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Quasicrystals and nano-quasicrystals in annealed ZrAlNiCuAg metallic glasses
Intermetallics 8 (2000) 493±498
Quasicrystals and nano-quasicrystals in annealed ZrAlNiCuAg metallic glasses M.W. Chen a,b,*, A. Inoue a, T. Zhang a, A. Sakai b, T. Sakurai a a
b
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Mesoscopic Materials Research Center, Kyoto University, Kyoto 606-8501, Japan
Abstract Quasicrystals and nano-quasicrystals are observed in annealed Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses. Through systematic transmission electron microscopy (TEM) analyses, selected area electron diraction (SAED) and nanobeam electron diraction (NBED) patterns corresponding to ®ve-, three- and twofold rotational symmetry are obtained, demonstrating that the precipitated phases in the ®rst stage devitri®cation of the alloys are icosahedral quasicrystalline phases. The discovery of the quasicrystals directly re¯ects the intrinsic relationship between the Zr-based bulk metallic glasses and the icosahedral structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: B. Phase transformations; C. Nanocrystals; C. Rapid solidi®cation processing; D. Microstructure; F. Electron microscopy transmission
1. Introduction Since the discovery of Zr-based bulk metallic glasses with excellent glass forming ability and low critical cooling rates [1,2], extensive research has been carried out on the new structural and functional materials. To understand the formation mechanism of the Zr-based bulk metallic glasses, various suggestions, such as the confusion principle [3] and empirical rules [4], were proposed in the last several years. However, the physical nature of the bulk metallic glasses is still unclear. Generally, an understanding of the ®rst stage of devitri®cation can re¯ect the formation mechanism of metallic glasses. However, the understanding of the devitri®cation of Zr-based bulk metallic glasses is very poor, even though some advanced techniques, such as small angle neutron scattering [5,6], atom probe (AP) [7], and electron-diraction intensity measurement [8], have been employed to investigate the decomposition behavior of Zr-based bulk metallic glasses. Recently, Koster et al. [9] reported that quasicrystalline phases were observed in the partially crystallized ZrCuAl and ZrCuNiAl * Corresponding author at present address: Department of Mechanical Engineering, Centre for Materials Science and Engineering, Navel Postgraduate School, Monterey, CA 93943, USA. Tel.: +1-831-656-2851; fax: +81-831-384-8834. E-mail address: [email protected] (M.W. Chen).
metallic glasses. However, such results were not observed in other work. Lately, Eckert et al. [10] demonstrated that the metastable quasicrystal phases in the reported alloys only appear at the early stage of crystallization, and that their formation is very sensitive to the concentration of impurity oxygen. By using atom probe, the existence of the oxygen enriched phases was con®rmed in a nano-crystallized ZrCuAlPd metallic glass [7]. However, since the particle size and volume fraction were too small and low to be identi®ed by Xray diraction (XRD) and electron diraction convincingly, it was dicult to determine that the oxygen enriched nano-particles were quasicrystals. It should be pointed out that all the results on the quasicrystalline phases in the partially devitri®ed ZrCuAl, ZrCuNiAl and ZrTiCuNiAl metallic glasses were based on X-ray diraction analyses [9±11]. Since the quasicrystalline phases only precipitate at the early stage of crystallization in these alloys, the low volume fraction and the small particle size of the quasicrystalline phases produce weak and broad X-ray diraction peaks [10,11]. In addition, the formation of quasicrystals in these alloys is susceptible to the impurity content and fabrication condition. There have been no systematic TEM studies to document the quasicrystalline symmetry in the reported alloys. Thus, the existence of quasicrystalline phases in Zr-based bulk metallic glasses is still debatable. However, whether or not the quasicrystalline
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00144-2
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M.W. Chen et al. / Intermetallics 8 (2000) 493±498
2. Experimental procedure
Fig. 1. DSC traces of as-quenched (a) Zr65Al7.5Ni10Cu12.5Ag5 and (b) Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses.
phases exist is very important to understand the formation and decomposition of Zr based bulk metallic glasses. In the present paper we report our TEM observations on the quasicrystals and nano-quasicrystals precipitating from Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses during the ®rst stage of devitri®cation.
Multicomponent alloys with nominal compositions of Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 (at%) were used in the present research. The alloy ingots were prepared from high-purity constituent elements in an argon atmosphere with an arc furnace. Glassy ribbons with a cross-section of about 0.031.5 mm were prepared by a single roller melt spinning apparatus with a wheel velocity of 4000 rpm. The endothermic and exothermic reactions associated with the glass transformation temperature (Tg ), and the crystallization start temperature, respectively, of the alloys were measured in a continuous heating mode by dierential scanning calorimetry (DSC) at a heating rate of 40 K/min. Two exothermic peaks appear in both DSC traces of the two alloys (Fig. 1), indicating that the devitri®cation of the two metallic glasses takes place in two stages. The values of Tg and the ®rst crystallization starting temperature (Tx1 ) are measured to be 656.3 and 734.4 K for the Zr65Al7.5Ni10Cu12.5Ag5 alloy, and 652.1 and 702.3 K for the Zr65Al7.5Ni10 Cu7.5Ag10 alloy, respectively. The glassy ribbons, encapsulated in quartz tubes with vacuum of about 10ÿ5 torr, were annealed at selected temperatures, which corresponds to dierent condensed states of the alloys according to the DSC
Fig. 2. Bright ®eld micrograph of the icosahedral grains and the small amount of intergranular residual amorphous (a) of a Zr65Al7.5Ni10Cu12.5Ag5 alloy annealed at 750 K for 5 min, and SAED patterns of the icosahedral phase along the directions of (b) ®ve-fold, (c) three-fold, and (d) two-fold.
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495
traces, and then were quenched into water with the quartz tubes. The foils for TEM observations were prepared by ion milling. TEM observations were performed in a Philips CM-12 transmission electron microscope equipped with nano-probe. X-ray diraction (XRD) was conducted by using conventional X-ray diractometry (Cu Ka). 3. Experimental results 3.1. Quasicrystals Fig. 2(a) shows a bright ®eld TEM image of the Zr65Al7.5Ni10Cu12.5Ag5 metallic glass annealed at 750 K for 5 min, in which the size of the precipitated phase with a weakly dendritic shape is around 500 nm and only a small amount of a residual amorphous phase remains among the grains. Systematic TEM rotation analysis on the precipitated phase was carried out by using a double tilting specimen holder. The SAED patterns shown from Fig. 2(b)±(d) reveal the characteristic sequence of ®ve-, three- and two-fold axes as expected from the simple icosahedral point group. The tilt angles from (b) to (c) and (c) to (d) are about 21 and 36 , respectively, which is basically consistent with the theoretical values [1] within the accuracy of the goniometer of the microscope. Meanwhile, no appreciable second phase is seen in the samples except the residual amorphous phase among the icosahedral grains. In addition, weak superlattice re¯ection spots can be observed in the SAED patterns, indicating that a certain kind of chemical ordering appears in the quasicrystalline phase. Fig. 3 shows XRD patterns from the glassy Zr65Al7.5Ni10Cu12.5Ag5 alloy annealed at temperatures ranging from 641 K (below Tg ) to 850 K (over the ®rst exothermic peak), which covers the entire supercooled liquid range of the alloy. In the spectra from Fig. 3(a)±(c), almost all the peaks can be indexed as the icosahedral phase with a quasi-lattice constant about 0.2533 nm, indicating that the quasicrystalline phase can be formed from both the glassy state and the supercooled liquid in a wide annealing temperature range. However, when the annealing temperature is higher than that of the ®rst exothermic peak, the icosahedral phase is not present in the XRD spectrum [Fig. 3(d)]. Therefore, it can be concluded that the ®rst exothermic peak in the DSC trace corresponds exclusively to the formation of the icosahedral phase in the metallic glass. 3.2. Nano-quasicrystals Since nano-particles precipitating from metallic glasses usually bene®t the strength of the metallic glasses, it is important for practical applications to develop nanostructures consisting of nano-crystals or nano-quasicrystals
Fig. 3. XRD patterns of an annealed Zr65Al7.5Ni10Cu12.5Ag5 metallic glass. The annealed conditions: (a) 641 K for 36 h, (b) 700 K for 15 min, (c) 750 K for 5 min, and (d) 800 K for 5 min.
embedding in glassy matrix. By optimizing the annealing conditions, icosahedral quasicrystals with size below 40 nm can be obtained in the Zr65Al7.5Ni10Cu12.5Ag5 alloy. However, the icosahedral grains in the alloy coarsen readily and the grain size is usually over hundreds of nano-meters in supercooled liquid range [12,13]. When the concentration of Ag is increased up to 10 at% in the alloy system, an icosahedral quasicrystalline phase with size around 20 nm is easily obtained during the ®rst stage of devitri®cation in the Zr65Al7.5Ni10Cu7.5Ag10 metallic glass. The microstructures of the Zr65Al7.5Ni10Cu7.5Ag10 metallic glass annealed at 700 K for 240 and 270 s are shown in Fig. 4(a) and (b). It can be observed that the precipitated phase with size around 20 nm can be formed in the samples annealed for 240 s [Fig. 4(a)]. Separate mechanical testing shows that, corresponding to the nano-structure, the strength of the metallic glass is signi®cantly improved without obvious brittleness with respect to the fully glassy state [14]. When the annealing time is increased to 270 s, the density of the nano-particles signi®cantly increases and only slight change on the maximum particle size is observed. Selected area electron diraction (SAED) patterns corresponding
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M.W. Chen et al. / Intermetallics 8 (2000) 493±498
Fig. 4. TEM micrographs and SAED patterns of a glassy Zr65Al7.5Ni10Cu7.5Ag10 alloy annealed at 700 K for 240 s (a) and (c), and for 270 s (b) and (d).
to Fig. 4(a) and (b) are shown in Fig. 4(c) and (d). The patterns are almost the same except the intensity of the diraction rings of the nano-phase, suggesting that only the volume fraction of the nano-phase increases and no other new phases appear during the annealing process. All the diraction rings corresponding to nano-particles can be indexed as an icosahedral quasicrystalline phase. To further con®rm the nano-phase, separate NBED with a nominal electron beam size of 5 nm was employed. The NBED patterns taken from the nanophase are shown in Fig. 5 (a)±(d), corresponding to ®ve-, three-, two-fold and mirror axes of a simple icosahedral point group. In addition, weak diraction halos are visible in the NBED patterns, indicating that the residual matrix phase is amorphous. Combining with the SAED results, it can be determined that the nano-particles in the annealed Zr65Al7.5Ni10Cu7.5Ag10 metallic glass are an icosahedral quasicrystalline phase. 4. Discussion and summary The present TEM and XRD results convincingly demonstrate the existence of an icosahedral quasicrystalline
phase in the partially devitri®ed ZrCuAlNiAg metallic glasses. Compared with the quasicrystalline phases precipitated from ZrCuNiAl and ZrTiCuNiAl metallic glasses, the icosahedral phases in the ZrAlNiCuAg alloys can be formed in a much wider annealing temperature range [13], suggesting that Ag has a bene®cial eect on the formation of quasicrystals in Zr based alloys. In addition, the high volume fraction of the quasicrystalline phases in the alloys suggests that the compositions of the alloys are very close to those of quasicrystalline phases. The atom probe and EDX analyses have demonstrated that the quasicrystalline phase contains all the alloying elements present in the alloy, and the composition dierence between the quasicrystalline phases and the residual amorphous phase is very small [15]. It reveals that the transformation from the supercooled liquid to the quasicrystalline phase is almost polymorphous. Generally, the formation of nano-crystals in crystallized metallic glasses requires that crystal growth should be suciently slow to provide time for further nucleation events to occur. Because of the necessity of volume diusion for nucleation and growth of primary crystals, primary crystallization is the most promising reaction
M.W. Chen et al. / Intermetallics 8 (2000) 493±498
497
Fig. 5. NBED patterns taken from the nano-phase in Fig. 4(a) corresponding to (a) ®ve-fold, (b) three-fold, (c) two-fold, and (d) mirror axes of a simple icosahedral point group.
for the formation of nano-crystalline microstructures [16]. Even though the nano-structure can be formed by primary devitri®cation in the Zr-based metallic glasses, however, no obvious partitioning of alloying elements takes place during the formation of nano-quasicrystals [15], indicating that the contribution of the long-range diusion on the formation of nano-quasicrystals is limited in this alloy system. The growth of the nano-quasicrystals may be controlled by other factors, such as phason strain [17], and energy and entropy related to the formation of the quasicrystalline phase [18]. However, based on the present TEM observations, it is clear that the formation of nano-quasicrystals is closely related to the high nucleation density and the high nucleation rate of the quasicrystalline phase. The excess addition of Ag may contribute to increase the nucleation sites, since the concentration of Ag in the nanoquasicrystals is slightly higher than that in the residual amorphous matrix [15].
The discovery of the quasicrystalline phase as the product of the ®rst stage crystallization directly re¯ects the natural relationship between the icosahedral structure and the possible atomic con®gurations of the Zrbased bulk metallic glasses; this has been well discussed in some simple alloying systems [17,19]. While there is no direct experimental evidence, various studies support the model of icosahedral clusters as elementary units in the icosahedral phases [20]. It is also postulated that the metallic glasses may consist of randomly oriented icosahedral clusters, because icosahedral clusters are extremely stable in liquids and glasses [19]. Even though we cannot conclude that the icosahedral cluster is one kind of the structure units in the Zr-based bulk metallic glasses only based on the present results, the discovery of the ordered icosahedral phase in the partially devitri®ed Zr-based metallic glasses directly demonstrates that the formation of icosahedral structure is favored over that of competing crystalline phases in the supercooled liquid
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M.W. Chen et al. / Intermetallics 8 (2000) 493±498
of the multicomponent metallic glasses because less atomic reconstruction may be required. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1990;31:177. Peker A, Johnson WL. Appl Phys Lett 1993;63:2342. Greer AL. Nature 1993;366:303. Inoue A. Sci Rep Res Inst Tohoku Univ 1996;A42:1. Schneider S, Thiyagarajan P, Johnson WL. Appl Phys Lett 1996;68:493. Liu JM. Appl Phys Lett 1997;70:1968. Chen MW, Inoue A, Sakurai T, Ping DH, Hono K. Appl Phys Lett 1999;74:812. Wang WH, Wei Q, Friedrich S. Phys Rev 1998;B57:8211. Koster U, Meinhardt J, Roos S, Liebertz H. Appl Phys Lett 1996;69:179.
[10] Eckert J, Mattern N, Zinkevitch M, Seidel M. Mater Trans JIM 1998;39:632. [11] Xing LQ, Eckert J, Loser W, Schultz L. Appl Phys Lett 1998;73:2110. [12] Chen MW, Zhang T, Inoue A, Sakai A, Sakurai T. Appl Phys Lett 1999;75:1697. [13] Chen MW, Inoue A, Zhang T, Sakai A, Sakurai T. Phil Mag Lett, in press. [14] Inoue A, Zhang T, Chen MW, Sakurai T. Mater Trans JIM 1999;40:1382. [15] Chen MW, Inoue A, Zhang T, Sakai A, Sakurai T, unpublished work. [16] Koster U, Meinhardt J. Mater Sci Enging 1994;A178:271. [17] Kelton KF. International Materials Reviews 1993;38:105. [18] Joseph D, Elser V. Phys Rev Lett 1997;79:1066. [19] Nelson DR, Spaepen F. Solid State Physics 1989;42:1. [20] Ebert Ph, Feuerbacher M, Tamura N, Wollgarten M, Urban K. Phys Rev Lett 1996;77:3827.
Quasicrystals and nano-quasicrystals in annealed ZrAlNiCuAg metallic glasses M.W. Chen a,b,*, A. Inoue a, T. Zhang a, A. Sakai b, T. Sakurai a a
b
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Mesoscopic Materials Research Center, Kyoto University, Kyoto 606-8501, Japan
Abstract Quasicrystals and nano-quasicrystals are observed in annealed Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses. Through systematic transmission electron microscopy (TEM) analyses, selected area electron diraction (SAED) and nanobeam electron diraction (NBED) patterns corresponding to ®ve-, three- and twofold rotational symmetry are obtained, demonstrating that the precipitated phases in the ®rst stage devitri®cation of the alloys are icosahedral quasicrystalline phases. The discovery of the quasicrystals directly re¯ects the intrinsic relationship between the Zr-based bulk metallic glasses and the icosahedral structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: B. Phase transformations; C. Nanocrystals; C. Rapid solidi®cation processing; D. Microstructure; F. Electron microscopy transmission
1. Introduction Since the discovery of Zr-based bulk metallic glasses with excellent glass forming ability and low critical cooling rates [1,2], extensive research has been carried out on the new structural and functional materials. To understand the formation mechanism of the Zr-based bulk metallic glasses, various suggestions, such as the confusion principle [3] and empirical rules [4], were proposed in the last several years. However, the physical nature of the bulk metallic glasses is still unclear. Generally, an understanding of the ®rst stage of devitri®cation can re¯ect the formation mechanism of metallic glasses. However, the understanding of the devitri®cation of Zr-based bulk metallic glasses is very poor, even though some advanced techniques, such as small angle neutron scattering [5,6], atom probe (AP) [7], and electron-diraction intensity measurement [8], have been employed to investigate the decomposition behavior of Zr-based bulk metallic glasses. Recently, Koster et al. [9] reported that quasicrystalline phases were observed in the partially crystallized ZrCuAl and ZrCuNiAl * Corresponding author at present address: Department of Mechanical Engineering, Centre for Materials Science and Engineering, Navel Postgraduate School, Monterey, CA 93943, USA. Tel.: +1-831-656-2851; fax: +81-831-384-8834. E-mail address: [email protected] (M.W. Chen).
metallic glasses. However, such results were not observed in other work. Lately, Eckert et al. [10] demonstrated that the metastable quasicrystal phases in the reported alloys only appear at the early stage of crystallization, and that their formation is very sensitive to the concentration of impurity oxygen. By using atom probe, the existence of the oxygen enriched phases was con®rmed in a nano-crystallized ZrCuAlPd metallic glass [7]. However, since the particle size and volume fraction were too small and low to be identi®ed by Xray diraction (XRD) and electron diraction convincingly, it was dicult to determine that the oxygen enriched nano-particles were quasicrystals. It should be pointed out that all the results on the quasicrystalline phases in the partially devitri®ed ZrCuAl, ZrCuNiAl and ZrTiCuNiAl metallic glasses were based on X-ray diraction analyses [9±11]. Since the quasicrystalline phases only precipitate at the early stage of crystallization in these alloys, the low volume fraction and the small particle size of the quasicrystalline phases produce weak and broad X-ray diraction peaks [10,11]. In addition, the formation of quasicrystals in these alloys is susceptible to the impurity content and fabrication condition. There have been no systematic TEM studies to document the quasicrystalline symmetry in the reported alloys. Thus, the existence of quasicrystalline phases in Zr-based bulk metallic glasses is still debatable. However, whether or not the quasicrystalline
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00144-2
494
M.W. Chen et al. / Intermetallics 8 (2000) 493±498
2. Experimental procedure
Fig. 1. DSC traces of as-quenched (a) Zr65Al7.5Ni10Cu12.5Ag5 and (b) Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses.
phases exist is very important to understand the formation and decomposition of Zr based bulk metallic glasses. In the present paper we report our TEM observations on the quasicrystals and nano-quasicrystals precipitating from Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 metallic glasses during the ®rst stage of devitri®cation.
Multicomponent alloys with nominal compositions of Zr65Al7.5Ni10Cu12.5Ag5 and Zr65Al7.5Ni10Cu7.5Ag10 (at%) were used in the present research. The alloy ingots were prepared from high-purity constituent elements in an argon atmosphere with an arc furnace. Glassy ribbons with a cross-section of about 0.031.5 mm were prepared by a single roller melt spinning apparatus with a wheel velocity of 4000 rpm. The endothermic and exothermic reactions associated with the glass transformation temperature (Tg ), and the crystallization start temperature, respectively, of the alloys were measured in a continuous heating mode by dierential scanning calorimetry (DSC) at a heating rate of 40 K/min. Two exothermic peaks appear in both DSC traces of the two alloys (Fig. 1), indicating that the devitri®cation of the two metallic glasses takes place in two stages. The values of Tg and the ®rst crystallization starting temperature (Tx1 ) are measured to be 656.3 and 734.4 K for the Zr65Al7.5Ni10Cu12.5Ag5 alloy, and 652.1 and 702.3 K for the Zr65Al7.5Ni10 Cu7.5Ag10 alloy, respectively. The glassy ribbons, encapsulated in quartz tubes with vacuum of about 10ÿ5 torr, were annealed at selected temperatures, which corresponds to dierent condensed states of the alloys according to the DSC
Fig. 2. Bright ®eld micrograph of the icosahedral grains and the small amount of intergranular residual amorphous (a) of a Zr65Al7.5Ni10Cu12.5Ag5 alloy annealed at 750 K for 5 min, and SAED patterns of the icosahedral phase along the directions of (b) ®ve-fold, (c) three-fold, and (d) two-fold.
M.W. Chen et al. / Intermetallics 8 (2000) 493±498
495
traces, and then were quenched into water with the quartz tubes. The foils for TEM observations were prepared by ion milling. TEM observations were performed in a Philips CM-12 transmission electron microscope equipped with nano-probe. X-ray diraction (XRD) was conducted by using conventional X-ray diractometry (Cu Ka). 3. Experimental results 3.1. Quasicrystals Fig. 2(a) shows a bright ®eld TEM image of the Zr65Al7.5Ni10Cu12.5Ag5 metallic glass annealed at 750 K for 5 min, in which the size of the precipitated phase with a weakly dendritic shape is around 500 nm and only a small amount of a residual amorphous phase remains among the grains. Systematic TEM rotation analysis on the precipitated phase was carried out by using a double tilting specimen holder. The SAED patterns shown from Fig. 2(b)±(d) reveal the characteristic sequence of ®ve-, three- and two-fold axes as expected from the simple icosahedral point group. The tilt angles from (b) to (c) and (c) to (d) are about 21 and 36 , respectively, which is basically consistent with the theoretical values [1] within the accuracy of the goniometer of the microscope. Meanwhile, no appreciable second phase is seen in the samples except the residual amorphous phase among the icosahedral grains. In addition, weak superlattice re¯ection spots can be observed in the SAED patterns, indicating that a certain kind of chemical ordering appears in the quasicrystalline phase. Fig. 3 shows XRD patterns from the glassy Zr65Al7.5Ni10Cu12.5Ag5 alloy annealed at temperatures ranging from 641 K (below Tg ) to 850 K (over the ®rst exothermic peak), which covers the entire supercooled liquid range of the alloy. In the spectra from Fig. 3(a)±(c), almost all the peaks can be indexed as the icosahedral phase with a quasi-lattice constant about 0.2533 nm, indicating that the quasicrystalline phase can be formed from both the glassy state and the supercooled liquid in a wide annealing temperature range. However, when the annealing temperature is higher than that of the ®rst exothermic peak, the icosahedral phase is not present in the XRD spectrum [Fig. 3(d)]. Therefore, it can be concluded that the ®rst exothermic peak in the DSC trace corresponds exclusively to the formation of the icosahedral phase in the metallic glass. 3.2. Nano-quasicrystals Since nano-particles precipitating from metallic glasses usually bene®t the strength of the metallic glasses, it is important for practical applications to develop nanostructures consisting of nano-crystals or nano-quasicrystals
Fig. 3. XRD patterns of an annealed Zr65Al7.5Ni10Cu12.5Ag5 metallic glass. The annealed conditions: (a) 641 K for 36 h, (b) 700 K for 15 min, (c) 750 K for 5 min, and (d) 800 K for 5 min.
embedding in glassy matrix. By optimizing the annealing conditions, icosahedral quasicrystals with size below 40 nm can be obtained in the Zr65Al7.5Ni10Cu12.5Ag5 alloy. However, the icosahedral grains in the alloy coarsen readily and the grain size is usually over hundreds of nano-meters in supercooled liquid range [12,13]. When the concentration of Ag is increased up to 10 at% in the alloy system, an icosahedral quasicrystalline phase with size around 20 nm is easily obtained during the ®rst stage of devitri®cation in the Zr65Al7.5Ni10Cu7.5Ag10 metallic glass. The microstructures of the Zr65Al7.5Ni10Cu7.5Ag10 metallic glass annealed at 700 K for 240 and 270 s are shown in Fig. 4(a) and (b). It can be observed that the precipitated phase with size around 20 nm can be formed in the samples annealed for 240 s [Fig. 4(a)]. Separate mechanical testing shows that, corresponding to the nano-structure, the strength of the metallic glass is signi®cantly improved without obvious brittleness with respect to the fully glassy state [14]. When the annealing time is increased to 270 s, the density of the nano-particles signi®cantly increases and only slight change on the maximum particle size is observed. Selected area electron diraction (SAED) patterns corresponding
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Fig. 4. TEM micrographs and SAED patterns of a glassy Zr65Al7.5Ni10Cu7.5Ag10 alloy annealed at 700 K for 240 s (a) and (c), and for 270 s (b) and (d).
to Fig. 4(a) and (b) are shown in Fig. 4(c) and (d). The patterns are almost the same except the intensity of the diraction rings of the nano-phase, suggesting that only the volume fraction of the nano-phase increases and no other new phases appear during the annealing process. All the diraction rings corresponding to nano-particles can be indexed as an icosahedral quasicrystalline phase. To further con®rm the nano-phase, separate NBED with a nominal electron beam size of 5 nm was employed. The NBED patterns taken from the nanophase are shown in Fig. 5 (a)±(d), corresponding to ®ve-, three-, two-fold and mirror axes of a simple icosahedral point group. In addition, weak diraction halos are visible in the NBED patterns, indicating that the residual matrix phase is amorphous. Combining with the SAED results, it can be determined that the nano-particles in the annealed Zr65Al7.5Ni10Cu7.5Ag10 metallic glass are an icosahedral quasicrystalline phase. 4. Discussion and summary The present TEM and XRD results convincingly demonstrate the existence of an icosahedral quasicrystalline
phase in the partially devitri®ed ZrCuAlNiAg metallic glasses. Compared with the quasicrystalline phases precipitated from ZrCuNiAl and ZrTiCuNiAl metallic glasses, the icosahedral phases in the ZrAlNiCuAg alloys can be formed in a much wider annealing temperature range [13], suggesting that Ag has a bene®cial eect on the formation of quasicrystals in Zr based alloys. In addition, the high volume fraction of the quasicrystalline phases in the alloys suggests that the compositions of the alloys are very close to those of quasicrystalline phases. The atom probe and EDX analyses have demonstrated that the quasicrystalline phase contains all the alloying elements present in the alloy, and the composition dierence between the quasicrystalline phases and the residual amorphous phase is very small [15]. It reveals that the transformation from the supercooled liquid to the quasicrystalline phase is almost polymorphous. Generally, the formation of nano-crystals in crystallized metallic glasses requires that crystal growth should be suciently slow to provide time for further nucleation events to occur. Because of the necessity of volume diusion for nucleation and growth of primary crystals, primary crystallization is the most promising reaction
M.W. Chen et al. / Intermetallics 8 (2000) 493±498
497
Fig. 5. NBED patterns taken from the nano-phase in Fig. 4(a) corresponding to (a) ®ve-fold, (b) three-fold, (c) two-fold, and (d) mirror axes of a simple icosahedral point group.
for the formation of nano-crystalline microstructures [16]. Even though the nano-structure can be formed by primary devitri®cation in the Zr-based metallic glasses, however, no obvious partitioning of alloying elements takes place during the formation of nano-quasicrystals [15], indicating that the contribution of the long-range diusion on the formation of nano-quasicrystals is limited in this alloy system. The growth of the nano-quasicrystals may be controlled by other factors, such as phason strain [17], and energy and entropy related to the formation of the quasicrystalline phase [18]. However, based on the present TEM observations, it is clear that the formation of nano-quasicrystals is closely related to the high nucleation density and the high nucleation rate of the quasicrystalline phase. The excess addition of Ag may contribute to increase the nucleation sites, since the concentration of Ag in the nanoquasicrystals is slightly higher than that in the residual amorphous matrix [15].
The discovery of the quasicrystalline phase as the product of the ®rst stage crystallization directly re¯ects the natural relationship between the icosahedral structure and the possible atomic con®gurations of the Zrbased bulk metallic glasses; this has been well discussed in some simple alloying systems [17,19]. While there is no direct experimental evidence, various studies support the model of icosahedral clusters as elementary units in the icosahedral phases [20]. It is also postulated that the metallic glasses may consist of randomly oriented icosahedral clusters, because icosahedral clusters are extremely stable in liquids and glasses [19]. Even though we cannot conclude that the icosahedral cluster is one kind of the structure units in the Zr-based bulk metallic glasses only based on the present results, the discovery of the ordered icosahedral phase in the partially devitri®ed Zr-based metallic glasses directly demonstrates that the formation of icosahedral structure is favored over that of competing crystalline phases in the supercooled liquid
498
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of the multicomponent metallic glasses because less atomic reconstruction may be required. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1990;31:177. Peker A, Johnson WL. Appl Phys Lett 1993;63:2342. Greer AL. Nature 1993;366:303. Inoue A. Sci Rep Res Inst Tohoku Univ 1996;A42:1. Schneider S, Thiyagarajan P, Johnson WL. Appl Phys Lett 1996;68:493. Liu JM. Appl Phys Lett 1997;70:1968. Chen MW, Inoue A, Sakurai T, Ping DH, Hono K. Appl Phys Lett 1999;74:812. Wang WH, Wei Q, Friedrich S. Phys Rev 1998;B57:8211. Koster U, Meinhardt J, Roos S, Liebertz H. Appl Phys Lett 1996;69:179.
[10] Eckert J, Mattern N, Zinkevitch M, Seidel M. Mater Trans JIM 1998;39:632. [11] Xing LQ, Eckert J, Loser W, Schultz L. Appl Phys Lett 1998;73:2110. [12] Chen MW, Zhang T, Inoue A, Sakai A, Sakurai T. Appl Phys Lett 1999;75:1697. [13] Chen MW, Inoue A, Zhang T, Sakai A, Sakurai T. Phil Mag Lett, in press. [14] Inoue A, Zhang T, Chen MW, Sakurai T. Mater Trans JIM 1999;40:1382. [15] Chen MW, Inoue A, Zhang T, Sakai A, Sakurai T, unpublished work. [16] Koster U, Meinhardt J. Mater Sci Enging 1994;A178:271. [17] Kelton KF. International Materials Reviews 1993;38:105. [18] Joseph D, Elser V. Phys Rev Lett 1997;79:1066. [19] Nelson DR, Spaepen F. Solid State Physics 1989;42:1. [20] Ebert Ph, Feuerbacher M, Tamura N, Wollgarten M, Urban K. Phys Rev Lett 1996;77:3827.