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Synchrotron X-ray scattering investigations of oxygen-induced nucleation in a Zr-based glass-forming alloy
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Scripta Materialia 61 (2009) 293–295 www.elsevier.com/locate/scriptamat
Synchrotron X-ray scattering investigations of oxygen-induced nucleation in a Zr-based glass-forming alloy J.J. Wall,a,* J.D. Almer,b S.C. Vogel,a P.K. Liaw,c H. Chooc,d and C.T. Liuc,d a
LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA c Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA d Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b
Received 6 February 2009; revised 2 April 2009; accepted 3 April 2009 Available online 9 April 2009
The metallic glass-forming alloy VIT-105 (Zr52.5Cu17.9Ni14.6Al10Ti5) was used to study the effect of oxygen on nucleation. Ex situ synchrotron X-ray scattering experiments performed on as-cast samples showed that oxygen leads to the formation of tetragonal and/or cubic phases, depending on oxygen content. The samples crystallized into either a primitive tetragonal phase or the so-called fcc ‘‘big cube” phase in a glassy matrix. A subsequent discussion on the role of oxygen in heterogeneous nucleation in Zr-based bulk metallic glasses is presented. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Heterogeneous nucleation; Glass forming ability; BMG; Nucleation
Crystallization of metallic-glass systems has been used to produce glassy-matrix composites [1,2], nanostructured materials [3,4] and quasicrystalline phases [5,6], and has been shown to result from shear deformation [7,8]. Altounian et al. [9] showed that the first nucleation product in amorphous stoichiometric NiZr2 during annealing above the glass transition temperature, Tg, is a metastable face-centered cubic (fcc) phase with a lattice ˚ in the presence of >1 at.% oxyparameter, a, of 12.27 A gen. Subsequent work has indicated that this fcc phase forms in a variety of Zr-based bulk metallic glass-forming alloys (BMGs) during annealing in the presence of oxygen [10–13]. During cooling from the liquid state, oxygen contamination results in reduced glass-forming ability in Zrbased BMGs, as was demonstrated directly in the VIT105 composition (Zr52.5Cu17.9Ni14.6Al10Ti5) by Lin et al. [14]. In addition, previous work by Wall et al. [15] showed evidence that oxygen-induced formation of intermetallic phases, which remained stable above the liquidus temperature, was responsible for the reduced glass-forming ability in VIT-105 materials containing oxygen. In this study, we look at the effect of oxygen addition on the
* Corresponding author. Present address: Electric Power Research Institute, Charlotte, NC 28262, USA. Tel.: +1 704 595 2162; fax: +1 704 595 2860; e-mail: [email protected]
crystallization products formed in the VIT-105 alloy during cooling from the melt. VIT-105 ingots with oxygen contents of 5000, 6250 and 14,000 ppm (‘‘atomic” herein) were produced by melting elemental constituents and ZrO2 in a water-cooled hearth in an arc-melter under an ultrahigh-purity argon cover. The ingots were then drop-cast into Ø 6.4 mm L 80 mm rods and sectioned for analyses. A description of the casting equipment and preliminary analyses can be found in [16] and [17], respectively. Disks of 1.0 mm in thickness were cut from each casting for ex situ synchrotron X-ray scattering (SXRS) measurements. SXRS measurements were performed on the disks of the as-cast materials at the Advanced Photon Source of the Argonne National Laboratory on the 1-ID beamline. The measurements were performed in transmission geometry using an incident X-ray beam energy, Ei, of 80.70 keV ˚ ) and a sample to detector (Mar-230 image(k = 0.1536 A plate) distance, z, of 547.3 mm. The X-ray wavelength, detector non-orthogonality and z were refined using a NIST Standard Reference Material (SRM) 674 [18] ceria Debye pattern taken under identical conditions. The X-ray beam cross-section was 500 lm 500 lm. The two-dimensional scattering data sets were integrated about the azimuth to obtain the scattering-intensity patterns, I(Q), where Q is the scattering momentum transfer, which is proportional to the diffraction-plane
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.04.004
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J. J. Wall et al. / Scripta Materialia 61 (2009) 293–295
spacing, d, by Q ¼ 2p=d. The data were reduced to this form using the Fit2D software [19]. The I(Q) patterns were fit by the whole powder-pattern-matching Rietveld method [20] using the General Structural Analysis Software (GSAS) [21]. The ex situ SXRS patterns showed that the samples containing 5000, 6250 and 14,000 ppm oxygen crystallized during casting. It was observed that increasing the oxygen content from 5000 to 6250 ppm resulted in an increased number of diffracting crystals, seen in the patterns as additional Laue diffraction spots (Fig. 1a and b). Rietveld fits of the integrated diffraction spectra revealed that the primary phase in the samples containing 5000 and 6250 ppm oxygen was a tetragonal Al3Zr2˚ and type phase (space group P42/mnm) with a = 7.40 A ˚ c = 6.66 A, coexisting with a small relative amount of a secondary phase. The structure of the primary phase is consistent with the electron-diffraction identification of Zr–Cu(Ni) crystalline inclusions in an as-cast, high-purity VIT-105 presented by Pekarskaya et al. [22]. Diffraction from the minor phase is consistent with fcc Zr2Ni (space group Fd-3m). The sample containing 14,000 ppm oxygen was found to have a fine-grained structure, as is apparent from the (relative) intensity and continuity of the Debye rings (Fig. 1c). Fitting of the integrated I(Q) pattern revealed coexisting fcc (Fd-3m) phases with lattice parameters of ˚ , respectively, after least-squares 11.93 and 12.16 A refinement. These phases are consistent with the Zr3Ni2Ti and Zr2Ni structures, both of which can be stabilized
at high temperatures by interstitial oxygen. Note that the Zr4Ni2OX and Zr6Ni4Ti2OX structures presented by Mackey et al. [23] were used as model structures for the fit. In addition, a significant amount of residual amorphous material was present, which was evident (qualitatively) from the relative amount of glass scattering in the integrated I(Q) pattern. The fitted diffraction patterns for the samples containing 5000 and 14,000 ppm oxygen are shown in Figure 2. Upon close inspection of the two-dimensional diffraction pattern of the as-cast sample containing 14,000 ppm oxygen, it was observed that, in a number of instances, intense Laue diffraction spots from planes in the fcc phases were coupled along a radial projection from the center. This finding is significant in that it reveals that large crystals of the two phases, which are similar in atomic structure, have exactly the same crystallographic orientation in the sample. Examples for several hkl planes are shown in Figure 3. These coupled Laue spots indicate diffraction from large (phase 1):(phase 2)-oriented bicrystals that appear to have formed via epitaxial heterogeneous nucleation. The tetragonal intermetallic phase found in the ascast samples containing 5000 and 6250 ppm oxygen was likely formed via heterogeneous nucleation on oxygen-stabilized intermetallic inoculants, per the nucleation catalysis criteria of Turnbull [24]. As such, it seems that the presence of such an oxygen-stabilized phase was directly responsible for crystallization. Note that this result is consistent with that of the electrostatic
27,000
a Observed I(d) data
22,000
Zr52.5 Cu17.9 Ni14.6 Al10Ti5 5,000 ppm oxygen synchrotron x-ray scattering
Reitveld Fit: Tet. + FCC Error (Iobs - Icalc)
I (counts)
17,000
12,000
7,000
2,000
1
8,500
1.5
2 2.5 3 d-Spacing (Angstroms)
4
b
7,500
Observed I(d) data 6,500
Zr52.5 Cu17.9 Ni14.6 Al10Ti5 14,000 ppm oxygen synchrotron x-ray scattering
Reitveld Fit: 2 x FCC Error (Iobs - Icalc)
5,500
I (counts)
3.5
4,500 3,500 2,500 1,500 500
1
1.5
2
2.5
3
3.5
4
d-Spacing (Angstroms)
Figure 1. Two-dimensional synchrotron X-ray scattering spectra from as-cast VIT-105 samples containing (a) 5000 ppm oxygen, (b) 6250 ppm oxygen and (c) 14,000 ppm oxygen, collected in transmission geometry by a MAR-230 image plate. The fourth-quadrant scattering data was enlarged to show details.
Figure 2. Rietveld fits of the integrated I(d) spectra for (a) the as-cast sample containing 5000 ppm oxygen and (b) 14,000 ppm oxygen. The open triangles indicate measured data points, and the solid line is the fit. The difference curve (fit error) is plotted below each spectrum.
J. J. Wall et al. / Scripta Materialia 61 (2009) 293–295
295
range of oxygen content and that the fcc oxygen-stabilized-phase is present in the superheated liquid in samples having oxygen concentrations outside this range. The results presented here reinforce the idea that glassforming ability in Zr-based BMGs is highly dependent on the mechanisms active in the superheated liquid. The authors J.J.W. and S.C.V. acknowledge the support of The Lujan Neutron Scattering Center at LANSCE, which is funded by the Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC5206NA25396. The authors J.J.W., P.K.L. and H.C. greatly acknowledge the support of the National Science Foundation International Materials Institutes (IMI) Program, DMR-0231320, with Dr. C. Huber as the Program Director.
Figure 3. Examples of coupled Laue diffraction spots in the fourthquadrant scattering pattern of the as-cast 14,000 ppm-oxygen-containing sample. The top image is the full Debye pattern for reference. The regions denoted ‘‘a”, ‘‘b”, and ‘‘c” were enlarged, and the intensity scales were reconfigured to highlight the coupled diffraction spots. This pairing of Laue spots along a radial projection from the center is indicative of oriented crystalline agglomerates formed by heterogeneous nucleation.
levitation study of Wall et al. [15], in which the same primitive tetragonal phase was found in a sample of high purity VIT-105 alloy that was cooled from below the socalled ‘‘threshold temperature”, resulting in heterogeneous nucleation. The highest levels of oxygen in the alloy facilitated the retention of a cubic structure after casting, showing [fcc]:[fcc] particles formed through catalyst–product epitaxy, but without a massive transformation of the amorphous material. This retention of the amorphous material was probably due to a very low thermodynamic driving force for the formation of fcc Zr2Ni, resulting in minimal growth. As such, it appears that at sufficiently high oxygen concentration an oxygen-stabilized fcc phase becomes stable in the superheated melt. The oxygen-stabilized fcc phase would not be a likely catalyst for heterogeneous nucleation of the equilibrium tetragonal CuZr2 or NiZr2 phases, or the primitive tetragonal phase formed in the samples containing 5000 and 6250 ppm oxygen. Thus, the fcc phase would not directly catalyze the tetragonal phase during crystallization. Note that the exact mechanism for the formation of the fcc + fcc + amorphous structure in the 14,000 ppm-oxygen-containing as-cast sample is not understood at present. It appears that the tetragonal oxygen-stabilized phase is only stable within a narrow
[1] J. Eckert, A. Reger-Leonhard, B. Weiss, M. Heilmaier, L. Schultz, Adv. Eng. Mater. 3 (2001) 41. [2] L.Q. Xing, C. Bertrand, J.-P. Dallas, M. Cornet, Mat. Sci. Eng. A 241 (1998) 216. [3] A.L. Greer, Nature 368 (1994) 688. [4] T. Kulik, J. Non-Cryst. Solids 287 (2001) 145. [5] D.V. Louzguine-Lugin, A. Inoue, J. Nanosci. Nanotechnol. 5 (2005) 999. [6] U. Ko¨ster, J. Meinhardt, S. Roos, H. Liebertz, Appl. Phys. Lett. 69 (1996) 179. [7] J.J. Kim, Y. Choi, S. Suresh, A.S. Argon, Science 295 (2002) 654. [8] M. Chen, A. Inoue, W. Zhang, T. Sakurai, Phys. Rev. Lett. 96 (2006) 245502. [9] Z. Altounian, E. Batalla, J.O. Strom-Olsen, J.L. Walter, J. Appl. Phys. 61 (1987) 149. [10] A. Gebert, J. Eckert, L. Schultz, Acta Mater. 56 (1998) 5475. [11] J. Saida, A. Inoue, J. Non-Cryst. Solids 317 (2003) 97. [12] M.F. de Oliveira, W.J. Botta, M.J. Kaufman, C.S. Kiminami, J. Non-Cryst. Solids 304 (2002) 51. [13] U. Ko¨ster, D. Zander, R. Janlewing, Mater. Sci. Forum 386 (2001) 89. [14] X.H. Lin, W.L. Johnson, W.-K. Rhim, Mater. Trans. JIM 38 (1997) 473. [15] J.J. Wall, C.T. Liu, W.-K. Rhim, J.J.Z. Li, P.K. Liaw, H. Choo, W.L. Johnson, Appl. Phys. Lett. 92 (2008) 244106. [16] J.J. Wall, C. Fan, P.K. Liaw, C.T. Liu, H. Choo, Rev. Sci. Instr. 77 (2006) 033902. [17] J.J. Wall, C. Fan, P.K. Liaw, H. Choo, Mater. Sci. Eng. A 472 (2008) 125. [18] C.R. Hubbard, NIST Standard Reference Material 674 Certificate of Analysis, 1983. [19] H.P. Hammersly, ESRF International Report No. EXP/ AH/93-02, 1993. [20] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [21] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR 86-748, 1994. [22] E. Pekarskaya, J.F. Loffler, W.L. Johnson, Acta Mater. 51 (2003) 4045. [23] R. Mackay, G.J. Miller, H.F. Franzen, J. Alloys Comp. 204 (1994) 109. [24] D. Turnbull, B. Vonnegut, Ind. Eng. Chem. 44 (1952) 1292.
Scripta Materialia 61 (2009) 293–295 www.elsevier.com/locate/scriptamat
Synchrotron X-ray scattering investigations of oxygen-induced nucleation in a Zr-based glass-forming alloy J.J. Wall,a,* J.D. Almer,b S.C. Vogel,a P.K. Liaw,c H. Chooc,d and C.T. Liuc,d a
LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA c Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA d Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b
Received 6 February 2009; revised 2 April 2009; accepted 3 April 2009 Available online 9 April 2009
The metallic glass-forming alloy VIT-105 (Zr52.5Cu17.9Ni14.6Al10Ti5) was used to study the effect of oxygen on nucleation. Ex situ synchrotron X-ray scattering experiments performed on as-cast samples showed that oxygen leads to the formation of tetragonal and/or cubic phases, depending on oxygen content. The samples crystallized into either a primitive tetragonal phase or the so-called fcc ‘‘big cube” phase in a glassy matrix. A subsequent discussion on the role of oxygen in heterogeneous nucleation in Zr-based bulk metallic glasses is presented. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Heterogeneous nucleation; Glass forming ability; BMG; Nucleation
Crystallization of metallic-glass systems has been used to produce glassy-matrix composites [1,2], nanostructured materials [3,4] and quasicrystalline phases [5,6], and has been shown to result from shear deformation [7,8]. Altounian et al. [9] showed that the first nucleation product in amorphous stoichiometric NiZr2 during annealing above the glass transition temperature, Tg, is a metastable face-centered cubic (fcc) phase with a lattice ˚ in the presence of >1 at.% oxyparameter, a, of 12.27 A gen. Subsequent work has indicated that this fcc phase forms in a variety of Zr-based bulk metallic glass-forming alloys (BMGs) during annealing in the presence of oxygen [10–13]. During cooling from the liquid state, oxygen contamination results in reduced glass-forming ability in Zrbased BMGs, as was demonstrated directly in the VIT105 composition (Zr52.5Cu17.9Ni14.6Al10Ti5) by Lin et al. [14]. In addition, previous work by Wall et al. [15] showed evidence that oxygen-induced formation of intermetallic phases, which remained stable above the liquidus temperature, was responsible for the reduced glass-forming ability in VIT-105 materials containing oxygen. In this study, we look at the effect of oxygen addition on the
* Corresponding author. Present address: Electric Power Research Institute, Charlotte, NC 28262, USA. Tel.: +1 704 595 2162; fax: +1 704 595 2860; e-mail: [email protected]
crystallization products formed in the VIT-105 alloy during cooling from the melt. VIT-105 ingots with oxygen contents of 5000, 6250 and 14,000 ppm (‘‘atomic” herein) were produced by melting elemental constituents and ZrO2 in a water-cooled hearth in an arc-melter under an ultrahigh-purity argon cover. The ingots were then drop-cast into Ø 6.4 mm L 80 mm rods and sectioned for analyses. A description of the casting equipment and preliminary analyses can be found in [16] and [17], respectively. Disks of 1.0 mm in thickness were cut from each casting for ex situ synchrotron X-ray scattering (SXRS) measurements. SXRS measurements were performed on the disks of the as-cast materials at the Advanced Photon Source of the Argonne National Laboratory on the 1-ID beamline. The measurements were performed in transmission geometry using an incident X-ray beam energy, Ei, of 80.70 keV ˚ ) and a sample to detector (Mar-230 image(k = 0.1536 A plate) distance, z, of 547.3 mm. The X-ray wavelength, detector non-orthogonality and z were refined using a NIST Standard Reference Material (SRM) 674 [18] ceria Debye pattern taken under identical conditions. The X-ray beam cross-section was 500 lm 500 lm. The two-dimensional scattering data sets were integrated about the azimuth to obtain the scattering-intensity patterns, I(Q), where Q is the scattering momentum transfer, which is proportional to the diffraction-plane
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.04.004
294
J. J. Wall et al. / Scripta Materialia 61 (2009) 293–295
spacing, d, by Q ¼ 2p=d. The data were reduced to this form using the Fit2D software [19]. The I(Q) patterns were fit by the whole powder-pattern-matching Rietveld method [20] using the General Structural Analysis Software (GSAS) [21]. The ex situ SXRS patterns showed that the samples containing 5000, 6250 and 14,000 ppm oxygen crystallized during casting. It was observed that increasing the oxygen content from 5000 to 6250 ppm resulted in an increased number of diffracting crystals, seen in the patterns as additional Laue diffraction spots (Fig. 1a and b). Rietveld fits of the integrated diffraction spectra revealed that the primary phase in the samples containing 5000 and 6250 ppm oxygen was a tetragonal Al3Zr2˚ and type phase (space group P42/mnm) with a = 7.40 A ˚ c = 6.66 A, coexisting with a small relative amount of a secondary phase. The structure of the primary phase is consistent with the electron-diffraction identification of Zr–Cu(Ni) crystalline inclusions in an as-cast, high-purity VIT-105 presented by Pekarskaya et al. [22]. Diffraction from the minor phase is consistent with fcc Zr2Ni (space group Fd-3m). The sample containing 14,000 ppm oxygen was found to have a fine-grained structure, as is apparent from the (relative) intensity and continuity of the Debye rings (Fig. 1c). Fitting of the integrated I(Q) pattern revealed coexisting fcc (Fd-3m) phases with lattice parameters of ˚ , respectively, after least-squares 11.93 and 12.16 A refinement. These phases are consistent with the Zr3Ni2Ti and Zr2Ni structures, both of which can be stabilized
at high temperatures by interstitial oxygen. Note that the Zr4Ni2OX and Zr6Ni4Ti2OX structures presented by Mackey et al. [23] were used as model structures for the fit. In addition, a significant amount of residual amorphous material was present, which was evident (qualitatively) from the relative amount of glass scattering in the integrated I(Q) pattern. The fitted diffraction patterns for the samples containing 5000 and 14,000 ppm oxygen are shown in Figure 2. Upon close inspection of the two-dimensional diffraction pattern of the as-cast sample containing 14,000 ppm oxygen, it was observed that, in a number of instances, intense Laue diffraction spots from planes in the fcc phases were coupled along a radial projection from the center. This finding is significant in that it reveals that large crystals of the two phases, which are similar in atomic structure, have exactly the same crystallographic orientation in the sample. Examples for several hkl planes are shown in Figure 3. These coupled Laue spots indicate diffraction from large (phase 1):(phase 2)-oriented bicrystals that appear to have formed via epitaxial heterogeneous nucleation. The tetragonal intermetallic phase found in the ascast samples containing 5000 and 6250 ppm oxygen was likely formed via heterogeneous nucleation on oxygen-stabilized intermetallic inoculants, per the nucleation catalysis criteria of Turnbull [24]. As such, it seems that the presence of such an oxygen-stabilized phase was directly responsible for crystallization. Note that this result is consistent with that of the electrostatic
27,000
a Observed I(d) data
22,000
Zr52.5 Cu17.9 Ni14.6 Al10Ti5 5,000 ppm oxygen synchrotron x-ray scattering
Reitveld Fit: Tet. + FCC Error (Iobs - Icalc)
I (counts)
17,000
12,000
7,000
2,000
1
8,500
1.5
2 2.5 3 d-Spacing (Angstroms)
4
b
7,500
Observed I(d) data 6,500
Zr52.5 Cu17.9 Ni14.6 Al10Ti5 14,000 ppm oxygen synchrotron x-ray scattering
Reitveld Fit: 2 x FCC Error (Iobs - Icalc)
5,500
I (counts)
3.5
4,500 3,500 2,500 1,500 500
1
1.5
2
2.5
3
3.5
4
d-Spacing (Angstroms)
Figure 1. Two-dimensional synchrotron X-ray scattering spectra from as-cast VIT-105 samples containing (a) 5000 ppm oxygen, (b) 6250 ppm oxygen and (c) 14,000 ppm oxygen, collected in transmission geometry by a MAR-230 image plate. The fourth-quadrant scattering data was enlarged to show details.
Figure 2. Rietveld fits of the integrated I(d) spectra for (a) the as-cast sample containing 5000 ppm oxygen and (b) 14,000 ppm oxygen. The open triangles indicate measured data points, and the solid line is the fit. The difference curve (fit error) is plotted below each spectrum.
J. J. Wall et al. / Scripta Materialia 61 (2009) 293–295
295
range of oxygen content and that the fcc oxygen-stabilized-phase is present in the superheated liquid in samples having oxygen concentrations outside this range. The results presented here reinforce the idea that glassforming ability in Zr-based BMGs is highly dependent on the mechanisms active in the superheated liquid. The authors J.J.W. and S.C.V. acknowledge the support of The Lujan Neutron Scattering Center at LANSCE, which is funded by the Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC5206NA25396. The authors J.J.W., P.K.L. and H.C. greatly acknowledge the support of the National Science Foundation International Materials Institutes (IMI) Program, DMR-0231320, with Dr. C. Huber as the Program Director.
Figure 3. Examples of coupled Laue diffraction spots in the fourthquadrant scattering pattern of the as-cast 14,000 ppm-oxygen-containing sample. The top image is the full Debye pattern for reference. The regions denoted ‘‘a”, ‘‘b”, and ‘‘c” were enlarged, and the intensity scales were reconfigured to highlight the coupled diffraction spots. This pairing of Laue spots along a radial projection from the center is indicative of oriented crystalline agglomerates formed by heterogeneous nucleation.
levitation study of Wall et al. [15], in which the same primitive tetragonal phase was found in a sample of high purity VIT-105 alloy that was cooled from below the socalled ‘‘threshold temperature”, resulting in heterogeneous nucleation. The highest levels of oxygen in the alloy facilitated the retention of a cubic structure after casting, showing [fcc]:[fcc] particles formed through catalyst–product epitaxy, but without a massive transformation of the amorphous material. This retention of the amorphous material was probably due to a very low thermodynamic driving force for the formation of fcc Zr2Ni, resulting in minimal growth. As such, it appears that at sufficiently high oxygen concentration an oxygen-stabilized fcc phase becomes stable in the superheated melt. The oxygen-stabilized fcc phase would not be a likely catalyst for heterogeneous nucleation of the equilibrium tetragonal CuZr2 or NiZr2 phases, or the primitive tetragonal phase formed in the samples containing 5000 and 6250 ppm oxygen. Thus, the fcc phase would not directly catalyze the tetragonal phase during crystallization. Note that the exact mechanism for the formation of the fcc + fcc + amorphous structure in the 14,000 ppm-oxygen-containing as-cast sample is not understood at present. It appears that the tetragonal oxygen-stabilized phase is only stable within a narrow
[1] J. Eckert, A. Reger-Leonhard, B. Weiss, M. Heilmaier, L. Schultz, Adv. Eng. Mater. 3 (2001) 41. [2] L.Q. Xing, C. Bertrand, J.-P. Dallas, M. Cornet, Mat. Sci. Eng. A 241 (1998) 216. [3] A.L. Greer, Nature 368 (1994) 688. [4] T. Kulik, J. Non-Cryst. Solids 287 (2001) 145. [5] D.V. Louzguine-Lugin, A. Inoue, J. Nanosci. Nanotechnol. 5 (2005) 999. [6] U. Ko¨ster, J. Meinhardt, S. Roos, H. Liebertz, Appl. Phys. Lett. 69 (1996) 179. [7] J.J. Kim, Y. Choi, S. Suresh, A.S. Argon, Science 295 (2002) 654. [8] M. Chen, A. Inoue, W. Zhang, T. Sakurai, Phys. Rev. Lett. 96 (2006) 245502. [9] Z. Altounian, E. Batalla, J.O. Strom-Olsen, J.L. Walter, J. Appl. Phys. 61 (1987) 149. [10] A. Gebert, J. Eckert, L. Schultz, Acta Mater. 56 (1998) 5475. [11] J. Saida, A. Inoue, J. Non-Cryst. Solids 317 (2003) 97. [12] M.F. de Oliveira, W.J. Botta, M.J. Kaufman, C.S. Kiminami, J. Non-Cryst. Solids 304 (2002) 51. [13] U. Ko¨ster, D. Zander, R. Janlewing, Mater. Sci. Forum 386 (2001) 89. [14] X.H. Lin, W.L. Johnson, W.-K. Rhim, Mater. Trans. JIM 38 (1997) 473. [15] J.J. Wall, C.T. Liu, W.-K. Rhim, J.J.Z. Li, P.K. Liaw, H. Choo, W.L. Johnson, Appl. Phys. Lett. 92 (2008) 244106. [16] J.J. Wall, C. Fan, P.K. Liaw, C.T. Liu, H. Choo, Rev. Sci. Instr. 77 (2006) 033902. [17] J.J. Wall, C. Fan, P.K. Liaw, H. Choo, Mater. Sci. Eng. A 472 (2008) 125. [18] C.R. Hubbard, NIST Standard Reference Material 674 Certificate of Analysis, 1983. [19] H.P. Hammersly, ESRF International Report No. EXP/ AH/93-02, 1993. [20] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [21] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR 86-748, 1994. [22] E. Pekarskaya, J.F. Loffler, W.L. Johnson, Acta Mater. 51 (2003) 4045. [23] R. Mackay, G.J. Miller, H.F. Franzen, J. Alloys Comp. 204 (1994) 109. [24] D. Turnbull, B. Vonnegut, Ind. Eng. Chem. 44 (1952) 1292.