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EBSD analysis of nano-structured copper processed by ECAP
Materials Science and Engineering A 528 (2011) 5348–5355
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
EBSD analysis of nano-structured copper processed by ECAP Farideh Salimyanfard a , Mohammad Reza Toroghinejad a,∗ , Fakhreddin Ashrafizadeh a , Meysam Jafari b a b
Isfahan University of Technology, Department of Materials Engineering, 84156-83111, Isfahan, Iran University of Tsukuba Graduate School of Pure and Applied Science, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 7 February 2011 Accepted 19 March 2011 Available online 25 March 2011 Keywords: Equal channel angular pressing (ECAP) Copper EBSD Nanostructure Tensile properties
a b s t r a c t In this study, copper was processed by equal channel angular pressing (ECAP) up to twenty passes at room temperature to obtain a mixture of submicron and nano-grained structure under different deformation routes. As the cross sections of the specimens were circular, the deformation routes were varied by rotating bars through 0◦ , 30◦ , 45◦ , 60◦ and 90◦ between each ECAP pass designated as routes A, B30 , B45 , B60 and BC , respectively. In order to study the orientation of the elongated crystallites, EBSD analysis was carried out and tensile tests were used to evaluate mechanical properties. The EBSD data were analysed for a statistical characterization of the deformed matrix. The effectiveness of different deformation routes with a 120◦ die in terms of grain refinement was in the order of B45 > B60 > B30 > A > BC . The mixing of ultrafine grains with nanocrystalline grains resulted in a combination of high strength and high elongation to failure after twenty pass ECAP with different routes at room temperature. The effectiveness of the five different routes in elongation was BC > A > B60 > B30 > B45 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Equal channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) processes suitable for producing submicrocrystalline to nanocrystalline bulk materials [1]. These materials are characterized by a very fine grain size and a large amount of grain boundary area. The presence of a large amount of grain boundary area results in unusual and extraordinary changes in both mechanical and physical properties [2]. As the name suggests, ECAP involves use of a die that contains two intersecting channels of equal cross sections [3]. A well lubricated billet is placed in one of the channels, and a punch then extrudes it into the second channel. In this process, the as-deformed dimensions are identical to the initial ones so that it is able to repeat the process for many cycles to accumulate large plastic strain [4]. The strain that the sample experiences is dependent on two parameters: the inner angle of intersection of the channels, ˚, and outer angle of curvature, . Microstructure after a certain number of passes is strongly dependent on the rotation scheme [5–7]. The most widely used rotation schemes are: route A, where the billet is not rotated between consecutive passes; route BA , where the billet is rotated 90◦ in alternate directions between consecutive passes; route BC , where the bar is rotated 90◦ in the same direction between consecutive passes; and route C, where the bar is rotated 180◦ between consecutive passes [8]. Microstructure and
grain size are changed during ECAP as a result of large plastic deformation that takes place in a narrow region at the intersection of two channels [9]. The so-obtained microstructure is quite complex and depends on the alloy and on the extrusion parameters (deformation route, die angle, total strain, temperature, etc.) [7]. As high strains are known to introduce large misorientations within subdivided grains, it is expected that multipass ECAP leads to grain refinement [10]. ECAP process has many advantages in terms of practical application in comparison with other severe plastic deformation process that can produce only nanocrystalline surface layers. However, the deformation and damage mechanism in the characteristic microstructure of ECAP-processed materials has not been clarified yet. For the detailed microscopic observation, electron backscattering diffraction (EBSD) technique is a useful tool that enables investigation based on crystallographic orientation. In the present study, new routes with rotation angles of 30◦ , 45◦ and 60◦ in the same direction between consecutive passes were defined. The EBSD analysis was made on pure copper processed by ECAP with different routes in order to investigate the mechanism of deformation and damage in the microstructure with nanoscopic and submicron structures and the most effective route to decrease the grain size was determined. Mechanical properties of the annealed sample and twenty pass ECAP processed samples with different routes were investigated by tensile testing. 2. Experimental procedure
∗ Corresponding author. Tel.: +98 311 3915726; fax: +98 311 3915726. E-mail address: [email protected] (M. Reza Toroghinejad). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.075
Processing was performed with a vertical hydraulic press by using a special ECAP die, which consisted of two channels identical
F. Salimyanfard et al. / Materials Science and Engineering A 528 (2011) 5348–5355
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Fig. 1. The geometry of ECAP is illustrated. The term ˚ is the intersecting angle of the two channels and is the outer curvature of the two channels.
in cross section. The channels intersected at an angle of 120◦ (Fig. 1). The initial samples of the oxygen-free high conductivity copper were taken as bars of 20 mm in diameter and 80 mm in length. Pressing was performed at room temperature. Prior to deformation by ECAP, the as-received materials were annealed for 2 h at 450 ◦ C in order to obtain a strain free microstructure with equiaxed grains. The maximum number of passes (N = 20) by different routes A, B30 , B45 , B60 and BC corresponded to a true deformation ε ∼ 12.6. The distribution of the grain diameters and the grain boundary misorientations were determined using EBSD technique. The EBSD scans were in areas of 50 m × 50 m with a 0.2 m step size. The scanning parameters were set in a manner such that a high angle grain boundary (HAGB) was defined when the misorientation between adjacent measurement points was higher than 15◦ . For tensile testing, the annealed sample and twenty passes ECAP samples with different routes A, B30 , B45 , B60 and BC were cut in the longitudinal direction into 2 pieces, then they were machined to 20 mm guage length and 4 mm diameter according to ASTM E 8 M standard, to make symmetrical samples, and then the aver-
Fig. 3. The EBSD pattern obtained from the longitudinal direction of the Cu ECAP samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC , (f) representation of the color code used to identify the crystallographic orientations on a standard stereographic projection (red: [0 0 1]; blue: [1 1 1]; green: [1 0 1]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 2. Optical microstructure of initial copper annealed at 450 ◦ C for 2 h.
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Fig. 4. Grain diameter distributions (from EBSD) for (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC .
age values of results were determined. All tests were carried out at room temperature using a HOUNSFIELD-H50KS machine. The initial strain rate was 5 mm/min. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the optical micrograph from the longitudinal section of the initial pure copper. The initial grain size measured by the linear intercept method (including annealing twin boundaries) was 50 m.
3.2. EBSD results ECAP specimens were examined by EBSD in the longitudinal direction. The EBSD/orientation imaging microscopy (OIM) patterns of ECAP processed Cu samples after 20 passes using five different routes A, B30 , B45 , B60 and BC are shown in Fig. 3. The evolution of microstructure in ECAP is dependent on the deformation path. Different processing routes lead to different final microstructures. In this work, five schemes were used: A (no rotation), B30 (30◦ rotation between passes), B45 (45◦ rotation between passes), B60 (60◦ rotation between passes), and BC (90◦ rotation between passes). As can be seen, routes A, B30 , B45 and BC result in elon-
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Fig. 5. Boundary maps obtained from the longitudinal direction of the Cu ECAP samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC , red lines mark boundaries with misorientation > 15◦ (HAGB), while black lines mark boundaries with misorientation between 2◦ and 15◦ (LAGB). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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gated grains while the distribution is fairly equiaxed in the sample processed via route B60 . The OIM images of the grains parallel to the pressing direction for each processing routes after twenty passes is shown in Fig. 3(a)–(e). The texture ranged from (1 0 0) to (1 0 1) and (1 1 1) in routes A, B30 , B45 and BC . This can be clearly seen as red, green and blue grains in the inverse pole figure (IPF) (Fig. 3(a)–(c) and (e)), but in route B60 the (1 0 1) is dominant texture that can be seen as green grains in IPF (Fig. 3d). The microstructures shown in Fig. 3 provide evidence for grain refinement and consist of both elongated and equiaxed grains. These small equiaxed grains may have resulted from continuous recrystallization. Many works have reported recrystallized grains in copper [11] and other FCC metals [12,13] after large deformations at room or low temperatures. Fig. 4(a)–(e) shows the corresponding histogram for grain size of the twenty passes ECAP processed samples with these five different routes. Fig. 4(a) shows the histogram variation of grain size for twenty passes ECAP with route A. It is found that only 3.37% of grains have the diameter of less than 100 nm, and the maximum diameter of grains in this route is 5 m that has the maximum area fraction too. In route B30 (Fig. 4(b)), it is calculated that 5.26% of grains are nanograins. The maximum area fraction in this route belongs to the diameter of 3 m that is the maximum size of grains. Fig. 4(c) shows the histogram for the grain size of twenty passes ECAP sample with route B45 . This figure indicates that 7.18% of grains have the diameter of nanometer and the maximum grain size is 1.4 m. The maximum diameter of grains in route B60 (Fig. 4(d)) is about 2 m. It is calculated that in this route 15.95% of grains are below 100 nm. Finally, Fig. 4(e) shows the result of grain size for route BC , which gives the maximum diameter of 4 m. The nano-grains in this route are in a minority (0.6%). These results are summarized in Table 1 and show that the samples processed by twenty passes exhibit microstructural inhomogeneity, with differences being observed in the features of grains lying in close vicinity. This inhomogeneity in deformation may be related to misorientation within a grain. The grains with heavy dislocation density break down to finer grains while the ones that are relatively low in dislocation density remain less affected [14]. This could be the reason why the final ECAP processed microstructure shows an inhomogeneous distribution of grain size. It is calculated from Figs. 3 and 4 that in 120◦ die, the effectiveness of grain refinement is route B45 > B60 > B30 > BC > A. Fig. 5(a)–(e) presents the corresponding grain boundary maps in which the high angle grain boundaries (HAGBs) with a misorientation angle above 15◦ are shown as red lines, and boundaries between 2◦ and 15◦ are shown as black lines. Fig. 6(a)–(e) shows the histograms for the misorientation angle of the five different routes. After twenty passes ECAP with route A (Figs. 5 and 6(a)) the microstructure mainly consists of HAGBs (70%). Like route A, in twenty passes ECAP sample with route B30 (Figs. 5 and 6(b)), the HAGBs are dominant (70%). The area fraction of HAGBs in route B45 (Figs. 5 and 6(c)) is 76%. Figs. 5(d) and 6(d) show that in route B60 the area fraction of HAGBs is 77%. It is clear from Figs. 5 and 6(a–d) that after twenty passes ECAP with routes A, B30 , B45 and B60 , the low angle grain boundaries (LAGBs) are in a minority. The crystallites are in fact surrounded in their majority by HAGBs, but in route BC (Figs. 5 and 6(e)), there is a balanced distribution of LAGBs and HAGBs. The area fraction of HAGBs in this route is 54%. These results are also shown in Table 1. Grain refinement during ECAP may be attributed to the deformation banding that inevitably occurs during the process and has two possible origins. According to the first concept [15], deformation bands originate due to the ambiguity associated with the selection of the operative slip systems. In many cases, the imposed strain can be accommodated by more than one set of slip systems and the different sets of slip systems lead to different lattice rotations. Another origin is associated to the inhomogeneous straining, i.e.
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Fig. 6. The misorientation angle distribution of ECAP Cu samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC .
different regions of a grain may experience different strains if the work done within the bands is less than that required for homogeneous deformation and if the bands can be arranged so that the net strain matches the overall deformation. These deformation bands are separated by geometrically necessary boundaries (GNBs). In
addition, boundaries are also formed by statistical trapping of dislocations, forming the so-called incidental dislocation boundaries (IDBs). In general, the misorientation of both types of boundaries increases with strain [16], the rate being larger for GNBs, i.e., GNBs accumulate dislocations at a higher rate than IDBs. Furthermore,
Table 1 The EBSD results for statistical characterization of the ECAP deformed matrix with five different routes; A, B30 , B45 , B60 and BC . Route
Maximum grain diameter (m)
Area fraction of grains with the size of less than 100 nm %
HAGB (>15◦ ) %
LAGB (2◦ –15◦ ) %
A B30 B45 B60 BC
5 3 1.4 2 4
3.4 5.3 7.2 15.9 0.6
70.3 70.09 76.45 76.7 53.9
28.3 11.32 22.45 21.7 44.8
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the spacing of both types of boundaries decreases with increasing deformation, indicating that grain subdivision continues to refine at high strain rate. By increasing the misorientation, this process leads to the formation of new high-angle grain boundaries, the percentage of which increases at high strains. Grain fragmentation depends on grain orientation. Because of the symmetry of special orientations, one slip system may cause an opposite lattice rotation to another. Grain fragmentation may be also different in the core and mantle of grains due to the geometric requirement for strain accommodation near grain boundaries. Provided grain boundary motion is slow, i.e., if the time a grain boundary moves across a grain distance is about the time a new boundary develops by the process described above, then a steady state results [17]. This mechanism of grain refinement is known as continuous recrystallization which is, as mentioned before, the mechanism of grain refinement in ECAP. 3.3. Mechanical properties Tensile tests were conducted on the annealed sample and twenty passes ECAP processed samples with different routes. As can be seen in Fig. 7, the initial sample with coarse grain (∼50 m) shows obvious strain hardening, lower tensile strength (237 MPa), and larger ductility (37%). In comparison, the ECAP-processed samples exhibit little strain hardening. Strain hardening is commonly defined as the increase of flow stress with increasing plastic strain. The uniform elongation before necking represents the ability of the steady plastic deformation. Work hardening in the process of SPD causes a large decrease in ductility of the ECAP samples, as can be seen in Fig. 7. The samples necked at strains of 3–5% and the distance between yield point and tensile strength is very small. This has been confirmed not only in UFG Cu [11], but also in many other UFG metals, such as Ti and Ni [18,19]. As expected, there was a significant increase in the strength of the ECAP samples over the initial sample. Fig. 8 shows the histogram of ultimate tensile strength for the annealed sample and twenty passes ECAP processed samples with five different routes. It seems that the effective strain path in tensile strength is A > B30 > BC > B60 > B45 > annealed sample. The histogram of elongation to failure for the annealed sample and ECAP processed samples with these five different routes is shown in Fig. 9. It is obvious from this figure that the maximum elongation belongs to the annealed sample. The effectiveness of the five different routes in elongation is BC > A > B60 > B30 > B45 .
Fig. 7. Plots of tensile engineering stress–strain curves for as-annealed and 20 pass ECAP-processed Cu samples with different routes.
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Fig. 8. The histogram of ultimate tensile strength vs processed conditions for the annealed sample and ECAP processed samples with five different routes.
For coarse-grained metals, dislocation movement and twinning are the primary deformation mechanisms. Ultrafine grains with high angle grain boundaries impede the motion of dislocations and consequently enhance strength [20], but in this work we have seen that the effectiveness of routes in HAGBs generation is not in agreement with their effectiveness in tensile strength. This may result from other factors that can affect strength of material during processing with ECAP. The grain boundaries generated by SPD are usually in a nonequilibrium state, with many dislocations that are not geometrically necessary to form the grain boundary [10,21]. These dislocations, as well as dislocations piled up near the grain boundaries, could move to facilitate grain boundary sliding [22] and grain rotation, and therefore increase the ductility. As a result, the effectiveness of different routes in LAGBs generation is in agreement with their effectiveness in ductility. 3.4. Fracture Fig. 10(a–f) shows the fracture surface of the initial and twenty passes ECAP processed samples with five different routes (tensile specimens). The as-annealed Cu is fractured with ductile features, and there are many big dimples. In Fig. 10(b–f), dimples are generated on a finer scale, and some ductile fracture features are similar to those observed in Fig. 10(a). After twenty passes ECAP with different routes, the number of dimples increases, the size of the dimples is finer and more diverse in local regions, as shown in Fig. 10(b–f). It is clearly seen that the twenty passes ECAP sample with route BC has the most ductility because the size of its dimples is larger. This result is in conformity with the histogram of Fig. 9.
Fig. 9. The histogram of elongation to failure vs processed conditions for the annealed sample and ECAP processed samples with five different routes.
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Fig. 10. SEM images of fracture surface of specimens after tensile test: (a) as-annealed and 20 passes with routes (b) A; (c) B30 ; (d) B45 ; (e) B60 and (f) BC .
The fracture morphologies also indicate that the microstructure is inhomogeneous. 4. Conclusions In this work, oxygen free high conductivity copper (OFHC) was subjected to severe plastic deformation at room temperature by ECAP. As the cross sections of specimens were circular, new routes with rotating angles of 30◦ , 45◦ and 60◦ in the same direction between passes were defined and the effect of deformation route on the microstructure and mechanical properties was investigated by EBSD and tensile test, respectively. Overall, the following conclusions can be drawn from the work. 1. The effectiveness of different deformation routes was evaluated with the 120◦ die. In terms of HAGBs generation, the efficiency is in the order of B60 ∼ B45 > B30 ∼A > BC . 2. Considering diameter of grains, the effectiveness of grain refinement in the 120◦ die is in the order of B45 > B60 > B30 > BC > A. 3. The effectiveness of producing nano-grain is in the order of B60 > B45 > B30 > A > BC . This study clearly shows that using different microstructural parameters such as grain refinement may give different results as to the effectiveness of deformation route. Therefore, care must be exercised if results from different sources are to be compared and contrasted.
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
EBSD analysis of nano-structured copper processed by ECAP Farideh Salimyanfard a , Mohammad Reza Toroghinejad a,∗ , Fakhreddin Ashrafizadeh a , Meysam Jafari b a b
Isfahan University of Technology, Department of Materials Engineering, 84156-83111, Isfahan, Iran University of Tsukuba Graduate School of Pure and Applied Science, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 7 February 2011 Accepted 19 March 2011 Available online 25 March 2011 Keywords: Equal channel angular pressing (ECAP) Copper EBSD Nanostructure Tensile properties
a b s t r a c t In this study, copper was processed by equal channel angular pressing (ECAP) up to twenty passes at room temperature to obtain a mixture of submicron and nano-grained structure under different deformation routes. As the cross sections of the specimens were circular, the deformation routes were varied by rotating bars through 0◦ , 30◦ , 45◦ , 60◦ and 90◦ between each ECAP pass designated as routes A, B30 , B45 , B60 and BC , respectively. In order to study the orientation of the elongated crystallites, EBSD analysis was carried out and tensile tests were used to evaluate mechanical properties. The EBSD data were analysed for a statistical characterization of the deformed matrix. The effectiveness of different deformation routes with a 120◦ die in terms of grain refinement was in the order of B45 > B60 > B30 > A > BC . The mixing of ultrafine grains with nanocrystalline grains resulted in a combination of high strength and high elongation to failure after twenty pass ECAP with different routes at room temperature. The effectiveness of the five different routes in elongation was BC > A > B60 > B30 > B45 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Equal channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) processes suitable for producing submicrocrystalline to nanocrystalline bulk materials [1]. These materials are characterized by a very fine grain size and a large amount of grain boundary area. The presence of a large amount of grain boundary area results in unusual and extraordinary changes in both mechanical and physical properties [2]. As the name suggests, ECAP involves use of a die that contains two intersecting channels of equal cross sections [3]. A well lubricated billet is placed in one of the channels, and a punch then extrudes it into the second channel. In this process, the as-deformed dimensions are identical to the initial ones so that it is able to repeat the process for many cycles to accumulate large plastic strain [4]. The strain that the sample experiences is dependent on two parameters: the inner angle of intersection of the channels, ˚, and outer angle of curvature, . Microstructure after a certain number of passes is strongly dependent on the rotation scheme [5–7]. The most widely used rotation schemes are: route A, where the billet is not rotated between consecutive passes; route BA , where the billet is rotated 90◦ in alternate directions between consecutive passes; route BC , where the bar is rotated 90◦ in the same direction between consecutive passes; and route C, where the bar is rotated 180◦ between consecutive passes [8]. Microstructure and
grain size are changed during ECAP as a result of large plastic deformation that takes place in a narrow region at the intersection of two channels [9]. The so-obtained microstructure is quite complex and depends on the alloy and on the extrusion parameters (deformation route, die angle, total strain, temperature, etc.) [7]. As high strains are known to introduce large misorientations within subdivided grains, it is expected that multipass ECAP leads to grain refinement [10]. ECAP process has many advantages in terms of practical application in comparison with other severe plastic deformation process that can produce only nanocrystalline surface layers. However, the deformation and damage mechanism in the characteristic microstructure of ECAP-processed materials has not been clarified yet. For the detailed microscopic observation, electron backscattering diffraction (EBSD) technique is a useful tool that enables investigation based on crystallographic orientation. In the present study, new routes with rotation angles of 30◦ , 45◦ and 60◦ in the same direction between consecutive passes were defined. The EBSD analysis was made on pure copper processed by ECAP with different routes in order to investigate the mechanism of deformation and damage in the microstructure with nanoscopic and submicron structures and the most effective route to decrease the grain size was determined. Mechanical properties of the annealed sample and twenty pass ECAP processed samples with different routes were investigated by tensile testing. 2. Experimental procedure
∗ Corresponding author. Tel.: +98 311 3915726; fax: +98 311 3915726. E-mail address: [email protected] (M. Reza Toroghinejad). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.075
Processing was performed with a vertical hydraulic press by using a special ECAP die, which consisted of two channels identical
F. Salimyanfard et al. / Materials Science and Engineering A 528 (2011) 5348–5355
5349
Fig. 1. The geometry of ECAP is illustrated. The term ˚ is the intersecting angle of the two channels and is the outer curvature of the two channels.
in cross section. The channels intersected at an angle of 120◦ (Fig. 1). The initial samples of the oxygen-free high conductivity copper were taken as bars of 20 mm in diameter and 80 mm in length. Pressing was performed at room temperature. Prior to deformation by ECAP, the as-received materials were annealed for 2 h at 450 ◦ C in order to obtain a strain free microstructure with equiaxed grains. The maximum number of passes (N = 20) by different routes A, B30 , B45 , B60 and BC corresponded to a true deformation ε ∼ 12.6. The distribution of the grain diameters and the grain boundary misorientations were determined using EBSD technique. The EBSD scans were in areas of 50 m × 50 m with a 0.2 m step size. The scanning parameters were set in a manner such that a high angle grain boundary (HAGB) was defined when the misorientation between adjacent measurement points was higher than 15◦ . For tensile testing, the annealed sample and twenty passes ECAP samples with different routes A, B30 , B45 , B60 and BC were cut in the longitudinal direction into 2 pieces, then they were machined to 20 mm guage length and 4 mm diameter according to ASTM E 8 M standard, to make symmetrical samples, and then the aver-
Fig. 3. The EBSD pattern obtained from the longitudinal direction of the Cu ECAP samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC , (f) representation of the color code used to identify the crystallographic orientations on a standard stereographic projection (red: [0 0 1]; blue: [1 1 1]; green: [1 0 1]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 2. Optical microstructure of initial copper annealed at 450 ◦ C for 2 h.
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Fig. 4. Grain diameter distributions (from EBSD) for (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC .
age values of results were determined. All tests were carried out at room temperature using a HOUNSFIELD-H50KS machine. The initial strain rate was 5 mm/min. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the optical micrograph from the longitudinal section of the initial pure copper. The initial grain size measured by the linear intercept method (including annealing twin boundaries) was 50 m.
3.2. EBSD results ECAP specimens were examined by EBSD in the longitudinal direction. The EBSD/orientation imaging microscopy (OIM) patterns of ECAP processed Cu samples after 20 passes using five different routes A, B30 , B45 , B60 and BC are shown in Fig. 3. The evolution of microstructure in ECAP is dependent on the deformation path. Different processing routes lead to different final microstructures. In this work, five schemes were used: A (no rotation), B30 (30◦ rotation between passes), B45 (45◦ rotation between passes), B60 (60◦ rotation between passes), and BC (90◦ rotation between passes). As can be seen, routes A, B30 , B45 and BC result in elon-
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Fig. 5. Boundary maps obtained from the longitudinal direction of the Cu ECAP samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC , red lines mark boundaries with misorientation > 15◦ (HAGB), while black lines mark boundaries with misorientation between 2◦ and 15◦ (LAGB). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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gated grains while the distribution is fairly equiaxed in the sample processed via route B60 . The OIM images of the grains parallel to the pressing direction for each processing routes after twenty passes is shown in Fig. 3(a)–(e). The texture ranged from (1 0 0) to (1 0 1) and (1 1 1) in routes A, B30 , B45 and BC . This can be clearly seen as red, green and blue grains in the inverse pole figure (IPF) (Fig. 3(a)–(c) and (e)), but in route B60 the (1 0 1) is dominant texture that can be seen as green grains in IPF (Fig. 3d). The microstructures shown in Fig. 3 provide evidence for grain refinement and consist of both elongated and equiaxed grains. These small equiaxed grains may have resulted from continuous recrystallization. Many works have reported recrystallized grains in copper [11] and other FCC metals [12,13] after large deformations at room or low temperatures. Fig. 4(a)–(e) shows the corresponding histogram for grain size of the twenty passes ECAP processed samples with these five different routes. Fig. 4(a) shows the histogram variation of grain size for twenty passes ECAP with route A. It is found that only 3.37% of grains have the diameter of less than 100 nm, and the maximum diameter of grains in this route is 5 m that has the maximum area fraction too. In route B30 (Fig. 4(b)), it is calculated that 5.26% of grains are nanograins. The maximum area fraction in this route belongs to the diameter of 3 m that is the maximum size of grains. Fig. 4(c) shows the histogram for the grain size of twenty passes ECAP sample with route B45 . This figure indicates that 7.18% of grains have the diameter of nanometer and the maximum grain size is 1.4 m. The maximum diameter of grains in route B60 (Fig. 4(d)) is about 2 m. It is calculated that in this route 15.95% of grains are below 100 nm. Finally, Fig. 4(e) shows the result of grain size for route BC , which gives the maximum diameter of 4 m. The nano-grains in this route are in a minority (0.6%). These results are summarized in Table 1 and show that the samples processed by twenty passes exhibit microstructural inhomogeneity, with differences being observed in the features of grains lying in close vicinity. This inhomogeneity in deformation may be related to misorientation within a grain. The grains with heavy dislocation density break down to finer grains while the ones that are relatively low in dislocation density remain less affected [14]. This could be the reason why the final ECAP processed microstructure shows an inhomogeneous distribution of grain size. It is calculated from Figs. 3 and 4 that in 120◦ die, the effectiveness of grain refinement is route B45 > B60 > B30 > BC > A. Fig. 5(a)–(e) presents the corresponding grain boundary maps in which the high angle grain boundaries (HAGBs) with a misorientation angle above 15◦ are shown as red lines, and boundaries between 2◦ and 15◦ are shown as black lines. Fig. 6(a)–(e) shows the histograms for the misorientation angle of the five different routes. After twenty passes ECAP with route A (Figs. 5 and 6(a)) the microstructure mainly consists of HAGBs (70%). Like route A, in twenty passes ECAP sample with route B30 (Figs. 5 and 6(b)), the HAGBs are dominant (70%). The area fraction of HAGBs in route B45 (Figs. 5 and 6(c)) is 76%. Figs. 5(d) and 6(d) show that in route B60 the area fraction of HAGBs is 77%. It is clear from Figs. 5 and 6(a–d) that after twenty passes ECAP with routes A, B30 , B45 and B60 , the low angle grain boundaries (LAGBs) are in a minority. The crystallites are in fact surrounded in their majority by HAGBs, but in route BC (Figs. 5 and 6(e)), there is a balanced distribution of LAGBs and HAGBs. The area fraction of HAGBs in this route is 54%. These results are also shown in Table 1. Grain refinement during ECAP may be attributed to the deformation banding that inevitably occurs during the process and has two possible origins. According to the first concept [15], deformation bands originate due to the ambiguity associated with the selection of the operative slip systems. In many cases, the imposed strain can be accommodated by more than one set of slip systems and the different sets of slip systems lead to different lattice rotations. Another origin is associated to the inhomogeneous straining, i.e.
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Fig. 6. The misorientation angle distribution of ECAP Cu samples after 20 passes using five different routes (a) route A, (b) route B30 , (c) route B45 , (d) route B60 and (e) route BC .
different regions of a grain may experience different strains if the work done within the bands is less than that required for homogeneous deformation and if the bands can be arranged so that the net strain matches the overall deformation. These deformation bands are separated by geometrically necessary boundaries (GNBs). In
addition, boundaries are also formed by statistical trapping of dislocations, forming the so-called incidental dislocation boundaries (IDBs). In general, the misorientation of both types of boundaries increases with strain [16], the rate being larger for GNBs, i.e., GNBs accumulate dislocations at a higher rate than IDBs. Furthermore,
Table 1 The EBSD results for statistical characterization of the ECAP deformed matrix with five different routes; A, B30 , B45 , B60 and BC . Route
Maximum grain diameter (m)
Area fraction of grains with the size of less than 100 nm %
HAGB (>15◦ ) %
LAGB (2◦ –15◦ ) %
A B30 B45 B60 BC
5 3 1.4 2 4
3.4 5.3 7.2 15.9 0.6
70.3 70.09 76.45 76.7 53.9
28.3 11.32 22.45 21.7 44.8
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the spacing of both types of boundaries decreases with increasing deformation, indicating that grain subdivision continues to refine at high strain rate. By increasing the misorientation, this process leads to the formation of new high-angle grain boundaries, the percentage of which increases at high strains. Grain fragmentation depends on grain orientation. Because of the symmetry of special orientations, one slip system may cause an opposite lattice rotation to another. Grain fragmentation may be also different in the core and mantle of grains due to the geometric requirement for strain accommodation near grain boundaries. Provided grain boundary motion is slow, i.e., if the time a grain boundary moves across a grain distance is about the time a new boundary develops by the process described above, then a steady state results [17]. This mechanism of grain refinement is known as continuous recrystallization which is, as mentioned before, the mechanism of grain refinement in ECAP. 3.3. Mechanical properties Tensile tests were conducted on the annealed sample and twenty passes ECAP processed samples with different routes. As can be seen in Fig. 7, the initial sample with coarse grain (∼50 m) shows obvious strain hardening, lower tensile strength (237 MPa), and larger ductility (37%). In comparison, the ECAP-processed samples exhibit little strain hardening. Strain hardening is commonly defined as the increase of flow stress with increasing plastic strain. The uniform elongation before necking represents the ability of the steady plastic deformation. Work hardening in the process of SPD causes a large decrease in ductility of the ECAP samples, as can be seen in Fig. 7. The samples necked at strains of 3–5% and the distance between yield point and tensile strength is very small. This has been confirmed not only in UFG Cu [11], but also in many other UFG metals, such as Ti and Ni [18,19]. As expected, there was a significant increase in the strength of the ECAP samples over the initial sample. Fig. 8 shows the histogram of ultimate tensile strength for the annealed sample and twenty passes ECAP processed samples with five different routes. It seems that the effective strain path in tensile strength is A > B30 > BC > B60 > B45 > annealed sample. The histogram of elongation to failure for the annealed sample and ECAP processed samples with these five different routes is shown in Fig. 9. It is obvious from this figure that the maximum elongation belongs to the annealed sample. The effectiveness of the five different routes in elongation is BC > A > B60 > B30 > B45 .
Fig. 7. Plots of tensile engineering stress–strain curves for as-annealed and 20 pass ECAP-processed Cu samples with different routes.
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Fig. 8. The histogram of ultimate tensile strength vs processed conditions for the annealed sample and ECAP processed samples with five different routes.
For coarse-grained metals, dislocation movement and twinning are the primary deformation mechanisms. Ultrafine grains with high angle grain boundaries impede the motion of dislocations and consequently enhance strength [20], but in this work we have seen that the effectiveness of routes in HAGBs generation is not in agreement with their effectiveness in tensile strength. This may result from other factors that can affect strength of material during processing with ECAP. The grain boundaries generated by SPD are usually in a nonequilibrium state, with many dislocations that are not geometrically necessary to form the grain boundary [10,21]. These dislocations, as well as dislocations piled up near the grain boundaries, could move to facilitate grain boundary sliding [22] and grain rotation, and therefore increase the ductility. As a result, the effectiveness of different routes in LAGBs generation is in agreement with their effectiveness in ductility. 3.4. Fracture Fig. 10(a–f) shows the fracture surface of the initial and twenty passes ECAP processed samples with five different routes (tensile specimens). The as-annealed Cu is fractured with ductile features, and there are many big dimples. In Fig. 10(b–f), dimples are generated on a finer scale, and some ductile fracture features are similar to those observed in Fig. 10(a). After twenty passes ECAP with different routes, the number of dimples increases, the size of the dimples is finer and more diverse in local regions, as shown in Fig. 10(b–f). It is clearly seen that the twenty passes ECAP sample with route BC has the most ductility because the size of its dimples is larger. This result is in conformity with the histogram of Fig. 9.
Fig. 9. The histogram of elongation to failure vs processed conditions for the annealed sample and ECAP processed samples with five different routes.
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Fig. 10. SEM images of fracture surface of specimens after tensile test: (a) as-annealed and 20 passes with routes (b) A; (c) B30 ; (d) B45 ; (e) B60 and (f) BC .
The fracture morphologies also indicate that the microstructure is inhomogeneous. 4. Conclusions In this work, oxygen free high conductivity copper (OFHC) was subjected to severe plastic deformation at room temperature by ECAP. As the cross sections of specimens were circular, new routes with rotating angles of 30◦ , 45◦ and 60◦ in the same direction between passes were defined and the effect of deformation route on the microstructure and mechanical properties was investigated by EBSD and tensile test, respectively. Overall, the following conclusions can be drawn from the work. 1. The effectiveness of different deformation routes was evaluated with the 120◦ die. In terms of HAGBs generation, the efficiency is in the order of B60 ∼ B45 > B30 ∼A > BC . 2. Considering diameter of grains, the effectiveness of grain refinement in the 120◦ die is in the order of B45 > B60 > B30 > BC > A. 3. The effectiveness of producing nano-grain is in the order of B60 > B45 > B30 > A > BC . This study clearly shows that using different microstructural parameters such as grain refinement may give different results as to the effectiveness of deformation route. Therefore, care must be exercised if results from different sources are to be compared and contrasted.
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