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SAXS studies of early stage crystallization of a Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass
Materials Science and Engineering A 375–377 (2004) 738–743
APFIM/TEM/SAXS studies of early stage crystallization of a Zr52.5Cu17.9Ni14.6Al10 Ti5 metallic glass K. Kajiwara a,b , M. Ohnuma b , T. Ohkubo b , D.H. Ping b , K. Hono b,a,∗ a
Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8573, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract The early stage of the crystallization process of a Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass has been investigated by three-dimensional atom probe (3DAP), high-resolution electron microscopy (HREM) and small angle X-ray scattering (SAXS) technique. HREM observation of the alloys annealed near Tg always showed the dispersion of nanocrystalline fcc big cube phase dispersed in the amorphous matrix whenever concentration fluctuations were detected by SAXS and 3DAP. The distribution of impurity oxygen was uniform in the amorphous state, while it was rejected from the big cube phase to the remaining amorphous phase after the primary crystallization. The time evolutions of primary crystals followed t0.5 , indicating the grain growth was controlled by the volume diffusion of solute atoms. © 2003 Elsevier B.V. All rights reserved. Keywords: Metallic glass; Amorphous; Crystallization; TEM; SAXS; Atom probe
1. Introduction Since phase separation was reported in a bulk-forming Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 alloy [1], many recent investigations proposed the presence of phase separations prior to the crystallization of various bulk-forming metallic glasses [2–9]. Long before the recently revived interest in this subject, the possibility of phase decomposition in supercooled melts was proposed in several Pd-base metallic glasses [10]. In these investigations, the tendency of phase separation in the supercooled liquid region above Tg was often discussed in connection with the glass forming abilities of bulk-forming metallic glasses [1]. In addition, the nanocrystalline microstructure observed in bulk-forming metallic glasses is often proposed to be the result of the crystallization from phase decomposed metallic glasses [4–6]. However, it is not easy to determine the occurrence of phase separation in the glassy state prior to the crystallization of metallic glasses experimentally, since the crystallization kinetics above glass transition temperature Tg is usually very fast. In addition, experimental techniques with a high
∗
Corresponding author. E-mail address: [email protected] (K. Hono).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.087
spatial resolution and a high sensitivity for nanometer-scale compositional deference must be employed to detect phase separation in metallic glasses. The small angle scattering (SAS) technique has an adequate spatial resolution to detect phase separation and it has been applied in many studies on liquid and amorphous phase separation [3,5,9–11]. However, it cannot determine which elements are fluctuating in multi-component alloys. In contrast, atom probe (AP) analysis can detect small concentration difference of almost all elements, but the detectable level of compositional fluctuation is limited by the statistical error originating from the limited number of atoms used to determine local composition. Because of this statistical error, the results reported on the phase separation in the same Zr-based alloy were not consistent [1,2]. It is also known the field ion microscope (FIM) image is not sensitive to the structural change such as crystallization, because not all atoms on the surface contribute to image formation. Hence, even if nanocrystal is present in the amorphous matrix phase, the FIM image often shows fully amorphous like contrast [12]. If only SAS and AP are employed, it is difficult to distinguish the two phase decomposed glass from the nanocrystal/amorphous microstructure that is commonly observed in many metallic glasses. The SAS/AP profiles interpreted as evidence for chemical decomposition can also be obtained from nanocrystal/amorphous composite. Therefore, X-ray diffraction or TEM observation must
K. Kajiwara et al. / Materials Science and Engineering A 375–377 (2004) 738–743
be employed with SAS/AP techniques, in order to distinguish phase separation from nanocrystallization. The phase separation in amorphous state was convincingly shown in a Fe–Si–B–Nb–Cu amorphous alloy by a combined use of HREM and AP [13], in which the formation of Cu-enriched amorphous phase was confirmed in the fully amorphous state. This result is thermodynamically predictable, because Fe and Cu have a large positive enthalpy of mixing, causing large driving force for phase decomposition in a supercooled liquid [14]. The aim of this study was to investigate the early stage crystallization process of the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass, which was reported to chemically decompose prior to the nanocrystallization. In order to detect phase separation, SAXS and 3DAP were employed. The same samples were observed with X-ray, TEM and HREM, in order to detect the onset of crystallization. Based on the results with these complementary techniques, the mechanism of nanocrystalline microstructure evolution of this alloy is discussed.
2. Experimental procedure An alloy ingot of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 was prepared from raw materials of 99.9% Zr, and 99.99% Cu, Ni, Al and Ti by arc melting in an argon atmosphere. Amorphous ribbons were prepared by rapidly solidifying the alloy by the single roller melt-spinning technique with a 20 cm Cu roll at a surface velocity of 20 m/s. Thermal analysis of the amorphous alloy was performed by a Perkin-Elmer Pyris 1 differential scanning calorimeter (DSC) at a heating rate of 20 K/min. Crystallization behavior was studied by X-ray diffraction (XRD) using Rigaku RINT 2500 with Cu K␣ radiation and small angle X-ray scattering (SAXS) using Rigaku PSAXS-3S with a Mo target. Transmission electron microscopy (TEM) observation was performed using a Philips CM 200 TEM for conventional observation and a JEOL JEM-4000EX TEM for high-resolution observation. Local chemical compositions were analyzed using a locally built energy compensated three-dimensional atom probe (EC-3DAP) equipped with CAMECA optical tomographic atom probe detection system [15].
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Fig. 1. DSC trace of a melt-spun Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 amorphous alloy scanned at a heating rate of 20 K/min.
lap. Fig. 2 shows XRD profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 annealed for 10 min at different temperatures. Peaks from a crystalline phase are observed with an amorphous halo in the sample annealed at 733 K near Tx . These peaks are identified as NiTi2 -type fcc big cube phase with the lattice parameter of 1.205 nm. The formation of this phase was previously reported by Baricco et al. in same alloy [16]. These peaks become stronger and the halo becomes weaker at 753 K because the amorphous phase almost disappears, indicating that the big cube phase crystallizes from the amorphous phase. The peaks from Zr67 Cu13 Ni20 -type bct phase (a = 0.655 nm, c = 0.523 nm, prototype: Al2 Cu-type) [16] are observed in the diffraction profiles of the samples annealed at 753, 793 and 873 K for 10 min in addition to the
3. Results and discussion 3.1. Overall crystallization process Fig. 1 shows DSC trace of as-spun Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass scanned at 20 K/min. Glass transition temperature, Tg , is observed at about 693 K. The exothermic reaction corresponding to the primary crystallization starts at about 753 K (Tx ). Hence, T is about 60 K. From the inflection point observed in the right shoulder of the exothermic reaction, we can see that two crystallization reactions over-
Fig. 2. X-ray diffraction profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed for 10 min at different temperatures.
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big cube phase. The intensity of bct phase becomes stronger with increasing annealing temperature. Fig. 3 shows TEM micrographs of the sample annealed for 10 min at (a) 693 K, (b) 753 K, (c) 793 K and (d) 873 K. The sample annealed at 693 K does not show any contrast from crystals as shown in Fig. 3(a), which means that the alloy is still fully amorphous in this stage. The microstructures of the samples annealed at 753 and 793 K consist of very fine nanocrystals (∼5 nm) corresponding to the big cube phase and the bct phase as shown in Fig. 3(b) and (c). The sample annealed at 873 K in Fig. 3(d) has the coarse microstructure (∼30 nm) perfectly crystallized into two phases. To evaluate the composition of each nanocrystalline phase, 3DAP measurements have been performed. The distribution of impurity oxygen which is expected to give a critical influence on the crystallization process [14] was also analyzed together with the other constituent elements. Fig. 4(a) shows the elemental mapping of Al and O in an analysis volume of approximately 8 × 8 × 49 nm3 obtained from fully crystallized sample after annealing at 793 K for 10 min. Small and large spots correspond to Al and O atoms respectively. Al lean and enriched regions correspond to the big cube phases and the bct phase, respectively. Fig. 4(b) shows elemental maps within a small volume selected from Fig. 4(a). Zr and Ni atoms partition in the big cube phases while Cu, Al, Ti and O atoms in the bct
Fig. 3. TEM micrographs of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed for 10 min at (a) 693 K, (b) 753 K, (c) 793 K and (d) 873 K.
Fig. 4. EC-3DAP analysis results for Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 793 K for 10 min, (a) elemental mapping of Al and O, (b) elemental mappings and (c) concentration depth profiles of solute elements.
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3.2. Early stage of crystallization
Fig. 5. SAXS profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for various periods of time.
phases. The estimated composition of the big cube phase is approximately Zr75 Cu10 Ni15 . Al and Ti are partitioned in the bct-Zr67 Cu13 Ni20 phase, making the approximate composition of Zr55 Cu16 Ni6 Al16 Ti7 . Note that this oxygen partitioning behavior is opposite to preferred partitioning of oxygen in the icosahedral phase particles in Zr65 Cu27.5 Al7.5 metallic glass [17]. It is known that increase of oxygen impurity often leads to the stabilization of the icosahedral phase, suppressing the formation of big cube phase. The observed oxygen partitioning behavior explains this tendency. From these results, the crystallization process of the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass is summarized as Amorphous → fcc-Zr 75 Cu10 Ni15 (big cube) + amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + bct-Zr67 Cu13 Ni20 (Al-Ti enriched)
To study the early stage of the primary crystallization, the microstructures of the samples annealed near Tg were investigated. Fig. 5 shows small angle X-ray scattering annealed at 693 K for different periods of time. After 120 min annealing, at which the XRD profiles showed clear peaks from crystalline phase (Fig. 2), strong scattering due to crystalline particles is observed in the SAXS profile. Although the XRD profiles showed only the diffuse halo in the sample annealed up to 60 min (the data not shown here), the SAXS profiles for the alloy annealed for 30 min shows weak scattering intensity indicating that some concentration fluctuation exists in the alloy. Fig. 6 shows HREM images of the sample annealed at 693 K for (a) 10 min, (b) 20 min and (c) 120 min. Only a maze pattern without any lattice fringes are observed in the alloy annealed for 10 min, indicating that the sample is fully amorphous in this stage. However, fringe contrasts from nanocrystals are observed in the alloy annealed for 20 min and 120 min. The average diameter of the particles evaluated from the TEM images are 1 and 3 nm for 20 and 120 min, which is in good agreement with that evaluated from Guinier plot in the SAXS profiles. These results indicate that the observed SAXS intensity in the alloy annealed for 30 min is not attributed to the concentration fluctuation in the glassy state, but to the dispersion of the nanocrystals of the big cube phase. As described above, a fully amorphous state is confirmed in the alloy annealed for 10 min. Fig. 7(a) shows 3DAP analysis results of the sample annealed at 693 K for 10 min. No concentration fluctuation beyond the statistic error is observed for all of the constituent elements. In order to compare the experimental data with the binominal distributions, χ2 statistical test of the concentration distribution of each solute element was carried out with the block size of 50 atoms. χ2 values, degrees of freedom (d.f.) and the χ2 values for the levels of significance of α = 0.05 are shown in Table 1. It is confirmed that χ2 values for each solute element are within 2 ), suggestthe acceptance regions for α = 0.005 (χ2 < χ0.05 ing that the experimentally observed concentration profile is
Fig. 6. HREM images of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for (a) 10 min, (b) 20 min and (c) 120 min.
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the primary crystal of the big cube phase and the remaining amorphous as shown in Fig. 7(b). In the big cube phase, Zr and Ni are enriched, and Al and Ti are rejected. The average composition of the big cube phase particle is estimated to be Zr75 Cu8 Ni15 Ti1 Al1 . SAXS profile observed in this stage is attributed to this compositional fluctuation as the result of the primary crystallization. Prior to the crystallization stage, neither 3DAP nor SAXS detected compositional fluctuations. Since 693 K is just above Tg , the sample was also annealed at a slightly higher temperature between Tg and Tx , 713 K. As the case of 693 K annealing, it was confirmed that the concentration is uniform in the fully amorphous state and the compositional fluctuation was observed only
Fig. 7. EC-3DAP analysis results for Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for (a) 10 min and (b) 120 min.
in good agreement with the binominal distributions, that is each element is randomly distributed in the amorphous state. After 120 min annealing, nanocrystals of the big cube phase is dispersed in the amorphous matrix as observed in HREM image in Fig. 6(c). In this stage, compositional fluctuation is observed by 3DAP due to the solute partitioning between Table 1 χ2 values of the solute elements in an analysis volume of 2 × 2 × 200 nm3
d.f. χ2 2 χ0.05
Zr
Cu
Ni
Al
Ti
O
16 21.0 26.2
9 7.3 16.9
10 16.0 18.3
9 3.9 16.9
10 4.0 18.3
3 1.2 7.8
2 d.f. is degree of freedom and χ0.05 is the χ2 value for the rejection region of 0.05. The block size of 50 atoms was chosen.
Fig. 8. (a) Time evolution of the grain growth on the primary crystallization reaction applied the exponent of time of 0.5 and (b) interdiffusion coefficient determined from (a).
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after the onset of the primary crystallization of the big cube phase. Löffler et al. [11] reported the time evolution of nanocrystallization in a Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 alloy follows t0.06 in the late stage based on their SANS measurement results, and explained this behavior by the spinodal decomposition mechanism. Fig. 8 shows the growth kinetics of the big cube phase particles estimated from the present SAXS results. As shown in Fig. 8(a), the exponent of time is about 0.5 which indicates that the grain growth can be explained as the primary crystallization mechanism whose growth kinetics is controlled by the volume diffusion of solute atoms. The interdiffusion constant at 693 K and the activation energy on the primary crystallization reaction were estimated to be 1.3 ± 1.1 × 10−20 m2 /s and 100 ± 15 kJ/mol. All the experimental observations suggest that the nanocrystalline microstructure evolution in the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass can be explained by the primary crystallization of the big cube phase, followed by the polymorphous crystallization of the remaining amorphous phase to the Zr67 Cu13 Ni20 -type bct phase. 4. Conclusion The crystallization process of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass has been investigated by 3DAP, HREM and SAXS techniques and the following results were obtained. 1. NiTi2 -type fcc big cube phase with the lattice parameter of 1.205 nm forms from amorphous phase directly by the primary crystallization mechanism. Zr and Ni atoms are enriched in the big cube phases, while Cu, Al and Ti atoms are rejected and enriched in the remaining amorphous phase. The Zr67 Cu13 Ni20 -type bct phase (a = 0.655 nm, c = 0.523 nm, prototype: Al2 Cu-type) with an approximate composition of Zr55 Cu16 Ni6 Al16 Ti7 appears in the late stage of the crystallization. Hence, the crystallization process is summarized as Amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + bct-Zr67 Cu13 Ni20 (Al-Ti enriched) 2. The present sample contained about 0.3 at.% of oxygen as impurity. Oxygen was uniformly dissolved in the amorphous state, and was rejected from the big cube phase during the primary crystallization. 3. The compositional fluctuation above the statistical error was observed only when the formation of nanocrystals
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was confirmed by HREM. The SAXS profiles can be explained from the uniform dispersion of nanocrystals. 4. The exponent of time of the grain growth was about 0.5 indicating that the grain growth was controlled by the volume diffusion of solute. The observed microstructural evolution can be explained by the primary crystallization mechanism from the metallic glass.
Acknowledgements This work was supported by the Special Coordination Funds for Promoting Science and Technology on “Nanohetero Metallic Materials” from the Ministry of Education, Culture, Sports, Science and Technology.
References [1] R. Busch, S. Schneider, A. Peker, W.L. Johnson, Appl. Phys. Lett. 67 (1995) 1544. [2] M.P. Macht, N. Wanderka, A. Wiedenmann, H. Wollenberger, Q. Wei, H.J. Fecht, S.E. Klose, Mater. Sci. Forum 225 (1996) 65. [3] S. Schneider, P. Thiyagarajan, W.L. Johnson, Appl. Phys. Lett. 68 (1996) 493. [4] T.A. Waniuk, R. Busch, A. Masuhr, W.L. Johnson, Acta Mater. 46 (1998) 5229. [5] J.F. Löffler, W.L. Johnson, Appl. Phys. Lett. 76 (2000) 3394. [6] S.C. Glade, J.F. Löffler, S. Bossuyt, W.L. Johnson, M.K. Miller, J. Appl. Phys. 89 (2001) 1573. [7] A.K. Gangopadhyay, T.K. Croat, K.F. Kelton, Acta Mater. 48 (2000) 4035. [8] C.C. Hays, C.P. Kim, W.L. Johnson, Appl. Phys. Lett. 75 (1999) 1089. [9] A.A. Kündig, J.F. Löffler, W.L. Johnson, P.J. Uggowitzer, P. Thiyagarajan, Scripta Mater. 44 (2001) 1269. [10] C.P. Chou, D. Turnbull, J. Non-Cryst. Solids 17 (1975) 169. [11] J.F. Löffler, S. Bossuyt, S.C. Glade, W.L. Johnson, W. Wagner, P. Thiyagarajan, Appl. Phys. Lett. 77 (2000) 525. [12] K. Hono, D.H. Ping, Mater. Charact. 44 (2000) 203. [13] K. Hono, D.H. Ping, M. Ohnuma, H. Onodera, Acta Mater. 47 (1999) 997. [14] M. Ohnuma, K. Hono, T. Abe, H. Onodera, S. Linderoth, J.S. Pedersen, Y. Yoshizawa, in: A.R. Dinesen, M. Dldrup, D. Juul Jensen, S. Linderoth, T.B. Pedersen, N.H. Pryds, A.S. Pedersen, J.A. Wert (Eds.), Proceedings of the 22nd Riso International Symposium on Materials Scinece: Science of Metastable and Nanocrystalline Alloys, Structure, Properties and Modelling, Riso National Laboratory, Roskilde, Denmark, 2001, p. 341. [15] B. Deconihout, L. Renaud, G. Da Costa, M. Bouet, A. Bostel, D. Blavette, Ultramicroscopy 73 (1998) 253. [16] M. Baricco, S. Spriano, I. Chang, M.I. Petrzhik, L. Battezzati, Mater. Sci. Eng. A304–A306 (2001) 305. [17] B.S. Murty, D.H. Ping, K. Hono, A. Inoue, Acta Mater. 48 (2000) 3985.
APFIM/TEM/SAXS studies of early stage crystallization of a Zr52.5Cu17.9Ni14.6Al10 Ti5 metallic glass K. Kajiwara a,b , M. Ohnuma b , T. Ohkubo b , D.H. Ping b , K. Hono b,a,∗ a
Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8573, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract The early stage of the crystallization process of a Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass has been investigated by three-dimensional atom probe (3DAP), high-resolution electron microscopy (HREM) and small angle X-ray scattering (SAXS) technique. HREM observation of the alloys annealed near Tg always showed the dispersion of nanocrystalline fcc big cube phase dispersed in the amorphous matrix whenever concentration fluctuations were detected by SAXS and 3DAP. The distribution of impurity oxygen was uniform in the amorphous state, while it was rejected from the big cube phase to the remaining amorphous phase after the primary crystallization. The time evolutions of primary crystals followed t0.5 , indicating the grain growth was controlled by the volume diffusion of solute atoms. © 2003 Elsevier B.V. All rights reserved. Keywords: Metallic glass; Amorphous; Crystallization; TEM; SAXS; Atom probe
1. Introduction Since phase separation was reported in a bulk-forming Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 alloy [1], many recent investigations proposed the presence of phase separations prior to the crystallization of various bulk-forming metallic glasses [2–9]. Long before the recently revived interest in this subject, the possibility of phase decomposition in supercooled melts was proposed in several Pd-base metallic glasses [10]. In these investigations, the tendency of phase separation in the supercooled liquid region above Tg was often discussed in connection with the glass forming abilities of bulk-forming metallic glasses [1]. In addition, the nanocrystalline microstructure observed in bulk-forming metallic glasses is often proposed to be the result of the crystallization from phase decomposed metallic glasses [4–6]. However, it is not easy to determine the occurrence of phase separation in the glassy state prior to the crystallization of metallic glasses experimentally, since the crystallization kinetics above glass transition temperature Tg is usually very fast. In addition, experimental techniques with a high
∗
Corresponding author. E-mail address: [email protected] (K. Hono).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.087
spatial resolution and a high sensitivity for nanometer-scale compositional deference must be employed to detect phase separation in metallic glasses. The small angle scattering (SAS) technique has an adequate spatial resolution to detect phase separation and it has been applied in many studies on liquid and amorphous phase separation [3,5,9–11]. However, it cannot determine which elements are fluctuating in multi-component alloys. In contrast, atom probe (AP) analysis can detect small concentration difference of almost all elements, but the detectable level of compositional fluctuation is limited by the statistical error originating from the limited number of atoms used to determine local composition. Because of this statistical error, the results reported on the phase separation in the same Zr-based alloy were not consistent [1,2]. It is also known the field ion microscope (FIM) image is not sensitive to the structural change such as crystallization, because not all atoms on the surface contribute to image formation. Hence, even if nanocrystal is present in the amorphous matrix phase, the FIM image often shows fully amorphous like contrast [12]. If only SAS and AP are employed, it is difficult to distinguish the two phase decomposed glass from the nanocrystal/amorphous microstructure that is commonly observed in many metallic glasses. The SAS/AP profiles interpreted as evidence for chemical decomposition can also be obtained from nanocrystal/amorphous composite. Therefore, X-ray diffraction or TEM observation must
K. Kajiwara et al. / Materials Science and Engineering A 375–377 (2004) 738–743
be employed with SAS/AP techniques, in order to distinguish phase separation from nanocrystallization. The phase separation in amorphous state was convincingly shown in a Fe–Si–B–Nb–Cu amorphous alloy by a combined use of HREM and AP [13], in which the formation of Cu-enriched amorphous phase was confirmed in the fully amorphous state. This result is thermodynamically predictable, because Fe and Cu have a large positive enthalpy of mixing, causing large driving force for phase decomposition in a supercooled liquid [14]. The aim of this study was to investigate the early stage crystallization process of the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass, which was reported to chemically decompose prior to the nanocrystallization. In order to detect phase separation, SAXS and 3DAP were employed. The same samples were observed with X-ray, TEM and HREM, in order to detect the onset of crystallization. Based on the results with these complementary techniques, the mechanism of nanocrystalline microstructure evolution of this alloy is discussed.
2. Experimental procedure An alloy ingot of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 was prepared from raw materials of 99.9% Zr, and 99.99% Cu, Ni, Al and Ti by arc melting in an argon atmosphere. Amorphous ribbons were prepared by rapidly solidifying the alloy by the single roller melt-spinning technique with a 20 cm Cu roll at a surface velocity of 20 m/s. Thermal analysis of the amorphous alloy was performed by a Perkin-Elmer Pyris 1 differential scanning calorimeter (DSC) at a heating rate of 20 K/min. Crystallization behavior was studied by X-ray diffraction (XRD) using Rigaku RINT 2500 with Cu K␣ radiation and small angle X-ray scattering (SAXS) using Rigaku PSAXS-3S with a Mo target. Transmission electron microscopy (TEM) observation was performed using a Philips CM 200 TEM for conventional observation and a JEOL JEM-4000EX TEM for high-resolution observation. Local chemical compositions were analyzed using a locally built energy compensated three-dimensional atom probe (EC-3DAP) equipped with CAMECA optical tomographic atom probe detection system [15].
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Fig. 1. DSC trace of a melt-spun Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 amorphous alloy scanned at a heating rate of 20 K/min.
lap. Fig. 2 shows XRD profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 annealed for 10 min at different temperatures. Peaks from a crystalline phase are observed with an amorphous halo in the sample annealed at 733 K near Tx . These peaks are identified as NiTi2 -type fcc big cube phase with the lattice parameter of 1.205 nm. The formation of this phase was previously reported by Baricco et al. in same alloy [16]. These peaks become stronger and the halo becomes weaker at 753 K because the amorphous phase almost disappears, indicating that the big cube phase crystallizes from the amorphous phase. The peaks from Zr67 Cu13 Ni20 -type bct phase (a = 0.655 nm, c = 0.523 nm, prototype: Al2 Cu-type) [16] are observed in the diffraction profiles of the samples annealed at 753, 793 and 873 K for 10 min in addition to the
3. Results and discussion 3.1. Overall crystallization process Fig. 1 shows DSC trace of as-spun Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass scanned at 20 K/min. Glass transition temperature, Tg , is observed at about 693 K. The exothermic reaction corresponding to the primary crystallization starts at about 753 K (Tx ). Hence, T is about 60 K. From the inflection point observed in the right shoulder of the exothermic reaction, we can see that two crystallization reactions over-
Fig. 2. X-ray diffraction profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed for 10 min at different temperatures.
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big cube phase. The intensity of bct phase becomes stronger with increasing annealing temperature. Fig. 3 shows TEM micrographs of the sample annealed for 10 min at (a) 693 K, (b) 753 K, (c) 793 K and (d) 873 K. The sample annealed at 693 K does not show any contrast from crystals as shown in Fig. 3(a), which means that the alloy is still fully amorphous in this stage. The microstructures of the samples annealed at 753 and 793 K consist of very fine nanocrystals (∼5 nm) corresponding to the big cube phase and the bct phase as shown in Fig. 3(b) and (c). The sample annealed at 873 K in Fig. 3(d) has the coarse microstructure (∼30 nm) perfectly crystallized into two phases. To evaluate the composition of each nanocrystalline phase, 3DAP measurements have been performed. The distribution of impurity oxygen which is expected to give a critical influence on the crystallization process [14] was also analyzed together with the other constituent elements. Fig. 4(a) shows the elemental mapping of Al and O in an analysis volume of approximately 8 × 8 × 49 nm3 obtained from fully crystallized sample after annealing at 793 K for 10 min. Small and large spots correspond to Al and O atoms respectively. Al lean and enriched regions correspond to the big cube phases and the bct phase, respectively. Fig. 4(b) shows elemental maps within a small volume selected from Fig. 4(a). Zr and Ni atoms partition in the big cube phases while Cu, Al, Ti and O atoms in the bct
Fig. 3. TEM micrographs of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed for 10 min at (a) 693 K, (b) 753 K, (c) 793 K and (d) 873 K.
Fig. 4. EC-3DAP analysis results for Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 793 K for 10 min, (a) elemental mapping of Al and O, (b) elemental mappings and (c) concentration depth profiles of solute elements.
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Fig. 5. SAXS profiles of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for various periods of time.
phases. The estimated composition of the big cube phase is approximately Zr75 Cu10 Ni15 . Al and Ti are partitioned in the bct-Zr67 Cu13 Ni20 phase, making the approximate composition of Zr55 Cu16 Ni6 Al16 Ti7 . Note that this oxygen partitioning behavior is opposite to preferred partitioning of oxygen in the icosahedral phase particles in Zr65 Cu27.5 Al7.5 metallic glass [17]. It is known that increase of oxygen impurity often leads to the stabilization of the icosahedral phase, suppressing the formation of big cube phase. The observed oxygen partitioning behavior explains this tendency. From these results, the crystallization process of the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass is summarized as Amorphous → fcc-Zr 75 Cu10 Ni15 (big cube) + amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + bct-Zr67 Cu13 Ni20 (Al-Ti enriched)
To study the early stage of the primary crystallization, the microstructures of the samples annealed near Tg were investigated. Fig. 5 shows small angle X-ray scattering annealed at 693 K for different periods of time. After 120 min annealing, at which the XRD profiles showed clear peaks from crystalline phase (Fig. 2), strong scattering due to crystalline particles is observed in the SAXS profile. Although the XRD profiles showed only the diffuse halo in the sample annealed up to 60 min (the data not shown here), the SAXS profiles for the alloy annealed for 30 min shows weak scattering intensity indicating that some concentration fluctuation exists in the alloy. Fig. 6 shows HREM images of the sample annealed at 693 K for (a) 10 min, (b) 20 min and (c) 120 min. Only a maze pattern without any lattice fringes are observed in the alloy annealed for 10 min, indicating that the sample is fully amorphous in this stage. However, fringe contrasts from nanocrystals are observed in the alloy annealed for 20 min and 120 min. The average diameter of the particles evaluated from the TEM images are 1 and 3 nm for 20 and 120 min, which is in good agreement with that evaluated from Guinier plot in the SAXS profiles. These results indicate that the observed SAXS intensity in the alloy annealed for 30 min is not attributed to the concentration fluctuation in the glassy state, but to the dispersion of the nanocrystals of the big cube phase. As described above, a fully amorphous state is confirmed in the alloy annealed for 10 min. Fig. 7(a) shows 3DAP analysis results of the sample annealed at 693 K for 10 min. No concentration fluctuation beyond the statistic error is observed for all of the constituent elements. In order to compare the experimental data with the binominal distributions, χ2 statistical test of the concentration distribution of each solute element was carried out with the block size of 50 atoms. χ2 values, degrees of freedom (d.f.) and the χ2 values for the levels of significance of α = 0.05 are shown in Table 1. It is confirmed that χ2 values for each solute element are within 2 ), suggestthe acceptance regions for α = 0.005 (χ2 < χ0.05 ing that the experimentally observed concentration profile is
Fig. 6. HREM images of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for (a) 10 min, (b) 20 min and (c) 120 min.
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the primary crystal of the big cube phase and the remaining amorphous as shown in Fig. 7(b). In the big cube phase, Zr and Ni are enriched, and Al and Ti are rejected. The average composition of the big cube phase particle is estimated to be Zr75 Cu8 Ni15 Ti1 Al1 . SAXS profile observed in this stage is attributed to this compositional fluctuation as the result of the primary crystallization. Prior to the crystallization stage, neither 3DAP nor SAXS detected compositional fluctuations. Since 693 K is just above Tg , the sample was also annealed at a slightly higher temperature between Tg and Tx , 713 K. As the case of 693 K annealing, it was confirmed that the concentration is uniform in the fully amorphous state and the compositional fluctuation was observed only
Fig. 7. EC-3DAP analysis results for Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glasses annealed at 693 K for (a) 10 min and (b) 120 min.
in good agreement with the binominal distributions, that is each element is randomly distributed in the amorphous state. After 120 min annealing, nanocrystals of the big cube phase is dispersed in the amorphous matrix as observed in HREM image in Fig. 6(c). In this stage, compositional fluctuation is observed by 3DAP due to the solute partitioning between Table 1 χ2 values of the solute elements in an analysis volume of 2 × 2 × 200 nm3
d.f. χ2 2 χ0.05
Zr
Cu
Ni
Al
Ti
O
16 21.0 26.2
9 7.3 16.9
10 16.0 18.3
9 3.9 16.9
10 4.0 18.3
3 1.2 7.8
2 d.f. is degree of freedom and χ0.05 is the χ2 value for the rejection region of 0.05. The block size of 50 atoms was chosen.
Fig. 8. (a) Time evolution of the grain growth on the primary crystallization reaction applied the exponent of time of 0.5 and (b) interdiffusion coefficient determined from (a).
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after the onset of the primary crystallization of the big cube phase. Löffler et al. [11] reported the time evolution of nanocrystallization in a Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 alloy follows t0.06 in the late stage based on their SANS measurement results, and explained this behavior by the spinodal decomposition mechanism. Fig. 8 shows the growth kinetics of the big cube phase particles estimated from the present SAXS results. As shown in Fig. 8(a), the exponent of time is about 0.5 which indicates that the grain growth can be explained as the primary crystallization mechanism whose growth kinetics is controlled by the volume diffusion of solute atoms. The interdiffusion constant at 693 K and the activation energy on the primary crystallization reaction were estimated to be 1.3 ± 1.1 × 10−20 m2 /s and 100 ± 15 kJ/mol. All the experimental observations suggest that the nanocrystalline microstructure evolution in the Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass can be explained by the primary crystallization of the big cube phase, followed by the polymorphous crystallization of the remaining amorphous phase to the Zr67 Cu13 Ni20 -type bct phase. 4. Conclusion The crystallization process of Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 metallic glass has been investigated by 3DAP, HREM and SAXS techniques and the following results were obtained. 1. NiTi2 -type fcc big cube phase with the lattice parameter of 1.205 nm forms from amorphous phase directly by the primary crystallization mechanism. Zr and Ni atoms are enriched in the big cube phases, while Cu, Al and Ti atoms are rejected and enriched in the remaining amorphous phase. The Zr67 Cu13 Ni20 -type bct phase (a = 0.655 nm, c = 0.523 nm, prototype: Al2 Cu-type) with an approximate composition of Zr55 Cu16 Ni6 Al16 Ti7 appears in the late stage of the crystallization. Hence, the crystallization process is summarized as Amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + amorphous → fcc-Zr75 Cu10 Ni15 (big cube) + bct-Zr67 Cu13 Ni20 (Al-Ti enriched) 2. The present sample contained about 0.3 at.% of oxygen as impurity. Oxygen was uniformly dissolved in the amorphous state, and was rejected from the big cube phase during the primary crystallization. 3. The compositional fluctuation above the statistical error was observed only when the formation of nanocrystals
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was confirmed by HREM. The SAXS profiles can be explained from the uniform dispersion of nanocrystals. 4. The exponent of time of the grain growth was about 0.5 indicating that the grain growth was controlled by the volume diffusion of solute. The observed microstructural evolution can be explained by the primary crystallization mechanism from the metallic glass.
Acknowledgements This work was supported by the Special Coordination Funds for Promoting Science and Technology on “Nanohetero Metallic Materials” from the Ministry of Education, Culture, Sports, Science and Technology.
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