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Local Structure Study in Zr-Based Metallic Glasses

Junji Saida

1;*

, Takashi Sanada

2

, Shigeo Sato

2

, Muneyuki Imafuku

3

,

Eiichiro Matsubara

4

and Akihisa Inoue

5

1

Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, Japan

2Research Department, Nissan ARC Ltd., Yokosuka 237-0061, Japan

3Materials Characterization Center, Nippon Steel Technoresearch Corp., Chiba 293-0011, Japan 4Department of Material Science and Engineering, Kyoto University, Kyoto 606-8501, Japan 5Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

The local structure in the glassy state is investigated in the Zr80Pt20and Zr70Cu30binary and Zr70Al10Ni20ternary alloys in correlation with the quasicrystalline (QC) phase formation. The Zr80Pt20alloy has a high QC-forming ability. It is easily formed in the as-quenched state with a considerably low cooling rate or by annealing the glassy alloy. The radial distribution function (RDF) and extended X-ray absorption fine structure (EXAFS) analysis clearly indicate the existence of icosahedral local structure around Pt atom. The Zr70Cu30binary and Zr70Al10Ni20 ternary metallic glasses have a QC-forming ability by the addition of a very small amount of noble metals such as Pd. We can also investigate the icosahedral local structure in these alloys. These results are realized that the icosahedral local structure can be applied as a dominant local atomic configuration in the supercooled liquid and/or glassy states in the QC-forming metallic glasses. [doi:10.2320/matertrans.MJ200753]

(Received November 27, 2006; Accepted January 29, 2007; Published June 20, 2007)

Keywords: zirconium-based metallic glass, radial distribution function, extended x-ray absorption fine structure, local structure, supercooled liquid state, icosahedral cluster

1. Introduction

It is well known that Zr-based metallic glasses have a high glass-forming ability (GFA), which enables us to produce the glassy alloy with bulky shape. To clarify the mechanism of such high GFA, a number of studies have been performed in the aspects of structural analysis, transformation behavior and so on.1,2)Recently, the icosahedral QC phase (I-phase) formation has been reported in several Zr-based metallic glasses with high GFA, leading to the structural correlation between the I-phase and local atomic configuration in the glassy state.3–5)In the recent studies, we have reported the investigation of icosahedral local structure in the Zr-Cu binary metallic glass as well as Zr-based multicomponent metallic glasses by the structural analysis using XRD and TEM.6,7) Since the I-phase is easily precipitated from the primary stage in the Zr-Cu metallic glass by the addition of a very small amount of noble metals, we speculate the strong correlation between the QC-formation and icosahedral local structure. In this paper, we have examined the local structure in several QC-forming Zr-based metallic glasses to inves-tigate the correlation above. At first, we have measured the RDF and EXAFS profiles for the glassy Zr80Pt20alloy, which

has a strong tendency to form the I-phase.8–10)Based on the results, we have tried to analyze the local structure investigation for the Zr-Cu and Zr-Al-Ni metallic glasses. These alloys are recognized as the important systems for the production of bulk metallic glasses (BMGs).11,12) In these studies, we investigate that the icosahedral local structure is very reasonable for QC-forming metallic glasses with the suggestion of the stabilization mechanism of the supercooled liquid and/or glassy state in these alloys.

2. Experimental Procedures

Ribbon samples with a cross section of0:031mm2were produced by melt spinning of arc-melted the Zr80Pt20,

Zr70Cu30 and Zr70Al10Ni20 alloy ingots in an argon

atmo-sphere. Thermal properties were measured with a differential scanning calorimeter (DSC) at a heating rate of 0.67 Ks1.

The structure of the as-quenched and annealed samples was examined by X-ray diffraction (XRD) measurements with Cu-Kradiation (40 kV-40 mA). The local atomic structure was studied by ordinary XRD measurements with

mono-chromatic Mo-K radiation operated at 45 kV-150 mA

produced by a rotating anode X-ray generator. The observed diffraction profiles were corrected by air scattering, polar-ization, absorption and Compton scattering and then, con-verted to electron units per atom by the Krog-Moe-Norman method13)to obtain an interference function,QiðQÞestimated from the coherent scattering intensity in absolute units. The ordinary RDF was led by Fourier transformation of the

QiðQÞ. The coordination number and interatomic distance between the constituent elements were calculated by fitting the QiðQÞ and RDF with non-linear least square fitting method. EXAFS measurements for the analysis of the local environments were performed on beam line BL-12C at the Photon Factory of the Institute of Materials Structure Science, High-Energy Accelerator Research Organization (KEK), Tsukuba, Japan and beam line BL-01B1 at SPring-8, Hyogo, Japan. All measurements were done in transmission geometry at room temperature. Measured spectra were analyzed using the program REX 2000 (Rigaku Corp.).

3. Results and Discussion

The authors have been reported that the Zr80Pt20 binary

alloy exhibits a high QC-forming ability.8,14)The I-phase is easily precipitated at the lower cooling rate or by annealing *Corresponding author, E-mail: jsaida@cir.tohoku.ac.jp

Special Issue on Bulk Metallic Glasses —Selected Papers from the Fifth International Conference on Bulk Metallic Glasses (BMGV)

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glassy alloy. Figure 1 shows XRD patterns of the Zr80Pt20

melt-spun ribbon at the roll speed of 60 m/s and the annealed ribbon sample for 300 s at 820 K. No obvious peaks corresponding to the crystalline structure except only a halo peak are observed in the as-quenched state. On the other hand, significant peaks characterized as the I-phase appear in the annealed sample. Figure 2 shows high-resolution TEM (HREM) image (a) and selected area electron diffraction pattern (SADP) (b) in the as-quenched Zr80Pt20 melt-spun

ribbon and bright-field TEM (BF) image (c) and nano beam electron diffraction (NBD) pattern (d) of the annealed sample. The HREM image clearly indicates homogeneous maze contrast. Considering that no fringe contrast contains in (a) and the SADP pattern (b) consists of only a halo ring, the glassy structure is obtained in the as-quenched state. The BF image in the annealed state reveals the precipitates of nano scale particles in the glassy matrix. The NBD pattern taken from the precipitated particles indicates the quasiperiodic five-fold symmetry, which is identified as the formation of the I-phase.

Interference function, QiðQÞ (a) and radial distribution function (RDF) (b) calculated through Fourier transformation ofQiðQÞfor the melt-spun Zr80Pt20glassy alloy are shown in

Fig. 3. The calculated curve (dotted line) agrees with the

experimental result (solid line) by considering the first nearest neighboring shell for the alloy in the QiðQÞprofile. The atomic distance of Pt-Pt pair in the RDF curve is calculated to be 0.326 nm, which is significantly longer than the expected value (0.278 nm) from the atomic radius. This result implies the formation of a unique local environment, i.e. some chemical short-range order (CSRO) for example, around Pt atom. Total coordination numbers around Zr and Pt are 11.1 and 12.0, respectively. The coordination number of 12.0 around Pt leads to the possibility of the presence of icosahedral like local atomic configuration. The authors have already reported the results of the RDF analysis in the QC-formed Zr80Pt20alloy, which is melt-spun at the roll speed of

Fig. 1 XRD patterns of the Zr80Pt20melt-spun ribbon at the roll speed of 60 m/s and the annealed ribbon sample for 300 s at 820 K.

Fig. 2 High-resolution TEM (HREM) image (a) and selected area electron diffraction pattern (SADP) (b) in the as-quenched Zr80Pt20 melt-spun ribbon and bright-field TEM (BF) image (c) and nano beam electron diffraction (NBD) pattern (d) of the annealed sample for 300 s at 820 K.

[image:2.595.61.279.68.244.2] [image:2.595.326.526.73.365.2] [image:2.595.118.483.613.747.2]
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40 m/s.15)The total coordination numbers around Zr and Pt atoms are 14.2 and 12.4, respectively. These results also support the suggestion of the icosahedral like environment around Pt atom. It also leads to the suggestion of a strong correlation between the local structure in the glassy state and the I-phase formation in the QC-forming metallic glasses.

Based on the suggestion, we have examined the local structure in several QC-forming Zr-based metallic glasses. Figure 4 shows XRD patterns of the primary crystallization stage in the Zr70Cu20 and Zr70Al10Ni20metallic glasses. The

XRD patterns of the primary stage in the metallic glasses in addition of 1 at% Pd are also denoted in the figure. It is well known that Zr-Cu and Zr-Al-Ni metallic glasses have a high

stability of supercooled liquid state and are the BMG alloys.11,12) The precipitation phase is identified as the tetragonal Zr2Cu phase (stable phase) in the Zr70Cu30 alloy

and the Zr6NiAl2 phase with a minor phase of tetragonal

Zr2Ni in the Zr70Al10Ni20 alloy. Meanwhile, the primary

phase is significantly changed into the I-phase by the addition of only 1 at% Pd in both alloys. The precipitation of the I-phase has been found in other additional elements of Au and Pt.16)The easy quasicrystallization in these alloys by a small amount of additional elements exhibits a possibility of the local structure correlation with the I-phase as the same as the case of the Zr80Pt20 QC-forming alloy. Thek3 weighted

EXAFS spectra and Fourier transformation curves of EXAFS measurements of the Zr K-edge ((a), (c)) and Cu K-edge ((b), (d)) in the Zr70Cu30 metallic glass are shown in Fig. 5. The

calculated results of the tetragonal Zr2Cu structure and

icosahedral cluster model defined by the atomic distances obtained from parameter fitting results by RDF measure-ment17)of the Zr K-edge and Cu K-edge are also shown in Fig. 5. In the EXFAS spectra, the measured results have a significantly different periodic oscillation comparing to those of the tetragonal Zr2Cu structure around both edges. On the

other hand, the icosahedral cluster model exhibits the similar oscillation with measured ones. Both of Fourier transforma-tion curves of the Zr K-edge and Cu K-edge in the as-quenched state can be fitted by the icosahedral cluster model well rather than the Zr2Cu structure model. It is in contrast in

the results of the Zr70Ni30 amorphous alloy, where the local

environment around Ni is very similar to that in the tetragonal Zr2Ni structure.17)Since we obtain the similar curves in the

icosahedral cluster model to the experimental ones in the Zr70Cu30 metallic glass, it can be concluded that the

icosahedral local environment is formed around the Zr and Cu atoms in the alloy. Recently, the local structural similarity

Fig. 4 XRD patterns of the primary crystallization stage in the Zr70Cu20 and Zr70Al10Ni20metallic glasses. The XRD patterns of the primary stage in the metallic glasses in addition of 1 at% Pd are also denoted in the figure.

[image:3.595.61.277.70.246.2] [image:3.595.127.468.507.761.2]
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has been clarified between the Zr70Cu30 and QC-formed

Zr70Cu29Pd1 metallic glasses by EXAFS measurements.18)

Figure 6 shows Fourier transformation curves of EXAFS measurements of the Zr K-edge (a) and Ni K-edge (b) of the as-quenched Zr70Al10Ni20 and QC-formed Zr70Al9Ni20Pd1

metallic glasses annealed for 900 s at 670 K. The curves of the as-quenched Zr70Al10Ni20and the QC-formed state of the

Zr70Al9Ni20Pd1 metallic glass annealed for 900 s at 670 K

have no significant differences around Zr K-edge. However, the peak intensities of the first and second shells for the annealed state of the Zr70Al9Ni20Pd1 sample is reversed

compared to that in the as-quenched Zr70Al10Ni20 metallic

glass. Here, the second shell consists of pairs of Zr-Zr (major) and Zr-Al (minor), considering their distances and coordina-tion numbers.19) The increase in the peak intensity of the second shell may be due to the slight rearrangement of Zr around Zr atoms during the quasicrystallization. Actually, we have already reported the RDF results for these alloys for supporting the suggestion. Table 1 shows the results of structural parameter fitting for RDF curves in the as-quenched Zr70Al10Ni20and QC-formed Zr70Al9Ni20Pd1

me-tallic glasses annealed for 900 s at 670 K. The coordination number of Zr around Zr in the nearest neighbor region,i.e. the first shell in the as-quenched Zr70Al10Ni20 alloy is 10.7,

which increases into the 12.0 in the QC-formed Zr70

-Al9Ni20Pd1 metallic glass. Since the coordination numbers

of Ni and Al around Zr in the as-quenched state as well as the distance of each pair remain during the QC formation, these results are completely consistent to the EXAFS

measure-ments. In the Ni K-edge data (d), no obvious peak shift is exhibited for the as-quenched Zr70Al10Ni20 and QC-formed

Zr70Al9Ni20Pd1metallic glasses. The sharpness of the profile

in the QC-formed state in the Zr70Al9Ni20Pd1 metallic glass

is attributed to QC-ordering, leading to the slight change in local environment around Ni atom following quasicrystalli-zation. In conclusion, the local environment in the glassy state of the Zr70Al10Ni20 alloy remains during the QC

formation. The results also indicate the icosahedral local order in the Zr-Al-Ni metallic glass.

4. Conclusions

We have performed the local structure analysis in the Zr-based metallic glasses in the aspects of the local structural similarity during the QC-formation. In the QC-forming Zr80Pt20 alloy, it is found that local environment around Pt

remains during the precipitation of the QC phase, leading to the conclusion that an icosahedral local structure can be investigated around Pt atom. Similarly, the Zr70Cu30 binary

and Zr70Al10Ni20 ternary metallic glasses with the high

stability of supercooled liquid state,i.e.BMG-forming ability have the icosahedral local structure, where the I-phase is easily precipitated by addition of a very small amount of noble metals. Therefore, we propose that the icosahedral local structure as a unique local environment in the glassy state is quite reasonable in the Zr-based metallic glasses in case of the observation of easy QC-formation.

Acknowledgements

This work has been supported by a Grant-in-Aid of the Ministry of Education, Sports, Culture, Science and Tech-nology, Japan, Priority Area on ‘‘Materials Science of Bulk Metallic Glasses’’ and Japan Society for the Promotion of Science (JSPS) Asian Core Program.

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[image:4.595.71.269.69.378.2]

K-edge (a) and Ni K-K-edge (b) of the as-quenched Zr70Al10Ni20 and QC-formed Zr70Al9Ni20Pd1metallic glasses annealed for 900 s at 670 K.

Table 1 Results of structural parameter fitting for RDF curves in the as-quenched Zr70Al10Ni20 and QC-formed Zr70Al9Ni20Pd1metallic glasses annealed for 900 s at 670 K.

Pair r/nm N

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Zr-Al 0.300 1.3

Zr70Al9Ni20Pd1 Zr-Zr 0.320 10.4

(670 K-900 s) 0.366 1.6

Zr-Ni 0.262 1.3

[image:4.595.304.550.102.219.2]
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Figure

Fig. 3Interference function, QiðQÞ (a) and radial distribution function(RDF) (b) calculated through Fourier transformation of QiðQÞ for the melt-spun Zr80Pt20 glassy alloy.
Fig. 5k3 weighted EXAFS spectra and Fourier transformation curves of EXAFS measurements of the Zr K-edge ((a), (c)) and Cu K-edge((b), (d)) in the Zr70Cu30 metallic glass, respectively.
Table 1Results of structural parameter fitting for RDF curves in the as-quenched Zr70Al10Ni20 and QC-formed Zr70Al9Ni20Pd1 metallic glassesannealed for 900 s at 670 K.

References

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