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Stability of the B19 martensite in rapidly solidified Ti–Ni–Cu alloys
Materials Science and Engineering A 438–440 (2006) 687–690
Stability of the B19 martensite in rapidly solidified Ti–Ni–Cu alloys Tae-hyun Nam a,∗ , Jae-hwa Lee a , Gyu-bong Cho a , Yeon-wook Kim b a
Division of Materials Engineering and ERI, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Republic of Korea b Department of Material Engineering, Keimyung University, 1000 Shindang-dong, Dalseo-gu, Taegu 704-710, Republic of Korea Received 18 April 2005; received in revised form 21 January 2006; accepted 14 February 2006
Abstract Stabilization of the B19 martensite by rapid solidification in a Ti–45Ni–5Cu (at.%) alloy has been investigated by means of differential scanning calorimetry and transmission electron microscopy. Rapid solidification process induced the B2–B19 transformation prior to formation of the B19 martensite, and thus the two-stage B2–B19–B19 transformation occurred in as-spun Ti–45Ni–5Cu alloy ribbons. Inducement of the B19 martensite was ascribed to the formation of Ti2 Ni particles coherent with the B2 matrix. With increasing Ti content from 49 to 51 at.%, the temperature range where the B19 martensite exists was expanded from 7 to 25 K, which was attributed to an increase in volume fraction of Ti2 Ni particles from 10 to 55% and a decrease in the size from 16 to 5 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: B19 martensite; Ti–45Ni–5Cu alloy ribbons; Melt spinning; Volume fraction of Ti2 Ni particles; Coherency
1. Introduction Ti–Ni–Cu shape memory alloys have been known to be very attractive for actuators because transformation hysteresis associated with the B2–B19 transformation in the alloys was small comparing with that associated with the B2–B19 transformation in Ti–Ni binary alloys [1]. The B2–B19 transformation was observed in Ti–Ni–Cu alloys only when Cu content was higher than 10 at.% [2]. Unfortunately, the alloys with Cu content more than 10 at.% were too brittle to be deformed in plastic manner. Therefore, from a practical point of view, it was desirable to induce the B2–B19 transformation in Ti–Ni–Cu alloys with Cu content less than 10 at.%. Rapid solidification has been known to form metastable Ti2 Ni particles with coherency strain in Ti–Ni alloys [3,4]. Coherent Ti2 Ni particles changed transformation behavior of Ti–Ni alloys [4,5]. Recently, it was found that melt-spun Ti–45Ni–5Cu alloy ribbons transformed in two-stage, i.e., B2–B19–B19 , while a Ti–45Ni–5Cu alloy prepared by a conventional casting did in one-stage, i.e., B2–B19 [6]. Ti2 Ni particles coherent with the B2 matrix were considered to play an important role for inducing the B2–B19 transformation [7]. It was expected that stability of
∗
Corresponding author. Tel.: +82 55 751 5307; fax: +82 55 759 1745. E-mail address: [email protected] (T.-h. Nam).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.155
the B19 martensite could be increased with increasing amount of coherent Ti2 Ni particles. Therefore, the purpose of the present study is to clarify a role of coherent Ti2 Ni particles for inducing the B2–B19 transformation and to investigate an effect of the amount of coherent Ti2 Ni particles on the stability of the B19 martensite. 2. Experimental procedure Alloy ribbons of xTi–(95 − x)Ni–5Cu (x = 49, 50 and 51 at.%) were prepared by melt spinning from pre-alloys fabricated by melting high purity Ni, Cu and sponge Ti in a high frequency vacuum induction furnace. Billets of 5 mm × 5 mm × 10 mm were cut from the pre-alloy ingot, and then encapsulated into quartz tubes. The chamber of the melt spinning system had been evacuated to less than 1 × 10−3 Pa before re-melting. The diameter of the orifice was 0.5 mm, the ejection pressure was 40 kPa and the distance from the tip of the nozzle to the wheel surface was 300 m. Linear velocity was 31 m/s. Some of as-spun ribbons were annealed at 1173 K for 3.6 ks in vacuum. Transformation behaviors of the ribbons were investigated by means of differential scanning calorimetry (DSC) with a heating and cooling rate of 0.17 K/s. Microstructures of the ribbons were investigated by transmission electron microscopy (TEM). Samples for TEM were prepared by twin-jet electropolishing with an electrolyte consisting of 95% CH3 COOH and 5% HClO4 in volume. TEM
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T.-h. Nam et al. / Materials Science and Engineering A 438–440 (2006) 687–690
Fig. 1. DSC curves of Ti–45Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K.
examination was made using a JEOL 2010 microscope operated at an accelerating voltage of 200 kV. 3. Results and discussion Fig. 1(a) shows DSC curves of as-spun Ti–45Ni–5Cu ribbons obtained at the melt spinning temperature of 1823 K. Two exothermic and endothermic peaks are found on each cooling and hearing curve, respectively. Those DSC peaks were ascribed to the two-stage B2–B19–B19 transformation [6]. Fig. 1(b) shows DSC curves obtained from the ribbons annealed at 1173 K for 3.6 ks. In contrast to the curves of (a), only one exothermic and endothermic DSC peak is found on each cooling and heating curve, respectively. DSC peaks in the curves of (b) were attributed to the B2–B19 transformation from X-ray diffraction. Therefore, it is concluded that transformation behavior of meltspun Ti–45Ni–5Cu alloy ribbons changes from the two-stage B2–B19–B19 to the one-stage B2–B19 , by annealing as-spun ribbons at 1173 K. In order to clarify the reason for the change in transformation behavior of Ti–45Ni–5Cu alloy ribbons by annealing, TEM observations were made on both as-spun and annealed ribbons, and then results obtained are shown in Fig. 2. Fig. 2(a)
Fig. 3. DSC curves of xTi–Ni–5Cu (x = 49 and 51) alloy ribbons fabricated at the liquid temperature of 1823 K.
is a bright field image of as-spun ribbon. Small spherical particles with the diameter of 5–7 nm are found. From the electron diffraction (ED) pattern presented on the upper right corner in Fig. 2(a), the particles are known to be Ti2 Ni coherent with the B2 matrix. Volume fraction of Ti2 Ni was measured to be about 40% from image analysis method. Fig. 2(b) is a bright field image of ribbon annealed at 1173 K for 3.6 ks. Comparing with Fig. 2(a), it is found that Ti2 Ni particles grow up to 50–70 nm by annealing. From the ED pattern presented on the upper right corner in Fig. 2(b), the particles are known to be incoherent with the B2 matrix. This means that Ti2 Ni particles loose coherency by annealing. Coherency strains around coherent particles have been known to depress the B2–B19 transformation in Ti–Ni alloys [8]. Therefore, it is concluded that the change in transformation behavior from the B2–B19–B19 to B2–B19 by annealing in melt-spun Ti–45Ni–5Cu alloy ribbons is ascribed to the coherency loss due to the growth of Ti2 Ni particles. According to the previous study [3], it is suggested that the amount of metastable Ti2 Ni particles will be increased with increasing Ti content in Ti–Ni alloys. Since coherency strains around Ti2 Ni particles induced the B2–B19 transfor-
Fig. 2. TEM observation results of Ti–45Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K: (a) as-spun and (b) heat treated at 1173 K.
T.-h. Nam et al. / Materials Science and Engineering A 438–440 (2006) 687–690
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Fig. 4. TEM observation results of the (a) 49Ti–Ni–5Cu and (b) 51Ti–Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K.
mation by suppressing the B2–B19 transformation as seen in Figs. 1 and 2, the stability of the B19 martensite in Ti–45Ni–5Cu alloy ribbons is expected to increase with increasing Ti content. Fig. 3(a and b) are DSC curves of melt-spun 49Ti–46Ni–5Cu and 51Ti–44Ni–5Cu alloy ribbons, respectively. Complex spits in DSC peaks are found in both alloys. In melt-spun Ti–25Ni–25Cu alloy ribbons, transformation temperatures of wheel side were different from those of free side because microstructures of each side were different [9]. Some splits in DSC peaks were observed on each cooling and heating curves due to the difference in transformation temperatures between wheel side and free side of the ribbons, even if only one-stage B2–B19 transformation occurred. DSC curves presented by dotted lines in Fig. 3 were obtained after removing wheel side of ribbons completely. By removing the wheel side of the ribbons, splits in DSC peaks originated from the microstructural difference between wheel side and free side of ribbons can be excluded. As seen in the curves, even after removing the wheel side completely, splits in DSC peaks still appear. Therefore, splits in DSC peaks are considered not to come from the microstructural difference between wheel side and free side of ribbons, but to be due to two-stage transformation behavior, i.e., B2–B19–B19 . Fig. 4(a) shows a bright field image of melt-spun 49Ti–46Ni–5Cu alloy ribbons. From the ED pattern presented in the upper right corner, spherical particles in the image are found to be Ti2 Ni being coherent with the B2 matrix. Average size of Ti2 Ni particles is about 16 nm. From image analysis method, volume fraction of Ti2 Ni is found to be about 10%. Fig. 4(b) shows a bright field image of melt-spun 51Ti–44Ni–5Cu alloy ribbons. From the ED pattern, spherical particles in the image are found to be Ti2 Ni being coherent with the B2 matrix. Average size of Ti2 Ni particles is about 5 nm. From image analysis method, volume fraction of Ti2 Ni is found to be about 55%. From Figs. 1 and 3, the temperature gap between the DSC peak temperatures associated with the B2–B19 transformation (Ms* ) and the B19–B19 transformation (Ms* ) were determined, and then plotted against Ti content in Fig. 5. It is found that the temperature gap between Ms* and Ms* increases from 7 to 25 K with increasing Ti content from 49 to 51 at.%. Com-
Fig. 5. Relationship between Ms* –Ms* and Ti content in xTi–Ni–5Cu (x = 49, 50 and 51) alloy ribbons fabricated at the liquid temperature of 1823 K.
paring Fig. 4 with Fig. 2, it is found that size of coherent Ti2 Ni particles decreases from 16 to 5 nm, volume fraction increases from 10 to 55% with increasing Ti content from 49 to 51 at.%. Therefore, it is considered that the increase in the stability of the B19 martensite with increasing Ti content is ascribed to the increase in the volume fraction and the decrease in size of coherent Ti2 Ni particles. 4. Conclusions Melt-spun xTi–(95 − x)Ni–5Cu (x = 49, 50 and 51 at.%) alloy ribbons showed the two-stage B2–B19–B19’ transformation behavior. Ti2 Ni particles coherent with the B2 matrix were formed in the all alloy ribbons. Size of coherent Ti2 Ni particles decreased from 16 to 5 nm and volume fraction of them increased from 10 to 55% with increasing Ti content from 49 to 51 at.%. Temperature gap between the DSC peak temperatures associated with the B2–B19 transformation (Ms* ) and the B19–B19 transformation (Ms* ) increased from 7 to 25 K with increasing Ti content from 49 to 51 at.%. The increase in the stability of the B19 martensite with increasing Ti content was
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T.-h. Nam et al. / Materials Science and Engineering A 438–440 (2006) 687–690
ascribed to the increase in the volume fraction and the decrease in size of coherent Ti2 Ni particles. Acknowledgements This work was supported by University IT Research Center Project and Engineering Research Foundation in Gyeongsang National University. References [1] T.H. Nam, T. Saburi, Y. Kawamura, K. Shimizu, Mater. Trans. JIM 31 (1990) 262.
[2] Y. Shugo, F. Hasegawa, T. Honma, Bull. Res. Inst. Miner. Dress. Metall. 37 (1981) 80. [3] R. Nagarajan, K. Chattopadhyay, Acta Metall. Mater. 42 (1994) 947. [4] A. Gyobu, Y. Kawamura, H. Horikawa, T. Saburi, Mater. Trans. JIM 37 (1996) 703. [5] A. Ishida, K. Ogawa, M. Sato, S. Miyazaki, Metall. Mater. Trans. 28A (1997) 1985. [6] T.H. Nam, J.H. Lee, T.Y. Kim, Y.W. Kim, Mater. Sci. Forum 449–452 (2004) 1093. [7] T.H. Nam, J.H. Lee, K.W. Kim, H.J. Ahn, Y.W. Kim, J. Mater. Sci. 40 (2005) 4925. [8] M. Nishida, T. Honma, J. Phys. Fr. 43 (Suppl.) (1982) C4–C75. [9] H. R¨osner, A.V. Shelyakov, A.M. Glezer, K. Feit, K.P. Schlomacher, Mater. Sci. Eng. A 273–275 (1999) 733.
Stability of the B19 martensite in rapidly solidified Ti–Ni–Cu alloys Tae-hyun Nam a,∗ , Jae-hwa Lee a , Gyu-bong Cho a , Yeon-wook Kim b a
Division of Materials Engineering and ERI, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Republic of Korea b Department of Material Engineering, Keimyung University, 1000 Shindang-dong, Dalseo-gu, Taegu 704-710, Republic of Korea Received 18 April 2005; received in revised form 21 January 2006; accepted 14 February 2006
Abstract Stabilization of the B19 martensite by rapid solidification in a Ti–45Ni–5Cu (at.%) alloy has been investigated by means of differential scanning calorimetry and transmission electron microscopy. Rapid solidification process induced the B2–B19 transformation prior to formation of the B19 martensite, and thus the two-stage B2–B19–B19 transformation occurred in as-spun Ti–45Ni–5Cu alloy ribbons. Inducement of the B19 martensite was ascribed to the formation of Ti2 Ni particles coherent with the B2 matrix. With increasing Ti content from 49 to 51 at.%, the temperature range where the B19 martensite exists was expanded from 7 to 25 K, which was attributed to an increase in volume fraction of Ti2 Ni particles from 10 to 55% and a decrease in the size from 16 to 5 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: B19 martensite; Ti–45Ni–5Cu alloy ribbons; Melt spinning; Volume fraction of Ti2 Ni particles; Coherency
1. Introduction Ti–Ni–Cu shape memory alloys have been known to be very attractive for actuators because transformation hysteresis associated with the B2–B19 transformation in the alloys was small comparing with that associated with the B2–B19 transformation in Ti–Ni binary alloys [1]. The B2–B19 transformation was observed in Ti–Ni–Cu alloys only when Cu content was higher than 10 at.% [2]. Unfortunately, the alloys with Cu content more than 10 at.% were too brittle to be deformed in plastic manner. Therefore, from a practical point of view, it was desirable to induce the B2–B19 transformation in Ti–Ni–Cu alloys with Cu content less than 10 at.%. Rapid solidification has been known to form metastable Ti2 Ni particles with coherency strain in Ti–Ni alloys [3,4]. Coherent Ti2 Ni particles changed transformation behavior of Ti–Ni alloys [4,5]. Recently, it was found that melt-spun Ti–45Ni–5Cu alloy ribbons transformed in two-stage, i.e., B2–B19–B19 , while a Ti–45Ni–5Cu alloy prepared by a conventional casting did in one-stage, i.e., B2–B19 [6]. Ti2 Ni particles coherent with the B2 matrix were considered to play an important role for inducing the B2–B19 transformation [7]. It was expected that stability of
∗
Corresponding author. Tel.: +82 55 751 5307; fax: +82 55 759 1745. E-mail address: [email protected] (T.-h. Nam).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.155
the B19 martensite could be increased with increasing amount of coherent Ti2 Ni particles. Therefore, the purpose of the present study is to clarify a role of coherent Ti2 Ni particles for inducing the B2–B19 transformation and to investigate an effect of the amount of coherent Ti2 Ni particles on the stability of the B19 martensite. 2. Experimental procedure Alloy ribbons of xTi–(95 − x)Ni–5Cu (x = 49, 50 and 51 at.%) were prepared by melt spinning from pre-alloys fabricated by melting high purity Ni, Cu and sponge Ti in a high frequency vacuum induction furnace. Billets of 5 mm × 5 mm × 10 mm were cut from the pre-alloy ingot, and then encapsulated into quartz tubes. The chamber of the melt spinning system had been evacuated to less than 1 × 10−3 Pa before re-melting. The diameter of the orifice was 0.5 mm, the ejection pressure was 40 kPa and the distance from the tip of the nozzle to the wheel surface was 300 m. Linear velocity was 31 m/s. Some of as-spun ribbons were annealed at 1173 K for 3.6 ks in vacuum. Transformation behaviors of the ribbons were investigated by means of differential scanning calorimetry (DSC) with a heating and cooling rate of 0.17 K/s. Microstructures of the ribbons were investigated by transmission electron microscopy (TEM). Samples for TEM were prepared by twin-jet electropolishing with an electrolyte consisting of 95% CH3 COOH and 5% HClO4 in volume. TEM
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Fig. 1. DSC curves of Ti–45Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K.
examination was made using a JEOL 2010 microscope operated at an accelerating voltage of 200 kV. 3. Results and discussion Fig. 1(a) shows DSC curves of as-spun Ti–45Ni–5Cu ribbons obtained at the melt spinning temperature of 1823 K. Two exothermic and endothermic peaks are found on each cooling and hearing curve, respectively. Those DSC peaks were ascribed to the two-stage B2–B19–B19 transformation [6]. Fig. 1(b) shows DSC curves obtained from the ribbons annealed at 1173 K for 3.6 ks. In contrast to the curves of (a), only one exothermic and endothermic DSC peak is found on each cooling and heating curve, respectively. DSC peaks in the curves of (b) were attributed to the B2–B19 transformation from X-ray diffraction. Therefore, it is concluded that transformation behavior of meltspun Ti–45Ni–5Cu alloy ribbons changes from the two-stage B2–B19–B19 to the one-stage B2–B19 , by annealing as-spun ribbons at 1173 K. In order to clarify the reason for the change in transformation behavior of Ti–45Ni–5Cu alloy ribbons by annealing, TEM observations were made on both as-spun and annealed ribbons, and then results obtained are shown in Fig. 2. Fig. 2(a)
Fig. 3. DSC curves of xTi–Ni–5Cu (x = 49 and 51) alloy ribbons fabricated at the liquid temperature of 1823 K.
is a bright field image of as-spun ribbon. Small spherical particles with the diameter of 5–7 nm are found. From the electron diffraction (ED) pattern presented on the upper right corner in Fig. 2(a), the particles are known to be Ti2 Ni coherent with the B2 matrix. Volume fraction of Ti2 Ni was measured to be about 40% from image analysis method. Fig. 2(b) is a bright field image of ribbon annealed at 1173 K for 3.6 ks. Comparing with Fig. 2(a), it is found that Ti2 Ni particles grow up to 50–70 nm by annealing. From the ED pattern presented on the upper right corner in Fig. 2(b), the particles are known to be incoherent with the B2 matrix. This means that Ti2 Ni particles loose coherency by annealing. Coherency strains around coherent particles have been known to depress the B2–B19 transformation in Ti–Ni alloys [8]. Therefore, it is concluded that the change in transformation behavior from the B2–B19–B19 to B2–B19 by annealing in melt-spun Ti–45Ni–5Cu alloy ribbons is ascribed to the coherency loss due to the growth of Ti2 Ni particles. According to the previous study [3], it is suggested that the amount of metastable Ti2 Ni particles will be increased with increasing Ti content in Ti–Ni alloys. Since coherency strains around Ti2 Ni particles induced the B2–B19 transfor-
Fig. 2. TEM observation results of Ti–45Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K: (a) as-spun and (b) heat treated at 1173 K.
T.-h. Nam et al. / Materials Science and Engineering A 438–440 (2006) 687–690
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Fig. 4. TEM observation results of the (a) 49Ti–Ni–5Cu and (b) 51Ti–Ni–5Cu alloy ribbons fabricated at the liquid temperature of 1823 K.
mation by suppressing the B2–B19 transformation as seen in Figs. 1 and 2, the stability of the B19 martensite in Ti–45Ni–5Cu alloy ribbons is expected to increase with increasing Ti content. Fig. 3(a and b) are DSC curves of melt-spun 49Ti–46Ni–5Cu and 51Ti–44Ni–5Cu alloy ribbons, respectively. Complex spits in DSC peaks are found in both alloys. In melt-spun Ti–25Ni–25Cu alloy ribbons, transformation temperatures of wheel side were different from those of free side because microstructures of each side were different [9]. Some splits in DSC peaks were observed on each cooling and heating curves due to the difference in transformation temperatures between wheel side and free side of the ribbons, even if only one-stage B2–B19 transformation occurred. DSC curves presented by dotted lines in Fig. 3 were obtained after removing wheel side of ribbons completely. By removing the wheel side of the ribbons, splits in DSC peaks originated from the microstructural difference between wheel side and free side of ribbons can be excluded. As seen in the curves, even after removing the wheel side completely, splits in DSC peaks still appear. Therefore, splits in DSC peaks are considered not to come from the microstructural difference between wheel side and free side of ribbons, but to be due to two-stage transformation behavior, i.e., B2–B19–B19 . Fig. 4(a) shows a bright field image of melt-spun 49Ti–46Ni–5Cu alloy ribbons. From the ED pattern presented in the upper right corner, spherical particles in the image are found to be Ti2 Ni being coherent with the B2 matrix. Average size of Ti2 Ni particles is about 16 nm. From image analysis method, volume fraction of Ti2 Ni is found to be about 10%. Fig. 4(b) shows a bright field image of melt-spun 51Ti–44Ni–5Cu alloy ribbons. From the ED pattern, spherical particles in the image are found to be Ti2 Ni being coherent with the B2 matrix. Average size of Ti2 Ni particles is about 5 nm. From image analysis method, volume fraction of Ti2 Ni is found to be about 55%. From Figs. 1 and 3, the temperature gap between the DSC peak temperatures associated with the B2–B19 transformation (Ms* ) and the B19–B19 transformation (Ms* ) were determined, and then plotted against Ti content in Fig. 5. It is found that the temperature gap between Ms* and Ms* increases from 7 to 25 K with increasing Ti content from 49 to 51 at.%. Com-
Fig. 5. Relationship between Ms* –Ms* and Ti content in xTi–Ni–5Cu (x = 49, 50 and 51) alloy ribbons fabricated at the liquid temperature of 1823 K.
paring Fig. 4 with Fig. 2, it is found that size of coherent Ti2 Ni particles decreases from 16 to 5 nm, volume fraction increases from 10 to 55% with increasing Ti content from 49 to 51 at.%. Therefore, it is considered that the increase in the stability of the B19 martensite with increasing Ti content is ascribed to the increase in the volume fraction and the decrease in size of coherent Ti2 Ni particles. 4. Conclusions Melt-spun xTi–(95 − x)Ni–5Cu (x = 49, 50 and 51 at.%) alloy ribbons showed the two-stage B2–B19–B19’ transformation behavior. Ti2 Ni particles coherent with the B2 matrix were formed in the all alloy ribbons. Size of coherent Ti2 Ni particles decreased from 16 to 5 nm and volume fraction of them increased from 10 to 55% with increasing Ti content from 49 to 51 at.%. Temperature gap between the DSC peak temperatures associated with the B2–B19 transformation (Ms* ) and the B19–B19 transformation (Ms* ) increased from 7 to 25 K with increasing Ti content from 49 to 51 at.%. The increase in the stability of the B19 martensite with increasing Ti content was
690
T.-h. Nam et al. / Materials Science and Engineering A 438–440 (2006) 687–690
ascribed to the increase in the volume fraction and the decrease in size of coherent Ti2 Ni particles. Acknowledgements This work was supported by University IT Research Center Project and Engineering Research Foundation in Gyeongsang National University. References [1] T.H. Nam, T. Saburi, Y. Kawamura, K. Shimizu, Mater. Trans. JIM 31 (1990) 262.
[2] Y. Shugo, F. Hasegawa, T. Honma, Bull. Res. Inst. Miner. Dress. Metall. 37 (1981) 80. [3] R. Nagarajan, K. Chattopadhyay, Acta Metall. Mater. 42 (1994) 947. [4] A. Gyobu, Y. Kawamura, H. Horikawa, T. Saburi, Mater. Trans. JIM 37 (1996) 703. [5] A. Ishida, K. Ogawa, M. Sato, S. Miyazaki, Metall. Mater. Trans. 28A (1997) 1985. [6] T.H. Nam, J.H. Lee, T.Y. Kim, Y.W. Kim, Mater. Sci. Forum 449–452 (2004) 1093. [7] T.H. Nam, J.H. Lee, K.W. Kim, H.J. Ahn, Y.W. Kim, J. Mater. Sci. 40 (2005) 4925. [8] M. Nishida, T. Honma, J. Phys. Fr. 43 (Suppl.) (1982) C4–C75. [9] H. R¨osner, A.V. Shelyakov, A.M. Glezer, K. Feit, K.P. Schlomacher, Mater. Sci. Eng. A 273–275 (1999) 733.