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Effect of strain rate on the tensile behavior of ultra-fine grained pure aluminum
Journal of Alloys and Compounds 455 (2008) L10–L14
Letter
Effect of strain rate on the tensile behavior of ultra-fine grained pure aluminum Mingliang Wang ∗ , Aidang Shan Key Laboratory for High Temperature Materials and High Temperature Tests, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 7 January 2007; received in revised form 24 January 2007; accepted 24 January 2007 Available online 8 February 2007
Abstract Ultra-fine grained (UFG) pure aluminum was produced through four-pass ECAP plus cold rolling at ambient temperature. Tensile test was performed under a strain rate range of 5 × 10−5 s−1 to 1 × 10−1 s−1 . The response of tensile properties to strain rate was analyzed. It was found that in the UFG Al the deformation mechanism operated by the dislocation interactions in the higher strain rate range, while in the lower strain rate range the deformation mechanism may be related to grain boundary sliding. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals; UFG; Strain rate effect; Grain boundaries
Polycrystalline materials with nanometer- or submicrometersized grains have attracted enormous interests in the past two decades because of their unusual mechanical properties [1–5]. For example, the classical Hall–Petch relation does not work in some submicro- and nano-crystalline (NC) structured materials, which has been proved by lots of experiments [6,7] and several computer simulations [8]. UFG materials are usually a grain size of tens and hundreds of nanometers [9]. And the UFG materials produced by severe plastic deformation (SPD) methods always have inhomogeneous deformation behaviors because of a high dislocation density and non-equilibrium grain boundaries [10]. In the grain size with 50–100 nm, NC structured materials deform via the slip of lattice dislocations; since the grain interior is largely dislocation-free [11], the grain boundaries act as both dislocation sources and dislocation sinks. The deformation mechanisms of UFG materials are influenced by grain size, strain rate, temperature, and so on [5]. Among the factors that affect the fracture mechanisms of UFG or NC materials, the stain rate has a great importance in understanding the inherent mechanical traits of materials. Lu et al. [7] observed an abnormal strain rate effect on the tensile
∗
Corresponding author. Tel.: +86 21 62932521. E-mail address: [email protected] (M. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.01.166
ductility of the electrodeposited NC Cu, in which the fracture strain increases successively with the elevation of strain rate. As a matter of fact, a careful analysis and comparison in UFG Al has not been made yet. In this study, the detailed analysis of strain rate effect on UFG Al was made. It was found that the UFG Al has shown a transitional deformation mechanism dependence of the strain rates. 1. Experimental In the present study, equal channel angular pressing (ECAP) was adopted to prepared UFG aluminum. ECAP [12] is a widely used SPD [13] method to produce bulk nano-structured materials and has received intensive investigation for years. Bulk commercial pure 1050 Al (99.6%), with the size of 12 mm × 12 mm × 60 mm, has been annealed at 530 ◦ C for 2 h. Then the sample was processed utilizing ECAP for four passes through route Bc [4,13], and plus cold rolling to 0.7 mm at room temperature. The cold rolling direction in the experiments was performed with longitude direction. It could be called UFG Al sample. Transmission electron microscopy (TEM) of a JEOL 2100F microscope was used to observe the microstructure of UFG Al. The Samples for TEM were prepared via silicon carbide papers with different grades to a thickness of approximate 60 m, and a double jet electro-polisher at −30 ◦ C was used for the final thinning and perforating until a small perforation appeared in the center. The electrolyte consists of 5% perchloric acid and 95% ethanol. For the tensile test, dogbone-shaped thin sheet specimens were made from the UFG Al sheets along the rolling direction using electro-discharge machining.
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
L11
Fig. 3. Strain rate change test of UFG Al.
Fig. 1. SAD and microstructures of UFG under TEM. The sample was with the overall length of 56 mm and thickness of 0.7 mm; the gauge length is 20 mm and width is 5 mm.
2. Results Fig. 1 shows the result of the TEM observation and selective area diffraction (SAD) of UFG Al. Statistics analysis shows the grains size is about 500–600 nm. Fig. 2 exhibits the true stress–strain curves for the UFG Al specimens at various tensile stain rates (˙ε) from 5 × 10−5 s−1 to 1 × 10−1 s−1 . It is evident that in the tensile tests (5 × 10−5 s−1 to 1 × 10−1 s−1 ), with the rise of the strain rate, the ultimate tensile strength (UTS) increase from 195 MPa to 234 MPa gradually, while the ductility do not follow this trend. From the true
Fig. 2. Room-temperature tensile true stress–strain curves for UFG Al with different stain rates: curve (A) 1 × 10−1 s−1 ; curve (B) 1 × 10−2 s−1 ; curve (C) 1 × 10−3 s−1 ; curve (D) 1 × 10−4 s−1 ; curve (E) 5 × 10−5 s−1 .
strain–stress curves shown in Fig. 2, the tensile properties can be roughly divided into two categories: the higher strain rate category and the lower strain rate category. The higher strain rate category, including 1 × 10−1 s−1 to 1 × 10−3 s−1 , these curves show an obvious strain-softening phenomenon with a sharp decrease in true stress after passing the true strain corresponding to the UTS. The lower strain rate category, including 1 × 10−4 s−1 and 5 × 10−5 s−1 , also exhibits an clear strainsoftening phenomenon, which is a slow decrease or near plateau process, after passing the true strain corresponding to the UTS when strain rate decreases from 1 × 10−1 s−1 to 1 × 10−3 s−1 , the fracture strain decreases a few from 10.5% to about 10%. When strain rate decreases from 1 × 10−4 s−1 to 5 × 10−5 s−1 , the fracture strain increase from 14% to nearly 15%. It is conceivably that for the UFG Al under different strain rate conditions, different deformation mechanism dominates. In general, samples tested at a lower strain rate show a better ductility than that at a higher strain rate. Fig. 3 exhibits the result of strain rate change test, or jump test, of UFG Al at room temperature. In this tensile test curve, two stages appear when the strain rate changes. When the strain rates increase, the strength also increases. It can be deemed that the strength of UFG Al is strain rate sensitive, which is in accordance with the tendency of acquired UTS values corresponding to respective strain rates in Fig. 2. Fig. 4 shows the fracture surface of UFG samples at the strain rate of (a) 1 × 10−1 s−1 ; (b) 1 × 10−2 s−1 ; (c) 1 × 10−3 s−1 ; (d) 1 × 10−4 s−1 ; (e) 5 × 10−5 s−1 , respectively. All the fracture surfaces showed as followed exhibited inhomogeneous fracture. The size of necking regions in the fracture surface of lower strain rate category (Fig. 4(d) and (e)) is a little wider than that of higher strain rate category (Fig. 4(a), (c), and (d)). However, the fracture surface was full of equiaxed dimples and cavities with different sizes and depth. No big difference exists between the fracture surfaces at different strain rates, and all the fracture surfaces belonged to the plastic fracture surface. In this study, a typical fracture sample with a fracture angle (the angle of the fracture line with respective to the tensile axis)
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M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
Fig. 4. Fracture Surface of UFG Al Samples at strain rate of: (a) 1 × 10−1 s−1 ; (b) 1 × 10−2 s−1 ; (c) 1 × 10−3 s−1 ; (d) 1 × 10−4 s−1 ; (e) 5 × 10−5 s−1 .
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
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Fig. 5. A typical fracture sample with a fracture angle of approximate 60◦ . Fig. 6. Calculating the m value for UFG Al.
of approximate 60◦ was exhibited in Fig. 5. Statistic of fracture degrees of tested UFG samples was shown in Table 1. It can be found that fracture angles are around 60◦ irrespective of the strain rates. 3. Discussion The UFG Al showed differences in deformation mechanism dependence of stain rates. As shown in Fig. 2, it is known that deformation at higher train rate has been considered as a strategy to improve the ductility of nano-structured materials [14]. Some studies have reported the tensile strength and ductility of ultrafine grained materials arise with the increase of strain rates [7]. The elevation of working hardening rates is the acceptable reason for improving ductility and tensile strength at higher strain rates [15], since working hardening delays necking, which is the onset of failure under tension for most metals and alloys. It is caused by the accumulation of crystalline defects, such as dislocations, which make the further deformation more difficult [16]. At the same time, the dynamic recovery can decrease the defect density so as to lower the working hardening rate. At a higher Table 1 Statistic of fracture degrees of testing UFG samples Strain rates, ε˙ (s−1 )
Sample no.
Fracture angle (◦ )
1 × 10−1
1 2 3
62 ± 1 62 ± 1 65 ± 1
1 × 10−2
4 5 6
65 ± 1 61 ± 1 63 ± 1
1 × 10−3
7 8 9
60 ± 1 65 ± 1 60 ± 1
1 × 10−4
10 11 12
64 ± 1 62 ± 1 65 ± 1
1 × 10−4
13 14
62 ± 1 63 ± 1
5 × 10−5
15 16
63 ± 1 62 ± 1
strain rate, a higher ductility can be lead by the more crystalline defects, which are produced to compete with dynamic recovery, and therefore increase the work hardening rate. This can explain the true stress–strain phenomenon in the higher strain rate category of UFG Al. The fracture mechanism for this category is involved with dislocations movements and interactions with grain boundaries and dislocations. However, the deformation mechanism in the lower strain rate category seems to be operated in a different way. Strain rate sensitivity (m) is a factor being the token of the hardening index of the material as the change of strain rates, which is defined as [17]: ∂ log σ m= ∂ log ε˙ ε,T It is said that increase the strain rate sensitivity would improve the ductility of nano-structured fcc metals [14]. As showed in Fig. 6, m is about 0.02 for the higher strain rate category while m equals to 0.039 for the lower strain category, which is approximately twice than the former one. It should mean the lower rate category possesses better ductility than the higher rate category, which is in compliance with what depicted in Fig. 3. The enhanced m for the lower strain rate category of UFG Al at RT is again implicit of a changed stimulated deformation mechanism. The strain rate sensitivity is likely to arise out of a mechanism linked with the ultra-fine grains and their grain boundaries, such as sliding/shear, or trailing atoms in the grain boundaries, which is also accompanied by atomic diffusion and relaxation [2,8]. In coexistence with the inter-grain activities [2], the processes dominates the flow stress level needed to adjust the given strain rate test. An acting mechanism involving slow activated process could convincingly become more preferred with decreasing strain rates [18]. In a typical uniaxial tensile test, the fracture angle is always 90◦ or 45◦ (along the shear force direction) in the typical fcc CG metals and alloys under a triaxial tensile stress state. This behavior also occurred in UFG Cu [19] with a 90◦ fracture angle of thin sheet samples, and 52◦ in NC Cu [20] of thin sheet samples. And also in the NC Cu a 90◦ fracture angle of thin sheet samples was observed [7]. In the UFG Al, 45◦ fracture angle
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M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
for cylinder samples [5] and 90◦ for thin sheet samples [9] were reported. Recently, Segal et al. [21] has established a mathematics model for tensile testing of UFG metals and predicted a 45◦ fracture angle for cylinder samples, and realized it in the UFG Al–0.5% Cu alloys. It could be confirmed that 45◦ fracture angle for cylinder samples in both theory and experiments of UFG and NC, and still to be discussed on the fracture angle in the thin sheet UFG and NC samples. 4. Conclusion Tensile tests was performed with UFG Al under strain rate of 5 × 10−5 s−1 to 1 × 10−1 s−1 at ambient temperature. It was found that in the UFG Al the deformation mechanism operated by the dislocation interactions in a higher strain rate range, while in a lower strain range the deformation mechanism may relate to grain boundary sliding or rotation. The fracture angles of UFG samples were about 60◦ regardless of the strain rates, which is different from the fracture angle degrees reported before. The reason for this phenomenon is still unclear. Acknowledgements This paper is supported by China National Natural Science Foundation under contract no. 50671062, and Shanghai nanoproject and New Century Scholarship of Ministry of education of China under contract no. 0452NM053.
References [1] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743–5774. [2] H. Conrad, Mater. Sci. Eng. A 341 (2003) 216–228. [3] H. Conrad, Metall. Mater. Trans. A 35 (2004) 2681–2695. [4] S. Cheng, J.A. Spencer, W.W. Milligan, Acta Mater. 51 (2003) 4505–4518. [5] P.C. Hung, P.L. Sun, C.Y. Yu, P.W. Gao, C.P. Chang, Scripta Mater. 53 (2005) 647–652. [6] R.W. Siegel, G.E. Fougere, Nanostruct. Mater. 6 (1995) 205–209. [7] L. Lu, S.X. Li, K. Lu, Scripta Mater. 45 (2001) 1163–1169. [8] P.M. Derlet, A. Hasnaoui, H. Van Swygenhoven, Scripta Mater. 49 (2003) 629–635. [9] R.Z. Valiev, Mater. Sci. Eng. A 234–236 (1997) 59–66. [10] Y.T. Zhu, T.G. Langdon, Mater. Sci. Eng. A 409 (2005) 234–242. [11] Y.T. Zhu, J.Y. Huang, J. Gubicza, T. Ungar, Y.M. Wang, E. Ma, R.Z. Valiev, J. Mater. Res. 18 (2003) 1908–1917. [12] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metal. (Metally) (1981) 1. [13] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [14] Y.M. Wang, E. Ma, Acta Mater. 52 (2004) 1699. [15] Y.T. Zhu, T.G. Langdon, JOM (2004) 58. [16] B.Q. Han, J.Y. Huang, Y.T. Zhu, E.J. Lavernia, Scripta Mater. 54 (2006) 1175. [17] M.A. Meyers, K.K. Chawla, Mechanical Metallurgy, Prentice-Hall, Englewood Cliffs, NJ, 1984, p. 570, 585. [18] Y.M. Wang, E. Ma, Appl. Phys. Lett. 83 (2003) 3165. [19] E. Ma, Scripta Mater. 49 (2003) 663. [20] S. Cheng, E. Ma, Y.M. Wang, L.J. Kecskes, K.M. Youssef, C.C. Koch, U.P. Trociewitz, K. Han, Acta Mater. 53 (2005) 1521. [21] V.M. Segal, S. Ferrasse, F. Alford, Mater. Sci. Eng. A 422 (2006) 321.
Letter
Effect of strain rate on the tensile behavior of ultra-fine grained pure aluminum Mingliang Wang ∗ , Aidang Shan Key Laboratory for High Temperature Materials and High Temperature Tests, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 7 January 2007; received in revised form 24 January 2007; accepted 24 January 2007 Available online 8 February 2007
Abstract Ultra-fine grained (UFG) pure aluminum was produced through four-pass ECAP plus cold rolling at ambient temperature. Tensile test was performed under a strain rate range of 5 × 10−5 s−1 to 1 × 10−1 s−1 . The response of tensile properties to strain rate was analyzed. It was found that in the UFG Al the deformation mechanism operated by the dislocation interactions in the higher strain rate range, while in the lower strain rate range the deformation mechanism may be related to grain boundary sliding. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals; UFG; Strain rate effect; Grain boundaries
Polycrystalline materials with nanometer- or submicrometersized grains have attracted enormous interests in the past two decades because of their unusual mechanical properties [1–5]. For example, the classical Hall–Petch relation does not work in some submicro- and nano-crystalline (NC) structured materials, which has been proved by lots of experiments [6,7] and several computer simulations [8]. UFG materials are usually a grain size of tens and hundreds of nanometers [9]. And the UFG materials produced by severe plastic deformation (SPD) methods always have inhomogeneous deformation behaviors because of a high dislocation density and non-equilibrium grain boundaries [10]. In the grain size with 50–100 nm, NC structured materials deform via the slip of lattice dislocations; since the grain interior is largely dislocation-free [11], the grain boundaries act as both dislocation sources and dislocation sinks. The deformation mechanisms of UFG materials are influenced by grain size, strain rate, temperature, and so on [5]. Among the factors that affect the fracture mechanisms of UFG or NC materials, the stain rate has a great importance in understanding the inherent mechanical traits of materials. Lu et al. [7] observed an abnormal strain rate effect on the tensile
∗
Corresponding author. Tel.: +86 21 62932521. E-mail address: [email protected] (M. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.01.166
ductility of the electrodeposited NC Cu, in which the fracture strain increases successively with the elevation of strain rate. As a matter of fact, a careful analysis and comparison in UFG Al has not been made yet. In this study, the detailed analysis of strain rate effect on UFG Al was made. It was found that the UFG Al has shown a transitional deformation mechanism dependence of the strain rates. 1. Experimental In the present study, equal channel angular pressing (ECAP) was adopted to prepared UFG aluminum. ECAP [12] is a widely used SPD [13] method to produce bulk nano-structured materials and has received intensive investigation for years. Bulk commercial pure 1050 Al (99.6%), with the size of 12 mm × 12 mm × 60 mm, has been annealed at 530 ◦ C for 2 h. Then the sample was processed utilizing ECAP for four passes through route Bc [4,13], and plus cold rolling to 0.7 mm at room temperature. The cold rolling direction in the experiments was performed with longitude direction. It could be called UFG Al sample. Transmission electron microscopy (TEM) of a JEOL 2100F microscope was used to observe the microstructure of UFG Al. The Samples for TEM were prepared via silicon carbide papers with different grades to a thickness of approximate 60 m, and a double jet electro-polisher at −30 ◦ C was used for the final thinning and perforating until a small perforation appeared in the center. The electrolyte consists of 5% perchloric acid and 95% ethanol. For the tensile test, dogbone-shaped thin sheet specimens were made from the UFG Al sheets along the rolling direction using electro-discharge machining.
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
L11
Fig. 3. Strain rate change test of UFG Al.
Fig. 1. SAD and microstructures of UFG under TEM. The sample was with the overall length of 56 mm and thickness of 0.7 mm; the gauge length is 20 mm and width is 5 mm.
2. Results Fig. 1 shows the result of the TEM observation and selective area diffraction (SAD) of UFG Al. Statistics analysis shows the grains size is about 500–600 nm. Fig. 2 exhibits the true stress–strain curves for the UFG Al specimens at various tensile stain rates (˙ε) from 5 × 10−5 s−1 to 1 × 10−1 s−1 . It is evident that in the tensile tests (5 × 10−5 s−1 to 1 × 10−1 s−1 ), with the rise of the strain rate, the ultimate tensile strength (UTS) increase from 195 MPa to 234 MPa gradually, while the ductility do not follow this trend. From the true
Fig. 2. Room-temperature tensile true stress–strain curves for UFG Al with different stain rates: curve (A) 1 × 10−1 s−1 ; curve (B) 1 × 10−2 s−1 ; curve (C) 1 × 10−3 s−1 ; curve (D) 1 × 10−4 s−1 ; curve (E) 5 × 10−5 s−1 .
strain–stress curves shown in Fig. 2, the tensile properties can be roughly divided into two categories: the higher strain rate category and the lower strain rate category. The higher strain rate category, including 1 × 10−1 s−1 to 1 × 10−3 s−1 , these curves show an obvious strain-softening phenomenon with a sharp decrease in true stress after passing the true strain corresponding to the UTS. The lower strain rate category, including 1 × 10−4 s−1 and 5 × 10−5 s−1 , also exhibits an clear strainsoftening phenomenon, which is a slow decrease or near plateau process, after passing the true strain corresponding to the UTS when strain rate decreases from 1 × 10−1 s−1 to 1 × 10−3 s−1 , the fracture strain decreases a few from 10.5% to about 10%. When strain rate decreases from 1 × 10−4 s−1 to 5 × 10−5 s−1 , the fracture strain increase from 14% to nearly 15%. It is conceivably that for the UFG Al under different strain rate conditions, different deformation mechanism dominates. In general, samples tested at a lower strain rate show a better ductility than that at a higher strain rate. Fig. 3 exhibits the result of strain rate change test, or jump test, of UFG Al at room temperature. In this tensile test curve, two stages appear when the strain rate changes. When the strain rates increase, the strength also increases. It can be deemed that the strength of UFG Al is strain rate sensitive, which is in accordance with the tendency of acquired UTS values corresponding to respective strain rates in Fig. 2. Fig. 4 shows the fracture surface of UFG samples at the strain rate of (a) 1 × 10−1 s−1 ; (b) 1 × 10−2 s−1 ; (c) 1 × 10−3 s−1 ; (d) 1 × 10−4 s−1 ; (e) 5 × 10−5 s−1 , respectively. All the fracture surfaces showed as followed exhibited inhomogeneous fracture. The size of necking regions in the fracture surface of lower strain rate category (Fig. 4(d) and (e)) is a little wider than that of higher strain rate category (Fig. 4(a), (c), and (d)). However, the fracture surface was full of equiaxed dimples and cavities with different sizes and depth. No big difference exists between the fracture surfaces at different strain rates, and all the fracture surfaces belonged to the plastic fracture surface. In this study, a typical fracture sample with a fracture angle (the angle of the fracture line with respective to the tensile axis)
L12
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
Fig. 4. Fracture Surface of UFG Al Samples at strain rate of: (a) 1 × 10−1 s−1 ; (b) 1 × 10−2 s−1 ; (c) 1 × 10−3 s−1 ; (d) 1 × 10−4 s−1 ; (e) 5 × 10−5 s−1 .
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
L13
Fig. 5. A typical fracture sample with a fracture angle of approximate 60◦ . Fig. 6. Calculating the m value for UFG Al.
of approximate 60◦ was exhibited in Fig. 5. Statistic of fracture degrees of tested UFG samples was shown in Table 1. It can be found that fracture angles are around 60◦ irrespective of the strain rates. 3. Discussion The UFG Al showed differences in deformation mechanism dependence of stain rates. As shown in Fig. 2, it is known that deformation at higher train rate has been considered as a strategy to improve the ductility of nano-structured materials [14]. Some studies have reported the tensile strength and ductility of ultrafine grained materials arise with the increase of strain rates [7]. The elevation of working hardening rates is the acceptable reason for improving ductility and tensile strength at higher strain rates [15], since working hardening delays necking, which is the onset of failure under tension for most metals and alloys. It is caused by the accumulation of crystalline defects, such as dislocations, which make the further deformation more difficult [16]. At the same time, the dynamic recovery can decrease the defect density so as to lower the working hardening rate. At a higher Table 1 Statistic of fracture degrees of testing UFG samples Strain rates, ε˙ (s−1 )
Sample no.
Fracture angle (◦ )
1 × 10−1
1 2 3
62 ± 1 62 ± 1 65 ± 1
1 × 10−2
4 5 6
65 ± 1 61 ± 1 63 ± 1
1 × 10−3
7 8 9
60 ± 1 65 ± 1 60 ± 1
1 × 10−4
10 11 12
64 ± 1 62 ± 1 65 ± 1
1 × 10−4
13 14
62 ± 1 63 ± 1
5 × 10−5
15 16
63 ± 1 62 ± 1
strain rate, a higher ductility can be lead by the more crystalline defects, which are produced to compete with dynamic recovery, and therefore increase the work hardening rate. This can explain the true stress–strain phenomenon in the higher strain rate category of UFG Al. The fracture mechanism for this category is involved with dislocations movements and interactions with grain boundaries and dislocations. However, the deformation mechanism in the lower strain rate category seems to be operated in a different way. Strain rate sensitivity (m) is a factor being the token of the hardening index of the material as the change of strain rates, which is defined as [17]: ∂ log σ m= ∂ log ε˙ ε,T It is said that increase the strain rate sensitivity would improve the ductility of nano-structured fcc metals [14]. As showed in Fig. 6, m is about 0.02 for the higher strain rate category while m equals to 0.039 for the lower strain category, which is approximately twice than the former one. It should mean the lower rate category possesses better ductility than the higher rate category, which is in compliance with what depicted in Fig. 3. The enhanced m for the lower strain rate category of UFG Al at RT is again implicit of a changed stimulated deformation mechanism. The strain rate sensitivity is likely to arise out of a mechanism linked with the ultra-fine grains and their grain boundaries, such as sliding/shear, or trailing atoms in the grain boundaries, which is also accompanied by atomic diffusion and relaxation [2,8]. In coexistence with the inter-grain activities [2], the processes dominates the flow stress level needed to adjust the given strain rate test. An acting mechanism involving slow activated process could convincingly become more preferred with decreasing strain rates [18]. In a typical uniaxial tensile test, the fracture angle is always 90◦ or 45◦ (along the shear force direction) in the typical fcc CG metals and alloys under a triaxial tensile stress state. This behavior also occurred in UFG Cu [19] with a 90◦ fracture angle of thin sheet samples, and 52◦ in NC Cu [20] of thin sheet samples. And also in the NC Cu a 90◦ fracture angle of thin sheet samples was observed [7]. In the UFG Al, 45◦ fracture angle
L14
M. Wang, A. Shan / Journal of Alloys and Compounds 455 (2008) L10–L14
for cylinder samples [5] and 90◦ for thin sheet samples [9] were reported. Recently, Segal et al. [21] has established a mathematics model for tensile testing of UFG metals and predicted a 45◦ fracture angle for cylinder samples, and realized it in the UFG Al–0.5% Cu alloys. It could be confirmed that 45◦ fracture angle for cylinder samples in both theory and experiments of UFG and NC, and still to be discussed on the fracture angle in the thin sheet UFG and NC samples. 4. Conclusion Tensile tests was performed with UFG Al under strain rate of 5 × 10−5 s−1 to 1 × 10−1 s−1 at ambient temperature. It was found that in the UFG Al the deformation mechanism operated by the dislocation interactions in a higher strain rate range, while in a lower strain range the deformation mechanism may relate to grain boundary sliding or rotation. The fracture angles of UFG samples were about 60◦ regardless of the strain rates, which is different from the fracture angle degrees reported before. The reason for this phenomenon is still unclear. Acknowledgements This paper is supported by China National Natural Science Foundation under contract no. 50671062, and Shanghai nanoproject and New Century Scholarship of Ministry of education of China under contract no. 0452NM053.
References [1] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743–5774. [2] H. Conrad, Mater. Sci. Eng. A 341 (2003) 216–228. [3] H. Conrad, Metall. Mater. Trans. A 35 (2004) 2681–2695. [4] S. Cheng, J.A. Spencer, W.W. Milligan, Acta Mater. 51 (2003) 4505–4518. [5] P.C. Hung, P.L. Sun, C.Y. Yu, P.W. Gao, C.P. Chang, Scripta Mater. 53 (2005) 647–652. [6] R.W. Siegel, G.E. Fougere, Nanostruct. Mater. 6 (1995) 205–209. [7] L. Lu, S.X. Li, K. Lu, Scripta Mater. 45 (2001) 1163–1169. [8] P.M. Derlet, A. Hasnaoui, H. Van Swygenhoven, Scripta Mater. 49 (2003) 629–635. [9] R.Z. Valiev, Mater. Sci. Eng. A 234–236 (1997) 59–66. [10] Y.T. Zhu, T.G. Langdon, Mater. Sci. Eng. A 409 (2005) 234–242. [11] Y.T. Zhu, J.Y. Huang, J. Gubicza, T. Ungar, Y.M. Wang, E. Ma, R.Z. Valiev, J. Mater. Res. 18 (2003) 1908–1917. [12] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metal. (Metally) (1981) 1. [13] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [14] Y.M. Wang, E. Ma, Acta Mater. 52 (2004) 1699. [15] Y.T. Zhu, T.G. Langdon, JOM (2004) 58. [16] B.Q. Han, J.Y. Huang, Y.T. Zhu, E.J. Lavernia, Scripta Mater. 54 (2006) 1175. [17] M.A. Meyers, K.K. Chawla, Mechanical Metallurgy, Prentice-Hall, Englewood Cliffs, NJ, 1984, p. 570, 585. [18] Y.M. Wang, E. Ma, Appl. Phys. Lett. 83 (2003) 3165. [19] E. Ma, Scripta Mater. 49 (2003) 663. [20] S. Cheng, E. Ma, Y.M. Wang, L.J. Kecskes, K.M. Youssef, C.C. Koch, U.P. Trociewitz, K. Han, Acta Mater. 53 (2005) 1521. [21] V.M. Segal, S. Ferrasse, F. Alford, Mater. Sci. Eng. A 422 (2006) 321.