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Substructural and phase transformations during plastic deformations of materials obtained by intensive deformation
Materials Science and Engineering A 410–411 (2005) 341–344
Substructural and phase transformations during plastic deformations of materials obtained by intensive deformation N.A. Koneva a,∗ , E.V. Kozlov a , Yu.F. Ivanov a , N.A. Popova a , A.N Zhdanov b a
b
Tomsk State University of Architecture and Buildings, Solyanaya Sq.2, Tomsk 634003, Russia Altaiskii State Technical University, 46 Lenin Prospect, Barnaul, 656099 Altaysky Region, Russia Received in revised form 20 June 2005
Abstract The paper presents the results of electron microscopy investigations of the defect structure evolution and the phase transformation of ultra fine-grained (UFG) Cu and Cu Al O alloy. The UFG Cu was prepared using the equal-channel angular (ECA) pressing method. The Cu Al O alloy was synthesized by mechanical milling and subsequent high temperature pressing. The initial grain size of the materials was 200 nm. The grain and dislocation structures were investigated qualitatively and quantitavely. In the Cu alloy, deformation twinning was revealed to occur in coarse grains together with slip deformation. The evolution of grain and dislocation micro structure in the presence or absence of solid solution and of dispersion hardening is analyzed, and differences are assessed. © 2005 Elsevier B.V. All rights reserved. Keywords: UFG Cu and Cu Al O alloy; Grains; Dislocations; Microtwins; Particles; Deformation
1. Introduction
2. Material and experimental procedure
The paper summarized the results obtained by the authors in investigation of structural transformations taking place under tensile or compression deformation at a room temperature in ultra fine-grained (UFG) metals and alloys. Samples of UFG Cu and UFG Cu alloy were investigated by transmission electron microscopy (TEM). The changes (1) in the grain structure (grain size, boundary density, boundary types), (2) in the dislocation structure (substructure transformations, scalar dislocation density) and (3) in the phase state (solid solution decomposition and deformation dissolving of fine particles) were studied quantitatively. Main attention is paid to a comparison of the substructure and phase transformations occurring upon plastic deformation of UFG Cu and its alloy. The average grain sizes of the chosen materials were almost equal to 200 nm. The changes in the grain and dislocation structures take place both in the pure Cu polycrystals and Cu Al O alloy. At the same time the deformation phase transformation occurs only in the Cu Al O alloy.
The UFG Cu [1] and UFG Cu Al O [2] alloy were taken as the investigation materials. The UFG Cu was produced by ECA pressing method. The Cu Al O was prepared from the powders of Cu and Al and special alloying additions containing oxygen by mechanical alloying and subsequent high-temperature (1073 K) pressing. The Cu Al O alloy consists of the aluminum solid solution in Cu grains, the two-phase Cu grains containing particles of Cu9 Al4 , CuAl2 , Cu2 O, with ␣-Al2 O3 particles located on the boundaries of initial and secondary grains. The aluminum concentration amounted to 1% of the Cu mass. In the initial state, the average grain size both in Cu and in Cu Al O alloy was equal to 200 nm. The material structural and phase composition in the initial and deformed states was studied by TEM using an EM-125 K microscope with goniometer and the microscope EM-125 that had higher resolution. The material structure was investigated by the thin foils method. The following characteristics were measured: the grain size d, the scalar dislocation density ρ, the sizes, density and volume fraction of the strengthening phase particles inside the grains and on their boundaries. While determining the phase volume fraction one used well-known methods based on measuring the linear sizes of the particles (including their forms) and the distances between them. The deformation
∗
Corresponding author. Tel.: +7 3822 654263; fax: +7 3822 654263. E-mail address: [email protected] (N.A. Koneva).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.114
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N.A. Koneva et al. / Materials Science and Engineering A 410–411 (2005) 341–344
change in material and phase state were investigated on samples 4 mm × 4 mm × 6 mm in size deformed by compression at a room temperature (Troom ) to ε = 3, 9, 29, 53 and 83% for Cu and ε = 3, 11, 23, 47% for Cu Al O alloy using INSTRON apparatus. The foils were prepared from plates cut from the central part of the deformed samples perpendicularly to the compression axis. 3. Results and their discussion At first, it should be mentioned that the strength of Cu Al O alloy is higher than that of pure Cu. The flow stress reaches 500 MPa in Cu and 800 MPa in Cu Al O alloy. The type of curves σ = f(ε) is similar for both materials [1,2]. Primarily let us consider the transformations in the grain and dislocation systems. It should be pointed out that initially the UFG Cu polycrystal contains three grain types [3]. In increasing average size they are: (1) dislocation-free grains; (2) grains with random and network dislocation structures; (3) grains with fragmentary substructure. In the UFG Cu Al O polycrystal, one sees two main grain types: (1) the copper solid solution grains; (2) the two-phase grains containing the copper solid solution and precipitates of Cu9 Al4 (CuAl2 ) [2]. The solid solution grains, in turn, are divided into two types: (1) dislocation-free grains and (2) grains with random dislocation arrangements. In other words, there are also three grain types in Cu Al O alloy, but they have somewhat different natures from those in relatively pure Cu. Under plastic deformation, the grain and dislocation structures of submicrometer grains change. The grain and dislocation structures observed by TEM are given in Fig. 1. We can see the qualitative changes in the grain and dislocation structures both in pure Cu and Cu Al O alloy. Under plastic deformation of UFG Cu, grain-boundary migration and grain growth take place. Fig. 1a,b shows electron microscopy images of UFG Cu at different degrees of deformation. One can see well that during deformation, the grain sizes increase. The dislocation density changes slightly. Different types of grain boundaries occur at different deformation degrees. A high density of defects in the structure of grain boundaries causes a decrease in activation energy of the deformation processes increasing grain boundaries. This value is equal to 0.70/0.72 eV for dynamic deformation [4], creep [5] and diffusion [6]. Therefore, at room temperature and for an average grain size of UFG Cu d = 210 nm, the sliding of grain boundaries and their migration take place together with the dislocation slide inside the grains. The results of these observations were published by the authors in [3,4]. The important role of diffusion processes in the grain boundary area of UFG Cu at room-temperature deformation was shown also experimentally in [6,7] and theoretically in [8,9]. The average grain size in pure Cu oscillates and increases during deformation (Fig. 2a, curve 1). At the same time the Cu Al O solid solution grain size decreases at the beginning of deformation and then the size remains constant (Fig. 2a, curve 2). Obviously, in pure Cu the grain boundary mobility is high while in the Cu alloy the grain boundaries are fixed due to formation on them of particles of secondary phase (Cu9 Al4 and CuAl2 ).
Fig. 1. Images of the UFG Cu and Cu Al O alloy grain structures: (a) Cu, ε = 29%; (b) Cu, ε = 83%; (c) Cu Al O alloy, ε = 0; (d) Cu Al O alloy, ε = 47%.
N.A. Koneva et al. / Materials Science and Engineering A 410–411 (2005) 341–344
Fig. 2. Quantative behaviour of the defect structure evolution: (a) the average grain size (1-Cu, 2-Cu Al O alloy) and the dislocation, free grain size (3-Cu, 4-Cu Al O); (b) the average scalar dislocation density (5-Cu, 6-Cu Al O alloy) and the scalar dislocation density inside the grains with dislocations (7Cu, 8-Cu Al O alloy); (c) the grain fraction with the with Cu Al solid solution Pv (9), the grain fraction with twins δ (10) and an average grain size with twins D (11).
It is interesting to compare the behavior of the average grain size with the size of dislocation-free grains [10] (Fig. 2a, curves 1–4). In pure Cu the dislocation-free grains are basically conserved. In the grains with the initial network dislocation substructure the fragmentary substructure is formed. The grains with the fragmentary substructure partly transform into new grains. On the grain boundaries the intensive processes connected with the boundary transformations take place. On the contrary in the UFG Cu Al O alloy the grain size decreases with deformation and average grain size with dislocations also decreases. Apart of the dislocation-free grains is filled with dislocations. Nevertheless the average dislocation-free grains size slightly changes. In the deformation interval their size lies near 100 nm both in pure Cu and in Cu Al O alloy (Fig. 2a, curves 3 and 4). The dislocations density increases and reaches a value of ρ ∼ 7 × 10−10 cm−2 at ε ∼ 47% (Fig. 2b, curve 6). When the deformation degree increases, the size of dislocation grains
343
decreases. In all deformation intervals, inside the Cu Al micrograins the network dislocation substructure is conserved. Formation and conservation of the network dislocation substructure at high dislocation densities can be explained by solid solution hardening. In coarse grains of the UFG Cu Al O alloy, alone with dislocation motion, microtwinning develops. This process is mainly developed in coarse grains of UFG material, the sizes of which exceed 300 nm (Fig. 2, curves 10 and 11). Microtwinning in UFG Cu was initially observed in the work reported in Ref. [11]. In pure UFG Cu, the scalar dislocation density ρ decreases at first and then increases under deformation (Fig. 2b, curve 5). The greatest part of dislocations in the pure UFG Cu is located in the grain boundaries. One of reasons of a considerable contribution of grain boundary area to deformation of UFG Cu at Troom is the high dislocation density in grain boundaries. Measurement of shear in grain boundaries during deformation of UFG Cu at Troom was carried out by the authors on replicas using TEM [3]. The direct measurement of shear confirmed this effect. Inside grains (Fig. 2a, curve 7) the dislocation density is less than the average dislocation density 2–4 times. In Cu Al alloy the grain boundaries are blocked by precipitations and the differences ρ and ρ inside the grains are small (Fig. 2b, curves 6 and 8). It is interesting that inside of the Cu Al O grains with solid solution Cu Al the dislocations are accumulated more than in pure Cu (compare the curves 7 and 8 in Fig. 2b) In the Cu Al alloy the phase composition changes. The intensive decomposition of the Cu Al solid solution is observed with the deformation increase on the grain boundaries and near them. The grain fraction where one sees only the solid solution sharply decreases (2c, curve 9). Decomposition has a heterogeneous character and is realized first of all on the grain boundaries. Thus, the changes of the grain and dislocation substructures in the UFG pure Cu and the alloy on its basis have different natures. 4. Conclusion In this work the comparative investigations of structural transformations during plastic deformation of Cu and Cu Al O alloy with the initial the average grain size of 200 nm were carried out. In the case of pure Cu the interactions of dislocations with one another and with the grain boundaries were the hardening mechanisms. In the case of the Cu alloy the solid solution hardening and the dispersion hardening were added. The quantitative electron microscopy investigations showed that the dislocation-free grain sizes do not change in both materials. At the same time in Cu the average grain size increases but in the Cu Al O alloy the average grain size decreases. In pure Cu the dislocations are mainly accumulated on the grain boundaries but inside the grains their density is less in several times then in the grain boundaries. In Cu Al O alloy, a little more dislocations are accumulated on the grain boundaries in comparison with the grain body. At the same time, the more dislocations are accumulated inside grains in alloy than in pure Cu because of solid solution hardening. In Cu Al O alloy the plastic deformation is realized not only by slipping but
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also by twinning. Simultaneously in this alloy the solid solution decomposition takes place near the grain boundaries. References [1] N.A. Koneva, N.A. Popova, A.N. Zhdanov, L.N. Ignatenko, E.V. Kozlov, Ultrafine Grained Materials III, Publication of TMS, Charlotte, 2004, pp. 397–400. [2] E.V. Kozlov, Yu.F. Ivanov, A.N. Zhdanov, N.A. Koneva, Ultrafine Grained Materials III, Publication of TMS, Charlotte, 2004, pp. 469–474. [3] E.V. Kozlov, A.N. Zhdanov, L.N. Ignatenko, N.A. Popova, Yu.F. Ivanov, N.A. Koneva, Ultrafine Grained Materials II, Publication of TMS, Seattle, 2002, pp. 419–428.
[4] R.Z. Valiev, E.V. Kozlov, Yu.F. Ivanov, et al., Acta Metal. Mater. 42 (1994) 2467. [5] B. Cai, Q.P. Kong, K. Lu, Mater. Sci. Eng. A 286 (2000) 188. [6] H. Conrad, Mater. Sci. Eng. A 341 (2003) 216. [7] Y.J. Li, X.H. Zeng, W. Blum, Acta Mater. 52 (2004) 5009. [8] H.S. Kim, Yu. Estrin, M.B. Bush, Acta. Metal. Mater. 48 (2000) 493. [9] H.S. Kim, Yu. Estrin, M.B. Bush, Mater. Sci. Eng. A 136 (2001) 195. [10] N.A. Koneva, A.N. Zhdanov, N.A. Popova, L.N. Ignatenko, E.E. Pekarskaya, E.V. Kozlov, in: M.J. Zehetbauer, R.Z. Valiev (Eds.), Nanomaterials by Severe Plastic Deformation, NANOSPD2, WILEY-VCH Verlag GMBH& Co. KGaA, Vienna, 2004, p. 357. [11] X.Z. Liao, Y.H. Zhao, S.G. Srinivasan, Y.T. Zhu, R.Z. Valiev, D.V. Gunderov, Appl. Phys. Lett. 84 (2004) 592.
Substructural and phase transformations during plastic deformations of materials obtained by intensive deformation N.A. Koneva a,∗ , E.V. Kozlov a , Yu.F. Ivanov a , N.A. Popova a , A.N Zhdanov b a
b
Tomsk State University of Architecture and Buildings, Solyanaya Sq.2, Tomsk 634003, Russia Altaiskii State Technical University, 46 Lenin Prospect, Barnaul, 656099 Altaysky Region, Russia Received in revised form 20 June 2005
Abstract The paper presents the results of electron microscopy investigations of the defect structure evolution and the phase transformation of ultra fine-grained (UFG) Cu and Cu Al O alloy. The UFG Cu was prepared using the equal-channel angular (ECA) pressing method. The Cu Al O alloy was synthesized by mechanical milling and subsequent high temperature pressing. The initial grain size of the materials was 200 nm. The grain and dislocation structures were investigated qualitatively and quantitavely. In the Cu alloy, deformation twinning was revealed to occur in coarse grains together with slip deformation. The evolution of grain and dislocation micro structure in the presence or absence of solid solution and of dispersion hardening is analyzed, and differences are assessed. © 2005 Elsevier B.V. All rights reserved. Keywords: UFG Cu and Cu Al O alloy; Grains; Dislocations; Microtwins; Particles; Deformation
1. Introduction
2. Material and experimental procedure
The paper summarized the results obtained by the authors in investigation of structural transformations taking place under tensile or compression deformation at a room temperature in ultra fine-grained (UFG) metals and alloys. Samples of UFG Cu and UFG Cu alloy were investigated by transmission electron microscopy (TEM). The changes (1) in the grain structure (grain size, boundary density, boundary types), (2) in the dislocation structure (substructure transformations, scalar dislocation density) and (3) in the phase state (solid solution decomposition and deformation dissolving of fine particles) were studied quantitatively. Main attention is paid to a comparison of the substructure and phase transformations occurring upon plastic deformation of UFG Cu and its alloy. The average grain sizes of the chosen materials were almost equal to 200 nm. The changes in the grain and dislocation structures take place both in the pure Cu polycrystals and Cu Al O alloy. At the same time the deformation phase transformation occurs only in the Cu Al O alloy.
The UFG Cu [1] and UFG Cu Al O [2] alloy were taken as the investigation materials. The UFG Cu was produced by ECA pressing method. The Cu Al O was prepared from the powders of Cu and Al and special alloying additions containing oxygen by mechanical alloying and subsequent high-temperature (1073 K) pressing. The Cu Al O alloy consists of the aluminum solid solution in Cu grains, the two-phase Cu grains containing particles of Cu9 Al4 , CuAl2 , Cu2 O, with ␣-Al2 O3 particles located on the boundaries of initial and secondary grains. The aluminum concentration amounted to 1% of the Cu mass. In the initial state, the average grain size both in Cu and in Cu Al O alloy was equal to 200 nm. The material structural and phase composition in the initial and deformed states was studied by TEM using an EM-125 K microscope with goniometer and the microscope EM-125 that had higher resolution. The material structure was investigated by the thin foils method. The following characteristics were measured: the grain size d, the scalar dislocation density ρ, the sizes, density and volume fraction of the strengthening phase particles inside the grains and on their boundaries. While determining the phase volume fraction one used well-known methods based on measuring the linear sizes of the particles (including their forms) and the distances between them. The deformation
∗
Corresponding author. Tel.: +7 3822 654263; fax: +7 3822 654263. E-mail address: [email protected] (N.A. Koneva).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.114
342
N.A. Koneva et al. / Materials Science and Engineering A 410–411 (2005) 341–344
change in material and phase state were investigated on samples 4 mm × 4 mm × 6 mm in size deformed by compression at a room temperature (Troom ) to ε = 3, 9, 29, 53 and 83% for Cu and ε = 3, 11, 23, 47% for Cu Al O alloy using INSTRON apparatus. The foils were prepared from plates cut from the central part of the deformed samples perpendicularly to the compression axis. 3. Results and their discussion At first, it should be mentioned that the strength of Cu Al O alloy is higher than that of pure Cu. The flow stress reaches 500 MPa in Cu and 800 MPa in Cu Al O alloy. The type of curves σ = f(ε) is similar for both materials [1,2]. Primarily let us consider the transformations in the grain and dislocation systems. It should be pointed out that initially the UFG Cu polycrystal contains three grain types [3]. In increasing average size they are: (1) dislocation-free grains; (2) grains with random and network dislocation structures; (3) grains with fragmentary substructure. In the UFG Cu Al O polycrystal, one sees two main grain types: (1) the copper solid solution grains; (2) the two-phase grains containing the copper solid solution and precipitates of Cu9 Al4 (CuAl2 ) [2]. The solid solution grains, in turn, are divided into two types: (1) dislocation-free grains and (2) grains with random dislocation arrangements. In other words, there are also three grain types in Cu Al O alloy, but they have somewhat different natures from those in relatively pure Cu. Under plastic deformation, the grain and dislocation structures of submicrometer grains change. The grain and dislocation structures observed by TEM are given in Fig. 1. We can see the qualitative changes in the grain and dislocation structures both in pure Cu and Cu Al O alloy. Under plastic deformation of UFG Cu, grain-boundary migration and grain growth take place. Fig. 1a,b shows electron microscopy images of UFG Cu at different degrees of deformation. One can see well that during deformation, the grain sizes increase. The dislocation density changes slightly. Different types of grain boundaries occur at different deformation degrees. A high density of defects in the structure of grain boundaries causes a decrease in activation energy of the deformation processes increasing grain boundaries. This value is equal to 0.70/0.72 eV for dynamic deformation [4], creep [5] and diffusion [6]. Therefore, at room temperature and for an average grain size of UFG Cu d = 210 nm, the sliding of grain boundaries and their migration take place together with the dislocation slide inside the grains. The results of these observations were published by the authors in [3,4]. The important role of diffusion processes in the grain boundary area of UFG Cu at room-temperature deformation was shown also experimentally in [6,7] and theoretically in [8,9]. The average grain size in pure Cu oscillates and increases during deformation (Fig. 2a, curve 1). At the same time the Cu Al O solid solution grain size decreases at the beginning of deformation and then the size remains constant (Fig. 2a, curve 2). Obviously, in pure Cu the grain boundary mobility is high while in the Cu alloy the grain boundaries are fixed due to formation on them of particles of secondary phase (Cu9 Al4 and CuAl2 ).
Fig. 1. Images of the UFG Cu and Cu Al O alloy grain structures: (a) Cu, ε = 29%; (b) Cu, ε = 83%; (c) Cu Al O alloy, ε = 0; (d) Cu Al O alloy, ε = 47%.
N.A. Koneva et al. / Materials Science and Engineering A 410–411 (2005) 341–344
Fig. 2. Quantative behaviour of the defect structure evolution: (a) the average grain size (1-Cu, 2-Cu Al O alloy) and the dislocation, free grain size (3-Cu, 4-Cu Al O); (b) the average scalar dislocation density (5-Cu, 6-Cu Al O alloy) and the scalar dislocation density inside the grains with dislocations (7Cu, 8-Cu Al O alloy); (c) the grain fraction with the with Cu Al solid solution Pv (9), the grain fraction with twins δ (10) and an average grain size with twins D (11).
It is interesting to compare the behavior of the average grain size with the size of dislocation-free grains [10] (Fig. 2a, curves 1–4). In pure Cu the dislocation-free grains are basically conserved. In the grains with the initial network dislocation substructure the fragmentary substructure is formed. The grains with the fragmentary substructure partly transform into new grains. On the grain boundaries the intensive processes connected with the boundary transformations take place. On the contrary in the UFG Cu Al O alloy the grain size decreases with deformation and average grain size with dislocations also decreases. Apart of the dislocation-free grains is filled with dislocations. Nevertheless the average dislocation-free grains size slightly changes. In the deformation interval their size lies near 100 nm both in pure Cu and in Cu Al O alloy (Fig. 2a, curves 3 and 4). The dislocations density increases and reaches a value of ρ ∼ 7 × 10−10 cm−2 at ε ∼ 47% (Fig. 2b, curve 6). When the deformation degree increases, the size of dislocation grains
343
decreases. In all deformation intervals, inside the Cu Al micrograins the network dislocation substructure is conserved. Formation and conservation of the network dislocation substructure at high dislocation densities can be explained by solid solution hardening. In coarse grains of the UFG Cu Al O alloy, alone with dislocation motion, microtwinning develops. This process is mainly developed in coarse grains of UFG material, the sizes of which exceed 300 nm (Fig. 2, curves 10 and 11). Microtwinning in UFG Cu was initially observed in the work reported in Ref. [11]. In pure UFG Cu, the scalar dislocation density ρ decreases at first and then increases under deformation (Fig. 2b, curve 5). The greatest part of dislocations in the pure UFG Cu is located in the grain boundaries. One of reasons of a considerable contribution of grain boundary area to deformation of UFG Cu at Troom is the high dislocation density in grain boundaries. Measurement of shear in grain boundaries during deformation of UFG Cu at Troom was carried out by the authors on replicas using TEM [3]. The direct measurement of shear confirmed this effect. Inside grains (Fig. 2a, curve 7) the dislocation density is less than the average dislocation density 2–4 times. In Cu Al alloy the grain boundaries are blocked by precipitations and the differences ρ and ρ inside the grains are small (Fig. 2b, curves 6 and 8). It is interesting that inside of the Cu Al O grains with solid solution Cu Al the dislocations are accumulated more than in pure Cu (compare the curves 7 and 8 in Fig. 2b) In the Cu Al alloy the phase composition changes. The intensive decomposition of the Cu Al solid solution is observed with the deformation increase on the grain boundaries and near them. The grain fraction where one sees only the solid solution sharply decreases (2c, curve 9). Decomposition has a heterogeneous character and is realized first of all on the grain boundaries. Thus, the changes of the grain and dislocation substructures in the UFG pure Cu and the alloy on its basis have different natures. 4. Conclusion In this work the comparative investigations of structural transformations during plastic deformation of Cu and Cu Al O alloy with the initial the average grain size of 200 nm were carried out. In the case of pure Cu the interactions of dislocations with one another and with the grain boundaries were the hardening mechanisms. In the case of the Cu alloy the solid solution hardening and the dispersion hardening were added. The quantitative electron microscopy investigations showed that the dislocation-free grain sizes do not change in both materials. At the same time in Cu the average grain size increases but in the Cu Al O alloy the average grain size decreases. In pure Cu the dislocations are mainly accumulated on the grain boundaries but inside the grains their density is less in several times then in the grain boundaries. In Cu Al O alloy, a little more dislocations are accumulated on the grain boundaries in comparison with the grain body. At the same time, the more dislocations are accumulated inside grains in alloy than in pure Cu because of solid solution hardening. In Cu Al O alloy the plastic deformation is realized not only by slipping but
344
N.A. Koneva et al. / Materials Science and Engineering A 410–411 (2005) 341–344
also by twinning. Simultaneously in this alloy the solid solution decomposition takes place near the grain boundaries. References [1] N.A. Koneva, N.A. Popova, A.N. Zhdanov, L.N. Ignatenko, E.V. Kozlov, Ultrafine Grained Materials III, Publication of TMS, Charlotte, 2004, pp. 397–400. [2] E.V. Kozlov, Yu.F. Ivanov, A.N. Zhdanov, N.A. Koneva, Ultrafine Grained Materials III, Publication of TMS, Charlotte, 2004, pp. 469–474. [3] E.V. Kozlov, A.N. Zhdanov, L.N. Ignatenko, N.A. Popova, Yu.F. Ivanov, N.A. Koneva, Ultrafine Grained Materials II, Publication of TMS, Seattle, 2002, pp. 419–428.
[4] R.Z. Valiev, E.V. Kozlov, Yu.F. Ivanov, et al., Acta Metal. Mater. 42 (1994) 2467. [5] B. Cai, Q.P. Kong, K. Lu, Mater. Sci. Eng. A 286 (2000) 188. [6] H. Conrad, Mater. Sci. Eng. A 341 (2003) 216. [7] Y.J. Li, X.H. Zeng, W. Blum, Acta Mater. 52 (2004) 5009. [8] H.S. Kim, Yu. Estrin, M.B. Bush, Acta. Metal. Mater. 48 (2000) 493. [9] H.S. Kim, Yu. Estrin, M.B. Bush, Mater. Sci. Eng. A 136 (2001) 195. [10] N.A. Koneva, A.N. Zhdanov, N.A. Popova, L.N. Ignatenko, E.E. Pekarskaya, E.V. Kozlov, in: M.J. Zehetbauer, R.Z. Valiev (Eds.), Nanomaterials by Severe Plastic Deformation, NANOSPD2, WILEY-VCH Verlag GMBH& Co. KGaA, Vienna, 2004, p. 357. [11] X.Z. Liao, Y.H. Zhao, S.G. Srinivasan, Y.T. Zhu, R.Z. Valiev, D.V. Gunderov, Appl. Phys. Lett. 84 (2004) 592.