Local structure of Al-La-Ni amorphous alloys

ELSMER

Journal

of Non-Crystalline

Solids 205-207

(1996)

Local structure of Al-La-Ni

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amorphous alloys

Itsuro Yamamoto a,* , John Van Zytveld b, Hirohisa Endo ’ of

a Department of Physics, Faculty Science, Kyoto University, Kyoto 606-01, Japan b Physics Deparhnent, Calvin College, Grand Rapids, MI 49546, USA ’ Faculty of Engineering, F&d lnsfitufe of Technology, At!& 910, Japan

Abstract Extended X-ray absorption fine structure (EXAFS) has been taken on the Ni K-edge for Al,La,,-,Ni,, (5 5 x I 75) and Al,La,,-,Ni,, (0 5 I I 45) amorphous alloys. The results deduced from the data fitting analysis suggest that two types of the chemical short-range order exist; i.e., the La,Ni,-like configuration in the La-rich compositional region and the Al,Ni-like one in the Al-rich region. The transformation from the La,Ni,-like to Al,Ni-like configuration with increasing Al content seems to correlate closely with the change of the electronic states.

1. Introduction It is known that Al-La-Ni amorphous alloys, which are formed by melt spinning over a wide compositional range, have a wide supercooledliquid region and a high glasstransition temperature [l]. In addition, since these amorphous alloys exhibit very superior mechanical properties of high strength and good ductility, their practical applications are expected. In fact, Inoue et al. [2] have recently produced the bulk amorphousalloys utilizing the high glass-forming ability. In a previous paper [3], we reported the data of XPS, electrical resistivity, optical reflectivity, etc. for Al,La,,-.Ni,, amorphousalloys, and the following results were obtained. (1) The peak of Ni 3d-band lies at a higher binding energy and has a narrower width than that of pure Ni, as a result of the hybridization of the Ni 3d-band with La and Al bands.

(2) The density of states at the Fermi level, N(E,), decreaseswith increasing Al content. (3) Above x = 30 the hybridization of Ni and Al becomesdominant rather than that of Ni and La, implying that the interaction between Ni and Al is comparatively strong. The decreaseof N(E,) is consistentwith the observation by Mizutani et al. [4], where they have measured the low-temperature specific heat for A1,Y(La33Ni67)100-,V amorphousalloys. It is very important to determine how the high stability in the Al-La-Ni amorphous alloys is related to the atomic structure and the electronic states. Extended X-ray absorption fine structure (EXAFS) gives useful information on the local environment around a particular atom. In this paper we report the results of EXAFS for the Al-La-Ni amorphous alloys with various compositions.

2. Experimental * Corresponding author. Present address: Faculty of Education, Hirosaki University, Hirosaki 036, Japan. Tel.: + 81-172 393 362; fax: + 81-172 393 362. 0022-3093/96/$15.00 Copyright PII SOO22-3093(96)00468-l

0 1996 Elsevier

Science

Alloy ingots were prepared by arc-melting appropriate amountsof 99.99% Al, 99.9% La and 99.96%

B.V. All rights reserved.

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et al./Journal

of Non-Crystalhe

Ni in a purified argon atmosphere. Arc melting was repeated several times to ensure homogeneity of the alloy. From the master alloy ingots, amorphous alloy ribbons with a size of about 0.03 mm X 2 mm were fabricated by a single-roller or double-roller meltspinning technique in a reduced argon atmosphere. The amorphous nature of the melt-spun ribbons was confirmed by X-ray diffraction and differential scanning calorimetry (DSC). Glass transition temperature (T,) and crystallization temperature (?“..I deduced from the DSC curves are in good agreement with the values reported by Inoue et al. [l]. In Fig. 1, the compositions of the Al-La-Ni amorphous alloys prepared in the present experiments are denoted by closed circles, together with the contour lines of rY [l]. The compositions are nominally expressed in atomic per cent. It should be noted that TX increases with increasing Al content. EXAFS experiments were performed with the spectrometer installed at BL-1OB of Photon Factory in National Laboratory for High Energy Physics. X-ray absorption spectra near Ni K-edge were measured at 80 K and room temperature. The energy region was from 8.07 to 9.44 keV. Further details of the experimental procedure and data analysis have been described elsewhere [5].

3. Results and analysis Fig. 2 shows Ni K-edge EXAFS oscillation x(k) for Al,La,,-,Ni,, amorphous alloys with n: = 10, 20, 25, 30, 40, 50 and 75 as a function of photoelectron wave vector k. The oscillation for each x(k) spectrum tampens strongly in the high k-region above 10 A-‘. Initially the amplitude of the oscillation decreases by the addition of Al. Above x = 25, however, the amplitude grows significantly, which is associated with the strong backward scattering from Al atoms around a central Ni atom. In addition, it is observed that the phase of the oscillation shifts continuously with increasing Al content. Fig. 3 shows the radial distribution function I F(r)] around a Ni atom for the Al,La,,-,Ni,, amorphous alloys. IF(r)] is obtained from Fourier transform of ,y(k) yeighted by k over the k-range, about 3.0-12.0 A-‘. For x= 10, there appear two peaks around 2.1 and 2.7 A. From the results of the curve

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Ni A

Al

10

20

30 40 Concentration

50

60 70 of Labt.%l

80

90

La

Fig. 1. Compositional dependence of crystallization temperature, T,z, for Al-La-M amorphous alloys [l]. Closed circles, the prepared compositions.

fitting analysis which is described later, these peaks correspond to the bond lengths of Ni-Al and Ni-La, respectively. For x = 20, the Ni-Al and Ni-La peaks form a broad peak. The Ni-Al peak grows rapidly and the Ni-La peak becomes small with the Al content above x = 25. As a result, the Ni-La peak is gradually immersed in the strong Ni-Al peak. In order to derive the structural parameters around a central Ni atom, we carried out a curve fitting analysis. First, we calculate back Fourier transform z(k) of F(r) in the region (about 1.2-3.0 A> around

/

2

”

I,

4

6

8

I.

I,

10e 12 k (A-')

1,

I,

14

16

16

Fig. 2. EXAFS oscillation x(k) near Ni K-edge A1,YLaso-,Ni,o amorphous alloys at room temperature.

for

I. Yamamoto

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et al./Joumal

of Non-Crysralline

Ni-La AIxLaao-xNi2o I

M-4 I

Fig. 3. Radial distribution function obtained by Fourier transform of amorphous alloys.

[F(r)/ kx(k),

around a Ni atom, for Al,La,,-,Ni,,

the peaks. Then, the filtered-Fourier z(k) curve weighted by k3 is fitted by means of the least square method with the following EXAFS formula based on a single scattering approximation [6]: k%(k)

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Al,Ni and AlNi. AEoj represents a small correction for the energy of absorption edge. Although we allow A Eoj to be free parameters, the values of A Eoj obtained by fitting are nearly constant and independent of the composition. A good quality of fit for each k3g(k> curve is obtained when we assume a two site model for Ni-Al and Ni-La. Even if we challenge to fit by a three site model taking account the Ni-Ni correlation, the contribution from the Ni-Ni correlation is negligible. Thisosuggests that no Ni-Ni bond exists within about 3 A. Fig. 4(a) shows the bond lengths of Ni-Al (open circles) and Ni-La (crosses) obtained from the fitting for the Al,La,+,Ni,, amorphous alloys as a function of Al content. With increasing Al conient, the Ni-La bond length decreases from 2.89 A at x = 5 to 2.80 A at x= 40, and is constant above x = 40: On the other hand, the Ni-Al bond length is 2.46 A and independent ?f the Al content. The Ni-La bond length, 2.84 A at x = 25 is in good agreement with that deduced from the anomalous

Aly,Lagg+Ni2(

= CSji?i;Bj(kj)k;exp(-2aj’kf) Xsin(2kjrj

+ $j(kj))/rj2,

where kj = ,/k2 - 0.2625A Eoj . The weighting factor k3 is introduced to compensate for the reduction of the g(k) amplitude in the high k-region. The subscript j specifies neighboring atom. Bj(k) is the backward scattering amplitude from j-atom and +j(k) is the phase shift experimented by the photoelectron. Numerical values of Bj(k) and $j(k) are adopted from the supplement table calculated by McKale et al. [7]. Nj represents the coordination number of j-atoms which are situated at an average distance of rj with the mean square displacement of 0;‘. Sj is the scaling factor which includes the decaying effects of the photoelectron wave due to the finite mean free path. In the present analysis the values of Sj are deduced from the fitting of X(k) for intermetallic compounds, La,Ni,, LaNi,

“23() :(*) v . r -x#XX

512.8 2 ; 2.6 i

20

31

Ni-AI

2.4 - ’ ’

z? 64-

# % x x x ;i-La

0,1as0008

0

NAI X%X>

E

2

54

00 8

O0 xx ogoo 1 ,u, I1 IX 10 20 30 40 50 60 70 10 Al (at.%)

Fig. 4. (a) Bond lengths of Ni-AI (open circles) and Ni-La (crosses), and (b) Al (open circles) and La (crosses) coordination numbers around a central Ni atom for Al,Las,-,Ni,, amorphous alloys as a function of Al content.

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et al./Joumal

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ALxLa70-xNi30 o:30 :la) v . -c “xxzxxx p2.8

Ni-La

-

X%83

' 2.6 -F 0 0 CJ 0 0 0 0 *N:-AL g 2.4 -

0

10

20

30

40

50

Fig. 5. (a) Bond lengths of N&Al and Ni-La, and (b) Al and La coordination numbers for A1,rLa,O-,Ni,, amorphous alloys.

X-ray scattering by Matsubara and Waseda [8]. Fig. 4(b) shows the average coordination numbers of Al, iVA, and La, NLa around a central Ni atom for the A1,YLa,,-,Nr,, amorphous alloys. N’r increases gradually with increasing Al content. It seemsthat NA, approaches8 at x = 80, where the amorphous alloy does not contain a La atom. NLa decreases continuously and a discrete decreaseof NLa is observed between x = 35 and 40. Above x = 40, NLa decreasesagain and approachesalmost 0 at x = 75. It is worthwhile to note that the sum of NALand NLa gives almost a constant value, 6 in the region from x=5 to x=35. Fig. 5 shows the bond lengths (a) and coordination numbers (b) of Ni-Al and Ni-La for the Al,La,a -xNi,, amorphousalloy as a function of Al

Fig. 6. (a) Trigonal

(b)

anti-prism.

(b) Archimedean

(1996)

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content. The changesof the bond lengths and coordination numbersfor N&Al and Ni-La are very similar to those for the Al,La,,-,Ni,, amorphous alloys, though the discrete decrease of N,, occurs between x = 30 and 35. NLa is 6 at x = 0.

4. Discussion

Al (at.%)

(a)

Solids 205-207

anti-prism.

The absenceof the Ni-Ni bond suggeststhat a Ni atom is surrounded by Al atoms and La atoms in preference to Ni atoms, the nearest neighbor in the inter-metallic compounds such as La,Ni,, Al& AlaNi, and AlNi is the Ni-La or Ni-Al bond, and the Ni-Ni bond length is far from the secondnearest neighbor distance [9-131. The above evidence is consistent with our results. The Ni-A1 bond length has a constant value of 2.46 A over the whole Al content. For the Al-Ni intermetallic compounds such as AlaNi, Al,Ni,, AlNi and AINi,, it is known that the Ni-Al bond length has0nearly the samevalues ranging from 2.46 to 2.52 A, despite the different crystal structure [ 1l-131. The Ni-Al bond length for the Al-La-Ni amorphousalloys is close to those values. The composition of x = 0 for Al,La,,-,Ni,, correspondsto the interrnetallic compound La,Ni,. has the Th,Fe,-type crystal structure La,Ni, (hexagonal, CzV (PG,mc)), and consistsof packings of trigonal anti-prisms [9,10]. In this structural unit, each Ni atom is coordinated by six La atoms at the apices of a trigonal anti-prism as shown in Fig. 6(a). Nta for the La,,Nr,, amorphous alloy as 6. In addition, the Ni-La bpnd length of 2.90 A agrees fairly well with 2.91 A which is deduced from our EXAFS analysis for La,Ni,. These results imply that the trigonal anti-prism is the structural unit in the La,,Ni,, amorphousalloy. Up to x = 30 for A1,La,O-xNi,, and up to x = 35 the sum of NLa and NA, has a for Al,La,,-,Ni,,, constant value 6. This suggeststhat the local structure for the La-rich amorphousalloys is composedof the trigonal anti-prism-like configuration. Since the atomic radius of Al (1.43 A) is rather small than that of La (1.87 A), the gradual change to the more close-packed configuration is expected when La atoms are replaced by Al atoms. This may be the

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reasonwhy the Ni-La bond length decreasesin the La-rich region as seenin Fig. 4(a) and Fig. 5(a). N,, decreases considerably above x = 35 for Al,La,,-,Ni,, and above x = 40 for Al,La,,-,N&, where the Al content contained in the samplesis higher than the La content. In these Al-rich compositional region the change of the local structure may occur. As a possibility. of the local structure for the Al-rich amorphousalloys, we may choose the structural unit which is found in the intermetallic compound Al,Ni. A1,Ni exists in the vicinity of the compositions where A1,rLa,,-,Ni,, and Al,La,,-,Ni,, contain no La atom. AlaNi is orthorhombic (D:; (Prima)) [ll], and its structural unit is characterized by a distorted Archimedean anti-prism shown in Fig. 6(b). In this Archimedean anti-prism, the coordination number of Al around a central Ni atom is 8. This value is in good agreement with the extrapolated value of NA,, 8 for Al,,Ni,,. Archimedean anti-prismatic polyhedra can be found in the denserandom packing model for amorphous metalsby Bernal [ 141.It is interesting to note that the transition from the ductile to brittle nature [ 11accompanies the discrete decreaseof N,,.

5. Conclusions We have carried out EXAFS measurementsfor the Al,La,,-,Ni,, and A1,xLa,,-,Ni,, amorphous alloys. The results obtained from the data fitting analysis suggestthat the chemical short-range order (CSRO) changesfrom the La,Ni,-like configuration to the more close-packedAl,Ni-like one. This structural transformation is accompaniedby the changes of the electronic states. The increase of the hybridization of Ni 3d-band and Al sp-bandcausesthe substantial decrease in N(E,) with increasing Al content. We can infer from Fig. 1 that the stability in these amorphous alloys increaseswith the Al content, because 7; can be taken as a measureof the

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stability for amorphous metals. The strong correlation amongthe glassstability, CSRO and N(E,) has been supportedin the theoretical study by Le et al. Ml. Acknowledgements The authors are grateful to ProfessorM. Yao and S. Kawakita and K. Okada for their collaboration in EXAFS measurements,and ProfessorsT. Okabe and S. Iida and Dr H. Ikemoto, Toyama University, for the use of double-roller melt-spinning apparatus. References [II

A. moue, T. Zhang and T. Masumoto, Mater. Trans. Jpn. Inst. Met. 30 (1989) 965. Dl A. Inoue, T. Nakamura, N. Nishiyama and T. Masumoto, Mater. Trans. Jpn. Inst. Met. 33 (1992) 937. J. Van Zytveld and H. Endo, J. Non-Cryst. 131 I. Yamamoto, Solids 156-158 (1993) 302. S. Ohashi, T. Matsuda, K. Fukamichi and K. [41 U. Mizutani, Tanaka, J. Phys.: Condens. Matter 2 (1990) 541. I51M. Inui, M. Yao and H. Endo, J. Phys. Sot. Jpn. 57 (1988) 553. 161B.K. Teo, EXAFS: Basic Principles and Data Analysis (Springer, Berlin, 1986) ch. 4. B.W. Veal, A.P. Paulikas, S-K. Chan and t71 A.G. McKale, G.S. Knapp, J. Am. Chem. Sot. 111 (1988) 3763. 181E. Matsubara and Y. Waseda, Sci. Rep. Res. Inst. Tohoku Univ. A36 (1991/1992) 187. PI P. Fischer, W. Halg, L. Schlapbach and K. Yvon, J. LessCommon Met. 60 (1978) 1. [IO] R.W.G. Wyckoff, Crystal Structures, 2nd Ed., Vol. 2 (Interscience, New York, 1964) p. 217. [ll] A.J. Bradley and A. Taylor, Philos. Mag. 23 (1937) 1049. (121 P. Villars and L.D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, Vols. l-3 (ASM, Metals Park, OH, 1985). 1131 W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vols. l&2 (Pergamon, London, 1958 and 1967). [14] J.D. Bemal, Proc. R. Sot. A280 (1964) 299. [15] D.H. Le, C. Colinet and A. Pasture& Philos. Mag. B63 (1991) 1299.