Characterization of amorphous alloy films

Mat. Res. Bull. Vol. 3, pp. 159-168, 1968.

P e r g a m o n Press, Inc. Printed

in the United States.

CHARACTERIZATION OF AMORPHOUS ALLOY FILMS B. G. Bagley , H. S. Chen and D. Turnbull Division of Engineering and Applied Physics, Harvard University, Cambridge, Massachusetts Now at Bell Telephone Laboratories, Murray Hill, New Jersey

(Received

December

Z~., 1967; R e f e r e e d )

ABSTRACT It is difficult to distinguish between the microcrystallite and c o n t i n u o u s random m o d e l s f o r amorphous a l l o y f i l m s . The application of X-ray diffraction, electron microscopy, calorimetry and v i s c o m e t r y t o t h i s p r o b l e m a r e d i s c u s s e d . In particular, the use of these techniques for the characterization o f amorphous n i c k e l - p h o s p h o r u s and g o l d - s i l i c o n - g e r m a n i u m a l l o y s a r e d e s c r i b e d .

Introduction In this

p a p e r we s h a l l

characterization ditions

discuss

o f amorphous a l l o y

c a n amorphous a l l o y

of this

films.

i n amorphous f i l m s ?

problems in the

The f i r s t

f i l m s be c l a s s i f i e d

The s e c o n d i s w h a t c o n f i g u r a t i o n a l arrangement

two i n t e r r e l a t e d

is under what con-

operationally

model b e s t c h a r a c t e r i z e s We s h a l l

p r o b l e m h a v i n g t o do w i t h s p a t i a l

give most attention

as glasses? the atomic to the aspect

a r r a n g e m e n t w i t h some comment on

component d i s t r i b u t i o n . T h e r e a r e p e r h a p s two l i m i t i n g solids.

One i s t h e o r y s t a l l i t e

models for the structure

model(1)and the other

random model s u c h a s t h e model o f Z a c h a r i a s e n B e r n a l model ~ )

as a p p l i e d

In view of the variety

(2)

o f amorphous

is the continuous

for oxide glasses

or the

t o monatomic o r s i m p l e m o l e c u l a r amorphous s o l i d s °

o f c o m p o s i t i o n and p r e p a r a t i o n

159

techniques

o f amorphous

160

AMORPHOUS

solids

there

structures

ts~ o f c o u r s e ,

ALLOY

FILMS

Vol. 3, No. 7

no r e a s o n t o s u p p o s e t h a t

would be c h a r a c t e r i z e d

exclusively

all

amorphous s o l i d

by J u s t one o f t h e s e two t y p e s

o f model. In contrast have rarely,

with undercooled

i f ever~ been p r e p a r e d

d e f i n e d or i n i n t e r n a l history

liquids,

equilibrium.

their

slow i s o t h e r m a l

in a state

which is thermodynamically

T h i s i s m a n i f e s t e d by t h e thermal

dependence of the properties

conditions#

it a p p e a r s t h a t amorphous s o l i d s

o f amorphous s o l i d s

drift with t i m e .

t h e r m o d y n a m i c a l l y d e f i n e d amorphous s t a t e ,

a n d , u n d e r some

The c o n c e p t o f an " i d e a l "

distinct

from t h e c r y s t a l l i n e ¢

was d e v e l o p e d f o r m o l e c u l a r l y

c o m p l e x s y s t e m s by G i b b s and Di M a r z i o ~4j and

f o r s i m p l e and m o n a t o m i c s y s t e m s by Cohen and T u r n b u l l ( 5 ) . the structure continuous

solidification process ture

is usually

of a liquid.

or increasing

pressure~

uished from crystallization To be c l a s s i f i e d

possible,

a liquid

from fluid

it

to solid

viscosity~

values.

be amorphous i n a d i f f r a c t i o n

preferably

the broad halos qualitatively

repeatable,

that

discontinuously. the solid

examination.

the liquid

tempera-

This is disting-

increases

is not sufficient

To c o n f i r m t h e c l a s s i f i c a t i o n

we mean a homogeneous

continuously~ with falling

in which the viscosity

as a glass

formed by c o n t i n u o u s

solidification

to an extremely fine grained crystalline

would e x h i b i t

structure.

states

increases

owing t o c o p i o u s n u c l e a t i o n I t h a t

continuously pattern

By c o n t i n u o u s

by a

or B e r n a l t y p e .

d e f i n e d a s a n amorphous s o l i d

in which the viscosity

by chilling

It appears that

o f an i d e a l amorphous s o l i d w o u l d be c h a r a c t e r i z e d

random model o f t h e Z a c h a r i a s e n

A glass

state,

%

formed

For it

solidified

is

dis-

s o l i d whose d i f f r a c t i o n associated

a continuous

between the metastable

with the glass

transition liquid

in

and g l a s s

s h o u l d be d e m o n s t r a t e d . By t h e u s u a l d e f i n i t i o n

vapor or electrodeposition,

many amorphous s o l i d s j w o u l d n o t be c l a s s i f i e d

e.g.

t h o s e formed by

as glasses.

It

seems

Vol. 3, No. Z

AMORPHOUS

ALLOY

FILMS

161

that the amorphous solid state of some substances, at least, would have essentially the same structure whether formed by vapor deposition or by chilling the liquid.

Actually there may be some merit in distinguishing

glasses from other amorphous solids for, as we shall see, the distinction in physical properties between different amorphous structures with the same composition may be subtle.

However, we think it reasonable to broaden the

definition of a glass to include those amorphous solids, however formed, which can be brought into a state in internal equilibrium having properties, viscosity, That is,

heat capacity, the transition,

etc.,

clearly

amorphous s o l i d

demonstrated.

We n o t e ,

not in itself

prove that

condensation

is different

though,

form.

However,

it

<

of a metastable

> metastable

liquid,

non-appearance of this

transition

the structure

o f an amorphous s o l i d

formed by

from that

liquid.

s h o u l d be

that

The E x i s t e n c e It appears that

characteristic

does

of a g l a s s . of Alloy Glasses

pure metals have rarely is well established

that

been prepared

i n amorphous s o l i d

the admixture of certain

impurities to metals greatly facilitates their formation in an amorphous solid

state.

the relative

Probably the principal stability

role of the impurities

o f t h e amorphous s t a t e

and t o r e t a r d

is to increase nucleation

of the

crystal. The t r a n s i t i o n fested

f r o m amorphous s o l i d

by t h e v i s c o s i t y

Cp, b e h a v i o r . equilibrium 1013 poise.

behavior,

to metastable

as noted earlier,

liquid

and by t h e h e a t c a p a c i t y ,

I n o r d e r f o r an amorphous s u b s t a n c e t o a c h i e v e in a relatively

short period

its

c a n be m a n i -

viscosity

internal

s h o u l d be l e s s

than

In the transition from the glass to the metastable liquid state

Cp usually rises abruptly. Amorphous alloys have not been explored extensively for the liquid : glass transition.

>

In the explorations which have been made the alloys often

16Z

AMORPHOUS

crystalllzed

ALLOY

FILMS

Vol. 3, No. Z

upon h e a t i n g w i t h o u t g i v i n g any c o n c l u s i v e i n d i c a t i o n s

transition.

However, t h e t r a n s i t i o n

of this

might have o c c u r r e d i n some o f t h e s e

s y s t e m s a t some h i g h e r t e m p e r a t u r e i f c r y s t a l

nucleation

and growth had n o t

intervened.

(6)

Duwez and a s s o c i a t e s

demonstrated that

a p p r o x i m a t e c o m p o s i t i o n 18-25 a t .

~ sillcon,

liquid gold-silicon

c a n be r a p i d l y c h i l l e d

form whlch was amorphous i n X - r a y e x a m i n a t i o n s .

warmed to J u s t below room t e m p e r a t u r e some r e l a x a t i o n

an i n t e r m e t a l l i c

~

~ liquid

transition

k~en t h e a l l o y was

effects

suggestive of

i n t h e a l l o y was

p r o c e s s was r e t a r d e d

was e x h i b i t e d

to

was f u l l y m a n i f e s t .

However~ Chen f o u n d t h a t when r o u g h l y h a l f o f t h e s i l i c o n

that the glass

to a solld

were observed but crystallization

p h a s e o c c u r r e d b e f o r e any t r a n s i t i o n

r e p l a c e d by germanium t h e c r y s t a l l i z a t i o n

of

Chen ( 7 ' 8 ) m e a s u r e d t h e h e a t

c a p a c i t y o f t h e s e amorphous a l l o y s a t low t e m p e r a t u r e s ,

the onset of a glass ~ liquid transition

alloys,

sufficiently

both theologically

so

and

thermally. In particular Ge0. 136Si0.09&. little

e x t e n s i v e m e a s u r e m e n t s w e r e made on t h e a l l o y Au0.77 At low t e m p e r a t u r e s Cp o f t h e amorphous a l l o y i s o n l y a

higher than that of the equilibrium mixture.

to rise

sharply with increasing

v a l u e s a b o u t 5. S c a l / g m , fall

t e m p e r a t u r e and t h e n l e v e l s n e a r 300°K t o

atom above Cp o f t h e e q u i l i b r i u m m i x t u r e and which

r e a s o n a b l y w e l l on t h e e x t r a p o l a t i o n

relation exhibit

for the liquid alloy. this

transition

o f t h e h i g h t e m p e r a t u r e Cp- T

I t was d e m o n s t r a t e d t h a t a specimen c o u l d

two o r more t i m e s when a f t e r

transition

i t was c h i l l e d

transition

t e m p e r a t u r e was s h i f t e d

factor

Near 295°K Cp b e g i n s

and a g a i n r e h e a t e d .

being taken through the

The t h e r m a l l y i n d i c a t e d g l a s s

o f t h e o r d e r o f 1-2 ° by changes o f a

o f 16 i n t h e r a t e o f b e a t i n g .

The a l l o y was h a r d and b r i t t l e and a f t e r

crystallization.

i n t h e amorphous s t a t e

However, i t

a t low t e m p e r a t u r e

i s s o f t and f l o w s e a s i l y

in the

Vol. 3, No.

Z

AMORPHOUS

ALLOY

FILMS

163

temperature range in which the glass transition is thermally manifested. Chen (7~8) measured the creep rate of the alloy over this temperature range, 12 to 32°C.

A steady creep rate, which increased linearly with the stress

over the range 3 to 140 Kg/mm 2, was reached almost instantaneously at the high temperatures and more slowly at the lower temperatures.

The viscosity

calculated from these results ranged from 1013 at the lowest temperature to 109 at the highest and its temperature variation was described by an equation of the Fulcher form: ffi 0.52 exp[1360/(T - 241.3)}

poise

The time constant 3 T~ for configurational relaxation was found to be T ffi I0-i0~ seconds.

Thus at viscosities lower than 1013 the liquid clearly

had achieved an internal equilibrium state in our experiments. Presumably the viscosity of the alloy will continue to fall rapidly with rising temperature to values in the range 10 -2 poise usually typical for molten metals.

This form of the viscosity variation with temperature,

especially the Fulcher behavlor I is qualitatively similar to that of other glass forming liquids. We think that these results clearly demonstrate the existence of a metallic glass.

They suggest also that many other amorphous solid alloys may

be in conflgurational states best characterized as glassy even though the glass ~

# liquid transition is unobserved because of the intervention of

rapid crystallization processes.

The binary amorphous alloy of gold-sillcon,

for example, probably belongs in this category. It is interesting that the metallic radii of silicon and germanium are not very different

(2 to 5%) from that of gold.

Also the metallic radii of

nickel and phosphorus~ which can be easily combined at certain compositions into an amorphous solid alloy 3 differ only by 3%.

These results indicate

that glassy and amorphous alloys can form when the atomic size disparities

164

AMORPHOUS

are rather tions

small.

ALLOY

Of c o u r s e ,

Vol. 3, No. Z

FILMS

the valences

o f t h e members o f t h e s e c o m b i n a -

are very different. Characterization

Of 9 o n f i ~ u r a t i q n s

i n Amorphous A l l o y s

Both c o n t i n u o u s random and m i c r o c r y s t a l l i n e exhibit

qualitatively

sJJuilar diffraction

by a few b r o a d h a l o s .

is very small it

resolution

to distinguish

It

is expected that

X-ray scattering density

is difficult

microcrystallites

have calculated

and t h e c r y s t a l l i t e

boundaries.

are packed like

in

Warren (9)

small angle scattering

from a silica

"pebbles in a pail".

full

of pebbles.

However,

B i e n e n s t o c k and B a g l e y (10)

t h e u p p e r bound o f t h e s m a l l a n g l e s c a t t e r i n g

gromcrystallite

assemblies with varying proportions

They showed~ a s an e x a m p l e , t h a t from a crystallite

model f o r f u s e d s i l i c a 1

and c r i s t o b a l i t e ~

t o be e x p e c t e d

of boundary void space.

t h e u p p e r bound o f s m a l l a n g l e s c a t t e r i n g

and a v o i d volume e q u a l t o t h e d i f f e r e n c e

with a crystallite

size of 8

i n volume (ca. 3~) b e t w e e n f u s e d

would be t o o s m a l l f o r d e t e c t i o n

with the present

resolution.

However, t o a s s e s s o f t h e minimum d e n s i t y continuities

of structure.

c a n be p a c k e d i n a s o l i d body w i t h a much s m a l l e r p r o p o r t i o n

of void space than in a pail

experimental

experimental

body due t o any n o n - u n i f o r m i t y

is indeed a large

gel in which small particles

characterized

s h o u l d be some e n h a n c e m e n t i n t h e s m a l l a n g l e

between the crystallites there

patterns

to achieve sufficient

b e t w e e n t h e two s o r t s there

i.e.

are expected to

size of the microcrystalline

from a mtcrocrystalline

demonstrated that

silica

patterns,

If the crystallite

structure

structures

the full deficit

significance

required

in the microcrystallite

types of glass

forming systems.

of these results

the problem

t o accommodate t h e s t r u c t u r a l

dis-

model n e e d s t o be s o l v e d f o r t h e v a r i o u s In a system of hard unattracting

spheres at

O°K the density d~f!c!t due t~ 5ound~r!e~ ~p~r~t~g ~!o~e-pe~e~ e r y . t = 1 ~ . m £ s o r i e n K e a t h r o u g h ~ a r g e a n g l e s mxgn~ De ~ p = ~ c u

~u u= u~ ~,= u~u=~ ux v~=

Vol. 3, No. Z

half

AMORPHOUS

o f a monoatomic l a y e r ( 1 1 ) .

m o d e l s o f such b o u n d a r i e s order of density

Inspection

suggests

that

of a m i c r o c r y s t a l l i t e

crystal.

evaluate

the density

measuring the density structure

associated

of solids

with a specified

Although its crystallite

deficit

utility

and c o n t i n u o u s

deficit

of the corresponding

is doubtful structures

deficit

with crystallite

that

the

s i z e 4 atoms a c r o s s 3 bulk single

becomes 15 t o

i t would be i n t e r e s t i n g

definitely

crystallite

is indeed of the

T h i s means, f o r e x a m p l e ,

3 a t o m s wide t h e d e n s i t y

In view of these considerations,

165

of soap-bubble or hard sphere

system 3 with a crystallite than the density

For c r y s t a l l i t e s

FILMS

the density

1/3 t o 1/2 o f a monoatomic l a y e r .

o u g h t t o be 12 t o 17% l e s s

23~.

ALLOY

to try

to

b o u n d a r i e s by

known t o h a v e a m i c r o c r y s t a l l i n e

size. for distinguishing in a s i n g l e

between micro-

phase system,

the small

angle scattering technique can be an extremely sensitive tool for determining whether or not a solid has a microcrystalline polyphase structure.

For

examplej Bagley (12) found a strong enhancement of the small angle X-ray scattering upon crystallizing an originally amorphous nlckel-phosphorus alloy.

These results indicate that~ as formed, the nlckel-phosphorus alloys,

whatever their state of structural continuity 3 were essentially homogeneous in composition. A mlcrocrystalline body might be expected to "crystallize" by a "normal" grain growth process in which many grains grow at the expense of a few. this process the average crystalllte size increases continuously during an isothermal anneal and concurrently the diffuse diffraction halos sharpen continuously.

However, the appearance, microscopically, of an extremely

flne-gralned crystallite structure is not, in itself, unequivocal evidence for a mlcrocrystallite model of the amorphous state for it might have r=cu!tcd fr=~ ce;i~u~ h .....o ....~uc nuc!~atien ~f ~ry~ta!~ in ~ cent!nuous ~K~UCEUr~.

In

166

AMORPHOUS

In contrast

Vol. 3, No. Z

o f an amorphous body i n w h i c h w e l l s e p a r a t e d

and grow t o r e l a t i v e l y

on t h e d i f f u s e

diffraction

crystallization

structure.

pattern

from t h e c r y s t a l s

halo of the matrlxwhich

progresses.

of crystallization

mode o f

crystals

appear

l a r g e s i z e s w i t h no a p p a r e n t c h a n g e s in t h e i n t e r v e n i n g

The s h a r p d i f f r a c t i o n

This nucleation

appear superimposed

remains unchanged as

and growth i s t h e e x p e c t e d mode

o f a n amorphous s o l i d w i t h a c o n t i n u o u s random t y p e o f

It

is not expected in a microcrystalline

analogous to secondary recrystallizatton preferential

FILMS

w i t h n o r m a l g r a i n growth i s t h e d i s c o n t i n u o u s

crystallization

matrix.

ALLOY

segregation

solid

unless a process

i s somehow e f f e c t e d

o f one o f t h e a l l o y

owing t o i n i t i a l

components at the crystallite

boundaries. B a g l e y (12) h a s o b s e r v e d w i t h t r a n s m i s s i o n interesting content,

differences

in the crystallization

of vapor deposited

heat treating diameter)

compositions

crystalline

with a well-defined to change.

crystals

was s u p p o s e d t h a t the crystalline very difficult

crystals

crystallizing,

At

appears not

( 0 . 2 atom ~) o f Ni t o t h e

during heat treatment,

The a p p e a r a n c e o f t h i s

t o a few w i d e l y

morphology.

m o r p h o l o g y was imposed by t h e r e j e c t i o n

to reconcile

(50

a p p e a r and grow

f o r m i n t o a m a t r t x whose s t r u c t u r e

of a very small excess

Ni3P p h a s e .

Upon

i s f o r m e d which t h e n c o a r s e n s u n i f o r m l y .

w h i c h grow w t t h a w e l l d e f i n e d d e n d r i t i c this

alloy.

a p p r o x i m a t i n g Ni4P , a v e r y f i n e - g r a i n e d

polyhedral

amorphous Ni3P l e d t o i t s separated

mode, d e p e n d i n g on p h o s p h o r u s

h o w e v e r , a few w i d e l y s e p a r a t e d

The a d d i t i o n

m i c r o s c o p y some

f i l m s o f amorphous n i c k e l - p h o s p h o r u s

structure

the N~P composition,

electron

dendritic

of nickel

It by

morphology is

w i t h any b u t a c o n t i n u o u s model f o r t h e amorphous

structure. Other observations alloy

of nucleation

films are those reported

formed f i l m s .

and g r o w t h m o r p h o l o g i e s i n amorphous (13) by Duwez a t t h i s c o n f e r e n c e on some s p l a t -

A l s o Nowick (14) and a s s o c i a t e s

have inferred

from t h e i r

Vol. 3, No. Z

kinetic alloys

analysis

AMORPHOUS

a nucleation

of copper-silver

ALLOY

FILMS

167

and g r o w t h mode o f c r y s t a l l i z a t i o n

deposited

o f amorphous

from the vapor.

Acknow led~ement The r e s e a r c h

at Harvard University

alluded

t o in t h i s

r e p o r t was

supported in part by the Office of Naval Research under Contract Nonr 1866(50) and by the Advanced Research Projects Agency under Contract ARPA SD-88. Also B. G. Bagley was a Xerox Corporation Fellow 1964-1966, and an A.S.T.M. Fellow 1966-1967. References I. (a)N. Valenkov and E.A. Poraj-Koslc, Z. Krlst. ~

196 (1936).

(b)N.F. Mott and R.W. Gurney, Rept. Progr. Phys. 5_j 46 (1938). 2.

W.H. Zachariasen, J. Am. Chem. Soc. ~

3841 (1932).

3.

J.D.

4.

J.H. Gibbs and E.A. Di Marzio~ J. Chem. Phys. ~

5.

M.H. Cohen and D. Turnbullj J. Chem. Phys. ~

Bernal, Nature 185, 68 (1960).

120 (1960);

Nature ~

373 (1958). 1164 (1959);

ibid. 34,

964 (1964).

6.

W. Klement~ Jr., R.H. Willens and P. Duwez, Nature 187, 869 (1960).

7.

H.S. Chen, Ph.D. thesis, Division of Engineering and Applied Physlcs~ Harvard University (1967).

8.

H.S. Chen and D. Turnbull~ in press~ J. Chem. Phys., 1968.

See also

Appl. Phys. Letters I0~ 284 (1967) and J. Appl. Phys. 38, 3646 (1967). 9.

B.E. Warren, J. Appl. Phys. 8~ 645 (1937).

I0.

A. Bienenstock and B.G. Bagley, J. Appl. Phys. ~

II.

D. Turnbull, pp. 6-25, in Liquids:

4840 (1966).

Structures, Properties, Solid Inter-

actions, ed. Thomas J. Hughel, Elsevier Publishing Company~ Amsterdam (1965). 12.

B.G. Bagley, Ph.D. thesis, Division of Engineering and Applied Physics, Harvard University (1967).

168

13.

AMORPHOUS

ALLOY

FILMS

Vol. 3, No. Z

P. Duwez, '~morphous Metallic Alloy Phases", 2nd Conference on Characterization of Materials, Rochester, N.Y., 1967.

14.

A.S. Nowlck, "Amorphous Alloys Produced by Vacuum Evaporation", 2nd Conference on Characterization of Materials, Rochester, N.Y., 1967.

This

paper

was

presented

at the Second

International

Conference

on the

Characterization of l~aterials, Rochester, N e w York, N o v e m b e r 8- 10, 1967.