Medium range structure of amorphous Fe44Ni36B20-alloys by means of neutron-scattering at small momentum transfer

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Medium range structure of amorphous

Fe44Ni36B20-alloys by means of neutron-scattering at small momentum transfer

H. Träuble, P. Lamparter, S. Steeb

To cite this version:

H. Träuble, P. Lamparter, S. Steeb. Medium range structure of amorphous Fe44Ni36B20-alloys by

means of neutron-scattering at small momentum transfer. Journal de Physique I, EDP Sciences, 1992,

2 (6), pp.1029-1041. �10.1051/jp1:1992194�. �jpa-00246583�

Classification

Physics

Abstracts

61.12E 61.40

Medium range ^structure of amorphous Fe44Ni~~B~o-alloys by

means

of neutron-scattering at small momentum transfer

H-

Trhuble,

P-

Lamparter

and S- Steeb

Max Planck Institute for Metal Research, Institute for Materials Science, Seestrasse 92, 7000

Stuttgart

1,

Germany

(Received 19 November 1991,

accepted

in

final form

27

January

1992)

Abstract. The medium range ^structure of amorphous Fe44N136B20-alloys was

investigated using

neutron small

angle scattering

with

isotopic

substitution and immersion

technique.

The bulk material contains

density

fluctuations but _no concentration fluctuations. In addition, surface

scattering

is caused

by

surface

roughness.

The

regions

with different densities within the bulk _as well _{as on} the surface _are

separated

by ^smooth boundaries. The structure of the bulk is the _same with all three melt spun

specimens,

whereas the surface

apparently

differs from

specimen

to

specimen.

1. Introduction,

Amorphous Fe-Ni-B-alloys

with boron concentrations between 10 and 25 atomic

percent

were

thoroughly investigated conceming

their

thermodynamic [I],

electrical

[2],

mechanical

[3],

and

magnetic [4] properties.

The atomic short range order _was

investigated by

neutron diffraction

[5]-

In the

present

^work we

investigated

the medium range ^structure

using

the small

angle

neutron diffraction method

(SANS)-

This work

mainly

is _a methodical work since _we combined for the first time

isotopic

substitution

technique

and immersion method.

Using

the

isotopic

substitution

technique

the

question

_was studied whether the

scattering

is caused

by density

fluctuations _or

by

concentration fluctuations.

Furthermore,

the

application

of the immersion method allowed the

separation

of

scattering

effects of the bulk and of the surface.

The

amorphous Fe-Ni-B-alloys

mentioned above _are

especially

convenient for the

present investigation

since the

nickel-isotopes

available allow to

apply

the

isotopic

substitution

method.

Furthermore,

the

amorphous Fe-Ni-B-alloys

_can be

produced by

^melt

spinning

_very

reproducible

and with

high efficiency,

I-e-

they

become

amorphous

_very

easily

as rather wide

ribbons.

Using

these wide

ribbons,

the

edges

_are not irradiated

by

neutrons and thus

refraction at the

edges

does not _occur.

In the

following

_we treat the

temary Fe-Ni-B-alloy

_{as a}

binary TM-B-alloy (TM

₌ transition

metal).

Note that the atomic diameters of the iron and the nickel _{atom are}

nearly

the _{same as} well _as the chemical behaviour.

2, Theoretical fundamentals.

2.I STRUCTURE FACTORS. For _a

binary alloy

three so-called Bhatia-Thomton

partial

structure factors

[6]

_are related to the differential

scattering

_cross section

[7]

via

equation (I)

S _{IQ )}

=

<b>~ SNNIQ ⁾

+ Ci

C21bi b2)~ Scc IQ )

+ 2

<b> ibi b2) SNCIQ ii)

with

Q

=

~£

sin 8

=

modulus of momentum transfer

A ₌

wavelength

2 8

=

scattering angle

b;

₌

scattering length

of component ^I ; I

=

I

corresponds

to

TM,

I

=

2

corresponds

to B.

(b)

₌

cibi

+ c~

b~

c, = atomic fraction of component ^I

S~~ IQ )

₌

partial

structure factor of the correlations between

density

fluctuations

Scc IQ )

₌

partial

structure factor of the correlations between concentration fluctuations

S~c(Q)

₌

partial

structure factor of the correlations between

density

and concentration fluctuations.

We _note that

equation (I)

_concems the bulk of _an

alloy.

In

practice

_a surface

scattering

effect may occur in addition to the

scattering

from the bulk.

2.2 SMALL _ANGLE SCATTERiNG

[8, 9].

If _a

specimen

consists of two

phases, namely

regions

r within _a matrix _m then the

scattering length density,

I-e- the

product

of

scattering length

and

density

^is

~r=

porlblr

_~w

=

pomlblm 12)

with

(b)r

= mean

scattering length

within _a

region (b)~

= mean

scattering lenght

within the matrix

p~~, p~~ ^{= mean} atomic number

density

of the

regions

and

matrix, respectively.

The difference of the

scattering length

densities is

AI~ _{= l~r ~m}

13)

For _a

disperse

system ^{which is}

isotropical

the differential _cross section for coherent

scattering

per ^{atom amounts to}

~"

IQ

= v~ v~

(A~

)~4 _w

lR

^~ y

(R )

^~~~

~~

dR

(4)

dD po

QR

where

v~, v~ = volume fractions of

regions

and matrix,

respectively

y(R )

₌ autocorrelation function of the

scattering length density.

Asymptotic

behaviour, If the

regions

within the matrix have _a well defined smooth

surface,

the

scattering

law at

larger Q-values

is

$ _{IQ )}

=

k iA~ )2( ) _Q

-~

15)

S ₌ total surface of the

regions

V = irradiated volume

S

V ₌

specific

surface.

This _means that in the

log ~(

_versus

log Q-plot

the

gradient

of _a

straight

line should be

d 4-

2.3 IMMERSION TECHNIQUE. In

[10j

the so-called immersion

technique

_was introduced

which allows _to separate ^{the effects of surface}

scattering

and of volume

scattering.

A

specimen

container with smooth inner surface contains _a so-called immersion

liquid

which surrounds the

specimen.

If the immersion

liquid

has the _same

scattering length density

_as the

specimen,

surface

scattering

effects _are eliminated and

only

volume

scattering

remains visible.

Of _course, the immersion

liquid

must

totally

wet the

specimen

surface. Mixtures of

deuterated and naturel

ethyl

alcohol

(e.a-)

_can be

adjusted

to

scattering length

densities

within the range 0-31 _x

1#° ~~

_{« ~ < +} 6-1 _x

1#° ~~ corresponding

to 0 vol, fb _w

cm cm

v~~~~_~_~_ w loo vol, fb- This follows from the

scattering lengths b~

= 0.37 _x 10~ ^~~ _cm and

b~

= + 0.67 _x 10~ ^~~ _cm-

It should be noted that the

complete suppression

of the surface effect is

only possible

for the

simple

_case of _a

rough

surface. Then the

scattering intensity

^scales with the square ^{of the} difference A~ ^{of the}

scattering length

densities of the

specimen

and of the immersion

liquid.

For the _case of concentration fluctuations at the

surface,

such _as

precipitates,

the surface

scattering

_can be

only

reduced.

3.

Experiments, results,

and discussion.

3.I SPECiMEN PREPARATION. Three

Fe44Ni~~li~o specimens

were

prepared using

_a melt-

spin

_{apparatus as} described in

[11]-

The

scattering lengths

for coherent

scattering

are

compiled

in table I-

The

following isotopic

combinations where chosen _:

I)

54Fe4462N136~~B20. ^{For this}

specimen (b)

=

0 which _means

according

to

equation (I)

that with _one

scattering experiment

_we obtain

directly

~~~ ~~

Cl

2(b~

~2)~ ~~ ^~~

~~~

ii)

natFe446°N136~~B20. ^{For this}

specimen (bi-b~)=0

which _means

according

to

equation (I)

that with _one

scattering experiment

_we obtain

directly

~~~~~

(b)

^{~' dD}

^~~

^~~~

iii) natFe44natNi36~~B2o.

For this

specimen

the differential

scattering

cross section

~"

IQ

contains all three

partial

structure factors.

dD

Table I-

Scattering lengths

b

for

coherent

scattering.

Isotope

^b

[10~

^~~

cml

Refe~~~~~

natfe 0.954 12

54Fe

_{0.412 13}

natNi 1,03 12

6°Ni

0.28 12

62Ni

0.87 12

~~B 0.666 12

3.2 SCATTERING APPARATUS. The neutron diffraction

experiments

_were

performed

at

ILL, Grenoble, using

the Dll instrument with _a

wavelength

of lo

I _[14,15]-

_The

Q

_range amounted to 3 _x

10~~ i~

«

Q

_< 2-3 _x

10~~i~~

3.3 EVALUATiON _{OF SCATTERiNG DATA.} The measured intensities _were corrected for

background, scattering

of the

container,

and

absorption.

The correction for

multiple scattering

_was done

according

to

[16]-

The corrected data _were normalized to the differential

cross section in units of

~~'~ ^using

^{the incoherent}

^scattering

^{of the}

^ethyl

^alcohol

stera atom

mixtures as reference standard. For the details of these

procedures

_see

[17]-

Amorphous

Fe-Ni-B

specimens

show

magnetic scattering

besides the nuclear

scattering.

Using

an extemal field for

magnetic

saturation of the

specimen

it _was

possible

to

separate magnetic

from nuclear

scattering [17]-

3.4 MODEL _{CURVE FOR} SEPARATION OF COHERENT AND INCOHERENT SCATTERING CONTRI-

BuTioNS- It tumed _out that the

log $ _{log Q-plot}

_{of the}

_scattering

_is

a

straight

line _on which the incoherent

scattering

of the

specimen

and of the immersion

liquid

is

superimposed.

Therefore the differential

scattering

cross section ~"

IQ

_can be

represented according

to dD

equation (8)

~~ al

$ ^~~ (8)

Q~~

~ ~~

The three parameters ai, a~, and a~ were calculated

according

to the

Levenberg-Marquard

method

[18, 19]-

_{a~ is} the _sum of the incoherent contributions of the immersion

liquid

and of the

specimen.

3.5 RESULTS.

3-5.I

Amorphous

~4Fe44~~Ni~~~~B~o ^with

(b)

=

0-

3.5.I-I Direct

Scc(Q )-determination

with 54Fe4462Ni~6~~B~o- ^It was

already

mentioned

(see Eq. (6))

that

by

a

SANS-experiment

with the

54Fe4462Ni~~~~B~o-specimen

in _vacuum

or air the

Scc(Q )-curve

follows

immediately

without any contribution of

S~~(Q)

and

S~c(Q

)- ^{Thus the}

experimental

result obtained with this

special specimen

is linked

directly

with the concentration fluctuations. However, the

corresponding intensity

_curve, not

presented

here, showed no small

angle scattering

effect and thus the

amorphous Fe44N136B20 specimen

contains _no concentration fluctuations in the bulk and also _no concentration

fluctuations at the surface.

3-5-1.2 SANS with

54Fe4462Ni~~llB~o

in immersion

liquids.

To obtain information

conceming

the surface

properties

of the

54Fe4462Ni~~llB~o-specimen

_we

performed

SANS-

experiments

with the

specimen

in five alcohol solutions which differed in their _content of undeuterated

ethyl

alcohol

(58

_; 74 _; 83 _;

91;

loo vol.

fb, respectively)

and thus in their

scattering length density (2-3

_; 1-3 _; 0-7 _; 0 0.3 _x 10~°

cm~~, respectively).

Figure

I shows the nuclear

scattering

for the

specimen plus

immersion

liquid

in _a

log

^~"

IQ )- log Q-presentation.

For

Q~6 x10~~i~~

_the

curves lie the

higher

the

dD

hydrogen

concentration in the immersion

liquid

is- This effect is caused

by

the incoherent

scattering

of the

hydrogen

atoms.

We

proved

in

chapter

3-5-1-1 that the bulk and the surface of the

Fe44Ni~~llB20-specimen

contains _no concentration fluctuations. Therefore the

scattering

effects observed in

figure

I have to be ascribed _to

scattering

caused

by

surface

roughness.

In

figure

2 _we show for the immersion

liquids

with

58, 91,

and 100 vol, fb the scattered

intensity

after subtraction of the incoherent

scattering.

2.4

~l

2. 2 t G

~ ~2.

( (

l. 8

$

~

~ loo cY

" 91 loo

l. 4 ₈₃ 4

Ii

~

74

~'~

-2.5 -2.0

~l.5

_{2 4 6 8 lo 12 14 16}

log (q

)I-i])

Q

lie-3A-']

Fig.

1.

Fig-

2.

Fig.

I.

Amorphous

5~Fe~~62Ni~~"B~o; Differential

scattering

cross section; Nuclear neutron

8cattering

(coherent

plus

incoherent

hydrogen 8cattering)

^Parameter : concentration of the immersion

liquid

in volume 9b of undeuterated

ethyl

alcohol.

Fig.

2.

Amorphous

5~Fe4462Ni~~"B~o ^~" IQ ^nuclear neutron

scattering

Parameter _: dl2

concentration in volume 9b of undeuterated

ethyl

alcohol.

In addition _we present ⁱⁿ

figure

3 the

magnetic scattering intensity

_as obtained from the

54Fe4462Ni~~llB~o-specimen

contained in _an immersion

liquid

with 58 vol. fb of undeuter- ated

ethyl

alcohol. The full line shows _a model _curve

according

to

equation (8).

The

magnetic scattering

_was found _to be

independent

from the

composition

of the alcohol solutions and thus is _a bulk effect.

JOURNAL DE PHYSIQUEI -T 2, N'6, JUNE 1992 39

-~100

j ~ (

₈₀

60

40

~~

djc

_fl

~ ~

5 lo 15 20

q [10-3A-1)

Fig.

3.

Amorphous

5~Fe4462N136~'B20, ^~" IQ ) (-) model _curve.

dl2

mag

The observed

integral

coherent

scattering

_cross section ~"

follows from the model dn _int.

curves

according

to

equation (8)

as

$

^~~

~

i~ ~( IQ dQ

"

~~ loiai~~ Q$ll~~l ₁₉₎

Qmm

Figure

4 shows the _run of

~(

versus the concentration of undeuterated

ethyl

alcohol in

int,

the alcohol solution. The numerical values also _are contained in table I- The

scattering

from _a

rough sample

surface is

expected

to scale with the _square of the difference of the

scattering length

densities of the

sample

and of the immersion

liquid.

A fit of _a

parabola

to the

points

in

figure

4 shows _a minimum at 92 vol-fb

C~HSOH, corresponding

to a

scattering length density

of the immersion

liquid equal

to zero, I-e-

equal

to that of air-

Therefore,

this

point corresponds

to _a diffraction

experiment performed

with the

specimen

in

air,

I-e- to the

observation

already

discussed in

chapter

3 5 1-

The Porod

gradients

_a~ follow _as

compiled

in table II and _are close to 4- The variations of the values _are within the 10 fb _error bar and _are of _no

significance.

0.24

/

fl I

^~~~

~ ~°

~ 0.16

~ ^"

[

0.12 a

fi 0.08

.~

u o. oq

#

jC4'~

o. Do

so 60 70 ao 90 loo

C2HSO H-concentration [volilij

Fig.4. Amorphous

5~Fe4462N136~~B201

Integrated intensity

_versus concentration of undeuterated

ethyl

alcohol with

parabola.

Table II-

-Amorphous

~~Fe44~~Ni~~~~B~o- ^Porod

gradient

_a~,

specific su~fiace

^~

,

and

integrated

V d«

~~~~~~~~

dn _int.

jon~ntration ) j

~~~~~ ~~~~~

~

1o3 ^~~~

[i~-16

~~

[V°I.f~l cm~

_{sr atom}

58 3-8 0-156 0-216

74 3-9 0.101 0.l19

83 3.9 0.206 0.030

91 4-0 0.007

loo 4-1 0-071 0.049

The observation of smalle

angle scattering

shows that the surface of the

specimen

is

rough

on a medium _range scale. The observation of the _run

according

to the Porod law means that this

roughness

has _no fractal

properties.

From

equations (5), (8)

the

specific

surface ~

follows _as V

'

= ~

lilt

~~

(lo)

Table II shows besides the Porod

gradient

_{a~ the} differential

scattering

_cross section and the

specific

surface

)

_in

_dependence

_from

the concentration of undeuterated

ethyl

alcohol in the alcoholic solution.

The

specific

surface ~

as obtained from

equation (lo)

is that share of _a

rough specimen

V

surface which

gives

rise to a small

angle scattering

effect. It should be

independent

from the

composition

of the

surrounding

alcoholic solution.

We

recognize

in table II rather wide

spread

^{values of ~ and} _propose an average value of V

about ~

= 0-13 cm~ for the

amorphous ~~Fe44~~Ni~~~~B~o-alloy.

V

3-5-2

Amorphous

~~~Fe44~°Ni~~~~B~o with Ab

= 0-

3-5-2-1 Direct

S~~ IQ )-determination

of ~~~Fe44~°Ni~~~~B~o- ^It was

already

^mentioned

(see Eq. 7))

that

by

a

SANS-experiment

with

amorphous

~~~Fe44~°Ni~~~~B~o ⁱⁿ vacuum or air the SN~N(Q

)-curve

follows

immediately.

In contrast to the

~~Fe44~~Ni~~~~B~o-alloy

as treated in

chapter

3-5-1-1 _we observed _a small

angle scattering

effect with the

natFe446°Ni~~llB~o- specimen.

In the

following

we will

discuss,

whether this effect is caused

by

bulk

scattering

or

by

surface

scattering.

3-5.2,2

Application

of the immersion method to

natFe446°Ni~~llB2o. Figure

5 shows the differential

scattering

_cross section _as measured with

natFe446°Ni~~llB~o

ⁱⁿ five alcohol solutions which differ in their content of

ethyl

alcohol

lo

9 _; 17 29 _; 42

vol-fb, respectively)

and thus in their

scattering length density (6

_; 5.49 5 _; 4-16 _; 3.38 x

I#°

_{cm~ ~,}

respectively).

2.

=

f2.0

~~

~l~i

5

j ⁴²

l ²⁹

#)@

Ii

9

bo 5

",,,,,

0.

-2.5 -2.0 -1.5

log jq

ii-ii)

Fig.5. Amorphous

5~Fe4462N136~~B20i ^~" (Q)I nuclear neutron

scatteRng;

Pararneter dl2

concentration in volume 9b undeuterated

ethyl

alcohol.

The

integrated

intensities _were calculated

according

to

equation (9)

and

plotted

in

figure

6

versus the concentration of

ethyl

alcohol,

Comparison

with

figure

4 shows that the minimum

of the _curve does not reach the _zero

intensity

line. This _means that the

intensity

_curve

obtained from the

specimen

in 17 vol-fb

ethyl

alcohol in

figure

5 represents ^{the SANS-curve} caused

merely by density

fluctuations within the bulk. Besides

S~~-bulk scattering

for the

other immersion

liquids

also surface

scattering

occurs. The Porod

gradients

and

~(

-values _are

compiled

in table III and amount to 4 _as in table II-

d _inn.

0.35

_

(

0.30

) _G

0.25

#

0.20

fi

~

~

0.15

,£

u

a d

jC4

°.~°

~o ~o o,05

0.00

0 lo 20 30 40 50

concentration of C2HSO H [volilij

Fig.

6.

Amorphous

natFe446°N136~~B20,

integrated intensity

versus concentration of undeuterated

ethyl

alcohol with

parabola.

Table III,

Amorphous

~~~Fe44~°Ni~~~ ~B~o- Porod

gradient

and

integrated differential scattering

cross section.

Concentration d«

of undeuterated dD _inn.

ethyl

alcohol ~

~~ ~~

l~°'.~l

^~~

sr atom

0 4-2 0-219

9 3-9 0.170

17 4-1 0-ill

29 4-2 0-142

42 3.8 0-271

Thus from the

investigation

of

amorphous natFe446°Ni~~llB~o

we obtain _{as a} result that the bulk contains

density

fluctuations _as well _as the surface.

3-5-3

Amorphous

~~~Fe44~~~Ni~~~~B~~

Figure

7 shows the differential _cross section _as

obtained with

amorphous natFe44natNi~~llB~o

^{in three different immersion}

liquids.

Since the

large scattering length density

of this

alloy

cannot be matched

by

_any convenient

immersion

medium,

the

parabola

method _was not

applied

in this _case. From the coincidence of the three

intensity

curves at small

Q-values

follows that _no surface

scattering

_occurs with this

specimen.

Table IV contains the Porod

gradients

and the

integrated

differential

scattering

cross sections, The Porod

gradients

amount

again

to 4-

2. 5

=

I (2.0

E

~_j

l 5 CY

bjC

~ ~

ii

bo ' g

O ',,

',,

""" 0

-2. 5 -2. 0 -1. 5

log (q [A-ij)

Fig.

7.

Amorphous

natFe44natNi36~'B2ol ^~" IQ) nuclear neutron

scattering;

Pararneter.

dl2 concentration in volume 9b of undeuterated

ethyl

alcohol.

From

chapters

3-5-1 and 3.5.2 _we

know,

that

amorphous Fe44Ni~~B~o

in the bulk shows _no concentration fluctuations but

only density

fluctuations. Furthermore _we know that its surface also contains

density

fluctuations in that _sense that it is

irregular.

The

experiments

with

amorphous

natFe44natNi36~~B2o ^thus

only

_were

performed

as a check for

consistency

of the results and this check will be discussed in connection with

figure

8-

Table IV.

Amorphous

~~~Fe44~~~Ni~~'~B~~ ^Porod

gradients

_a~ and

~(

-values.

d _int.

Concentration ^d"

of undeuterated ^dD inn.

ethyl

alcohol °~

l~°~'~l

^{~~ ~~} ~~

sr atom

0 4,0 0-271

9 4-3 0-261

17 4-2 0.285

3.6 DlscussioN-

3-6-1 ^~" _versus

(A~)~.

_The

integrated

total

scattering

_cross section ~"

is

dfl int. dfl _int.

d b lk

composed

from the contribution of the bulk

)

^~ _{and a} contribution of the

specimen

~

surface ~" ~~ ~~~

The Flrst of _course is

independent

of any immersion

liquid surrounding

the dD

specimen

and the second is

according

to

equation (5) proportional

to

(A~

_)~- Thus _we obtain

d~

omax

^{d~ bulk}

^~d~

^sudace

= + COnSt. (A7~

dQ Ill)

dfl _int

~~,~ dfl dfl

and _we

expect

^{for each of the}

specimens

_a linear behaviour of ~"

versus

IA

~ )~ ^where the

dD _inn,

ordinate section is

proportional

to the bulk

scattering

and the

gradient proportional

to the surface

scattering.

0.30

fi

_,m

~

°

fl I

^~~~

~ ~°

~ 0.20

~ ^"

~

0.15 fi

0. lo

~

4 0. 05 b C$

~d ~d 0. 00

0 30 60 90 120 150

(Aq)~ _[10~~cm~~]

Fig.

8.

Amorphous Fe44Ni~6B~ol integrated intensity

_versus

(A1i)~.

ID) 54Fe4462Ni~~"B~o.

j/~) nat~~~60~i~~ll~~~_ jQ) nat~~~nat~i~~ll~~~_

For the three

specimens investigated

in the

present

^{work the} run of ~"

versus

dD jut.

(A~

)~ ^is

plotted

in

figure

8 and _we state the

following points.

I) Amorphous 54Fe4462Ni~~l 'B20 16 )

=

0 _;

chapter

3 5.

As

already

derived in

chapter 3-5-1,

this

specimen

shows _no bulk

scattering,

but _a certain amount of surface

scattering

caused

by density

fluctuations.

ii) Amorphous natFe446°Ni~~"B~o

_; ^Ab

= 0 _;

chapter

3,5-2-

As

already

derived in

chapter 3-5-2,

this

specimen

shows _a certain amount of bulk

scattering

caused

by density

fluctuations. Furthermore it shows also surface

scattering

caused

by density

fluctuations.

iii) Amorphous natFe44natNi~~"B~o

;

chapter

3-5-3-

This

specimen

shows rather

large

bulk

scattering

and

only

_a small contribution of surface

scattering.

The

scattering

from

density

fluctuations in the bulk is

according

to

equation (I) proportional

to

(b)~-

The

corresponding

values _are

compiled

in table V

together

with the

ordinate sections.

Table V- Ordinate sections

jfom figure

8 and

(b)~.

Specimen

da

bulk j~j2

dfl _int.

I 0.02 0

11 0-11 0.41

iii 0.26 0.85

d~ ^bulk

The

scaling

between and

(b)

~ indeed

corresponds

to the

expectation.

dfl _jnt.

From

figure

⁸ we

recognize

furthermore that the surface

scattering,

which is not the

topic

of the present

study,

but _a side

effect,

differs from

specimen

to

specimen.

This shows that

irregularities

^of the surface _may

depend strongly

_on the actual conditions of the

sample preparation

in the melt

spin

_process, such _as the _state of the surface of the copper wheel.

3-6-2

Origin of

the

density fluctuations. Density

variations within _an

amorphous

solid _can be caused

by

two effects.

First,

the _so called free volume _can be frozen into the solid _non

uniformly

and second the

density

_can be varied

by

residual mechanical stress.

3.6-3 Dimensions

of

the

fluctuations.

3-6.3-1Small scale fluctuations. In

amorphous N181B19120], Ni8oP2ol2l],

and

Fe~oB2o 122]

we observed

by

small

angle scattering

besides fluctuations with

large

dimensions

(~

l

000i)

_also fluctuations with very small dimensions

(~10i)- Designing

R _as the

dimension of _a

fluctuation,

the

relationship

QR

_~ l

(12)

is valid for the calculation of the

Q-region

in which _{one can} observe the Guinier

regime

of the

corresponding scattering

effect.

According

to

equation (12)

small

angle scattering

caused

by

small scale fluctuations should _occur at about

Q

MO- I

i~

_where

no effect is observed with the

present specimens.

Thus _we conclude that the temary Fe,Ni-B

-alloy

which is _an easy

glass

former contains _no small scale fluctuations. The

binary amorphous alloys Ni~iBi~, Ni~oP~o,

and

Fe80B20

form _a

glass

less

easily

and contain small scale fluctuations which

probably

act as

nuclei for

crystallisation already during

the _{one or} other melt

spin

_run-

3,6.3,2

Large

scale fluctuations. The small

angle scattering

effects observed in the

present

work

belong

to the Porod

tail,

I-e- the

larger-Q

end of the small

angle scattering

_curve. The

Guinier,region

for which

equation (12)

is

valid,

lies below the minimum

Q

of the

present experiments,

I,e- at

Q ~10~~ i~~- According

to

equation (12)

this _means that the

density

fluctuations observed here _are of the dimension _m 000

I-

_Thus

they correspond

in their size to the

large

scale fluctuations _as observed in

amorphous Ni~iBi~ [20], Ni~oP2o121],

and

Fe~oB~o [20]

3-6-4

Specific su~fiace.

The thickness of the

specimen

amounts to t

= 14 _~Lm- This

yields

_a

geometrical specific

surface

)

=

~

= l.4 _x

lli

_{cm~ if}

we

neglect

the

edge

surface of the

G t

ribbon which is also

experimentally

masked.

Comparison

with the

experimental

values for

)

_{in table II shows that this}

geometrical

value is

larger

than the

experimental

value

giving

rise to small

angle scattering.

4. Conclusion.

The medium range ^structure of the

amorphous Fe44N136B20-alloy

was

investigated using

neutron small

angle scattering.

We treated the

Fe-Ni-B,alloy

_as

binary

transition metal,

boron,alloy-

The

isotopic

substitution

technique

allowed to decide between the contribution to the total

scattering

of concentration fluctuations _on the _one side and of

density

fluctuations

on the other side. The immersion

technique

was used _to decide between the contributions of the bulk and the

surface, respectively,

_to the total

scattering.

The result _was that the bulk

scattering

is caused

by density

fluctuations

only

and _not

by

concentration fluctuations. The same stands for the surface

scattering.

The

scattering

behaviour

always

_can be described

by

Porod's law which _means that the borders between the

regions

with different densities _are smooth.

The

density

fluctuations _{are so} called

large

scale fluctuations with dimensions of at least 000

/k-

_Such fluctuations also _were observed in

amorphous Ni-B-, Fe-B-,

and

Ni-P-alloys- Amorphous Fe-Ni-B-alloys

contain _no small scale fluctuations which

probably

is connected with the fact that

they

can be

produced

_very

easily

in the form of wide ribbons

by

melt

spinning.

Within the bulk the

density

fluctuations _are caused either

by

the free volume _or

by

mechanical _stress. The surface

scattering

^is caused

by

the variations of

density

which _are

coupled

with medium range surface

roughness.

The method of

preparation

was

melt-spinning

and the results show that different

specimens

have the _same bulk _structure but rather different surface structure.

Acknowledgements.

Thanks _are due _to the

ILL, Grenoble,

for allocation of beam time and to Dr. P-

Chieux, ILL,

for his substantial

help during

the

performance

^{of the}

experiments.

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