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Nano-grained copper strip produced by accumulative roll bonding process
Materials Science and Engineering A 473 (2008) 28–33
Nano-grained copper strip produced by accumulative roll bonding process Mahnoosh Shaarbaf, Mohammad Reza Toroghinejad ∗ Department of Materials Engineering, Isfahan University of Technology, 8415683111 Isfahan, Iran Received 4 February 2007; received in revised form 11 March 2007; accepted 15 March 2007
Abstract Accumulative roll bonding (ARB) process is a severe plastic deformation (SPD) process that has been used for pure copper (99.9%). The ARB process up to 8 cycles was performed at ambient temperature under unlubricated conditions. Microstructural characterizations were done by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). It was found that continuous recrystallization resulted in microstructure covered with small recrystallized grains with an average diameter below 100 nm. The tensile strength and hardness of the ARB processed copper has become two times higher than initial value. On the other hand, the elongation dropped abruptly at the first cycle and then increased slightly. Strengthening in ARB processed copper may be attributed to strain hardening and grain refinement. In order to clarify the failure mode, fracture surfaces after tensile tests were observed by scanning electron microscopy (SEM). Observations revealed that failure mode in ARB processed copper is shear ductile rupture with elongated small dimples. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-grained copper; Accumulative roll bonding; Severe plastic deformation
1. Introduction In recent years research on the processing, structure and mechanical behavior of nanocrystalline (d < 100 nm) and ultrafine grained (100 nm < d < 1 m) materials has thrived. Two basic and complementary approaches have been developed for the synthesis of ultra fine grain (UFG) materials and these are known as the “bottom-up” and the “top-down” approaches. In the “bottom-up” approach, UFG materials are fabricated by assembling individual atoms or by consolidating nanoparticulate solids. Examples of these techniques include inert gas condensation, electrodeposition, ball milling with subsequent consolidation, etc. In practice, these techniques are often limited to the production of fairly small samples. The “top-down” approach is different because it is dependent upon taking a bulk solid with a relatively coarse grain size and processing the solid to produce a UFG microstructure through heavy straining or shock loading. This approach avoids the small product sizes and the contamination, which are inherent features of materials produced using the “bottom-up” approach. The “top-down”
∗
Corresponding author. Tel.: +98 311 3915726; fax: +98 311 3912752. E-mail address: [email protected] (M.R. Toroghinejad).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.03.065
approach includes severe plastic deformation (SPD) processing techniques [1,2]. SPD can be explained as deformation to large strains below recrystallization temperature without intermediate thermal treatments that can result in UFG structures [3]. A number of techniques, such as equal channel angular pressing (ECAP) [2,4], cyclic extrusion-compression (CEC) [5,6], high pressure torsion (HPT) [7,8], repetitive corrugation straightening (RCS) [9,10], hydrostatic extrusion [11] and accumulative roll bonding (ARB) have been developed. Very high strains have been successfully obtained by means of these methods in many different metals and alloys, and significant structural refinement has been obtained. Among these processes, the ARB process has been successfully applied to various kinds of metallic materials. The advantage of this process is its applicability to large bulky materials [12–14]. The accumulative roll bonding (ARB) is a relatively new method of severe plastic deformation proposed by Saito et al. [15]. The basic goal of ARB is to impose an extremely high plastic strain on the material, which results in structural refinement and strength increase without changing specimen dimensions. In accumulative roll bonding, the surfaces of the strips to be joined are roughened and cleaned and then stacked. After stacking, the specimen is roll-bonded by rolling. The rolled
M. Shaarbaf, M.R. Toroghinejad / Materials Science and Engineering A 473 (2008) 28–33
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specimen is cut into two halves, and the above-mentioned procedure is repeated for several times. The stacked strips are bonded during the rolling process, creating bulk material. To achieve good bonding, surface treatments such as degreasing and wire brushing of the sheet surface are required before stacking. The objective of the present study is to evaluate the microstructural changes, mechanical properties and fracture surfaces of pure copper during the ARB process.
The microstructural evaluations were done by PHILIPS CM200 transmission electron microscopy (TEM) for the specimens after ARB. Thin foils parallel to the rolling plane were prepared so that the observed position was about 300 m below surface. Selected area diffraction (SAD) patterns also were taken. The electron backscattered diffraction (EBSD) analysis was carried out on the normal direction–rolling direction planes of the ARB processed sheets. The EBSD measurement was performed at a step size of 50 nm. The tensile tests were conducted at ambient temperature on a Hounsfield H50KS testing machine at an initial strain rate of 1.67 × 10−4 s−1 . The ARB processed sheets were machined to the size of the ASTM E8M tensile specimen, whose dimensions were 12.5 mm in gage width and 50 mm in gage length. The specimens were prepared so that the tensile direction was parallel to the rolling direction (RD) of the sheets. The values reported for hardness Vickers represent average of seven separate measurements taken at randomly selected points using a load of 5 kg for 20 s. In order to determine the failure mode, fracture surfaces of the tensile samples were examined by a PHILIPS XL30 scanning electron microscopy (SEM).
2. Experimental procedure
3. Results and discussion
The material used in this study was tough pitch copper (99.9%). The initial sheets with 1 mm in thickness were cut into the dimensions 30 mm wide and 300 mm long, and then subjected to the ARB process. Fig. 1 illustrates the principle of ARB process. One side of the surface of the sheets was degreased by acetone and wire-brushed. After the surface treatment, two pieces of the sheets were stacked so that the brushed surfaces were in contact and were fixed to each other tightly by copper wires and then rolled. The roll diameter was 127 mm and the rolling speed was about 6 m/min. In the present study, the ARB process up to 8 cycles was performed at ambient temperature without lubrication. The sheets were air-cooled after roll bonding. To evaluate bonding condition, the optical examination of the samples was conducted. All optical microstructures were observed at the rolling direction–normal direction (RD–ND) plane of specimens.
3.1. The cross section of roll-bonded strips
Fig. 1. Schematic diagram of accumulative roll bonding process [16].
The cross section of the ARB processed strips are shown in Fig. 2. In the case of 1-cycle processed material (Fig. 2a), the interface introduced in the first cycle is seen clearly. If the rolling reduction was insufficient for bonding, the bonded interfaces between the sheets would be seen clearly. After 8 cycles ARB (Fig. 2b), the interfaces are visible only slightly. In case of this specimen, 255 (28 − 1) interfaces must be observed. However, only a few unbonded parts of interfaces are seen in the specimen. This indicates that the subsequent rolling sufficiently improves the bonding of interfaces introduced in a previous cycle. 3.2. Microstructure In order to evaluate the microstructure changes during ARB process both TEM and EBSD analysis were applied.
Fig. 2. The cross-section of roll-bonded strips after (a) 1 cycle and (b) 8 cycles.
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M. Shaarbaf, M.R. Toroghinejad / Materials Science and Engineering A 473 (2008) 28–33
Fig. 3. Typical TEM micrographs and the matching SAD patterns from RD–TD planes of ARB processed copper by (a) 1 cycle, (b) 4 cycles and (c) 8 cycles.
Fig. 3 shows the TEM microstructures and corresponding SAD patterns observed at rolling plane of ARB specimens produced by 1, 4 and 8 cycles. After 1 cycle of the ARB process the microstructure showed a mixture of non-deformed and deformed grains with some dislocation tangles (Fig. 3a). It is not surprising because plastic deformation is inherently an inhomogeneous process. The SAD pattern of this sample was taken from a single crystal that shows no deformation has occurred in this grain. For the specimen after 4 cycles, the dislocation density increased and cell structures were observed. Also the microstructure became more uniform and some grains with an average grain size of 200 nm have formed. The SAD patterns of the 4 cycles ARB
sample is more diffused than that of the single cycle sample and gradually evolves into ring pattern consisting of discrete spots (Fig. 3b). This may indicates that examined area has subdivided into small domains with wide orientation spread. With increasing the strain up to 8 cycles, the dislocation density at grain interior seemed lower than those after 4 ARB cycles. It is noteworthy that small recrystallized grains were observed along with the ultrafine deformation microstructures, as seen in Fig. 3c. The average grain size of these equiaxed recrystallized grains is below 100 nm, smaller than that after 4 cycles. The mean grain size of pure copper fabricated by ARB process has been reported about 260–300 nm by other researchers [13,17,18]. Also the SAD pattern became more ring-like with
Fig. 4. Gray scale map of (a) 4 cycles and (b) 8 cycles ARB processed copper.
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Fig. 5. Schematic diagram showing the continuous recrystallization of a highly deformed lamellar microstructure. (a) Initial structure, (b) collapse of the lamellar boundaries, (c) spheroidisaton begins and (d) further spheroidisation and growth [11].
increasing strain up to 8 cycles, indicating the increment of a portion of high angle boundary. In order to investigate the microstructure evolutions of the ARBed copper with more details, EBSD analysis was also performed at the thickness centre of ARB processed samples by 4 and 8 cycles. Fig. 4 shows the gray scale map of the samples. After 4 ARB cycles a lamellar structure elongated along rolling direction has developed in most of the observed regions in the sample (Fig. 4a) but at higher strain an equiaxed structure has formed as shown in Fig. 4b. The formation mechanism of the UFGs during SPD is still an issue under discussion. However, recent investigations suggest that the formation process of the UFGs is not conventional discontinuous recrystallization but continuous recrystallization (or in situ recrystallization) characterized by ultrafine grain subdivision and short range grain boundary migration [13]. For the larger initials grain size and lower strains, normal discontinuous recrystallization occurs but for larger strains and smaller grain sizes, a predominantly high angle boundary microstructure is performed with only minor boundary movements by a process of continuous recrystallization [19]. On the continuous recrystallization of a highly deformed lamellar microstructure, it is thought the energy is lowered by localized boundary migration as shown in Fig. 5, and this can be considered to occur in two stages. The first stage is collapse of the lamellar microstructure. The lamellar structure will tend to collapse due to the surface tension at the node points as A, where the boundaries (of energy γ R ) aligned in the rolling plane, are pulled by the boundaries (of energy γ N ) aligned in the normal direction as shown in Fig. 5b. The critical condition for collapse of the structure is when the nodes A and A touch. This depends on the grain length in the rolling direction (L) and the normal direction (H) and on the relative boundary energies. If γ R = γ N then the critical aspect ratio (L/H) for impingement is ∼2. However, the boundaries in the normal direction are usually of lower angle and if for example γ R = 2γ N , the criti-
cal aspect ratio is ∼4. The second stage is spheroidisation and growth. When the node A and A touch, node switching will occur, and two new nodes A1 and A2 will form and be pulled apart by the boundary tensions as shown in Fig. 5c. Further spheroidisation and growth will occur due to boundary tensions, as shown in Fig. 5d, leading to a more equiaxed grain structure [19]. 3.3. Mechanical properties The tensile strength and the elongation have been determined in standard tensile tests. The results are shown in Fig. 6, plotted against the number of cycles. The major increase was observed to occur in the first cycle and then increased slightly. The tensile strength increased from ∼210 to ∼430 MPa, so the tensile strength of the ARB processed copper (after 8 cycles) is about two times higher than initial value. On the other hand, the elongation value greatly decreased from ∼50% to ∼2% in the first cycle of ARB and then by proceeding the ARB cycles it increased up to ∼6%, suggesting that strain hardening mechanism alone could not explain the present phe-
Fig. 6. The variation of tensile strength and elongation with number of cycles.
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M. Shaarbaf, M.R. Toroghinejad / Materials Science and Engineering A 473 (2008) 28–33
rolling. Hardness increased rapidly by the first two passes and then saturated by further rolling. The hardness of the as-received copper, prior to rolling, was ∼50 HV. At the end of the eighth pass, the hardness has become ∼150 HV, indicating significant hardening. It is observed that the largest increase in the hardness is created when the two-layered strip was rolled. The hardening behavior showing saturation at large strains was commonly reported in UFG materials fabricated by severe plastic straining [21]. The rapid increase of hardness at relatively low strains seems to be attributed to strain hardening as a result of subgrain formation. 3.4. Fractography Fig. 7. The variation of hardness with number of cycles.
nomenon. Also indicates a very pronounced loss of formability. That is a typical phenomenon in the SPD materials; therefore, it is necessary to increase the elongation for practical use of the ARB processed materials. It seems that strain hardening plays a main role in the strength increase in initial stage of ARB process; then by progress of ARB it has less and less effect on strength and evolution of the grain structure begin to dominate. Because the number of ultra-fine grains with high angle grain boundaries increase with increasing the cycles up to 8 cycles. This result is in agreement with results reported by other researchers [18,20]. The hardness on the ARB processed samples was measured in the rolling direction–transverse direction plane, and the averages of the measurements are shown in Fig. 7. A remarkable increase of hardness, which was almost three times than that of the alloy before ARB, was achieved by the present eight-passes
A scanning electron microscopic (SEM) study was under taken in order to clarify the failure mechanisms in the ARB processed copper. The fracture surfaces after tensile test that were observed by SEM are shown in Fig. 8. It shows that the initial material exhibited a typical ductile fracture showing deep equiaxed dimples (Fig. 8a). Ductile tensile fractures in most materials have a gray fibrous appearance with equiaxed or hemispheroidal dimples [22]. This kind of fracture occurs by microvoid formation and coalescence. Clearly there are two governing ductile rupture mechanisms, which will either compete or co-operate under different situation leading to ductile failure. Near a generalized shear state of stress the presence and growth of voids does not play a significant role. Here rupture occurs by internal shearing between voids and seems to be governed by a simple shear deformation. This mechanism is often referred to as shear dimple rupture, where final failure take place by shearing of the intervoid ligaments [23].
Fig. 8. Tensile fracture surface of ARB processed copper after: (a) 0 cycle, (b) 4 cycles and (c) 8 cycles.
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After ARB process, the samples also show a ductile fracture having dimples but these dimples were not as deep as those in initial material. These are shear dimples. Fig. 8b and c clearly shows that the failure mode is shear ductile rupture. This mode is characterized by shallow small elongated shear dimples. 4. Conclusions The ARB process up to 8 cycles has been performed successfully at ambient temperature for pure copper. The optical microstructures were observed at the thickness of the specimens showed that a good bonding between sheets was achieved at each pass. Small recrystallized grains with average grain size of below 100 nm partly appeared after 8 cycles of ARB in the present copper specimens. Formation mechanism of the UFGs during ARB process was explained by continuous recrystallization. Moreover the high purity of the present copper contributes to recrystallization occurrence. It was found that tensile strength of pure copper increased with the increasing number of ARB cycles due to strain hardening in the initial stage. The strain hardening mechanism role would then weakened and fine grain boundary mechanism would begin to dominate. SEM observations on fracture surfaces showed that failure mode in ARB processed copper is shear ductile rupture. Characterization of this mode is elongated shear dimples. Acknowledgement The authors would like to acknowledge Prof. Dr. Reinhard Pippan (Erich Schmid Institute of Materials Science) for performing the EBSD analysis.
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References [1] M.T. P´erez-Prado, J.A. Del Valle, O.A. Ruano, Scripta Mater. 51 (2004) 1093. [2] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881. [3] B. Cherukuri, T.S. Nedkova, R. Srinivasan, Mater. Sci. Eng. A 410–411 (2005) 394. [4] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 45 (1997) 4733. [5] M. Richert, Q. Liu, N. Hansen, Mater. Sci. Eng. A 260 (1999) 275. [6] M. Richert, H.P. Stu¨we, M.J. Zehetbauer, J. Richert, R. Pippan, Ch. Motz, E. Schafler, Mater. Sci. Eng. A 355 (2003) 180. [7] G. Sakai, K. Nakamura, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 406 (2005) 268. [8] A. Vorhauer, R. Pippan, Scripta Mater. 51 (2004) 921. [9] J.Y. Huang, Y.T. Zhu, H. Jiang, C. Lowet, Acta Mater. 49 (2001) 1497. [10] J. Huang, Y.T. Zhu, D.J. Alexander, X. Liao, T.C. Lowe, R.J. Asaro, Mater. Sci. Eng. A 371 (2004) 35. [11] K.J. Kurzydlowski, Mater. Sci. Forum 503–504 (2006) 341. [12] X. Huang, N. Tsuji, Y. Minamino, N. Hansen, The 22nd Risø International Symposium on Materials Science, Roskilde, Denmark, 2001, p. 255. [13] N. Tsuji, Y. Saito, S.H. Lee, Y. Minamino, Adv. Eng. Mater. 5 (2003) 338. [14] N. Tsuji, Y. Saito, H. Utsunomiya, S. Tanigawa, Scripta Mater. 40 (1999) 795. [15] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Mater. 47 (1999) 579. [16] Y.S. Kim, S.H. Kang, D.H. Shin, Mater. Sci. Forum 503–504 (2006) 681. [17] Y.H. Jang, S.S. Kim, S.Z. Han, C.Y. Lim, C.J. Kim, M. Goto, Scripta Mater. 52 (2005) 21. [18] S.Z. Han, C. Lim, C.J. Kim, S. Kim, Mater. Sci. Forum 475–479 (2005) 3497. [19] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, 2nd ed., Elsevier Science Ltd., United Kingdom, 2004. [20] Z.P. Xing, S.B. Kang, H.W. Kim, J. Mater. Sci. 37 (2002) 717. [21] K.T. Park, H.J. Kwon, W.J. Kim, Y.S. Kim, Mater. Sci. Eng. A 316 (2001) 145. [22] B.L. Gabriel, Scanning Electron Microscopy, vol. 12, ASM Handbooks, 1998, p. 173. [23] I. Barsoum, J. Faleskog, Int. J. Solids Struct. 44 (2007) 1768.
Nano-grained copper strip produced by accumulative roll bonding process Mahnoosh Shaarbaf, Mohammad Reza Toroghinejad ∗ Department of Materials Engineering, Isfahan University of Technology, 8415683111 Isfahan, Iran Received 4 February 2007; received in revised form 11 March 2007; accepted 15 March 2007
Abstract Accumulative roll bonding (ARB) process is a severe plastic deformation (SPD) process that has been used for pure copper (99.9%). The ARB process up to 8 cycles was performed at ambient temperature under unlubricated conditions. Microstructural characterizations were done by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). It was found that continuous recrystallization resulted in microstructure covered with small recrystallized grains with an average diameter below 100 nm. The tensile strength and hardness of the ARB processed copper has become two times higher than initial value. On the other hand, the elongation dropped abruptly at the first cycle and then increased slightly. Strengthening in ARB processed copper may be attributed to strain hardening and grain refinement. In order to clarify the failure mode, fracture surfaces after tensile tests were observed by scanning electron microscopy (SEM). Observations revealed that failure mode in ARB processed copper is shear ductile rupture with elongated small dimples. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-grained copper; Accumulative roll bonding; Severe plastic deformation
1. Introduction In recent years research on the processing, structure and mechanical behavior of nanocrystalline (d < 100 nm) and ultrafine grained (100 nm < d < 1 m) materials has thrived. Two basic and complementary approaches have been developed for the synthesis of ultra fine grain (UFG) materials and these are known as the “bottom-up” and the “top-down” approaches. In the “bottom-up” approach, UFG materials are fabricated by assembling individual atoms or by consolidating nanoparticulate solids. Examples of these techniques include inert gas condensation, electrodeposition, ball milling with subsequent consolidation, etc. In practice, these techniques are often limited to the production of fairly small samples. The “top-down” approach is different because it is dependent upon taking a bulk solid with a relatively coarse grain size and processing the solid to produce a UFG microstructure through heavy straining or shock loading. This approach avoids the small product sizes and the contamination, which are inherent features of materials produced using the “bottom-up” approach. The “top-down”
∗
Corresponding author. Tel.: +98 311 3915726; fax: +98 311 3912752. E-mail address: [email protected] (M.R. Toroghinejad).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.03.065
approach includes severe plastic deformation (SPD) processing techniques [1,2]. SPD can be explained as deformation to large strains below recrystallization temperature without intermediate thermal treatments that can result in UFG structures [3]. A number of techniques, such as equal channel angular pressing (ECAP) [2,4], cyclic extrusion-compression (CEC) [5,6], high pressure torsion (HPT) [7,8], repetitive corrugation straightening (RCS) [9,10], hydrostatic extrusion [11] and accumulative roll bonding (ARB) have been developed. Very high strains have been successfully obtained by means of these methods in many different metals and alloys, and significant structural refinement has been obtained. Among these processes, the ARB process has been successfully applied to various kinds of metallic materials. The advantage of this process is its applicability to large bulky materials [12–14]. The accumulative roll bonding (ARB) is a relatively new method of severe plastic deformation proposed by Saito et al. [15]. The basic goal of ARB is to impose an extremely high plastic strain on the material, which results in structural refinement and strength increase without changing specimen dimensions. In accumulative roll bonding, the surfaces of the strips to be joined are roughened and cleaned and then stacked. After stacking, the specimen is roll-bonded by rolling. The rolled
M. Shaarbaf, M.R. Toroghinejad / Materials Science and Engineering A 473 (2008) 28–33
29
specimen is cut into two halves, and the above-mentioned procedure is repeated for several times. The stacked strips are bonded during the rolling process, creating bulk material. To achieve good bonding, surface treatments such as degreasing and wire brushing of the sheet surface are required before stacking. The objective of the present study is to evaluate the microstructural changes, mechanical properties and fracture surfaces of pure copper during the ARB process.
The microstructural evaluations were done by PHILIPS CM200 transmission electron microscopy (TEM) for the specimens after ARB. Thin foils parallel to the rolling plane were prepared so that the observed position was about 300 m below surface. Selected area diffraction (SAD) patterns also were taken. The electron backscattered diffraction (EBSD) analysis was carried out on the normal direction–rolling direction planes of the ARB processed sheets. The EBSD measurement was performed at a step size of 50 nm. The tensile tests were conducted at ambient temperature on a Hounsfield H50KS testing machine at an initial strain rate of 1.67 × 10−4 s−1 . The ARB processed sheets were machined to the size of the ASTM E8M tensile specimen, whose dimensions were 12.5 mm in gage width and 50 mm in gage length. The specimens were prepared so that the tensile direction was parallel to the rolling direction (RD) of the sheets. The values reported for hardness Vickers represent average of seven separate measurements taken at randomly selected points using a load of 5 kg for 20 s. In order to determine the failure mode, fracture surfaces of the tensile samples were examined by a PHILIPS XL30 scanning electron microscopy (SEM).
2. Experimental procedure
3. Results and discussion
The material used in this study was tough pitch copper (99.9%). The initial sheets with 1 mm in thickness were cut into the dimensions 30 mm wide and 300 mm long, and then subjected to the ARB process. Fig. 1 illustrates the principle of ARB process. One side of the surface of the sheets was degreased by acetone and wire-brushed. After the surface treatment, two pieces of the sheets were stacked so that the brushed surfaces were in contact and were fixed to each other tightly by copper wires and then rolled. The roll diameter was 127 mm and the rolling speed was about 6 m/min. In the present study, the ARB process up to 8 cycles was performed at ambient temperature without lubrication. The sheets were air-cooled after roll bonding. To evaluate bonding condition, the optical examination of the samples was conducted. All optical microstructures were observed at the rolling direction–normal direction (RD–ND) plane of specimens.
3.1. The cross section of roll-bonded strips
Fig. 1. Schematic diagram of accumulative roll bonding process [16].
The cross section of the ARB processed strips are shown in Fig. 2. In the case of 1-cycle processed material (Fig. 2a), the interface introduced in the first cycle is seen clearly. If the rolling reduction was insufficient for bonding, the bonded interfaces between the sheets would be seen clearly. After 8 cycles ARB (Fig. 2b), the interfaces are visible only slightly. In case of this specimen, 255 (28 − 1) interfaces must be observed. However, only a few unbonded parts of interfaces are seen in the specimen. This indicates that the subsequent rolling sufficiently improves the bonding of interfaces introduced in a previous cycle. 3.2. Microstructure In order to evaluate the microstructure changes during ARB process both TEM and EBSD analysis were applied.
Fig. 2. The cross-section of roll-bonded strips after (a) 1 cycle and (b) 8 cycles.
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Fig. 3. Typical TEM micrographs and the matching SAD patterns from RD–TD planes of ARB processed copper by (a) 1 cycle, (b) 4 cycles and (c) 8 cycles.
Fig. 3 shows the TEM microstructures and corresponding SAD patterns observed at rolling plane of ARB specimens produced by 1, 4 and 8 cycles. After 1 cycle of the ARB process the microstructure showed a mixture of non-deformed and deformed grains with some dislocation tangles (Fig. 3a). It is not surprising because plastic deformation is inherently an inhomogeneous process. The SAD pattern of this sample was taken from a single crystal that shows no deformation has occurred in this grain. For the specimen after 4 cycles, the dislocation density increased and cell structures were observed. Also the microstructure became more uniform and some grains with an average grain size of 200 nm have formed. The SAD patterns of the 4 cycles ARB
sample is more diffused than that of the single cycle sample and gradually evolves into ring pattern consisting of discrete spots (Fig. 3b). This may indicates that examined area has subdivided into small domains with wide orientation spread. With increasing the strain up to 8 cycles, the dislocation density at grain interior seemed lower than those after 4 ARB cycles. It is noteworthy that small recrystallized grains were observed along with the ultrafine deformation microstructures, as seen in Fig. 3c. The average grain size of these equiaxed recrystallized grains is below 100 nm, smaller than that after 4 cycles. The mean grain size of pure copper fabricated by ARB process has been reported about 260–300 nm by other researchers [13,17,18]. Also the SAD pattern became more ring-like with
Fig. 4. Gray scale map of (a) 4 cycles and (b) 8 cycles ARB processed copper.
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Fig. 5. Schematic diagram showing the continuous recrystallization of a highly deformed lamellar microstructure. (a) Initial structure, (b) collapse of the lamellar boundaries, (c) spheroidisaton begins and (d) further spheroidisation and growth [11].
increasing strain up to 8 cycles, indicating the increment of a portion of high angle boundary. In order to investigate the microstructure evolutions of the ARBed copper with more details, EBSD analysis was also performed at the thickness centre of ARB processed samples by 4 and 8 cycles. Fig. 4 shows the gray scale map of the samples. After 4 ARB cycles a lamellar structure elongated along rolling direction has developed in most of the observed regions in the sample (Fig. 4a) but at higher strain an equiaxed structure has formed as shown in Fig. 4b. The formation mechanism of the UFGs during SPD is still an issue under discussion. However, recent investigations suggest that the formation process of the UFGs is not conventional discontinuous recrystallization but continuous recrystallization (or in situ recrystallization) characterized by ultrafine grain subdivision and short range grain boundary migration [13]. For the larger initials grain size and lower strains, normal discontinuous recrystallization occurs but for larger strains and smaller grain sizes, a predominantly high angle boundary microstructure is performed with only minor boundary movements by a process of continuous recrystallization [19]. On the continuous recrystallization of a highly deformed lamellar microstructure, it is thought the energy is lowered by localized boundary migration as shown in Fig. 5, and this can be considered to occur in two stages. The first stage is collapse of the lamellar microstructure. The lamellar structure will tend to collapse due to the surface tension at the node points as A, where the boundaries (of energy γ R ) aligned in the rolling plane, are pulled by the boundaries (of energy γ N ) aligned in the normal direction as shown in Fig. 5b. The critical condition for collapse of the structure is when the nodes A and A touch. This depends on the grain length in the rolling direction (L) and the normal direction (H) and on the relative boundary energies. If γ R = γ N then the critical aspect ratio (L/H) for impingement is ∼2. However, the boundaries in the normal direction are usually of lower angle and if for example γ R = 2γ N , the criti-
cal aspect ratio is ∼4. The second stage is spheroidisation and growth. When the node A and A touch, node switching will occur, and two new nodes A1 and A2 will form and be pulled apart by the boundary tensions as shown in Fig. 5c. Further spheroidisation and growth will occur due to boundary tensions, as shown in Fig. 5d, leading to a more equiaxed grain structure [19]. 3.3. Mechanical properties The tensile strength and the elongation have been determined in standard tensile tests. The results are shown in Fig. 6, plotted against the number of cycles. The major increase was observed to occur in the first cycle and then increased slightly. The tensile strength increased from ∼210 to ∼430 MPa, so the tensile strength of the ARB processed copper (after 8 cycles) is about two times higher than initial value. On the other hand, the elongation value greatly decreased from ∼50% to ∼2% in the first cycle of ARB and then by proceeding the ARB cycles it increased up to ∼6%, suggesting that strain hardening mechanism alone could not explain the present phe-
Fig. 6. The variation of tensile strength and elongation with number of cycles.
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rolling. Hardness increased rapidly by the first two passes and then saturated by further rolling. The hardness of the as-received copper, prior to rolling, was ∼50 HV. At the end of the eighth pass, the hardness has become ∼150 HV, indicating significant hardening. It is observed that the largest increase in the hardness is created when the two-layered strip was rolled. The hardening behavior showing saturation at large strains was commonly reported in UFG materials fabricated by severe plastic straining [21]. The rapid increase of hardness at relatively low strains seems to be attributed to strain hardening as a result of subgrain formation. 3.4. Fractography Fig. 7. The variation of hardness with number of cycles.
nomenon. Also indicates a very pronounced loss of formability. That is a typical phenomenon in the SPD materials; therefore, it is necessary to increase the elongation for practical use of the ARB processed materials. It seems that strain hardening plays a main role in the strength increase in initial stage of ARB process; then by progress of ARB it has less and less effect on strength and evolution of the grain structure begin to dominate. Because the number of ultra-fine grains with high angle grain boundaries increase with increasing the cycles up to 8 cycles. This result is in agreement with results reported by other researchers [18,20]. The hardness on the ARB processed samples was measured in the rolling direction–transverse direction plane, and the averages of the measurements are shown in Fig. 7. A remarkable increase of hardness, which was almost three times than that of the alloy before ARB, was achieved by the present eight-passes
A scanning electron microscopic (SEM) study was under taken in order to clarify the failure mechanisms in the ARB processed copper. The fracture surfaces after tensile test that were observed by SEM are shown in Fig. 8. It shows that the initial material exhibited a typical ductile fracture showing deep equiaxed dimples (Fig. 8a). Ductile tensile fractures in most materials have a gray fibrous appearance with equiaxed or hemispheroidal dimples [22]. This kind of fracture occurs by microvoid formation and coalescence. Clearly there are two governing ductile rupture mechanisms, which will either compete or co-operate under different situation leading to ductile failure. Near a generalized shear state of stress the presence and growth of voids does not play a significant role. Here rupture occurs by internal shearing between voids and seems to be governed by a simple shear deformation. This mechanism is often referred to as shear dimple rupture, where final failure take place by shearing of the intervoid ligaments [23].
Fig. 8. Tensile fracture surface of ARB processed copper after: (a) 0 cycle, (b) 4 cycles and (c) 8 cycles.
M. Shaarbaf, M.R. Toroghinejad / Materials Science and Engineering A 473 (2008) 28–33
After ARB process, the samples also show a ductile fracture having dimples but these dimples were not as deep as those in initial material. These are shear dimples. Fig. 8b and c clearly shows that the failure mode is shear ductile rupture. This mode is characterized by shallow small elongated shear dimples. 4. Conclusions The ARB process up to 8 cycles has been performed successfully at ambient temperature for pure copper. The optical microstructures were observed at the thickness of the specimens showed that a good bonding between sheets was achieved at each pass. Small recrystallized grains with average grain size of below 100 nm partly appeared after 8 cycles of ARB in the present copper specimens. Formation mechanism of the UFGs during ARB process was explained by continuous recrystallization. Moreover the high purity of the present copper contributes to recrystallization occurrence. It was found that tensile strength of pure copper increased with the increasing number of ARB cycles due to strain hardening in the initial stage. The strain hardening mechanism role would then weakened and fine grain boundary mechanism would begin to dominate. SEM observations on fracture surfaces showed that failure mode in ARB processed copper is shear ductile rupture. Characterization of this mode is elongated shear dimples. Acknowledgement The authors would like to acknowledge Prof. Dr. Reinhard Pippan (Erich Schmid Institute of Materials Science) for performing the EBSD analysis.
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