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Devitrification of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass studied by XRD, SANS, and NMR
Journal of Non-Crystalline Solids 317 (2003) 118–122 www.elsevier.com/locate/jnoncrysol
Devitrification of the Zr41:2Ti13:8Cu12:5Ni10:0Be22:5 bulk metallic glass studied by XRD, SANS, and NMR €ffler X.-P. Tang a, J.F. Lo a
b,c
, W.L. Johnson c, Y. Wu
a,*
Department of Physics and Astronomy, Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill, NC 27599-3255, USA b Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA c W. M. Keck Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
Abstract The crystallization process of the Zr41:2 Ti13:8 Cu12:5 Ni10:0 Be22:5 bulk metallic glass was investigated by X-ray diffraction (XRD), small-angle neutron scattering, and 9 Be nuclear magnetic resonance. Upon annealing near the glass transition and up to a temperature near 673 K, a Be-containing crystalline phase forms after sufficient decomposition of the supercooled liquid. The XRD pattern of this Be-containing phase is consistent with icosahedral symmetry. The small length scale of the decomposed supercooled liquid structure limits the grain size of this phase, which gives rise to broad XRD peaks. Above 673 K, annealing produces Be2 Zr and Zr2 Cu, along with other crystalline phases. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 71.23.Cq; 76.60.)k; 61.18.Fs; 61.10.)I; 61.12.Ex; 81.05.Kf; 64.75.þg
1. Introduction Bulk metallic glasses (BMG) possess significantly improved thermal stability against crystallization in the deeply undercooled liquid region compared to conventional metallic glasses [1,2]. Over the past several years, a large number of experiments have been performed to study the effects of thermal treatments on BMG. In particular, the vitreloy BMG Zr41:2 Ti13:8 Cu12:5 Ni10:0 Be22:5 (Vit1) has been investigated extensively, using a
*
Corresponding author. Tel.: +1-919 962 0307; fax: +1-919 962 0480. E-mail address: [email protected] (Y. Wu).
variety of techniques, including atom-probe field ion microscopy (APFIM) [3–6], differential scanning calorimetry (DSC) [4,7,8], small-angle neutron scattering (SANS) [7–12], X-ray diffraction (XRD) [4], and transmission electron microscopy (TEM) [3–5,7,13]. Upon annealing above 613 K (near the DSC glass transition temperature Tg ) and below a critical temperature, Tc , of around 673 K, Vit1 decomposes into two supercooled liquid phases, revealed by the appearance of an interference peak in SANS [7–12]. Time-resolved SANS experiments show [12] that this decomposition triggers the formation of nanocrystals. Indeed, nanocrystalline precipitates were detected by TEM in such decomposed systems [7,13]. From these TEM observations, however, it is not clear
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)01991-9
X.-P. Tang et al. / Journal of Non-Crystalline Solids 317 (2003) 118–122
whether the crystalline precipitates are correlated with the spatial correlation developed in the decomposed structure [7] or are distributed randomly [13]. There are also uncertainties regarding the nature of the precipitated nanocrystalline phases and it is difficult to determine quantitatively the volume fraction of nanocrystals in the amorphous matrix by TEM. Here, we report a 9 Be nuclear magnetic resonance (NMR) study of the crystallization process in Vit1 along with XRD and SANS experiments. 9 Be NMR is used to determine the fraction of Be remaining in the glassy phase upon annealing. The XRD and NMR data show that the formation of a Be-containing phase plays an important role in the devitrification process of Vit1 below Tc . Above Tc , Be2 Zr forms along with Zr2 Cu and other crystalline phases.
2. Experimental Vit1 was prepared as a 25 g ingot by arc melting the constituents ðpurity > 99:9%Þ in a titaniumgettered argon atmosphere. The ingot was melted again in a silica tube of inner diameter 10 mm ðvacuum 105 mbarÞ and subsequently waterquenched. The resulting glassy rod was cut into several 1 mm thick disks. Such disks were then annealed in vacuum at various temperatures for various times. This study focuses on four such samples, including sample A (as-prepared), sample B (annealed at 623 K for 15 h), sample C (annealed at 663 K for 1 h), and sample D (annealed at 683 K for 1 h). First, SANS and XRD measurements were carried out on these samples. The XRD measurements were performed with Co Ka radiation. The SANS measurements are described in detail elsewhere [8]. Subsequently, these samples were cut either into thin slabs of a few hundred micrometers thickness or broken up into small pieces of a few hundred micrometers in dimension to overcome the skin-depth effect in NMR measurements. The 9 Be NMR measurements were conducted in a magnetic field of 9.4 T. The 9 Be spectrum of the Vit1 glass has been shown previously [14]. The 9 Be nuclear spin-lattice relaxation curves were measured by the saturation recovery method. The 9 Be magnetization M was first satu-
119
rated to zero with a comb rf pulse sequence ð90°x 1 ms 90°y 1 ms 90°x 1 ms 90°y Þ, followed by a time interval, t, for magnetization recovery before the detection of 9 Be magnetization, MðtÞ, using a quadrupole echo sequence ð90°x s 90°y Þ.
3. Results The XRD patterns of samples A, B, C, and D are displayed in Fig. 1. Sample A shows the typical broad diffraction pattern of amorphous Vit1. Sample B shows a few narrow features at 2h ¼ 43:6°, 45.8°, and 78.4° (marked by dotted lines) indicating the occurrence of crystallization. The XRD pattern of sample C is similar to that of sample B except that the narrow features become stronger and sharper. A qualitatively different XRD pattern is observed in sample D, which exhibits many sharp diffraction peaks along with the previously observed peaks at 2h ¼ 43:6°, 45.8°, and 78.4°. A large number of diffraction peaks in
Fig. 1. XRD data of Vit1 in the as-prepared state (sample A) and after annealing for several hours (annealing temperature and time are shown in the plot). The peaks marked by dots and crosses correspond to XRD peaks of Be2 Zr and Zr2 Cu, respectively. The dotted lines positioned at 2h ¼ 43:6°, 45.8°, and 78.4° mark the (1 0 0 0 0 0), (1 1 0 0 0 0), and (1 0 1 0 0 0) diffraction peaks, respectively, of the simple icosahedral phase.
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sample D can be attributed to Be2 Zr and Zr2 Cu as shown in Fig. 1. Additional XRD peaks originate presumably from Ti2 Ni, Zr2 Ni, or a related ternary phase. It is interesting to note that the peak at 2h ¼ 43:6° also appears in the XRD pattern of Zr2 Cu and that the peak at 2h ¼ 45:8° also appears in the XRD pattern of Be2 Zr. However, attempts to generate the peak at 2h ¼ 78:4° with known binary crystalline phases and some ternary crystalline phases failed. Furthermore, the strong (1 0 0) peak of Be2 Zr at 2h ¼ 31:1° is completely missing in the XRD patterns of samples B and C indicating the absence of Be2 Zr in these samples. On the other hand, the three peaks at 2h ¼ 43:6°, 45.8°, and 78.4° in samples B and C can be indexed well as the (1 0 0 0 0 0), (1 1 0 0 0 0), and (1 0 1 0 0 0) diffraction peaks, respectively, of the primitive icosahedral structure with a quasilattice constant . Evidence of icosahedral phase of 4:58 0:02 A formation upon annealing was also observed in the Zr46:8 Ti8:2 Cu7:5 Ni10:0 Be27:5 BMG (Vit4), with a [15], and in other quasilattice constant of 4.779 A Zr-based BMGs [16,17]. The width of the diffraction peaks associated with the icosahedral phase in Vit1 are much broader than observed in Vit4 [15]. This implies that either the icosahedral phase is more disordered or that the grain size of the icosahedral phase is much smaller in Vit1 than in Vit4. This can indeed be explained by the finegrained structure developed in Vit1 as a result of the decomposition process in the supercooled liquid, and is consistent with SANS results shown in Fig. 2. Similar to as-prepared Vit1, a sample annealed at 593 K for 30 h does not show any scattering. In contrast, an interference peak develops in sample B becomes much stronger in sample C. In sample D, however, very strong scattering occurs at small Q, without the appearance of an interference maximum, indicating a different crystallization mechanism upon annealing above Tc . The development of the strong interference peak is related to decomposition with respect to Ti (and presumably Be) [4,7,12]. No icosahedral phase can be detected in Vit1 by XRD until a significant SANS interference peak develops upon annealing. Conversely, a Vit1 sample annealed at 613 K for 15 h exhibits a clear interference peak in the SANS spectrum, whereas the XRD pattern remains the
Fig. 2. SANS data of Vit1 in the as-prepared state and after annealing for several hours, as indicated in the figure (Q ¼ 4psinh=k, with h being half the scattering angle and k being the wavelength of neutrons). For a better comparison, the intensity of some samples is reduced by the scaling factor S, given in parentheses.
same as that of the amorphous sample [18]. Thus, in agreement with Ref. [12], the formation of the icosahedral phase in Vit1 is not polymorphic and the spatial extension of the decomposition process in Vit1 limits the grain size of the icosahedral phase. Fig. 3 displays the room temperature (RT) saturation recovery curve M ðtÞ ¼ ½Mð1Þ MðtÞ = ½Mð1Þ Mð0Þ versus the recovery time t for sample A. The M ðtÞ curve of sample A can be fit well by a single-exponential function M ðtÞ ¼ expðt=T1 Þ with T1 ¼ ð3:8 0:2Þ s. As reported earlier [14], T1 in the glassy phase is dominated by the Korringa relaxation mechanism, which is induced by the coupling of the 9 Be nuclear spins with the unpaired conduction electron spins near the Fermi level. Here, T1 is a measure of the density of states near the Fermi level and the characteristics of the associated wave function. In this respect, Vit1 and Vit4 are very similar since the T1 value of as-prepared Vit4 is identical to that of Vit1 [14]. As another example, the in situ b phase/BMG composite [19] was studied. The b phase dendrite has a
X.-P. Tang et al. / Journal of Non-Crystalline Solids 317 (2003) 118–122
M ðtÞ ¼ aet=T1a þ ð1 aÞet=T1b
1
121
ð1Þ
ann. 683 K, 1h as-prepared in-situ composite
M*(t)
0.1
1
as-prepared ann. 623 K, 15 h ann. 663 K, 1h
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M*(t)
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0
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t (s) Fig. 3. Measured 9 Be M ðtÞ curves for sample A (as-prepared) and D (annealed at 683 K for 1 h). The M ðtÞ curve of the in situ composite material is also shown. The solid lines are singleexponential function fits for sample A and for the in situ composite material and a double-exponential fit for sample D. The inset shows M ðtÞ curves for samples A, B (annealed at 623 K for 15 h) and C (annealed at 663 K for 1 h). The solid lines are a single-exponential function fit for sample A and a doubleexponential fit for sample C.
composition of Zr71 Ti16:3 Nb10 Cu1:8 Ni0:9 and the amorphous matrix has a composition of Zr47 Ti12:9 Nb2:8 Cu11 Ni9:6 Be16:7 [19]. The M ðtÞ curve of this in situ composite material is also shown in Fig. 3. It follows a perfect exponential decay with T1 ¼ ð3:4 0:1Þ s. Again, this value is very similar to the result in Vit1. These observations indicate that the variation of 9 Be T1 is small in these Zr-based BMGs. Thus, it is expected that the decomposition of Vit1 into two amorphous phases upon annealing below Tc would not be detected through changes of the M ðtÞ decay curve, unless the changes of composition and structure are very dramatic. Changes of composition and structure are expected to be significant upon crystallization. This could lead to a significant change of T1 . Fig. 3 shows the M ðtÞ curve of sample D where a clearly non-exponential decay of M ðtÞ is observed. Here, M ðtÞ can be fit well with a double-exponential function
with fitting parameters of a ¼ 0:480:05, T1a ¼ ð3:6 0:5Þ s, and T1b ¼ ð17 3Þ s. T1a is very similar to that of sample A suggesting that half of the Be atoms remain in the amorphous matrix. The other half of the Be nuclei give rise to T1b which is about five times longer than T1a . Since the XRD pattern of sample D reveals the presence of Be2 Zr in agreement with previous TEM and APFIM studies [5,6,13], the component associated with T1b is attributed to Be2 Zr. This assignment is confirmed by NMR measurement of pure Be2 Zr. M ðtÞ of pure Be2 Zr at RT exhibits a single exponential decay with T1 ¼ 16 s which is nearly identical to T1b of sample D. The inset of Fig. 3 shows the M ðtÞ curves of samples B and C, which exhibit an interference peak in SANS. Although the M ðtÞ curve of sample B shows a visible change from that of sample A, the deviation is too small to permit an accurate quantitative analysis. Nevertheless, it is clear that almost all 9 Be nuclei in sample B have the same T1 as those in sample A and that a very small fraction of Be (less than 2% of Be) gives rise to a longer T1 . The M ðtÞ curve of sample C is clearly not single-exponential. The M ðtÞ curve of sample C can be fit by a doubleexponential function as described by Eq. (1). The fit parameters are a ¼ 0:89 0:05, T1a ¼ ð3:8 0:2Þ s, and T1b ¼ ð15 5Þ s. Within experimental errors, these T1a and T1b values are the same as those measured in sample D. This suggests that about 89% of Be atoms remain in the amorphous regions in sample C while 11% of Be atoms belong to a crystalline or icosahedral phase.
4. Discussion The estimated specific gravity of Be2 Zr is 4.4 g/ cm3 and that of Vit1 is 6.1 g/cm3 . Since 50% of the Be atoms in sample D belong to Be2 Zr, the volume fraction of Be2 Zr in sample D is about 15%. Of course, other crystalline phases are also present in sample D. Besides Zr2 Cu, there are presumably Ti2 Ni, Zr2 Ni, or a related ternary phase. Thus, the total volume fraction of all crystalline phases in sample D can be several times higher than 15%.
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Since the strong (1 0 0) diffraction peak of Be2 Zr at 2h ¼ 31:1° is completely missing in the XRD pattern of sample C (see Fig. 1), the 11% Be associated with T1b cannot be attributed to Be2 Zr. A natural assignment of this component associated with T1b is the icosahedral phase observed by XRD in sample C. It is well known that a pseudogap exists in the electronic density-of-states at the Fermi level in icosahedral phases and that the T1 observed in icosahedral phases is significantly longer than in regular metals [20]. Therefore, it is expected that T1b is significantly longer than T1a if the phase associated with T1b is an icosahedral phase. The fact that T1b is very similar to the T1 value of Be2 Zr indicates the similarity in local electronic structure at Be sites between the icosahedral phase and Be2 Zr. This might also be related to the fact that the XRD peak at 45.8° appears in both the icosahedral phase and Be2 Zr. The participation of Be in the icosahedral phase also supports the assumption that the icosahedral phase forms in the Be-rich and Ti-poor region in the decomposed Vit1 supercooled liquid.
5. Conclusions More than 15% volume fraction of Be2 Zr forms in Vit1 upon annealing above the critical temperature Tc of 673 K as determined by both XRD and NMR. Long-time annealing below Tc did not produce Be2 Zr detectable by XRD. Instead, a phase with structure consistent with icosahedral symmetry forms. NMR results show that a large number of Be atoms participated in this devitrification process. Both XRD and NMR indicate that there is a close structural relationship between the icosahedral phase and Be2 Zr. XRD and SANS show that the formation of the icosahedral phase in Vit1 is not polymorphic and presumably occurs in a Be-rich (Ti-poor) decomposed region, as supported by the NMR results. The spatial extension of the decomposition process in the supercooled liquid state of Vit1 limits the grain size of the icosahedral phase, and leads to broader XRD diffraction peaks than are observed in Vit4.
Acknowledgements We thank C.C. Hays for providing the in situ composite sample and M. Conradi and V.D. Kodibagkar for providing the Be2 Zr sample. This work was supported by the US Army Research Office (grant no. DAAD19-99-1-0328), by the US Department of Energy (grant no. DEFG-03-86ER 45242), and by the Alexander von Humboldt Foundation via the Feodor Lynen Program (J.F.L.).
References [1] T. Zhang, A. Inoue, T. Masumoto, Mater. Trans., JIM 32 (1991) 1005. [2] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [3] R. Busch, S. Schneider, A. Peker, W.L. Johnson, Appl. Phys. Lett. 67 (1995) 1544. [4] M.-P. Macht, N. Wanderka, A. Wiedenmann, H. Wollenberger, Q. Wei, H.J. Fecht, S.G. Klose, Mater. Sci. Forum 225–227 (1996) 65. [5] N. Wanderka, U. Czubayko, P. Schubert-Bischoff, M.-P. Macht, Mater. Sci. Eng. A 270 (1999) 44. [6] M.K. Miller, Mater. Sci. Eng. A 250 (1998) 133. [7] S. Schneider, P. Thiyagarajan, W.L. Johnson, Appl. Phys. Lett. 68 (1996) 493. [8] J.F. L€ offler, P. Thiyagarajan, W.L. Johnson, J. Appl. Crystallogr. 33 (2000) 500. [9] A. Wiedenmann, U. Keiderling, M.-P. Macht, H. Wollenberger, Mater. Sci. Forum 225–227 (1996) 71. [10] S. Schneider, U. Geyer, P. Thiyagarajan, W.L. Johnson, Mater. Sci. Forum 235–238 (1997) 337. [11] J.-M. Liu, A. Wiedenmann, U. Gerold, U. Keiderling, H. Wollenberger, Phys. Stat. Sol. (b) 199 (1997) 379. [12] J.F. L€ offler, W.L. Johnson, Appl. Phys. Lett. 76 (2000) 3394. [13] N. Wanderka, Q. Wei, R. Doole, M. Jenkins, S. Friedrich, M.-P. Macht, H. Wollenberger, Mater. Sci. Forum 269– 272 (1998) 773. [14] X.-P. Tang, R. Busch, W.-L. Johnson, Y. Wu, Phys. Rev. Lett. 81 (1998) 5358. [15] N. Wanderka, M.-P. Macht, M. Seidel, S. Mechler, K. St ahl, J.Z. Jiang, Appl. Phys. Lett. 77 (2000) 3935. [16] L.Q. Xing, J. Eckert, W. Loser, L. Schultz, Appl. Phys. Lett. 73 (1998) 2110. [17] M.W. Chen, T. Zhang, A. Inoue, A. Sakai, T. Sakurai, Appl. Phys. Lett. 75 (1999) 1697. [18] J.F. L€ offler, X.-P. Tang, Y. Wu, W.L. Johnson, Mater. Res. Soc. Proc. 644 (2001) L1.9.1. [19] C.C. Hays, C.P. Kim, W.L. Johnson, Phys. Rev. Lett. 84 (2001) 2901. [20] X.-P. Tang, E.A. Hill, S.K. Wonnell, S.J. Poon, Y. Wu, Phys. Rev. Lett. 79 (1997) 1070.
Devitrification of the Zr41:2Ti13:8Cu12:5Ni10:0Be22:5 bulk metallic glass studied by XRD, SANS, and NMR €ffler X.-P. Tang a, J.F. Lo a
b,c
, W.L. Johnson c, Y. Wu
a,*
Department of Physics and Astronomy, Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill, NC 27599-3255, USA b Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA c W. M. Keck Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
Abstract The crystallization process of the Zr41:2 Ti13:8 Cu12:5 Ni10:0 Be22:5 bulk metallic glass was investigated by X-ray diffraction (XRD), small-angle neutron scattering, and 9 Be nuclear magnetic resonance. Upon annealing near the glass transition and up to a temperature near 673 K, a Be-containing crystalline phase forms after sufficient decomposition of the supercooled liquid. The XRD pattern of this Be-containing phase is consistent with icosahedral symmetry. The small length scale of the decomposed supercooled liquid structure limits the grain size of this phase, which gives rise to broad XRD peaks. Above 673 K, annealing produces Be2 Zr and Zr2 Cu, along with other crystalline phases. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 71.23.Cq; 76.60.)k; 61.18.Fs; 61.10.)I; 61.12.Ex; 81.05.Kf; 64.75.þg
1. Introduction Bulk metallic glasses (BMG) possess significantly improved thermal stability against crystallization in the deeply undercooled liquid region compared to conventional metallic glasses [1,2]. Over the past several years, a large number of experiments have been performed to study the effects of thermal treatments on BMG. In particular, the vitreloy BMG Zr41:2 Ti13:8 Cu12:5 Ni10:0 Be22:5 (Vit1) has been investigated extensively, using a
*
Corresponding author. Tel.: +1-919 962 0307; fax: +1-919 962 0480. E-mail address: [email protected] (Y. Wu).
variety of techniques, including atom-probe field ion microscopy (APFIM) [3–6], differential scanning calorimetry (DSC) [4,7,8], small-angle neutron scattering (SANS) [7–12], X-ray diffraction (XRD) [4], and transmission electron microscopy (TEM) [3–5,7,13]. Upon annealing above 613 K (near the DSC glass transition temperature Tg ) and below a critical temperature, Tc , of around 673 K, Vit1 decomposes into two supercooled liquid phases, revealed by the appearance of an interference peak in SANS [7–12]. Time-resolved SANS experiments show [12] that this decomposition triggers the formation of nanocrystals. Indeed, nanocrystalline precipitates were detected by TEM in such decomposed systems [7,13]. From these TEM observations, however, it is not clear
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)01991-9
X.-P. Tang et al. / Journal of Non-Crystalline Solids 317 (2003) 118–122
whether the crystalline precipitates are correlated with the spatial correlation developed in the decomposed structure [7] or are distributed randomly [13]. There are also uncertainties regarding the nature of the precipitated nanocrystalline phases and it is difficult to determine quantitatively the volume fraction of nanocrystals in the amorphous matrix by TEM. Here, we report a 9 Be nuclear magnetic resonance (NMR) study of the crystallization process in Vit1 along with XRD and SANS experiments. 9 Be NMR is used to determine the fraction of Be remaining in the glassy phase upon annealing. The XRD and NMR data show that the formation of a Be-containing phase plays an important role in the devitrification process of Vit1 below Tc . Above Tc , Be2 Zr forms along with Zr2 Cu and other crystalline phases.
2. Experimental Vit1 was prepared as a 25 g ingot by arc melting the constituents ðpurity > 99:9%Þ in a titaniumgettered argon atmosphere. The ingot was melted again in a silica tube of inner diameter 10 mm ðvacuum 105 mbarÞ and subsequently waterquenched. The resulting glassy rod was cut into several 1 mm thick disks. Such disks were then annealed in vacuum at various temperatures for various times. This study focuses on four such samples, including sample A (as-prepared), sample B (annealed at 623 K for 15 h), sample C (annealed at 663 K for 1 h), and sample D (annealed at 683 K for 1 h). First, SANS and XRD measurements were carried out on these samples. The XRD measurements were performed with Co Ka radiation. The SANS measurements are described in detail elsewhere [8]. Subsequently, these samples were cut either into thin slabs of a few hundred micrometers thickness or broken up into small pieces of a few hundred micrometers in dimension to overcome the skin-depth effect in NMR measurements. The 9 Be NMR measurements were conducted in a magnetic field of 9.4 T. The 9 Be spectrum of the Vit1 glass has been shown previously [14]. The 9 Be nuclear spin-lattice relaxation curves were measured by the saturation recovery method. The 9 Be magnetization M was first satu-
119
rated to zero with a comb rf pulse sequence ð90°x 1 ms 90°y 1 ms 90°x 1 ms 90°y Þ, followed by a time interval, t, for magnetization recovery before the detection of 9 Be magnetization, MðtÞ, using a quadrupole echo sequence ð90°x s 90°y Þ.
3. Results The XRD patterns of samples A, B, C, and D are displayed in Fig. 1. Sample A shows the typical broad diffraction pattern of amorphous Vit1. Sample B shows a few narrow features at 2h ¼ 43:6°, 45.8°, and 78.4° (marked by dotted lines) indicating the occurrence of crystallization. The XRD pattern of sample C is similar to that of sample B except that the narrow features become stronger and sharper. A qualitatively different XRD pattern is observed in sample D, which exhibits many sharp diffraction peaks along with the previously observed peaks at 2h ¼ 43:6°, 45.8°, and 78.4°. A large number of diffraction peaks in
Fig. 1. XRD data of Vit1 in the as-prepared state (sample A) and after annealing for several hours (annealing temperature and time are shown in the plot). The peaks marked by dots and crosses correspond to XRD peaks of Be2 Zr and Zr2 Cu, respectively. The dotted lines positioned at 2h ¼ 43:6°, 45.8°, and 78.4° mark the (1 0 0 0 0 0), (1 1 0 0 0 0), and (1 0 1 0 0 0) diffraction peaks, respectively, of the simple icosahedral phase.
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sample D can be attributed to Be2 Zr and Zr2 Cu as shown in Fig. 1. Additional XRD peaks originate presumably from Ti2 Ni, Zr2 Ni, or a related ternary phase. It is interesting to note that the peak at 2h ¼ 43:6° also appears in the XRD pattern of Zr2 Cu and that the peak at 2h ¼ 45:8° also appears in the XRD pattern of Be2 Zr. However, attempts to generate the peak at 2h ¼ 78:4° with known binary crystalline phases and some ternary crystalline phases failed. Furthermore, the strong (1 0 0) peak of Be2 Zr at 2h ¼ 31:1° is completely missing in the XRD patterns of samples B and C indicating the absence of Be2 Zr in these samples. On the other hand, the three peaks at 2h ¼ 43:6°, 45.8°, and 78.4° in samples B and C can be indexed well as the (1 0 0 0 0 0), (1 1 0 0 0 0), and (1 0 1 0 0 0) diffraction peaks, respectively, of the primitive icosahedral structure with a quasilattice constant . Evidence of icosahedral phase of 4:58 0:02 A formation upon annealing was also observed in the Zr46:8 Ti8:2 Cu7:5 Ni10:0 Be27:5 BMG (Vit4), with a [15], and in other quasilattice constant of 4.779 A Zr-based BMGs [16,17]. The width of the diffraction peaks associated with the icosahedral phase in Vit1 are much broader than observed in Vit4 [15]. This implies that either the icosahedral phase is more disordered or that the grain size of the icosahedral phase is much smaller in Vit1 than in Vit4. This can indeed be explained by the finegrained structure developed in Vit1 as a result of the decomposition process in the supercooled liquid, and is consistent with SANS results shown in Fig. 2. Similar to as-prepared Vit1, a sample annealed at 593 K for 30 h does not show any scattering. In contrast, an interference peak develops in sample B becomes much stronger in sample C. In sample D, however, very strong scattering occurs at small Q, without the appearance of an interference maximum, indicating a different crystallization mechanism upon annealing above Tc . The development of the strong interference peak is related to decomposition with respect to Ti (and presumably Be) [4,7,12]. No icosahedral phase can be detected in Vit1 by XRD until a significant SANS interference peak develops upon annealing. Conversely, a Vit1 sample annealed at 613 K for 15 h exhibits a clear interference peak in the SANS spectrum, whereas the XRD pattern remains the
Fig. 2. SANS data of Vit1 in the as-prepared state and after annealing for several hours, as indicated in the figure (Q ¼ 4psinh=k, with h being half the scattering angle and k being the wavelength of neutrons). For a better comparison, the intensity of some samples is reduced by the scaling factor S, given in parentheses.
same as that of the amorphous sample [18]. Thus, in agreement with Ref. [12], the formation of the icosahedral phase in Vit1 is not polymorphic and the spatial extension of the decomposition process in Vit1 limits the grain size of the icosahedral phase. Fig. 3 displays the room temperature (RT) saturation recovery curve M ðtÞ ¼ ½Mð1Þ MðtÞ = ½Mð1Þ Mð0Þ versus the recovery time t for sample A. The M ðtÞ curve of sample A can be fit well by a single-exponential function M ðtÞ ¼ expðt=T1 Þ with T1 ¼ ð3:8 0:2Þ s. As reported earlier [14], T1 in the glassy phase is dominated by the Korringa relaxation mechanism, which is induced by the coupling of the 9 Be nuclear spins with the unpaired conduction electron spins near the Fermi level. Here, T1 is a measure of the density of states near the Fermi level and the characteristics of the associated wave function. In this respect, Vit1 and Vit4 are very similar since the T1 value of as-prepared Vit4 is identical to that of Vit1 [14]. As another example, the in situ b phase/BMG composite [19] was studied. The b phase dendrite has a
X.-P. Tang et al. / Journal of Non-Crystalline Solids 317 (2003) 118–122
M ðtÞ ¼ aet=T1a þ ð1 aÞet=T1b
1
121
ð1Þ
ann. 683 K, 1h as-prepared in-situ composite
M*(t)
0.1
1
as-prepared ann. 623 K, 15 h ann. 663 K, 1h
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40
50
t (s) Fig. 3. Measured 9 Be M ðtÞ curves for sample A (as-prepared) and D (annealed at 683 K for 1 h). The M ðtÞ curve of the in situ composite material is also shown. The solid lines are singleexponential function fits for sample A and for the in situ composite material and a double-exponential fit for sample D. The inset shows M ðtÞ curves for samples A, B (annealed at 623 K for 15 h) and C (annealed at 663 K for 1 h). The solid lines are a single-exponential function fit for sample A and a doubleexponential fit for sample C.
composition of Zr71 Ti16:3 Nb10 Cu1:8 Ni0:9 and the amorphous matrix has a composition of Zr47 Ti12:9 Nb2:8 Cu11 Ni9:6 Be16:7 [19]. The M ðtÞ curve of this in situ composite material is also shown in Fig. 3. It follows a perfect exponential decay with T1 ¼ ð3:4 0:1Þ s. Again, this value is very similar to the result in Vit1. These observations indicate that the variation of 9 Be T1 is small in these Zr-based BMGs. Thus, it is expected that the decomposition of Vit1 into two amorphous phases upon annealing below Tc would not be detected through changes of the M ðtÞ decay curve, unless the changes of composition and structure are very dramatic. Changes of composition and structure are expected to be significant upon crystallization. This could lead to a significant change of T1 . Fig. 3 shows the M ðtÞ curve of sample D where a clearly non-exponential decay of M ðtÞ is observed. Here, M ðtÞ can be fit well with a double-exponential function
with fitting parameters of a ¼ 0:480:05, T1a ¼ ð3:6 0:5Þ s, and T1b ¼ ð17 3Þ s. T1a is very similar to that of sample A suggesting that half of the Be atoms remain in the amorphous matrix. The other half of the Be nuclei give rise to T1b which is about five times longer than T1a . Since the XRD pattern of sample D reveals the presence of Be2 Zr in agreement with previous TEM and APFIM studies [5,6,13], the component associated with T1b is attributed to Be2 Zr. This assignment is confirmed by NMR measurement of pure Be2 Zr. M ðtÞ of pure Be2 Zr at RT exhibits a single exponential decay with T1 ¼ 16 s which is nearly identical to T1b of sample D. The inset of Fig. 3 shows the M ðtÞ curves of samples B and C, which exhibit an interference peak in SANS. Although the M ðtÞ curve of sample B shows a visible change from that of sample A, the deviation is too small to permit an accurate quantitative analysis. Nevertheless, it is clear that almost all 9 Be nuclei in sample B have the same T1 as those in sample A and that a very small fraction of Be (less than 2% of Be) gives rise to a longer T1 . The M ðtÞ curve of sample C is clearly not single-exponential. The M ðtÞ curve of sample C can be fit by a doubleexponential function as described by Eq. (1). The fit parameters are a ¼ 0:89 0:05, T1a ¼ ð3:8 0:2Þ s, and T1b ¼ ð15 5Þ s. Within experimental errors, these T1a and T1b values are the same as those measured in sample D. This suggests that about 89% of Be atoms remain in the amorphous regions in sample C while 11% of Be atoms belong to a crystalline or icosahedral phase.
4. Discussion The estimated specific gravity of Be2 Zr is 4.4 g/ cm3 and that of Vit1 is 6.1 g/cm3 . Since 50% of the Be atoms in sample D belong to Be2 Zr, the volume fraction of Be2 Zr in sample D is about 15%. Of course, other crystalline phases are also present in sample D. Besides Zr2 Cu, there are presumably Ti2 Ni, Zr2 Ni, or a related ternary phase. Thus, the total volume fraction of all crystalline phases in sample D can be several times higher than 15%.
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Since the strong (1 0 0) diffraction peak of Be2 Zr at 2h ¼ 31:1° is completely missing in the XRD pattern of sample C (see Fig. 1), the 11% Be associated with T1b cannot be attributed to Be2 Zr. A natural assignment of this component associated with T1b is the icosahedral phase observed by XRD in sample C. It is well known that a pseudogap exists in the electronic density-of-states at the Fermi level in icosahedral phases and that the T1 observed in icosahedral phases is significantly longer than in regular metals [20]. Therefore, it is expected that T1b is significantly longer than T1a if the phase associated with T1b is an icosahedral phase. The fact that T1b is very similar to the T1 value of Be2 Zr indicates the similarity in local electronic structure at Be sites between the icosahedral phase and Be2 Zr. This might also be related to the fact that the XRD peak at 45.8° appears in both the icosahedral phase and Be2 Zr. The participation of Be in the icosahedral phase also supports the assumption that the icosahedral phase forms in the Be-rich and Ti-poor region in the decomposed Vit1 supercooled liquid.
5. Conclusions More than 15% volume fraction of Be2 Zr forms in Vit1 upon annealing above the critical temperature Tc of 673 K as determined by both XRD and NMR. Long-time annealing below Tc did not produce Be2 Zr detectable by XRD. Instead, a phase with structure consistent with icosahedral symmetry forms. NMR results show that a large number of Be atoms participated in this devitrification process. Both XRD and NMR indicate that there is a close structural relationship between the icosahedral phase and Be2 Zr. XRD and SANS show that the formation of the icosahedral phase in Vit1 is not polymorphic and presumably occurs in a Be-rich (Ti-poor) decomposed region, as supported by the NMR results. The spatial extension of the decomposition process in the supercooled liquid state of Vit1 limits the grain size of the icosahedral phase, and leads to broader XRD diffraction peaks than are observed in Vit4.
Acknowledgements We thank C.C. Hays for providing the in situ composite sample and M. Conradi and V.D. Kodibagkar for providing the Be2 Zr sample. This work was supported by the US Army Research Office (grant no. DAAD19-99-1-0328), by the US Department of Energy (grant no. DEFG-03-86ER 45242), and by the Alexander von Humboldt Foundation via the Feodor Lynen Program (J.F.L.).
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