LOCAL ORDER IN Fe-B METALLIC GLASSES STUDIED BY HIGH-RESOLUTION NEUTRON DIFFRACTION

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LOCAL ORDER IN Fe-B METALLIC GLASSES

STUDIED BY HIGH-RESOLUTION NEUTRON

DIFFRACTION

E. Sváb, G. Faigel, G. Mészáros, S. Ishmaev, I. Sadikov, A. Chernyshov

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, supplCment au n012, Tome 46, dCcernbre 1985 page C8-267

LOCAL ORDER IN Fe-B METALLIC GLASSES STUDIED

BY

HIGH-RESOLUTION NEUTRON DIFFRACTION

E . Svkb, G . Faigel, G . M6szBros, S . N . ~shmaev*, I.P. ~adikov* and A.A. ~hernyshov*

Central Research I n s t i t u t e for Physics, H-1525 Budapest 114, P.O. BOX 49, Hungary

*I.V. Kurchatov Atomic Energy I n s t i t u t e , 123182 Moscow, U.S.S.R.

R6sum&

-

Le facteur de structure du systeme amorphe Fe-B, avec une concentration en bore allant de 14 5 25%, a &t6 obtenu par diffraction des neutrons d'une source puls&e. Le domaine de vecteurs de diffusion explor6 va jusqu'z 230 nm-I ce qui permet d'atteindre une trSs bonne r6solution. On en a d&duit les distances entre proches voisins Fe-B et Fe-re, ainsi que la largeur de distribution de ces distances et les nombres de coordination. Les rgsultats sont interpr&t&s sur la base d'un modSle de quasicristal.

Abstract

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The structure factor of amorphous Fe-B system for 14-25 at% boron was measured by pulsed neutron diffraction up to

230 nm-l resulting in high resolution in the distribution function. The first neighbour Fe-B and Fe-Fe distances, the width of their distributions and the coordination numbers are derived. The results are interpreted by quasI-crystalline model calculations.

I

-

INTRODUCTION

The Fe-B metallic glasses belong to the most widely Pnvestigated amorphous systems (see review /I/)-, but even so a complete understand- ing of the structure is still lacking. The main task of diffraction measurements is to obtain information on part2al atomic correlations. The three partial correlation functions of PegoB20 /2/ were determined using combined isotopic substitution neutron and X-ray diffraction methods. From a high resolution time-of-flight neutron diffraction study /3/ the Fe-B and Fe-Fe first neighbour distr2butions were re- solved for the FeglBlg glass. The Fe75B25 /4/ glass was investigated by energy dispersive X-ray diffraction and the Feg3B17 /5/ composition was studied by anomalous X-ray diffraction.

The structure of the Fe-B glasses as a function of the B content has not, as yet, been discussed on the level of the partials. The aim of the present work is to study the local atomic environment In the amorphous Fe-B system in dependence of the boron by means of high resolution time-of-flight neutron diffraction ( ' ) and to interpret the experimental data in the framework of quasi-crystalline model /6/

calculations. 11

-

EXPERIMENTAL

Amorphous FelOO-xBx samples with x=14, 19 and 25 at% boron content were prepared by melt spinning in the form of ribbons approximately

( I ) Our previous results obtained on PeglBlg glass /3/ are included

in this paper.

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C8-268 JOURNAL DE PHYSIQUE

20 um thick and 2

mm

wide. For sample preparation, "B isotope enriched

to 99.2% was used in order to avoid high absorption of thermal neutrons by 1 0 ~ isotope. The ribbons, of about 15 g, were wound on a vanadium

tube of 5 mm diameter and 0.3 mm wall thickness, forming a cylindrical

sample of 40 mm height and 10 mrn diameter.

Neutron diffraction experiments were performed combining the facilities of two neutron diffractomet rs. In the scattering vector range,

Q=4rr-sini3/X, of 4<Q<100 nm-y the measurements were carried out on a

double axis neutron diffractometer at the FnVR-SM reactor in Budapest using a monochromatic beam of ho=0.105 nm. The magnetic scattering of the samples was separated by using a 1.8 T magnetic field. The measurements were extended to relatively high Q range using the time-of- flight diffractometer /7/ at the 60 MeV electron LINAC in Moscow. The 16 detectors of the instrument were placed in ihe region of

29=95.5-164.5°covering a range of 48dQ<480 nm-

.

The structure factor S(Q) was determined from both kinds of measurements and the S(Q) functions were connected at the maximum position of

the oscillation at about 80 nm-l. The physical oscillations in the S(Q) for Q>230 nm-I are buried in the background and thus the measured data are informative for further calculation up to this limit.

I11

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RESULTS

Figure 1 shows the structure factors up to 230 nm-I. Oscillations clearly exist up to high scattering vectors, although their amplitudes become very small; less than 2% above 200 nm-l. The character of the S(Q) functions is similar for the three samples, however, slight dif- ferences can be observed. These deviations appear in the position of the oscillations and in the full width of half maximum of the first

peak at 31 nrn-lvarying from 5.2 nm-1 to 4.2 nm-l with decreastng B

content. The amplitude of the oscillations at hlgh Q-values is some- what larger for the stoichiometric Pe75B25 glass than for the two other

samples.

Fig. 1

-

Structure factors S(Q) Fig. 2

-

Reduced distribution func-

of Fe-B metallic glasses tions G(r) of Fe-B metallic glasses

Figure 2 shows the reduced distribution functions G(r) obtained by

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Qmax

The upper limit of Fourier transformation at about Q = 230 nm-l is taken at the position where the S(Q) is unity, in or%? to minimize the non-physical oscillations at small r-values.

The peculiarity of the G(r) functions is the splitting of the first peak into two subpeaks. These subpeaks can be related to the Fe-B and Fe-Fe first neighbour distributions on the basis of physical conside- rations and with regard to the weighting factors of the partial

GFeB (r) functions:

From the split first peak of G(r) the first neighbour Fe-B and Fe-Fe distances and the width of their distribution were determined. The results are summarized in Table 1. The first'neighbour distances are rFeB=0.215+0.003 nm and rFeFe=0.255+0.002 nm for the three samples independently of the composltlon.

he-real

width (Area1) of the distribution was determined from the measured 3ne (Ameas) using Gaussian ap- proximation: (Area1) = ( Ameas)

-

(aFT)

,

where

AFT

= 3. 8/Qmax is the broadening caused by the limited Fourier transformation. The width of the Fe-B distribution proved to be significant smaller than that of the Fe-Fe distribution. The value Ag'& = 0.019+0.003 nm was obtained for all three samples independently of the concentration, while the width of the Fe-Fe distribution is areal = 0.036+0.002 nm for the Fe75B25 glass and 0.04+0.003 nm for %Zeoff-stoichiometric compositions. Table 1

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Structural parameters for Fe-B metallic glass from high resolution neutron diffraction measurement: Fe-B and Fe-Fe first neighbour distances (nm),their distribution width ( n r n ) , partial coordination numbers (atom) and the Cargill /11/ chemical short range order parameter.

~ Z ~ / ' Pmeas real e B meas real e / n 'F~B 'BF~ 'FeFe r F e B'F~B ='F~B e ' ~ ~

The first neighbour partial coordination numbers ZFeB were calculated from the radial distribution function, SDF (r) = 4 ~ r ~ ~ ~ + r ~ (r) as described in /3/. The average atomic densities poc95.1, 92.0, 89.0 at/nm3 /lo/ were used for the 25 at%, 19 at% and 14 at% boron content samples, respectively. The results are summarized in Table 1, the error of ZF data is about 10 %. The number of Fe-Fe first neighbours and the num%er of Fe atoms surrounding the B atoms seem to be independent of the con-

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(28-270 JOURNAL DE PHYSIQUE

centration. As an average we get ZFeFe=ll+l atoms and ZBFe=7.5+0.8 atoms for the investigated samples. On the other hand the number of B atoms around an Fe atom decreases from 2.5 atoms to 1.1 atoms with decreasing B content. From the Z F e ~ values the short range order coeffi- cients introduced by Cargill /11/ were calculated. The normalized coefficient

no

is near to unity for all three samples indicating a chemically or%?ed arrangement with preference for unlike atomic neighbours. The slight deviation from unity for the FeglBlg and Fe86B14 samples probably originates from the inaccuracy of ZFeB data.

IV

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QUASI-CRYSTALLINE MODEL CALCULATIONS

For the interpretation of the experimental G(r1 functions, calculations were performed on the basis of the quasi-crystalline model /6/. The QC- model is based on the assumption that the first neiqhbour environment of an atom in an amorphous alloy is similar to that of a crystalline compound existing at'the composition investigated. However, it is not always trivial to find the corresponding crystalline phase, the study of the crystallization process may give important clue to it.

In the case of the amorphous Fe75B25 alloy the first step of crystalliza-

6 _{tion /12/ is the transformation into}

metastable tetragonal Fe3B which has

4 _{a Ni3P type structure /13/. The ato-}

mic position parameters of metasta-

2 _{ble Fe3B have not yet been determin-}

ed, thus we had to use those of the

0 _{crystalline NijP in the model calcu-}

4 _{lation. The inaccuracy of the posi-}

tion parameters does not cause sig-

2 _{nificant changes in the first coor-}

dination shell, but it may modify

0 _{drastically the shape of the second}

coord2nation shell.

4 The model calculation was extended to the description of the local

2 structure of the off-stoichiometric Pe-B samples. It was supposed that

0 _{the structure preserves the local}

atomic arrangement of the stoichio-

- 2 _{metric Fe75B25 glass and the Fe}

surplus forms iron rich clusters. Thus the G(r) function for the off-

r[nml stoichiometric compositions is the sum of two spectra. One corresponds Fig. 3

-

G(r) functions from ex- to the Fe75B25 glass as described periment (-) and from quasi- above and the other one to the pure crystalline model calculations iron. The best agreement between

( - - - ) calculation and experiment was ob-

tained when the close packed fcc structure of the iron phase stable at high temperature, was taken into consideration. The experimental and calculated G(r) functions are shown in Fig. 3. The main features of the experimental G(r) are reproduced by the calculation, namely the position and the relative maxima of the subpeaks in the first peak.

V

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CONCLUSIONS

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first neighbour distances and the partial coordination numbers do not show any dependence on the B content, except that the number of B atoms around an Fe atom decreases with decreasing boron concentration. The width of the Fe-B first neighbour distance distribution is narrow, and it is significantly smaller than the width of the Fe-Fe distribution. The latter is slightly smaller for the stoichiometric Fe75B25 glass than for the off-stoichiometric compositions.

The quasi-crystalline model has been used to interpret the experimental results. The general features of the split first coordination shell of the experimental G(r) are reflected by the model calculation using the tetragonal Fe3B type short range order in the case of the Fe75B25 glass. For the off-stoichiometric samples it was supposed that two cha- racteristic environments of the Fe atoms exist in the glasses: one with short range order of the tetragonal Fe3B and another one with that of the close packed pure Fe.

This idea is in agreement with the results of small angle neutron scattering measurements /6/ which show the presence of small inhomoge- neities of gyration radius Rg=0.6 nm. These small clusters may be related to Fe rich surroundings while the overall structure of the glass preserves the Fe75B25 type structure in accordance with the model calculation.

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