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Formation of nanocrystals in metallic Zr–Cu–Ni glass
Journal of Alloys and Compounds 347 (2002) 101–104
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Formation of nanocrystals in metallic Zr–Cu–Ni glass a, b a a,c a Huan-Rong Wang *, Xi-Dong Hui , Guang-Hui Min , Yi-Fu Ye , Xin-Ying Teng , a Zhi-Qiang Shi a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, South Campus of Shandong University, Shandong University, Jinan 250061, PR China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China c College of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 13 March 2002; received in revised form 17 April 2002; accepted 17 April 2002
Abstract Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are employed to investigate the crystallization behavior in metallic Zr 70 Cu 20 Ni 10 glass during continuous heating. It is found that the DSC curves of metallic Zr 70 Cu 20 Ni 10 glass exhibit two exothermic peaks, indicating that the crystallization process proceeds via a double stage mode. TEM microstructural analysis confirms that the first exothermic reaction in the DSC traces of metallic Zr 70 Cu 20 Ni 10 glass mainly corresponds to the precipitation of the Zr 2 Cu phase, while the second one corresponds to the formation of nano-scale Zr 2 Ni particles. Ni plays an important role in the stability of the nanocrystals, and its effect is also discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous materials; Nanostructured materials; Rapid solidification; Calorimetry; Transmission electron microscopy (TEM)
1. Introduction In recent years, nano-materials have received increasing attention due to their particular properties, i.e. high strength, high hardness, high toughness, etc. More recently, a novel class of bulk metallic glasses characterized by the appearance of a wide supercooled liquid regime exceeding 60 K, as exemplified by La–Al–Ni [1], Zr–Ti– Ni–Cu–Be [2], Zr–Al–Ni [3], Zr–Cu–Ni–Al [4] and Zr–Ti–Ni–Cu–Al [5], has received increasing attention due to exceptional good glass forming ability (GFA). It is of interest to note that all the above-mentioned alloy systems contain Ni, implying that Ni plays an important role in the synthesis of these bulk metallic glasses. Maret et al. [6,7] attributed the prepeak in the structure factor of metallic Ni 33 Y 67 glass and liquid Al 80 Ni 20 alloy to the interactions between Ni atoms. The author of this paper has found a distinct prepeak in the structure factor of liquid Cu 70 Ni 30 alloy and also believes that it is strongly associated with the Ni–Ni interactions [8]. In addition, the formation of ternary amorphous Zr–Cu–Ni alloy does not satisfy the widely accepted empirical rules of glass forma*Corresponding author. E-mail address: [email protected] (H.R. Wang).
tion proposed by Inoue [9], i.e. the requirement of a significant difference in atomic size ratios above 12% among the three main constituent elements (only 3% between Cu and Ni atoms) and the requirement of negative heats of mixing among them (positive value of 4 KJ / mol between Cu and Ni atoms) [10]. Therefore, it remains an urgent problem to conduct an in-depth study on the GFA and crystallization behavior of metallic Zr–Cu–Ni glasses, which will provide some useful information for alloy design to achieve new bulk metallic glasses. Unfortunately, few experimental investigations have been carried out on such materials except for some thermodynamic calculations [11–13]. The aim of this study is to investigate the crystallization behavior of metallic Zr 70 Cu 20 Ni 10 glass.
2. Experimental An alloy ingot with a nominal composition of Zr 70 Cu 20 Ni 10 was prepared by arc melting a mixture of pure zirconium (3 N), high-purity copper (5 N) and nickel (5 N) under an argon atmosphere of 99.999% purity. The alloy ingot was remelted several times to ensure homogeneity. A melt-spun ribbon with a thickness of |60 mm was fabricated by vacuum melt-spinning. The diameter of
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00677-1
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the copper roller and the surface velocity were 20 cm and 20 m / s, respectively. The experiment was performed under a high-purity argon atmosphere after the chamber had been evacuated in a vacuum of 3310 23 Pa. The amorphicity of the melt-spun ribbon was studied by a D/ max-rB diffractometer with Cu Ka radiation. The thermal property of the melt-spun ribbon was examined using a Netzsch DSC404 calorimeter. Transmission electron microscopy (TEM) experiments were carried out with an H-800 type microscope, operated at 150 kV. High-resolution TEM (HRTEM) experiments were performed using a JEOL200CX instrument with an accelerating voltage of 200 kV. The samples for TEM observations were heat treated in a CARBOLITE CWF / 1300-5 muffle furnace in a vacuum of 10 23 Pa.
3. Results and discussion The melt-spun ribbon with a thickness of 60 mm was observed to be fully amorphous by XRD as well as by high-resolution TEM. Fig. 1 shows the HRTEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at room temperature and 385 8C, respectively. From Fig. 1(a) only a bright diffraction halo can be seen, suggesting that the microstructure of the metallic Zr 70 Cu 20 Ni 10 glass is fully disordered. When the temperature reaches 385 8C, which is located in the supercooled liquid region, structural relaxation takes place and many atomic clusters start to precipitate, as shown in Fig. 1(b). These atomic clusters may act as nucleation sites, which plays an important role in the subsequent crystallization. Typical continuous heating DSC curves of
Fig. 2. DSC traces of metallic Zr 70 Cu 20 Ni 10 glass at various heating rates.
metallic Zr 70 Cu 20 Ni 10 glass at different heating rates are shown in Fig. 2, in which two distinct separable exothermic peaks can be observed, indicating that the crystallization proceeds through a double stage mode. It is known from Ref. [14] that the binary Zr 70 Cu 30 amorphous alloy crystallizes through predominantly one exothermic reaction corresponding to the precipitation of the stable Zr 2 Cu phase. That is to say, the addition of Ni to the Zr–Cu binary alloy alters its crystallization pathway. It is of interest to note that the value of endothermic heat effect related to the appearance of supercooled liquid region becomes lower at lower heating rate used in relation to the value of the exothermic heat effect which is related to crystallization of the amorphous phase. The activation energy for crystallization Ea can be estimated based on the Kissinger equation [15]:
Fig. 1. High-resolution TEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at (a) room temperature and (b) 385 8C, respectively.
H.R. Wang et al. / Journal of Alloys and Compounds 347 (2002) 101–104
Fig. 3. Relationship Zr 70 Cu 20 Ni 10 glass.
between
F S DGY S D
f d ln ]2 TX
1 /T X
Ea 1 d ] 5 2] TX R
and
ln(f /T 2X )
of
metallic
(1)
where f, T X and R are the heating rate, onset crystallization temperature (or peak temperature) and gas constant, respectively. A plot of ln(f /T 2X ) versus 1 /T X yields a straight line with slope 2 Ea /R. The relationship between ln(f /T 2X ) and 1 /T X of Zr 70 Cu 20 Ni 10 metallic glass is shown in Fig. 3. In the light of the slope of the fitted line we can calculate that the activation energy for crystallization of metallic Zr 70 Cu 20 Ni 10 glass is |388 KJ / mol. Compared with binary metallic Zr 70 Cu 30 glass, both the crystallization temperature and activation energy for crystallization of metallic Zr 70 Cu 20 Ni 10 glass are much higher, indicating that the thermal stability of Zr–Cu–Ni alloy is greatly improved by the introduction of Ni [14]. To determine the origin of the two exothermic peaks in the continuous heating DSC curves of metallic Zr 70 Cu 20 Ni 10 glass, corresponding TEM experiments were carried out for the specimens. The samples were prepared by continuously heating the melt-spun ribbons to 425 and
103
450 8C at a heating rate of 20 K / min in the DSC calorimeter, and then rapidly cooled to room temperature. Fig. 4 shows the bright-field TEM images and selectedarea diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at 425 and 450 8C. It is seen that at the first stage being associated with the exothermic reaction at lower temperature, the main crystallization product is the tetragonal Zr 2 Cu phase, which is distributed homogeneously in the matrix. When the temperature reaches 450 8C, some nanoscale particles start to precipitate, which is determined as tetragonal Zr 2 Ni crystalline phase, indicating that the second exothermic peak in the DSC curves mainly corresponds to the precipitation of nano-scale Zr 2 Ni particles. It is known that the formation of a nanostructure from an amorphous phase requires the following factors: (i) a multistage crystallization mode leading to the precipitation of a primary crystalline phase, (ii) ease of homogeneous nucleation of the primary phase, (iii) difficulty of the subsequent crystal growth reaction, and (iv) high thermal stability of the remaining amorphous phase [16]. The addition of Ni with much larger negative heats of mixing with Zr causes a change of the crystallization mode involving two distinct stages. Theoretically, due to the much larger negative heat of mixing between Ni and Zr atoms than that between Cu and Zr atoms, the Zr 2 Ni crystalline phase should precipitate first instead of the Zr 2 Cu phase. However, it is known from Ref. [8] that some medium-range order atomic clusters exist in the liquid Cu–Ni alloy consisting of Cu atoms with centered Ni atoms, which exist up to temperatures of 1400 8C. During rapid quenching conditions some atomic clusters might be preserved in the amorphous state and exert an influence on the properties of the melt-spun Zr–Cu–Ni alloy. These atomic clusters may act as nucleation sites, which decrease the nucleation barrier and facilitate the crystallization process. As far as the Ni atoms surrounded by Cu atoms are concerned, it is necessary to break through the restriction of Cu–Ni atomic clusters to make a long-range redistribution to form the Zr 2 Ni phase. As
Fig. 4. Bright-field TEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at (a) 425 8C and (b) 450 8C, respectively.
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H.R. Wang et al. / Journal of Alloys and Compounds 347 (2002) 101–104
regards the Cu atoms it is much easier to form the Zr 2 Cu phase. It is the above-mentioned reason that partly results in the formation of nano-scale Zr 2 Ni particles, which is in accordance with experimental fact. On the other hand, this behavior can also be explained based on the Kirkendall effect between Ni and Cu atoms. It is generally known that Cu and Ni atoms can form a solid solution with infinite solubility. In fact, the Cu atoms can diffuse into Ni atoms during solid diffusion but the inversion does not work because of the Kirkendall effect. It is known that the inter-diffusion coefficients between Cu and Ni are 3.37310 218 and 5.22310 224 cm 2 / s at 350 8C, respectively. From this we know that the diffusion coefficient of Cu / Ni is much larger than that of Ni / Cu, which partly results in the preferential nucleation and growth of the Zr 2 Cu phase than the Zr 2 Ni phase. For this reason, nano-scale Zr 2 Ni particles during the crystallization are formed.
4. Conclusions The crystallization of metallic Zr 70 Cu 20 Ni 10 glass under continuous heating conditions proceeds via a double stage mode. Addition of Ni to the Zr–Cu binary alloys may effectively improve the glass formability. The first exothermic peak in the DSC scan of metallic Zr 70 Cu 20 Ni 10 glass mainly corresponds to the nucleation and growth of Zr 2 Cu phase, while the second peak mainly corresponds to the formation of Zr 2 Ni nanoparticles. Ni plays an important role in the stability of the metallic Zr 70 Cu 20 Ni 10 glass and the crystallization process of metallic Zr 70 Cu 20 Ni 10 glass is mainly controlled by the diffusion of the Cu atoms.
Acknowledgements One of the authors (H.R. Wang) would like to thank Professor Y.F. Deng for help in performing the HRTEM
experiments. Financial support by the (1) National Natural Science Foundation of China under grant No. 59871025, 50171006, (2) Hi-tech Research and Development Program of China (863) under grant No. 2001AA331010, and (3) National Major Basic Research Project of China (973) ‘Scientific Foundation of Advanced Preparation, Forming and Processing for Materials’ with grant No. G2000 67201-3 are gratefully acknowledged.
References [1] A. Inoue, K. Kita, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 722. [2] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [3] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1990) 177. [4] T. Zhang, A. Inoue, T. Masumoto, Mater. Trans. JIM 32 (1991) 1005. ¨ [5] L.Q. Xing, J. Eckert, W. Loser, L. Schultz, Appl. Phys. Lett. 74 (1999) 664. [6] M. Maret, P. Chieux, P. Hicter, M. Atzmon, W.L. Johnson, J. Phys. F: Met. Phys. 17 (1987) 315. [7] M. Maret, P. Chieux, T. Pomme, A. Pasturel, Phys. Rev. B 42 (1990) 1598. [8] H.R. Wang, X.L. Tian, X.Y. Teng, Z.Q. Shi, Y.F. Ye, G.H. Min, J.Y. Qin, Chin. Phys. Lett. 19 (2002) 233. [9] A. Inoue, Acta Mater. 48 (2000) 279. [10] A. Takeuchi, A. Inoue, Mater. Trans. JIM 41 (2000) 1372. [11] V.T. Witusiewicz, F. Sommer, Metall. Mater. Trans. 31B (2000) 277. [12] X.D. Hui, G.L. Chen, G. He, Z. Bian, X.M. Wang, Trans. Nonferrous Met. Soc. China 11 (2001) 684. [13] A. Takeuchi, A. Inoue, Mater. Trans. JIM 42 (2001) 1435. [14] C. Li, L. Wang, A. Inoue, J. Phys.: Condens. Matter 13 (2001) L803. [15] Z. Altounian, G.H. Tu, J.O. Strom-Olsen, J. Appl. Phys. 53 (1982) 4755. [16] A. Inoue, C. Fan, A. Takeuchi, Mater. Sci. Forum 307 (1999) 1.
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Formation of nanocrystals in metallic Zr–Cu–Ni glass a, b a a,c a Huan-Rong Wang *, Xi-Dong Hui , Guang-Hui Min , Yi-Fu Ye , Xin-Ying Teng , a Zhi-Qiang Shi a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, South Campus of Shandong University, Shandong University, Jinan 250061, PR China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China c College of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 13 March 2002; received in revised form 17 April 2002; accepted 17 April 2002
Abstract Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are employed to investigate the crystallization behavior in metallic Zr 70 Cu 20 Ni 10 glass during continuous heating. It is found that the DSC curves of metallic Zr 70 Cu 20 Ni 10 glass exhibit two exothermic peaks, indicating that the crystallization process proceeds via a double stage mode. TEM microstructural analysis confirms that the first exothermic reaction in the DSC traces of metallic Zr 70 Cu 20 Ni 10 glass mainly corresponds to the precipitation of the Zr 2 Cu phase, while the second one corresponds to the formation of nano-scale Zr 2 Ni particles. Ni plays an important role in the stability of the nanocrystals, and its effect is also discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous materials; Nanostructured materials; Rapid solidification; Calorimetry; Transmission electron microscopy (TEM)
1. Introduction In recent years, nano-materials have received increasing attention due to their particular properties, i.e. high strength, high hardness, high toughness, etc. More recently, a novel class of bulk metallic glasses characterized by the appearance of a wide supercooled liquid regime exceeding 60 K, as exemplified by La–Al–Ni [1], Zr–Ti– Ni–Cu–Be [2], Zr–Al–Ni [3], Zr–Cu–Ni–Al [4] and Zr–Ti–Ni–Cu–Al [5], has received increasing attention due to exceptional good glass forming ability (GFA). It is of interest to note that all the above-mentioned alloy systems contain Ni, implying that Ni plays an important role in the synthesis of these bulk metallic glasses. Maret et al. [6,7] attributed the prepeak in the structure factor of metallic Ni 33 Y 67 glass and liquid Al 80 Ni 20 alloy to the interactions between Ni atoms. The author of this paper has found a distinct prepeak in the structure factor of liquid Cu 70 Ni 30 alloy and also believes that it is strongly associated with the Ni–Ni interactions [8]. In addition, the formation of ternary amorphous Zr–Cu–Ni alloy does not satisfy the widely accepted empirical rules of glass forma*Corresponding author. E-mail address: [email protected] (H.R. Wang).
tion proposed by Inoue [9], i.e. the requirement of a significant difference in atomic size ratios above 12% among the three main constituent elements (only 3% between Cu and Ni atoms) and the requirement of negative heats of mixing among them (positive value of 4 KJ / mol between Cu and Ni atoms) [10]. Therefore, it remains an urgent problem to conduct an in-depth study on the GFA and crystallization behavior of metallic Zr–Cu–Ni glasses, which will provide some useful information for alloy design to achieve new bulk metallic glasses. Unfortunately, few experimental investigations have been carried out on such materials except for some thermodynamic calculations [11–13]. The aim of this study is to investigate the crystallization behavior of metallic Zr 70 Cu 20 Ni 10 glass.
2. Experimental An alloy ingot with a nominal composition of Zr 70 Cu 20 Ni 10 was prepared by arc melting a mixture of pure zirconium (3 N), high-purity copper (5 N) and nickel (5 N) under an argon atmosphere of 99.999% purity. The alloy ingot was remelted several times to ensure homogeneity. A melt-spun ribbon with a thickness of |60 mm was fabricated by vacuum melt-spinning. The diameter of
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00677-1
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H.R. Wang et al. / Journal of Alloys and Compounds 347 (2002) 101–104
the copper roller and the surface velocity were 20 cm and 20 m / s, respectively. The experiment was performed under a high-purity argon atmosphere after the chamber had been evacuated in a vacuum of 3310 23 Pa. The amorphicity of the melt-spun ribbon was studied by a D/ max-rB diffractometer with Cu Ka radiation. The thermal property of the melt-spun ribbon was examined using a Netzsch DSC404 calorimeter. Transmission electron microscopy (TEM) experiments were carried out with an H-800 type microscope, operated at 150 kV. High-resolution TEM (HRTEM) experiments were performed using a JEOL200CX instrument with an accelerating voltage of 200 kV. The samples for TEM observations were heat treated in a CARBOLITE CWF / 1300-5 muffle furnace in a vacuum of 10 23 Pa.
3. Results and discussion The melt-spun ribbon with a thickness of 60 mm was observed to be fully amorphous by XRD as well as by high-resolution TEM. Fig. 1 shows the HRTEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at room temperature and 385 8C, respectively. From Fig. 1(a) only a bright diffraction halo can be seen, suggesting that the microstructure of the metallic Zr 70 Cu 20 Ni 10 glass is fully disordered. When the temperature reaches 385 8C, which is located in the supercooled liquid region, structural relaxation takes place and many atomic clusters start to precipitate, as shown in Fig. 1(b). These atomic clusters may act as nucleation sites, which plays an important role in the subsequent crystallization. Typical continuous heating DSC curves of
Fig. 2. DSC traces of metallic Zr 70 Cu 20 Ni 10 glass at various heating rates.
metallic Zr 70 Cu 20 Ni 10 glass at different heating rates are shown in Fig. 2, in which two distinct separable exothermic peaks can be observed, indicating that the crystallization proceeds through a double stage mode. It is known from Ref. [14] that the binary Zr 70 Cu 30 amorphous alloy crystallizes through predominantly one exothermic reaction corresponding to the precipitation of the stable Zr 2 Cu phase. That is to say, the addition of Ni to the Zr–Cu binary alloy alters its crystallization pathway. It is of interest to note that the value of endothermic heat effect related to the appearance of supercooled liquid region becomes lower at lower heating rate used in relation to the value of the exothermic heat effect which is related to crystallization of the amorphous phase. The activation energy for crystallization Ea can be estimated based on the Kissinger equation [15]:
Fig. 1. High-resolution TEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at (a) room temperature and (b) 385 8C, respectively.
H.R. Wang et al. / Journal of Alloys and Compounds 347 (2002) 101–104
Fig. 3. Relationship Zr 70 Cu 20 Ni 10 glass.
between
F S DGY S D
f d ln ]2 TX
1 /T X
Ea 1 d ] 5 2] TX R
and
ln(f /T 2X )
of
metallic
(1)
where f, T X and R are the heating rate, onset crystallization temperature (or peak temperature) and gas constant, respectively. A plot of ln(f /T 2X ) versus 1 /T X yields a straight line with slope 2 Ea /R. The relationship between ln(f /T 2X ) and 1 /T X of Zr 70 Cu 20 Ni 10 metallic glass is shown in Fig. 3. In the light of the slope of the fitted line we can calculate that the activation energy for crystallization of metallic Zr 70 Cu 20 Ni 10 glass is |388 KJ / mol. Compared with binary metallic Zr 70 Cu 30 glass, both the crystallization temperature and activation energy for crystallization of metallic Zr 70 Cu 20 Ni 10 glass are much higher, indicating that the thermal stability of Zr–Cu–Ni alloy is greatly improved by the introduction of Ni [14]. To determine the origin of the two exothermic peaks in the continuous heating DSC curves of metallic Zr 70 Cu 20 Ni 10 glass, corresponding TEM experiments were carried out for the specimens. The samples were prepared by continuously heating the melt-spun ribbons to 425 and
103
450 8C at a heating rate of 20 K / min in the DSC calorimeter, and then rapidly cooled to room temperature. Fig. 4 shows the bright-field TEM images and selectedarea diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at 425 and 450 8C. It is seen that at the first stage being associated with the exothermic reaction at lower temperature, the main crystallization product is the tetragonal Zr 2 Cu phase, which is distributed homogeneously in the matrix. When the temperature reaches 450 8C, some nanoscale particles start to precipitate, which is determined as tetragonal Zr 2 Ni crystalline phase, indicating that the second exothermic peak in the DSC curves mainly corresponds to the precipitation of nano-scale Zr 2 Ni particles. It is known that the formation of a nanostructure from an amorphous phase requires the following factors: (i) a multistage crystallization mode leading to the precipitation of a primary crystalline phase, (ii) ease of homogeneous nucleation of the primary phase, (iii) difficulty of the subsequent crystal growth reaction, and (iv) high thermal stability of the remaining amorphous phase [16]. The addition of Ni with much larger negative heats of mixing with Zr causes a change of the crystallization mode involving two distinct stages. Theoretically, due to the much larger negative heat of mixing between Ni and Zr atoms than that between Cu and Zr atoms, the Zr 2 Ni crystalline phase should precipitate first instead of the Zr 2 Cu phase. However, it is known from Ref. [8] that some medium-range order atomic clusters exist in the liquid Cu–Ni alloy consisting of Cu atoms with centered Ni atoms, which exist up to temperatures of 1400 8C. During rapid quenching conditions some atomic clusters might be preserved in the amorphous state and exert an influence on the properties of the melt-spun Zr–Cu–Ni alloy. These atomic clusters may act as nucleation sites, which decrease the nucleation barrier and facilitate the crystallization process. As far as the Ni atoms surrounded by Cu atoms are concerned, it is necessary to break through the restriction of Cu–Ni atomic clusters to make a long-range redistribution to form the Zr 2 Ni phase. As
Fig. 4. Bright-field TEM images and selected-area diffraction patterns of metallic Zr 70 Cu 20 Ni 10 glass at (a) 425 8C and (b) 450 8C, respectively.
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H.R. Wang et al. / Journal of Alloys and Compounds 347 (2002) 101–104
regards the Cu atoms it is much easier to form the Zr 2 Cu phase. It is the above-mentioned reason that partly results in the formation of nano-scale Zr 2 Ni particles, which is in accordance with experimental fact. On the other hand, this behavior can also be explained based on the Kirkendall effect between Ni and Cu atoms. It is generally known that Cu and Ni atoms can form a solid solution with infinite solubility. In fact, the Cu atoms can diffuse into Ni atoms during solid diffusion but the inversion does not work because of the Kirkendall effect. It is known that the inter-diffusion coefficients between Cu and Ni are 3.37310 218 and 5.22310 224 cm 2 / s at 350 8C, respectively. From this we know that the diffusion coefficient of Cu / Ni is much larger than that of Ni / Cu, which partly results in the preferential nucleation and growth of the Zr 2 Cu phase than the Zr 2 Ni phase. For this reason, nano-scale Zr 2 Ni particles during the crystallization are formed.
4. Conclusions The crystallization of metallic Zr 70 Cu 20 Ni 10 glass under continuous heating conditions proceeds via a double stage mode. Addition of Ni to the Zr–Cu binary alloys may effectively improve the glass formability. The first exothermic peak in the DSC scan of metallic Zr 70 Cu 20 Ni 10 glass mainly corresponds to the nucleation and growth of Zr 2 Cu phase, while the second peak mainly corresponds to the formation of Zr 2 Ni nanoparticles. Ni plays an important role in the stability of the metallic Zr 70 Cu 20 Ni 10 glass and the crystallization process of metallic Zr 70 Cu 20 Ni 10 glass is mainly controlled by the diffusion of the Cu atoms.
Acknowledgements One of the authors (H.R. Wang) would like to thank Professor Y.F. Deng for help in performing the HRTEM
experiments. Financial support by the (1) National Natural Science Foundation of China under grant No. 59871025, 50171006, (2) Hi-tech Research and Development Program of China (863) under grant No. 2001AA331010, and (3) National Major Basic Research Project of China (973) ‘Scientific Foundation of Advanced Preparation, Forming and Processing for Materials’ with grant No. G2000 67201-3 are gratefully acknowledged.
References [1] A. Inoue, K. Kita, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 722. [2] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [3] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1990) 177. [4] T. Zhang, A. Inoue, T. Masumoto, Mater. Trans. JIM 32 (1991) 1005. ¨ [5] L.Q. Xing, J. Eckert, W. Loser, L. Schultz, Appl. Phys. Lett. 74 (1999) 664. [6] M. Maret, P. Chieux, P. Hicter, M. Atzmon, W.L. Johnson, J. Phys. F: Met. Phys. 17 (1987) 315. [7] M. Maret, P. Chieux, T. Pomme, A. Pasturel, Phys. Rev. B 42 (1990) 1598. [8] H.R. Wang, X.L. Tian, X.Y. Teng, Z.Q. Shi, Y.F. Ye, G.H. Min, J.Y. Qin, Chin. Phys. Lett. 19 (2002) 233. [9] A. Inoue, Acta Mater. 48 (2000) 279. [10] A. Takeuchi, A. Inoue, Mater. Trans. JIM 41 (2000) 1372. [11] V.T. Witusiewicz, F. Sommer, Metall. Mater. Trans. 31B (2000) 277. [12] X.D. Hui, G.L. Chen, G. He, Z. Bian, X.M. Wang, Trans. Nonferrous Met. Soc. China 11 (2001) 684. [13] A. Takeuchi, A. Inoue, Mater. Trans. JIM 42 (2001) 1435. [14] C. Li, L. Wang, A. Inoue, J. Phys.: Condens. Matter 13 (2001) L803. [15] Z. Altounian, G.H. Tu, J.O. Strom-Olsen, J. Appl. Phys. 53 (1982) 4755. [16] A. Inoue, C. Fan, A. Takeuchi, Mater. Sci. Forum 307 (1999) 1.