strain rate has a great influence on the mechanical properties of materials. Static tensile properties have been intensively studied in recent years [1]. Nowadays, Kolsky bar equipment has been used to study material dynamic properties in the strain rate range ε˙ N 1000 s−1 [9]. For the purpose of strain rate investigation in the range ε˙ ~ 100–1000 s−1 very limited studies exist in literature. The influence of these strain rates can be studied by a rotating flywheel machine [10,11]. The main advantage of a rotating flywheel is the kinetic energy which is created during the process. The kinetic energy is greater than the work done on the sample deformation this means that the constant velocity of deformation can be assumed [10]. 2. Material and methods The starting material for this study was OFHC copper in rod shape after drawing and ECAP processes at room temperature. The first series of samples was drawn from diameter 15 mm to 10 mm. The
⁎ Corresponding author. Tel./fax: +421 55 6024198. E-mail address: [email protected] (T. Donič).
second one (diameter 10 mm, length 100 mm) was processed by ECAP die (φ = 90°) with 5 passes and rotated by route C. The samples with CG and UFG microstructure were investigated at room temperature in consideration of their mechanical tensile properties and fracture morphology. The classical tensile testing machine LabTest 5.20 ST and rotating flywheel machine offer the possibility of obtaining a wide range of strain rates (from 10−3 to 10−1 s−1 for static conditions and~102 s−1 for dynamic ones). The geometrical parameters of the samples for static and dynamic tensile tests were L0 = 28 mm, d0 = 4 mm and L0 = 8, d0 = 4 mm, respectively. The achieved values from measurements were applied for mathematical analysis by the software product “R”. The fractured surface morphology of samples after tensile tests was investigated using scanning electron microscopy JEOL 7000F. The dimple sizes of ECAP samples after dynamic tensile tests were measured from a statistical file of 400 features. 3. Results and discussion 3.1. Strength properties The ultimate tensile stress (UTS) dependence on strain rate is illustrated in Fig. 1. Because of strain hardening the increase of UTS on strain rate is visible in both cases (drawing, ECAP). In the dynamic regime, the increase of UTS is steep what implies higher strain rate sensitivity in comparison with the static regime, what was confirmed by authors [9], too. These results can be explained by faster strain rate hardening elevation in the dynamic regime than in the static regime. Based on the literary source [12] as well as our experiment, the reached results could be explained by different deformation
0167-577X/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.047
T. Kvačkaj et al. / Materials Letters 64 (2010) 2344–2346
Fig. 1. The ultimate tensile stress progress in dependence on strain rate.
mechanism performance during plastic deformation. The higher strain rates caused a great amount of dislocation pile-up generation and their higher speed of movement resulting in greater strain hardening. Fig. 1 also shows little influence of UFG and CG microstructure on UTS. As noted previously, strain rate research on the tensile mechanical properties of UFG copper has not been described in the literature enough. The authors [9] investigated strain rate influence on flow stress progress however in experimental conditions based on dynamic compression tests. Comparing results from our experiment and results presented by authors [9] different data were achieved as is illustrated on Fig. 1. Using compression tests, lower dynamic strength properties in compare with tensile tests were obtained owing to different stress states. The results shown by authors [9] have only informative status due to only three point existence in every regime (without ECAP, 2 and 8 ECAP passes). Our results (5 ECAP passes) show a slow increase of strength properties until ε˙ ~ 0, 4 s−1 was reached. On the other hand, a steep increase of strength properties was shown in the strain rate range ε˙ = 〈313−563〉 [s−1], Fig. 1 Our presented results have confirmed the author's [9] idea about linear strength property dependence on strain rate. At the same time, the results defined UTS dependence on strain rate more precisely in compare with authors [9]. The obtained experimental results which are described above were processed by the mathematical method of nonlinear regression (method of least squares). The relation between ultimate tensile ˙ is described by Eqs. (1) and (2) for the stress (UTS) and strain rate (ε)
Fig. 2. The measured and calculated ultimate tensile stress data of the drawing state.
2345
Fig. 3. The measured and calculated ultimate tensile stress data of the ECAP state.
drawing and ECAP states, respectively. UTSDrawing = 408 + 32:53 × 1:45 ˙
UTSECAP = 436 + 0:78 × 2:72
lnε
ε˙
ln
ð1Þ ð2Þ
The calculated data using Eqs. (1) and (2) and measured data were plotted to Figs. 2 and 3. From graphical dependences, a close conformity between date was found what it also implied by the correlation index (I). The correlation index is 0.998 for Eq. (1) and 0.997 for Eq. (2).
Fig. 4. SEM images of a) drawn b) ECAP samples after static tensile tests (~10−3 s−1).
2346
T. Kvačkaj et al. / Materials Letters 64 (2010) 2344–2346
fracture mode with typical dimple morphology in both strain rate regimes independent from applied experimental methods (drawing, ECAP) was found. The fractured surface of ECAP samples after dynamic tensile tests was investigated in detail. The decrease of the dimple size with strain rate increasing from 2.3 μm to 1.2 μm (200–400 s−1) was found. 4. Conclusion The ultimate tensile stress of OFHC Cu depends on strain rate. This dependence is stronger at higher strain rates (~102 s−1) than at lower ones (from 10−3 to 10−1 s−1) and is described by the following equations ε˙ ε˙ UTSDrawing = 408 + 32:53 × 1:45ln and UTSECAP = 436 + 0:78 × 2:72ln . The results revealed negligible differences in UTS progress in the whole strain rate range for the drawing and ECAP states. The fracture investigation revealed no strain rate and microstructure influence on the failure mechanism. However, the decrease of the dimple size from 2.3 μm to 1.2 μm with strain rate increasing was observed. Acknowledgements This work is supported by the Slovak Agency APVV-20-027205 and the Bilateral Project SK-PL-0011-09. References [1] [2] [3] [4] [5] [6] Fig. 5. SEM images of a) drawn b) ECAP samples after dynamic tensile tests (~102 s−1).
3.2. Fractography
[7] [8] [9]
After the static and dynamic tensile tests, the fractured surfaces of samples were investigated. The typical micrographs are shown in Figs. 4 and 5. Based on the fractured surface investigation, a ductile
[10] [11] [12]
Zhu YT, Langdon TG. JOM 2004;56:58–63. Valiev RZ, Langdon TG. Prog Mater Sci 2006;51:881–981. Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103–89. Valiev RZ, Korznikov AV, Mulyukov RR. Mater Sci Eng 1993;A168:141–8. Bidulsky R, Bidulska J, Actis Grande M. High Temp Mater Process 2009;28:337–42. Kvackaj T, Bidulska J, Fujda M, Kocisko R, Pokorny I, Milkovic O. Mater Sci Forum 2010;633–634:273–302. Bidulska J, Kocisko R, Bidulsky R, Actis Grande M, Donic T, Martikan M. Acta Metall Slovaca 2010;16:4–11. Kocisko R, Kvackaj T, Bidulska J, Molnarova M. Acta Metall Slovaca 2009;15(4): 228–33. Mishra A, Martin M, Thadhani NN, Kad BK, Kenik EA, Meyers MA. Acta Mater 2008;56:2770–83. Niechajowicz A, Tobota A. ACME 2008;8(2):129–37. Pawlicki J. Rudy i Metale Niezelazne 2009;11:798–802. Wang M, Shan A. J Alloys Compd 2008;455(1–2):L10–4.