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Mechanical properties of a nanocrystalline Co–Cu alloy with a high-density fine nanoscale lamellar structure
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Scripta Materialia 58 (2008) 731–734 www.elsevier.com/locate/scriptamat
Mechanical properties of a nanocrystalline Co–Cu alloy with a high-density fine nanoscale lamellar structure Yoshiaki Nakamoto, Motohiro Yuasa,* Youqing Chen, Hiromu Kusuda and Mamoru Mabuchi Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 6 November 2007; revised 1 December 2007; accepted 11 December 2007 Available online 14 January 2008
The mechanical properties of a nanocrystalline Co–Cu alloy containing a high density of in-growth nanoscale lamellar structure with a narrow spacing of 3 nm were investigated. The Co–Cu alloy exhibited a high 0.2% proof stress of 1420 MPa and an ultimate tensile strength of 1875 MPa. Also, the activation volume was 3.3b3. This low value is attributed to the emission of dislocations from boundaries of the nanoscale lamellar structure due to the stress concentration. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cobalt; Nanocrystalline; Nanoscale lamellar structure; Mechanical properties; Activation volume
Co/Cu multilayer systems have been extensively investigated for giant magnetoresistance (GMR) [1], and Co–Cu alloy film provides an alternative for potential GMR applications such as magnetic sensors [2]. For these applications, it is necessary to improve the physical and mechanical properties of Co–Cu alloy film to ensure high performance and reliability. Grain size reduction is one of the effective ways to strengthen metals and alloys because grain boundaries act as obstacles to dislocation motion. The mechanical properties of nanocrystalline metals and alloys have been extensively studied for over a decade, and extremely high strength and hardness have been found in metals and alloys with the grain size in the nanometer regime [3–6]. However, these nanocrystalline metals were very brittle, with a ductility of less than a few percent in tensile tests [7–9], due to the absence of dislocation activity [10,11]. It is known that twin boundaries behave similarly to grain boundaries as an obstacle to plastic deformation [12]. On the other hand, it has been demonstrated from transmission electron microscopy (TEM) observation that twin boundaries can act as dislocation sources [13,14]. Therefore, the presence of nanoscale twins can lead to high strength and high ductility. Recently, this has been verified in nanocrystalline metals with * Corresponding author. Tel.: +81 75 753 5421; fax: +81 75 753 5428; e-mail: [email protected]
nanoscale twins [15–17]. Thus, interfaces such as grain boundaries and twin boundaries have a great influence on mechanical properties of metals. The present paper describes the mechanical properties of a nanocrystalline Co–Cu alloy containing a high density of in-growth nanoscale lamellar structure with a narrow spacing of 3 nm. The activation volume decreases with decreasing twin spacing in face-centered cubic (fcc) metals [18]. Hence, it is worthwhile to investigate the activation volume of the nanocrystalline Co alloy with a nanoscale lamellar structure with narrow spacing of 3 nm. The nanocrystalline Co–Cu alloy with in-growth nanoscale lamellar structure was processed by electrodeposition. The electrolyte composition was CoSO4 7H2O (1 M) and CuSO4 5H2O (0.025 M). The pH of the electrolyte was adjusted to 5.0 using H2SO4. The current density was 40 mA cm2 with direct current. The film specimen was electrodeposited on an amorphous Fe substrate plate. The bath temperature was maintained at 19 °C. The microstructure of the Co–Cu alloy was investigated by TEM. The TEM observation was carried out with a JEOL JEM-2010 at an operating voltage of 200 kV. The specimen observed by TEM was thinned using a dimple grinder and by Ar ion milling. Energydispersive X-ray (EDX) analysis was also carried out at 200 kV using the transmission electron microscope with EDX equipment (Noran Instruments VANTAGE),
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.12.013
Y. Nakamoto et al. / Scripta Materialia 58 (2008) 731–734
Figure 1. Transmission electron micrograph of the Co–Cu alloy.
a A
50 nm
b 1200 Co
Intensity (a. u.)
to investigate the chemical composition of the Co–Cu alloy. The hardness and Young’s modulus were investigated from indentation data following the procedure outlined in Refs. [19–21]. A Shimadzu DUH-W201 hardness tester equipped with a diamond Berkovich tip was used to test the samples over a wide range of loading rates. Experiments at constant loading rates of 13.24, 1.324 and 0.378 mN s1 were performed. The hardness tests were carried out 10 times at each loading rate. The tip was brought into contact with the material. Subsequently, the specimen was indented at a constant loading rate to a depth of 600 nm. The load was held constant at its maximum value for 10 s. Finally, the samples were unloaded. Dogbone-shaped specimens, 7.0 mm in gage length, 2.0 mm in gage width and 10 lm in gage thickness, were prepared for the tensile tests by an electrodischarge machine. The tensile tests were conducted at a strain rate of 102 s1 and at room temperature by a Shimadzu AUTOGRAGH AG50kN-G. A transmission electron micrograph of the Co–Cu alloy is shown in Figure 1. The grain size of the Co–Cu alloy was 110 nm. Most of the grains contained a high-density fine nanoscale lamellar structure. The average lamellar spacing was about 3 nm. In previous studies [15,18], the twin spacing was more than tens of nanometers in nanocrystalline metals with nanoscale twins. Note that the Co–Cu alloy contained a high-density fine nanoscale lamellar structure with a narrow spacing of 3 nm. The result of EDX analysis is shown in Figure 2. The EDX analysis showed that the Co content was 93 wt.% and the Cu content was 7 wt.%. The solid solubility limit of Cu in Co at room temperature is almost 0% in the equilibrium state. Therefore, it is suggested that the Cu is forced to dissolve into the Co because electrodeposition tends to cause the nonequilibrium state. It is known that an allotropic phase transformation occurs in Co [22]. The presence of an fcc Co phase was not verified from the XRD measurement. This indicates that the nanoscale lamellar structure consists of nanotwins. However, because the Co–Cu alloy showed strong ferromagnetic characteristics, evidence for nanotwins could not be obtained by high resolution TEM. Further research is needed to investigate the nanoscale lamellar structure. Figure 3 shows a typical nominal stress–strain curve obtained from the tensile test for the Co–Cu alloy.
900
600 Co
300
Co Cu
Cu Cu
0
0
2
4 6 Energy (keV)
8
10
Figure 2. EDX spectrum of Co–Cu alloy: (a) TEM photograph and (b) EDX profile at point A in (a).
2000
Nominal stress, σ/ MPa
732
1000
0
0
1
2 3 Nominal strain, ε (%)
4
Figure 3. Nominal stress–nominal strain curve obtained from the tensile test for the Co–Cu alloy.
Apparent strain hardening was found, suggesting that the dominant deformation process is related to dislocation activity [23]. The 0.2% proof stress and ultimate tensile strength were 1420 and 1875 MPa, respectively. The yield strength is much higher than that of nanocrystalline Co with a grain size of 12 nm (=1002 MPa) [24]. In nanocrystalline Cu with nanotwins [15,18,25], a twin boundary interacts with dislocations. For example, a ð1 1 1Þ slip interacts with a (1 1 1) twin [25]. Also, a 1=2½0 1 1 dislocation propagates across the twin, whereas the motion of 1/2[1 1 0] and 1/2[1 0 1] dislocations is blocked at the twin boundaries, however, these dislocations propagate across twins if they undergo dislocation dissociation reactions such as 1=2½1 0 1 ! 1=6½1 2 1 þ 1=3½1 1 1 [25]. Dissociations of dislocations are energetically unfavorable and require the concentration of stress at twin-slip band intersections, resulting in strengthening [25]. Similarly, in nanocrystalline Co with
Y. Nakamoto et al. / Scripta Materialia 58 (2008) 731–734 40 13.239 (mN/s)
Load (mN)
30
1.3239 (mN/s) 0.3783 (mN/s)
20
10
0
0
0.2 0.4 Displacement (μm)
0.6
Figure 4. Load–displacement curves obtained from the hardness tests at three different loading rates for the Co–Cu alloy.
Assuming that the boundary thickness of lamellar structures is 2b, the volume ratio of the lamellar structure boundaries is calculated to be about 0.2 in the Co–Cu alloy. Therefore, such a large volume fraction of lamellar structure boundaries is suggested to be responsible for the reduction in Young’s modulus for the Co–Cu alloy. From the hardness test results, the hardnesses of the Co–Cu alloy were 4.12–5.02 GPa. As shown in Figure 4, a higher load was required at a higher loading rate to impose the same displacement. The loading rate dependence was significantly greater than the experimental scatter associated with the specimens. This trend of the large loading rate dependence is the same as that for Cu with nanotwins [16–18]. The variation in hardness as a function of loading rate is shown in Figure 5. The strain rate sensitivity of the Co–Cu alloy can be given by [35] pffiffiffi pffiffiffi 3kT 3 3kT ð1Þ m¼ ¼ vr vH where m is the strain rate sensitivity, k is the Boltzmann constant, T is the absolute temperature, r is the flow stress, H is the hardness (which is generally assumed to be three times the flow stress) and v* is the activation volume. Hence, the activation volume can be given by pffiffiffi o ln e_ ð2Þ v ¼ 3kT or where e_ is the strain rate. From the results in Figure 5, the values of m and v* were 0.055 and 3.3b3 for the 6
Hardness (GPa)
nanotwins, deformation proceeds by the interaction between (0 0 0 2) or ð1 1 2 1Þ twins and 1=3½ 12 1 0 dislocations in the basal plane and by dislocation dissociation reactions such as 1=3½ 12 1 0 ! 1=3½0 1 1 0 þ 1=3½1 1 0 0 [26,27]. Furthermore, the strengthening due to twin boundaries acting as obstacles to dislocation motion has been demonstrated by in situ TEM observation [28]. Therefore, twin boundaries behave similarly to grain boundaries as an obstacle to plastic deformation [12]. It is likely that these mechanisms in twin boundaries are responsible for the high strength of the Co– Cu alloy. The Co–Cu alloy showed an elongation to fracture of 3.3%. Although this value is lower than that for bulk Co [24], it is much larger than those for nanocrystalline metals with a grain size of less than 10 nm and containing no nanotwins [7–9]. High tensile ductility has been obtained in nanocrystalline Cu with nanotwins [15,25]. Molecular dynamics simulations [29] have indicated that when a perfect glide dislocation with b = 1/2[1 0 1] crosses a symmetric (1 1 1) twin boundary, a Shockley partial with b ¼ 1=6½ 1 1 2 is left behind at the twin boundary. Also, it has been demonstrated by TEM observation that a Shockley partial moves along the twin boundary [15,23]. In post-deformation Cu with nanotwins, highly concentrated dislocation activities were observed in the vicinity of twin boundaries [16,18]. Hence, the high capacity of dislocation accumulation at the twin boundaries results in enhanced ductility. Besides, twin boundaries can act as dislocation sources [30]. Hence, not only do twin boundaries behave as obstacles to dislocation motion, but they also serve as dislocation sources during further deformation. Thus, high ductility for the Co–Cu alloy is likely to be attributed to boundaries of the nanoscale lamellar structures acting as nucleation/accumulation sites of dislocations as well as the twin boundaries. A high-angle grain boundary plays an important role as a dislocation source when the grain size is less than about 100 nm [31–33]; however, it cannot serve as a dislocation source when the grain size is less than about 10 nm, and therefore nanocrystalline metals with a grain size of less than 10 nm are very brittle due to the absence of dislocation activity [10,11]. The fact that the Co–Cu alloy with the narrow lamellar spacing of 3 nm showed much larger elongation than nanocrystalline metals with a grain size of less than 10 nm and containing no nanotwins indicates that dislocation activity plays an important role, irrespective of the nanoscale spacing of 3 nm, in the Co–Cu alloy. This may be because it is difficult to induce boundary-mediated deformations such as grain boundary sliding at low energy boundaries. Typical load-displacement curves obtained from the hardness tests at the three loading rates are shown in Figure 4. The Young’s modulus for the Co–Cu alloy was 160 GPa from the hardness tests. This value is lower than that for polycrystalline cobalt (=212–223 GPa [24]). A large volume fraction of interfaces such as grain boundaries and triple junctions results in a reduction in Young’s modulus in a nanocrystalline metal with a grain size of a few nanometers [34]. However, it was reported that the Young’s modulus of Cu with nanotwins whose spacing is 20 nm is comparable to that for bulk Cu [18].
733
5
4
3 -1 10
100 101 Loading rate (mN/s)
102
Figure 5. Variation in hardness as a function of loading rate for the Co–Cu alloy.
734
Y. Nakamoto et al. / Scripta Materialia 58 (2008) 731–734
Co–Cu alloy, respectively. The activation volume is 12–22b3 for Cu with nanotwins whose spacing is 20–90 nm [18]. Note that the activation volume for the Co–Cu alloy is lower than that of Cu. When the grain size decreases to the nm range, grain boundary diffusion plays an important role in the deformation mechanism [36,37]. Also, the abundant grain boundary networks contribute to the enhanced rate sensitivity of nanocrystalline metals [38]. Molecular dynamics simulations have shown that partial dislocations and disordered atom segments dominate the deformation process in nanocrystalline Co [26,27]. On the other hand, it was reported that the presence of nanotwins leads to an increase in strain rate sensitivity and a reduction in activation volume [16–18]. In metals with nanotwins, in addition to the classical deformation by dislocation slips, the interactions between slip dislocations and twin boundaries play a critical role in the plastic deformation process, namely, twin boundaries act as both obstacles to dislocation motion and nucleation/ accumulation sites of dislocations. Asaro and Suresh [35] showed that the emission of partial dislocations at twin boundaries is responsible for the reduced activation volume. When partial dislocations and loops are initiated from the tip of the area of stress concentration such as twin boundaries, the activation volume is pb3 [35]. This value is agreement with our value of v* = 3.3b3 for the Co–Cu alloy. Therefore, it is likely that the low activation volume for the Co–Cu alloy is attributed to the emission of dislocations from boundaries of the lamellar structure due to the stress concentration. Thus, the Co–Cu alloy showed distinguishing mechanical properties, such as the high strength of 1875 MPa and low activation volume of 3.3b3. These are attributed to the high-density fine nanoscale lamellar structure. In general, the formation of lamellar structure such as twin is suppressed in fine-grained Co [22]. In the Co–Cu alloy, as shown in Figure 2, Cu is forced to dissolve into Co, indicating that intense lattice strain is caused. Such intense lattice strain may be related to the formation of the high-density fine nanoscale lamellar structure and the emission of dislocations from the boundaries. [1] M. Shima, L. Salamanca-Riba, T.P. Moffat, R.D. McMichael, J. Magn. Magn. Mater. 198–199 (1999) 52. [2] S. Kainuma, K. Takayanagi, K. Hisatake, T. Watanabe, J. Magn. Magn. Mater. 246 (2002) 207. [3] F.D. Torre, H.V. Swygenhoven, M. Victoria, Acta Mater. 50 (2002) 3957. [4] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Mater. 51 (2003) 431. [5] K.S. Kumar, H.V. Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743. [6] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427.
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Scripta Materialia 58 (2008) 731–734 www.elsevier.com/locate/scriptamat
Mechanical properties of a nanocrystalline Co–Cu alloy with a high-density fine nanoscale lamellar structure Yoshiaki Nakamoto, Motohiro Yuasa,* Youqing Chen, Hiromu Kusuda and Mamoru Mabuchi Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 6 November 2007; revised 1 December 2007; accepted 11 December 2007 Available online 14 January 2008
The mechanical properties of a nanocrystalline Co–Cu alloy containing a high density of in-growth nanoscale lamellar structure with a narrow spacing of 3 nm were investigated. The Co–Cu alloy exhibited a high 0.2% proof stress of 1420 MPa and an ultimate tensile strength of 1875 MPa. Also, the activation volume was 3.3b3. This low value is attributed to the emission of dislocations from boundaries of the nanoscale lamellar structure due to the stress concentration. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cobalt; Nanocrystalline; Nanoscale lamellar structure; Mechanical properties; Activation volume
Co/Cu multilayer systems have been extensively investigated for giant magnetoresistance (GMR) [1], and Co–Cu alloy film provides an alternative for potential GMR applications such as magnetic sensors [2]. For these applications, it is necessary to improve the physical and mechanical properties of Co–Cu alloy film to ensure high performance and reliability. Grain size reduction is one of the effective ways to strengthen metals and alloys because grain boundaries act as obstacles to dislocation motion. The mechanical properties of nanocrystalline metals and alloys have been extensively studied for over a decade, and extremely high strength and hardness have been found in metals and alloys with the grain size in the nanometer regime [3–6]. However, these nanocrystalline metals were very brittle, with a ductility of less than a few percent in tensile tests [7–9], due to the absence of dislocation activity [10,11]. It is known that twin boundaries behave similarly to grain boundaries as an obstacle to plastic deformation [12]. On the other hand, it has been demonstrated from transmission electron microscopy (TEM) observation that twin boundaries can act as dislocation sources [13,14]. Therefore, the presence of nanoscale twins can lead to high strength and high ductility. Recently, this has been verified in nanocrystalline metals with * Corresponding author. Tel.: +81 75 753 5421; fax: +81 75 753 5428; e-mail: [email protected]
nanoscale twins [15–17]. Thus, interfaces such as grain boundaries and twin boundaries have a great influence on mechanical properties of metals. The present paper describes the mechanical properties of a nanocrystalline Co–Cu alloy containing a high density of in-growth nanoscale lamellar structure with a narrow spacing of 3 nm. The activation volume decreases with decreasing twin spacing in face-centered cubic (fcc) metals [18]. Hence, it is worthwhile to investigate the activation volume of the nanocrystalline Co alloy with a nanoscale lamellar structure with narrow spacing of 3 nm. The nanocrystalline Co–Cu alloy with in-growth nanoscale lamellar structure was processed by electrodeposition. The electrolyte composition was CoSO4 7H2O (1 M) and CuSO4 5H2O (0.025 M). The pH of the electrolyte was adjusted to 5.0 using H2SO4. The current density was 40 mA cm2 with direct current. The film specimen was electrodeposited on an amorphous Fe substrate plate. The bath temperature was maintained at 19 °C. The microstructure of the Co–Cu alloy was investigated by TEM. The TEM observation was carried out with a JEOL JEM-2010 at an operating voltage of 200 kV. The specimen observed by TEM was thinned using a dimple grinder and by Ar ion milling. Energydispersive X-ray (EDX) analysis was also carried out at 200 kV using the transmission electron microscope with EDX equipment (Noran Instruments VANTAGE),
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.12.013
Y. Nakamoto et al. / Scripta Materialia 58 (2008) 731–734
Figure 1. Transmission electron micrograph of the Co–Cu alloy.
a A
50 nm
b 1200 Co
Intensity (a. u.)
to investigate the chemical composition of the Co–Cu alloy. The hardness and Young’s modulus were investigated from indentation data following the procedure outlined in Refs. [19–21]. A Shimadzu DUH-W201 hardness tester equipped with a diamond Berkovich tip was used to test the samples over a wide range of loading rates. Experiments at constant loading rates of 13.24, 1.324 and 0.378 mN s1 were performed. The hardness tests were carried out 10 times at each loading rate. The tip was brought into contact with the material. Subsequently, the specimen was indented at a constant loading rate to a depth of 600 nm. The load was held constant at its maximum value for 10 s. Finally, the samples were unloaded. Dogbone-shaped specimens, 7.0 mm in gage length, 2.0 mm in gage width and 10 lm in gage thickness, were prepared for the tensile tests by an electrodischarge machine. The tensile tests were conducted at a strain rate of 102 s1 and at room temperature by a Shimadzu AUTOGRAGH AG50kN-G. A transmission electron micrograph of the Co–Cu alloy is shown in Figure 1. The grain size of the Co–Cu alloy was 110 nm. Most of the grains contained a high-density fine nanoscale lamellar structure. The average lamellar spacing was about 3 nm. In previous studies [15,18], the twin spacing was more than tens of nanometers in nanocrystalline metals with nanoscale twins. Note that the Co–Cu alloy contained a high-density fine nanoscale lamellar structure with a narrow spacing of 3 nm. The result of EDX analysis is shown in Figure 2. The EDX analysis showed that the Co content was 93 wt.% and the Cu content was 7 wt.%. The solid solubility limit of Cu in Co at room temperature is almost 0% in the equilibrium state. Therefore, it is suggested that the Cu is forced to dissolve into the Co because electrodeposition tends to cause the nonequilibrium state. It is known that an allotropic phase transformation occurs in Co [22]. The presence of an fcc Co phase was not verified from the XRD measurement. This indicates that the nanoscale lamellar structure consists of nanotwins. However, because the Co–Cu alloy showed strong ferromagnetic characteristics, evidence for nanotwins could not be obtained by high resolution TEM. Further research is needed to investigate the nanoscale lamellar structure. Figure 3 shows a typical nominal stress–strain curve obtained from the tensile test for the Co–Cu alloy.
900
600 Co
300
Co Cu
Cu Cu
0
0
2
4 6 Energy (keV)
8
10
Figure 2. EDX spectrum of Co–Cu alloy: (a) TEM photograph and (b) EDX profile at point A in (a).
2000
Nominal stress, σ/ MPa
732
1000
0
0
1
2 3 Nominal strain, ε (%)
4
Figure 3. Nominal stress–nominal strain curve obtained from the tensile test for the Co–Cu alloy.
Apparent strain hardening was found, suggesting that the dominant deformation process is related to dislocation activity [23]. The 0.2% proof stress and ultimate tensile strength were 1420 and 1875 MPa, respectively. The yield strength is much higher than that of nanocrystalline Co with a grain size of 12 nm (=1002 MPa) [24]. In nanocrystalline Cu with nanotwins [15,18,25], a twin boundary interacts with dislocations. For example, a ð1 1 1Þ slip interacts with a (1 1 1) twin [25]. Also, a 1=2½0 1 1 dislocation propagates across the twin, whereas the motion of 1/2[1 1 0] and 1/2[1 0 1] dislocations is blocked at the twin boundaries, however, these dislocations propagate across twins if they undergo dislocation dissociation reactions such as 1=2½1 0 1 ! 1=6½1 2 1 þ 1=3½1 1 1 [25]. Dissociations of dislocations are energetically unfavorable and require the concentration of stress at twin-slip band intersections, resulting in strengthening [25]. Similarly, in nanocrystalline Co with
Y. Nakamoto et al. / Scripta Materialia 58 (2008) 731–734 40 13.239 (mN/s)
Load (mN)
30
1.3239 (mN/s) 0.3783 (mN/s)
20
10
0
0
0.2 0.4 Displacement (μm)
0.6
Figure 4. Load–displacement curves obtained from the hardness tests at three different loading rates for the Co–Cu alloy.
Assuming that the boundary thickness of lamellar structures is 2b, the volume ratio of the lamellar structure boundaries is calculated to be about 0.2 in the Co–Cu alloy. Therefore, such a large volume fraction of lamellar structure boundaries is suggested to be responsible for the reduction in Young’s modulus for the Co–Cu alloy. From the hardness test results, the hardnesses of the Co–Cu alloy were 4.12–5.02 GPa. As shown in Figure 4, a higher load was required at a higher loading rate to impose the same displacement. The loading rate dependence was significantly greater than the experimental scatter associated with the specimens. This trend of the large loading rate dependence is the same as that for Cu with nanotwins [16–18]. The variation in hardness as a function of loading rate is shown in Figure 5. The strain rate sensitivity of the Co–Cu alloy can be given by [35] pffiffiffi pffiffiffi 3kT 3 3kT ð1Þ m¼ ¼ vr vH where m is the strain rate sensitivity, k is the Boltzmann constant, T is the absolute temperature, r is the flow stress, H is the hardness (which is generally assumed to be three times the flow stress) and v* is the activation volume. Hence, the activation volume can be given by pffiffiffi o ln e_ ð2Þ v ¼ 3kT or where e_ is the strain rate. From the results in Figure 5, the values of m and v* were 0.055 and 3.3b3 for the 6
Hardness (GPa)
nanotwins, deformation proceeds by the interaction between (0 0 0 2) or ð1 1 2 1Þ twins and 1=3½ 12 1 0 dislocations in the basal plane and by dislocation dissociation reactions such as 1=3½ 12 1 0 ! 1=3½0 1 1 0 þ 1=3½1 1 0 0 [26,27]. Furthermore, the strengthening due to twin boundaries acting as obstacles to dislocation motion has been demonstrated by in situ TEM observation [28]. Therefore, twin boundaries behave similarly to grain boundaries as an obstacle to plastic deformation [12]. It is likely that these mechanisms in twin boundaries are responsible for the high strength of the Co– Cu alloy. The Co–Cu alloy showed an elongation to fracture of 3.3%. Although this value is lower than that for bulk Co [24], it is much larger than those for nanocrystalline metals with a grain size of less than 10 nm and containing no nanotwins [7–9]. High tensile ductility has been obtained in nanocrystalline Cu with nanotwins [15,25]. Molecular dynamics simulations [29] have indicated that when a perfect glide dislocation with b = 1/2[1 0 1] crosses a symmetric (1 1 1) twin boundary, a Shockley partial with b ¼ 1=6½ 1 1 2 is left behind at the twin boundary. Also, it has been demonstrated by TEM observation that a Shockley partial moves along the twin boundary [15,23]. In post-deformation Cu with nanotwins, highly concentrated dislocation activities were observed in the vicinity of twin boundaries [16,18]. Hence, the high capacity of dislocation accumulation at the twin boundaries results in enhanced ductility. Besides, twin boundaries can act as dislocation sources [30]. Hence, not only do twin boundaries behave as obstacles to dislocation motion, but they also serve as dislocation sources during further deformation. Thus, high ductility for the Co–Cu alloy is likely to be attributed to boundaries of the nanoscale lamellar structures acting as nucleation/accumulation sites of dislocations as well as the twin boundaries. A high-angle grain boundary plays an important role as a dislocation source when the grain size is less than about 100 nm [31–33]; however, it cannot serve as a dislocation source when the grain size is less than about 10 nm, and therefore nanocrystalline metals with a grain size of less than 10 nm are very brittle due to the absence of dislocation activity [10,11]. The fact that the Co–Cu alloy with the narrow lamellar spacing of 3 nm showed much larger elongation than nanocrystalline metals with a grain size of less than 10 nm and containing no nanotwins indicates that dislocation activity plays an important role, irrespective of the nanoscale spacing of 3 nm, in the Co–Cu alloy. This may be because it is difficult to induce boundary-mediated deformations such as grain boundary sliding at low energy boundaries. Typical load-displacement curves obtained from the hardness tests at the three loading rates are shown in Figure 4. The Young’s modulus for the Co–Cu alloy was 160 GPa from the hardness tests. This value is lower than that for polycrystalline cobalt (=212–223 GPa [24]). A large volume fraction of interfaces such as grain boundaries and triple junctions results in a reduction in Young’s modulus in a nanocrystalline metal with a grain size of a few nanometers [34]. However, it was reported that the Young’s modulus of Cu with nanotwins whose spacing is 20 nm is comparable to that for bulk Cu [18].
733
5
4
3 -1 10
100 101 Loading rate (mN/s)
102
Figure 5. Variation in hardness as a function of loading rate for the Co–Cu alloy.
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Co–Cu alloy, respectively. The activation volume is 12–22b3 for Cu with nanotwins whose spacing is 20–90 nm [18]. Note that the activation volume for the Co–Cu alloy is lower than that of Cu. When the grain size decreases to the nm range, grain boundary diffusion plays an important role in the deformation mechanism [36,37]. Also, the abundant grain boundary networks contribute to the enhanced rate sensitivity of nanocrystalline metals [38]. Molecular dynamics simulations have shown that partial dislocations and disordered atom segments dominate the deformation process in nanocrystalline Co [26,27]. On the other hand, it was reported that the presence of nanotwins leads to an increase in strain rate sensitivity and a reduction in activation volume [16–18]. In metals with nanotwins, in addition to the classical deformation by dislocation slips, the interactions between slip dislocations and twin boundaries play a critical role in the plastic deformation process, namely, twin boundaries act as both obstacles to dislocation motion and nucleation/ accumulation sites of dislocations. Asaro and Suresh [35] showed that the emission of partial dislocations at twin boundaries is responsible for the reduced activation volume. When partial dislocations and loops are initiated from the tip of the area of stress concentration such as twin boundaries, the activation volume is pb3 [35]. This value is agreement with our value of v* = 3.3b3 for the Co–Cu alloy. Therefore, it is likely that the low activation volume for the Co–Cu alloy is attributed to the emission of dislocations from boundaries of the lamellar structure due to the stress concentration. Thus, the Co–Cu alloy showed distinguishing mechanical properties, such as the high strength of 1875 MPa and low activation volume of 3.3b3. These are attributed to the high-density fine nanoscale lamellar structure. In general, the formation of lamellar structure such as twin is suppressed in fine-grained Co [22]. In the Co–Cu alloy, as shown in Figure 2, Cu is forced to dissolve into Co, indicating that intense lattice strain is caused. Such intense lattice strain may be related to the formation of the high-density fine nanoscale lamellar structure and the emission of dislocations from the boundaries. [1] M. Shima, L. Salamanca-Riba, T.P. Moffat, R.D. McMichael, J. Magn. Magn. Mater. 198–199 (1999) 52. [2] S. Kainuma, K. Takayanagi, K. Hisatake, T. Watanabe, J. Magn. Magn. Mater. 246 (2002) 207. [3] F.D. Torre, H.V. Swygenhoven, M. Victoria, Acta Mater. 50 (2002) 3957. [4] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Mater. 51 (2003) 431. [5] K.S. Kumar, H.V. Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743. [6] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427.
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