ELSEVIER Physica B 234-236 (1997) 416-417
Short-range order in Nis3La17 metallic glass
M.I. Mendelev a'*, S.N. Ishmaev b, F. Hajdu c, Gy. Meszaros c, E. Svab c
"Moscow Steel & Alloys Institute, 117936 Moscow, Russian Federation bRussian Research Centre Kurchatov Institute, 123182 Moscow, Russian Federation
CResearch Institute for Solid State Physics, H-1525 Budapest, POB 49, Hungary
Abstract
Short-range order in amorphous Ni83Lalv was examined by neutron and X-ray diffraction. Computer modelling was based on a new method using the molecular dynamics technique and the Born-Green-Bogoljubov equation. All three partial pair correlation functions and characteristic structure parameters are derived.
Keywords: Diffraction; Computer modelling; Amorphous materials
In structure studies of amorphous systems the most detailed information on the partial atomic correlations may be obtained by neutron diffrac- tion employing the isotope substitution method (see e.g. Refs. [1-3]). However, isotopic amorphous samples cannot be obtained in every case. In such cases other methods have to be used. The aim of this work was to obtain information on the partial correlations in the Nia3La17 amorphous alloy. To do this, neutron and X-ray diffraction experiments and computer modelling were performed.
The amorphous Nis3La17 alloy was produced by ion-plasma sputtering. In the case of this sample preparation procedure the loss of material is very high, thus the preparation of isotopic specimens is not reliable. Neutron scattering investigations were performed using a multidetector time-of-flight diffractometer [4] at the pulsed source of the Linac Fakel (Kurchatov Institute). The total structure factor S(Q) was measured in a large range of momentum transfer Q = 7-250 nm-1. The X-ray diffraction measurement was carried out in the scattering vector range of Q = 1-140 nm-1, with
* Corresponding author.
monochromatic M o - K a radiation in a non-focusing transmission arrangement [5]. The total pair cor- relation functions (TPCF) 9(r) were determined by the usual sine Fourier transformation of the S(Q).
An iterative procedure of computer modelling based on the molecular dynamics (MD) tech- nique and the Born-Green-Bogoljubov equation (without the superposition approximation) was described [6]. To apply this procedure to the prob- lem, when experimental 9(r) data serve as a starting point for modelling of a two component system, it is necessary to elaborate the transition from total 9(0 to partial 9,b(r) pair correlation functions. The initial model was created by the MD method with partial pair potentials of the FezTb amorphous alloy taken from Ref. [7]. The model contains 1992 particles in the cubic cell with edge length of 3.0752 nm. The TPCF's calculated from g~°)(r) func- tions of the initial model differ from the experi- mental ones. The new set of 9ab(r) was calculated using the equation system:
(1) (o)
g,b (r),
gab (r) = (1)
Z W(P) "' ab ~4ab (r) = g(P)(r), ~(1) a,b
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 1 0 4 9 - 6
5"LL Mendelev et al. / Physica B 234 236 (1997) 416-417 417
where p is the number of the T P C F and w(p) is the " " a b
weight with which the gab(r) function enter into the total functions.
This system was solved by least-squares method.
Thereafter from the initial model, the next model
~(1)(r, was created by iterative procedure on wab, ) and thereby
9'~lb)(r)
were obtained. As a followingg(r)
4
3
2
I
0
2
I
0 0.I
~ t
... e x p e r i m e n t
. . . . m o d e l
neug. !~ o • ,
r , , , | • , , i , , , ,
, ,
, ~ , , , ~X-ray
f • ! • l • !
0.3 0,5 0.7 0.9 1,1 1.3 1.5 r s
4 3 2 I 0 3 2 t 0 3 2 t o
0,1
g~,( r ) b,
n
, I
: : Nt - N t
'i/ '.:
. . ., " . . . . . . . . , , , , ,
o . 1
i~ s J - ~ - ,
t *
:..,,/,-...,, ...
• ; . ".~ . . .
: ; L a - L a
a i " ° ' ' " ~ . , o * - . . . . , - . - ° - . . - - D
, "tl • ~ • ~ , ! • s • w • i
O,) 0,5 0,7 0 , 9 1,1 1.3 1.5
1:, r ~
Fig. 1. Total (a) and partial (b) pair correlation functions of Ni83Lal 7 metallic glass.
step
g(,Z)(r)
were calculated from ,qab~-'(t)¢r) functions, using equation system (1), and the procedure was repeated.The experimental and model
g(r)
functions are shown in Fig. l(a). They are in good agreement with each other. Fig. l(b) shows the calculated partial distributions,g~,b(r).
The distributions of the first neighbour atomic coordination numbers, angle distributions and the size distribution of cavi- ties were determined. The first neighbour atomic distances are: rNiNi = 0.244nm, ryiLa = 0.306nm and rLaLa = 0.353nm. The average coordination numbers are: ZNiNi = 9.9, ZNiLa = 2.3, ZLaNI = 11.6 and ZL,L, = 5.6. The results show that the atomic distribution of this amorphous system is somewhat heterogenous. For example, whereas the average number of La atoms surrounding a Ni atom is ZNiLa = 2.3, there are single Ni atoms possessing 7, 8 or even 9 neighbouring La atoms. The greatest cavity radius is equal to 0.0823nm, which is 1.5 times less than the atomic radius of Ni. Therefore, it can be concluded that the structure of Ni83Lat7 metallic glass is topologically dense.Acknowledgements
The authors greatly acknowledge the courtesy of Prof. G. Palinkas permitting the use of the X-ray laboratory of Central Research Inst. for Chemistry, Hungarian Academy of Sciences. This work was supported by grant O T K A T017129.
References
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