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Intermediate annealing of pure copper during cyclic equal channel angular pressing
Materials Science and Engineering A 483–484 (2008) 430–432
Intermediate annealing of pure copper during cyclic equal channel angular pressing W.Z. Han a , S.D. Wu a,∗ , S.X. Li a , Y.D. Wang b a
Shengyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Department of Materials Science and Engineering, Northeastern University, Shenyang 110004, PR China Received 6 June 2006; received in revised form 18 October 2006; accepted 18 October 2006
Abstract The influence of proper intermediate heat treatment on mechanical properties of the ultrafine-grained copper processed by ECAP was investigated. The tensile experiment showed that the ultimate strength is elevated about 50 MPa compared with the sample without intermediate annealing. X-ray diffraction peak broadening analyses have demonstrated that the coherent domain size decreased in some ways with the intermediate annealing, which is the reason for the increase in strength of the copper specimen. © 2007 Published by Elsevier B.V. Keywords: Equal channel angular pressing; Intermediate annealing; Recovery/recrystallization; X-ray diffraction
1. Introduction Since Gleiter presented the first concept for developing nanocrystalline materials with special properties [1], nanostructured materials have attracted much attention, and relative fields have developed rapidly owing to tremendous interest in this topic of scientific and technological importance. Most of earlier studies only focused on fabricating nanostructured materials by inert gas condensation [1]. With the development of severe plastic deformation (SPD) methods, various bulk nanostructured metals and alloys have been successfully fabricated. Equal channel angular pressing (ECAP), as one of the important SPD methods, is at present the most promising technique to produce bulk nanostructured or ultrafine-grained materials for structural application [2,3]. During the ECAP process, a metal billet is pressed through a die consisting of two channels with equal cross-section and intersecting at an angle φ (generally, 90◦ or 120◦ ) [4,5]. The billet undergoes essentially severe plastic deformation but retains the same cross-sectional geometry, so that it is possible to repeat the pressings for a number of passes. By multiple pressings, a very large effective shear plastic deformation can be developed in bulk products [4,5]. ∗
Corresponding author. Tel.: +86 24 8397 8271; fax: +86 24 2397 1215. E-mail address: [email protected] (S.D. Wu).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2006.10.179
After this kind of process became a main technology for making ultrafine-grianed materials with different kinds of metals and alloys, an interesting phenomenon has been found when multiple presses were operated. For instance, a maximum hardness can be obtained for copper specimen subjected to four passes, then its hardness decreases with further pressing [6]. The average grain size would keep around 300 nm, independent of the ECAP pass [6]. Many researchers believe that the reasons for the reduction in strength and the associated slightly changes in grain size are associated with the operation of recovery and recrystallization mechanisms [6]. Chang et al. [8] have applied intermediate annealing to pure aluminum during cyclic ECAP in order to investigate the microstructure evolution. However, their annealing temperature is too high compared with the recrystallization temperature of pure aluminum to avoid the quick grain growth during annealing. It is obviously that the annealing with high temperature and long time is not beneficial to the grain refinement process. The present paper uses an intermediate annealing with a lower temperature and relatively shorter time during cyclic ECAP to improve the strength of ultrafine-grained copper. 2. Experimental procedures Samples of commercially pure copper (99.9%) with an average grain size of about 100 m were processed at room
W.Z. Han et al. / Materials Science and Engineering A 483–484 (2008) 430–432
431
temperature. The tooling parameters of the die used for ECAP are φ = 90◦ and ψ = 0◦ ; the cross-section of the work piece is 10 mm in diameter. Two types of sample were fabricated: one was pressed to eight passes using route Bc with intermediate annealing between each pressing, denoted as 8P + A; another was pressed to eight passes using route Bc without intermediate annealing, denoted as 8P. The annealing process was conducted at 180 ◦ C for 30 min. This temperature is lower than the recrystilization temperature of copper (200 ◦ C). The specimens with the gauge dimension of 2 mm × 4 mm × 16 mm were cut from the extruded billets. Tensile test at a strain rate of 1 × 10−3 s−1 was performed on an MTS 858 testing machine. The size of coherent domain and the microstrain in the samples were measured experimentally by the analysis of X-ray diffraction (XRD) patterns [9,10]. The local microstructure information was established by scanning electron microscopy (SEM). 3. Results and discussions Fig. 1 presents the true stress–strain curves for the ECAPprocessed pure copper billets with and without intermediate annealing, respectively. It is obvious that the ultimate strength for the 8P + A is higher than the 8P, while the elongation is approximately identical for the two types of specimens. The tensile experiment showed that the strength is elevated about 50 MPa compared with the sample without intermediate annealing. The X-ray diffraction intensities including the 1 1 1, 2 0 0, 2 2 0, 3 1 1, 2 2 2, 4 0 0 and 3 3 1 Bragg reflections are plotted on a logarithmic scale for ultrafine-grained (UFG) Cu samples 8P + A and 8P as well as the reference course-grained (CG) Cu sample, respectively, as shown in Fig. 2. The Bragg-reflection broadenings from the samples 8P + A and 8P are significantly larger than that from the reference CG Cu sample. Simultaneously, the degree of the broadening for 8P + A sample is larger than that for 8P sample. Full width at half maximum (FWHM) of the Bragg reflections from the UFG and reference Cu samples were measured from each Bragg reflection, as demonstrated in the Fig. 3. The larger FWHM for the UFG Cu specimens suggests that a physical broadening caused by the small crystallite
Fig. 1. True stress–strain curves for the copper specimens: 8P + A and 8P.
Fig. 2. The intensity data (on a logarithmic scale of intensities) for the UFG Cu samples 8P + A and 8P as well as the reference CG Cu sample plotted against the diffraction angle 2θ.
and structural defects in these samples occurred. The broadening width difference between 8P + A and 8P reflects the microstructure changes for these specimens due to the different thermal mechanical processing methods. The average size of coherent domain and microstrain were calculated using the XRD broadening analysis method [9,10]. The coherent domain size for 8P + A and 8P are 42.1 and 45.1 nm, respectively. It indicates that the coherent domain size is slightly refined for the copper processed by ECAP with intermediate annealing. The values of microstrain are 0.145 for 8P + A and 0.125 for 8P, respectively. It is clear that a relatively large lattice distortion occurred for specimen 8P + A. The strength enhancement is due to the microstructure refinement during thermal mechanical processing. Fig. 4 shows the SEM images of as-pressed and annealed samples after eight passes, respectively. The microstructure features of the as pressed samples display ultrafine morphology with some directional elongation structures, as demonstrated in Fig. 4a. After annealing for 30 min at 180 ◦ C, the elongation microstructures disappeared and became steadied by almost equiaxed grains, as shown in Fig. 4b. These observations are con-
Fig. 3. The FWHM data for the UFG Cu sample 8P + A and 8P as well as the reference CG Cu sample with different Bragg reflections.
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W.Z. Han et al. / Materials Science and Engineering A 483–484 (2008) 430–432
large number of subgrain boundaries, which store the plastic energy. During the next pressing, the stored plastic energy can be activated as the driving force for recovery and recrystallization, which is a dragging force for continually refining the microstructures. For copper, one can anneal an as-received specimen for 30 min at 180 ◦ C, which is lower than its recrystallization temperature. The recovery and recrystallization processes will take place while the new formed grains located at the subgrain boundaries had no enough time for their apparent growth. As a result, the driving force for recovery and recrystallization decreases during the following ECAP process. Therefore, it is possible to get finer microstructures through intermediate annealing during ECAP. Our experimental results show that the strength of specimen 8P + A has been improved about 50 MPa after the thermal mechanical process for eight times. 4. Conclusions
Fig. 4. SEM images showing the microstructure features of copper specimen processed by different thermal–mechanical method: (a) by eight passes ECAP; (b) eight passes ECAP with 30 min annealing at 180 ◦ C.
We conducted intermediate annealing during cyclic equal channel angular pressing for the copper specimens. The experimental results indicate that the strength of specimen with a low temperature intermediate annealing has been improved about 50 MPa compared with the specimen without intermediate annealing. The reason for the strength improvement is the further refinement of the microstructures owing to decrease in the dragging force for grain refinement by intermediate annealing. Acknowledgements
sistent with the experimental results of Chang et al. [8]. They conducted annealing treatment on as-received aluminum with ultrafine-grained and found that the shape of grains changes from elongation to equiaxed forms after annealing. Annealing process consists of recovery and recrystallization (grain nucleation and growth). Recovery involves the polygonization occurring by rearrangement of boundary dislocations, which can produce low-angle boundaries with more stable dislocation configurations. Even though such change in subgrain boundary configurations cannot produce high-angle boundaries, it may act as an effective nucleation site for recrystallization [7,11]. It is well known that recrystallization can produce strain free grains with high-angle boundaries through nucleation and growth process. The driving force for such process is the stored plastic energy and can be activated by heat or deformation [7]. Based on the principles of recovery and recrystallization mentioned above, the intermediate annealing with low temperature and short time during cyclic ECAP can further refine the microstructures and acquire some more homogeneous microstructure. The material processed by ECAP contains a
This work was financially supported by the National Natural Sciences Foundation of China (NSFC) under grant nos. 50371090, 50571102 and 50471082. References [1] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223–330. [2] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [3] R.Z. Valiev, Nat. Mater. 3 (2004) 511–516. [4] V.M. Segal, Mater. Sci. Eng. A197 (1995) 157–164. [5] V.M. Segal, Mater. Sci. Eng. A386 (2004) 269–276. [6] F. Dalla Torre, R. Lapovok, J. Sandlin, P.E. Thomson, C.H.J. Davies, E.V. Pereloma, Acta Mater. 52 (2004) 4819–4832. [7] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Galliard Ltd., Great Yarmouth, Great Britain, 1995. [8] J.Y. Chang, A.D. Shan, J. Mater. Sci. 38 (2003) 2613–2617. [9] T. Ungar, A. Borbely, Appl. Phys. Lett. 69 (1996) 3173–3175. [10] T. Ungar, Scr. Mater. 51 (2004) 777–781. [11] G. Wang, S.D. Wu, L. Zuo, C. Esling, Z.G. Wang, G.Y. Li, Mater. Sci. Eng. A 346 (2003) 83–90.
Intermediate annealing of pure copper during cyclic equal channel angular pressing W.Z. Han a , S.D. Wu a,∗ , S.X. Li a , Y.D. Wang b a
Shengyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Department of Materials Science and Engineering, Northeastern University, Shenyang 110004, PR China Received 6 June 2006; received in revised form 18 October 2006; accepted 18 October 2006
Abstract The influence of proper intermediate heat treatment on mechanical properties of the ultrafine-grained copper processed by ECAP was investigated. The tensile experiment showed that the ultimate strength is elevated about 50 MPa compared with the sample without intermediate annealing. X-ray diffraction peak broadening analyses have demonstrated that the coherent domain size decreased in some ways with the intermediate annealing, which is the reason for the increase in strength of the copper specimen. © 2007 Published by Elsevier B.V. Keywords: Equal channel angular pressing; Intermediate annealing; Recovery/recrystallization; X-ray diffraction
1. Introduction Since Gleiter presented the first concept for developing nanocrystalline materials with special properties [1], nanostructured materials have attracted much attention, and relative fields have developed rapidly owing to tremendous interest in this topic of scientific and technological importance. Most of earlier studies only focused on fabricating nanostructured materials by inert gas condensation [1]. With the development of severe plastic deformation (SPD) methods, various bulk nanostructured metals and alloys have been successfully fabricated. Equal channel angular pressing (ECAP), as one of the important SPD methods, is at present the most promising technique to produce bulk nanostructured or ultrafine-grained materials for structural application [2,3]. During the ECAP process, a metal billet is pressed through a die consisting of two channels with equal cross-section and intersecting at an angle φ (generally, 90◦ or 120◦ ) [4,5]. The billet undergoes essentially severe plastic deformation but retains the same cross-sectional geometry, so that it is possible to repeat the pressings for a number of passes. By multiple pressings, a very large effective shear plastic deformation can be developed in bulk products [4,5]. ∗
Corresponding author. Tel.: +86 24 8397 8271; fax: +86 24 2397 1215. E-mail address: [email protected] (S.D. Wu).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2006.10.179
After this kind of process became a main technology for making ultrafine-grianed materials with different kinds of metals and alloys, an interesting phenomenon has been found when multiple presses were operated. For instance, a maximum hardness can be obtained for copper specimen subjected to four passes, then its hardness decreases with further pressing [6]. The average grain size would keep around 300 nm, independent of the ECAP pass [6]. Many researchers believe that the reasons for the reduction in strength and the associated slightly changes in grain size are associated with the operation of recovery and recrystallization mechanisms [6]. Chang et al. [8] have applied intermediate annealing to pure aluminum during cyclic ECAP in order to investigate the microstructure evolution. However, their annealing temperature is too high compared with the recrystallization temperature of pure aluminum to avoid the quick grain growth during annealing. It is obviously that the annealing with high temperature and long time is not beneficial to the grain refinement process. The present paper uses an intermediate annealing with a lower temperature and relatively shorter time during cyclic ECAP to improve the strength of ultrafine-grained copper. 2. Experimental procedures Samples of commercially pure copper (99.9%) with an average grain size of about 100 m were processed at room
W.Z. Han et al. / Materials Science and Engineering A 483–484 (2008) 430–432
431
temperature. The tooling parameters of the die used for ECAP are φ = 90◦ and ψ = 0◦ ; the cross-section of the work piece is 10 mm in diameter. Two types of sample were fabricated: one was pressed to eight passes using route Bc with intermediate annealing between each pressing, denoted as 8P + A; another was pressed to eight passes using route Bc without intermediate annealing, denoted as 8P. The annealing process was conducted at 180 ◦ C for 30 min. This temperature is lower than the recrystilization temperature of copper (200 ◦ C). The specimens with the gauge dimension of 2 mm × 4 mm × 16 mm were cut from the extruded billets. Tensile test at a strain rate of 1 × 10−3 s−1 was performed on an MTS 858 testing machine. The size of coherent domain and the microstrain in the samples were measured experimentally by the analysis of X-ray diffraction (XRD) patterns [9,10]. The local microstructure information was established by scanning electron microscopy (SEM). 3. Results and discussions Fig. 1 presents the true stress–strain curves for the ECAPprocessed pure copper billets with and without intermediate annealing, respectively. It is obvious that the ultimate strength for the 8P + A is higher than the 8P, while the elongation is approximately identical for the two types of specimens. The tensile experiment showed that the strength is elevated about 50 MPa compared with the sample without intermediate annealing. The X-ray diffraction intensities including the 1 1 1, 2 0 0, 2 2 0, 3 1 1, 2 2 2, 4 0 0 and 3 3 1 Bragg reflections are plotted on a logarithmic scale for ultrafine-grained (UFG) Cu samples 8P + A and 8P as well as the reference course-grained (CG) Cu sample, respectively, as shown in Fig. 2. The Bragg-reflection broadenings from the samples 8P + A and 8P are significantly larger than that from the reference CG Cu sample. Simultaneously, the degree of the broadening for 8P + A sample is larger than that for 8P sample. Full width at half maximum (FWHM) of the Bragg reflections from the UFG and reference Cu samples were measured from each Bragg reflection, as demonstrated in the Fig. 3. The larger FWHM for the UFG Cu specimens suggests that a physical broadening caused by the small crystallite
Fig. 1. True stress–strain curves for the copper specimens: 8P + A and 8P.
Fig. 2. The intensity data (on a logarithmic scale of intensities) for the UFG Cu samples 8P + A and 8P as well as the reference CG Cu sample plotted against the diffraction angle 2θ.
and structural defects in these samples occurred. The broadening width difference between 8P + A and 8P reflects the microstructure changes for these specimens due to the different thermal mechanical processing methods. The average size of coherent domain and microstrain were calculated using the XRD broadening analysis method [9,10]. The coherent domain size for 8P + A and 8P are 42.1 and 45.1 nm, respectively. It indicates that the coherent domain size is slightly refined for the copper processed by ECAP with intermediate annealing. The values of microstrain are 0.145 for 8P + A and 0.125 for 8P, respectively. It is clear that a relatively large lattice distortion occurred for specimen 8P + A. The strength enhancement is due to the microstructure refinement during thermal mechanical processing. Fig. 4 shows the SEM images of as-pressed and annealed samples after eight passes, respectively. The microstructure features of the as pressed samples display ultrafine morphology with some directional elongation structures, as demonstrated in Fig. 4a. After annealing for 30 min at 180 ◦ C, the elongation microstructures disappeared and became steadied by almost equiaxed grains, as shown in Fig. 4b. These observations are con-
Fig. 3. The FWHM data for the UFG Cu sample 8P + A and 8P as well as the reference CG Cu sample with different Bragg reflections.
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W.Z. Han et al. / Materials Science and Engineering A 483–484 (2008) 430–432
large number of subgrain boundaries, which store the plastic energy. During the next pressing, the stored plastic energy can be activated as the driving force for recovery and recrystallization, which is a dragging force for continually refining the microstructures. For copper, one can anneal an as-received specimen for 30 min at 180 ◦ C, which is lower than its recrystallization temperature. The recovery and recrystallization processes will take place while the new formed grains located at the subgrain boundaries had no enough time for their apparent growth. As a result, the driving force for recovery and recrystallization decreases during the following ECAP process. Therefore, it is possible to get finer microstructures through intermediate annealing during ECAP. Our experimental results show that the strength of specimen 8P + A has been improved about 50 MPa after the thermal mechanical process for eight times. 4. Conclusions
Fig. 4. SEM images showing the microstructure features of copper specimen processed by different thermal–mechanical method: (a) by eight passes ECAP; (b) eight passes ECAP with 30 min annealing at 180 ◦ C.
We conducted intermediate annealing during cyclic equal channel angular pressing for the copper specimens. The experimental results indicate that the strength of specimen with a low temperature intermediate annealing has been improved about 50 MPa compared with the specimen without intermediate annealing. The reason for the strength improvement is the further refinement of the microstructures owing to decrease in the dragging force for grain refinement by intermediate annealing. Acknowledgements
sistent with the experimental results of Chang et al. [8]. They conducted annealing treatment on as-received aluminum with ultrafine-grained and found that the shape of grains changes from elongation to equiaxed forms after annealing. Annealing process consists of recovery and recrystallization (grain nucleation and growth). Recovery involves the polygonization occurring by rearrangement of boundary dislocations, which can produce low-angle boundaries with more stable dislocation configurations. Even though such change in subgrain boundary configurations cannot produce high-angle boundaries, it may act as an effective nucleation site for recrystallization [7,11]. It is well known that recrystallization can produce strain free grains with high-angle boundaries through nucleation and growth process. The driving force for such process is the stored plastic energy and can be activated by heat or deformation [7]. Based on the principles of recovery and recrystallization mentioned above, the intermediate annealing with low temperature and short time during cyclic ECAP can further refine the microstructures and acquire some more homogeneous microstructure. The material processed by ECAP contains a
This work was financially supported by the National Natural Sciences Foundation of China (NSFC) under grant nos. 50371090, 50571102 and 50471082. References [1] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223–330. [2] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [3] R.Z. Valiev, Nat. Mater. 3 (2004) 511–516. [4] V.M. Segal, Mater. Sci. Eng. A197 (1995) 157–164. [5] V.M. Segal, Mater. Sci. Eng. A386 (2004) 269–276. [6] F. Dalla Torre, R. Lapovok, J. Sandlin, P.E. Thomson, C.H.J. Davies, E.V. Pereloma, Acta Mater. 52 (2004) 4819–4832. [7] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Galliard Ltd., Great Yarmouth, Great Britain, 1995. [8] J.Y. Chang, A.D. Shan, J. Mater. Sci. 38 (2003) 2613–2617. [9] T. Ungar, A. Borbely, Appl. Phys. Lett. 69 (1996) 3173–3175. [10] T. Ungar, Scr. Mater. 51 (2004) 777–781. [11] G. Wang, S.D. Wu, L. Zuo, C. Esling, Z.G. Wang, G.Y. Li, Mater. Sci. Eng. A 346 (2003) 83–90.