MEASUREMENTS OF COMPOSITIONAL ORDER IN BINARY METALLIC GLASSES

(1)

HAL Id: jpa-00220356

https://hal.archives-ouvertes.fr/jpa-00220356

Submitted on 1 Jan 1980

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

MEASUREMENTS OF COMPOSITIONAL ORDER

IN BINARY METALLIC GLASSES

M. Sakata, N. Cowlam, H. Davies

To cite this version:

(2)

JOURNAL DE PHYSIQUE

CoZZoque

C 8 ,

suppLlment

au n08, Tome

4 1 ,

aoat

1980,

page

C8-190

MEASUREElENTS OF C O M P O S I T I O N A L ORDER I N B I N A R Y M E T A L L I C GLASSES

M. Sakata, N . Cowlam and H.A. ~ a v i e s '

Department o f Physics

%epartment o f MetalZurgy, U n i v e r s i t y

of

S h e f f i e Z d , S h e f f i e

M,

53 7RH,

U.

K.

I n o r d e r t o have a f u l l u n d e r s t a n d i n g of S(Q)

=

x SNN

+

( I - x ) s ~ ~ / ( c ~ c ~ )

...

2 t h e s t r u c t u r e i n b i n a r y m e t a l l l c g l a s s e s , both

t h e c o n f i g u r a t i o n a l and t h e compositional

o r d e r l n g (1. e. chemlcal s h o r t range o r d e r (CSRO))

have t o be known. For a substitution a l l o y , i n < b > = c b cBbB, Ab = b A

-

bg

...

4

which atomlc s i z e d i f f e r e n c e e f f e c t s can be i g n o r e d , the c o n f i g u r a t i o n a l o r d e r i n g means t h e d e n s i t y fluctuations and t h e c o m p o s i t i o n a l

o r d e r i n g i s r e p r e s e n t e d by t h e c o n c e n t r a t i o n where bA, b r e p r e s e n t e i t h e r t h e atomic form B

f l u c t u a t ~ o n s . I n such a n a l l o y , t h e t o t a l f a c t o r s f o r x-rays o r t h e s c a t t e r i n g l e n g t h s f o r s t r u c t u r e f a c t o r s ( Q ) , i s i n g e n e r a l , a mixture neutron d i f f r a c t i o n . c A , cg a r e t h e

of two components

Sm

and SCC a r i s l n g from c o n c e n t r a t i o n s of A and B atoms.

d e n s i t y and concentration f l u c t u a t i o n s I n t h e p r e s e n t b r i e f paper, we w i l l

r e s p e c t i v e l y ( ' ) . I n p r i n c i p l e , t h e s e components c o n c e n t r a t e on a s t u d y of amorphous a l l o y s i n t h e can be obtalned by a combination of x-ray and CuTi system, and p r e s e n t d e t a l l s of measurements neutron diffraction data. I t i s , however,

extremely d i f f i c u l t t o do t h i s u n l e s s t h e r e i s a l a r g e enough d i f f e r e n c e between t h e atomic form f a c t o r s ( o r t h e n e u t r o n s c a t t e r i n g

amplitudes)

of t h e c o n s t i t u e n t atoms. Indeed ~ f t h e s e f a c t o r s have s l r n l l a r magnitudes, t h e n t h e t o t a l s t r u c t u r e f a c t o r r e l a t e s almost wholly t o d e n s i t y f l u c t u a t i o n s , and ~t 1s t h e r e f o r e p r a c t i c a l l y ~ n p o s s l b l e t o g e t l n f o m a t i o n about t h e

c o n c e n t r a t i o n f l u c t u a t i o n s from x-ray and n e u t r o n d l f f r a c t l o n i n such a case. Thls s i t u a t i o n can be e a s i l y understood by t h e f o l l o w i n g e q u a t i o n s , ( 2 ) which dehine S(Q) i n terms of t h e observed s c a t t e r i n g , and t h e s t r u c t u r e f a c t o r s SNN and SCC

of t h e chemical s h o r t range o r d e r parameter f o r a l l o y s of t h r e e d i f f e r e n t compositions, Cu Ti 50 50' Cu58Ti42 and Cu

₆₆

T i 54' I n o r d e r t o show t h e e x t e n t of c o n t r i b u t i o n from and ScC t o t h e e x p e r i m e n t a l l y observed s ( Q ) , f o r t h e s e a l l o y s , t h e v a l u e s of x and (I-x) a r e g i v e n i n Table 1 b o t h f o r x-rays ( a t Q = 0) and f o r n e u t r o n s .

Table 1 Values of t h e p a r a m e t e r s x , I-x i n e q u a t i o n 3 f o r CuTi a l l o y s . Cu Ti 50 50 Cu Ti

58

42 Cu Ti

66

34

-

X-ray x 0.982 0.983 0.985 Neutron (1-x) 0.018 0.017 .0.015 x 0.128 0.233 0.355 (1-x) 0.872 0.767 0.645

(3)

a

In

A-'

F . z u r e 1 S(Q) f o r Cu58T142 u s l n g x-rays.

Because of t h e n e g a t i v e scattering l e n g t h of TI f o r n e u t r o n s t h e c o n t r l b u t l o n from SCC i s domlnant I n t h e neutron d i f f r a c t i o n case.

A l l t h e specimens were produced by t h e

conventional c h l l l - b l o c k melt-spinnlng method and t h e rlbbon dimensions were approximately 1 mm wide and 20 microns t h l c k . These have been examlned by x-ray diffraction u s i n g c r y s t a l monochromatised MoKa r a d l a t l o n , and by neutron

a

In

A

F i g u r e 2 S(Q) f o r Cu Ti u s i n g neutrons.

58

5 2

d i f f r a c t i o n u s i n g t h e CURRAN and 10H d i f f r a c t o - meters a t A.E.R.E., Harwell, u s i n g t h e sample geometry described by us p r e v i o u s l y (3).

The observed S(Q) f o r t h e Cu58Ti42 a l l o y f o r example a r e shown i n F l g u r e s 1 and 2, f o r x-ray and n e u t r o n d i f f r a c t i o n . With x-rays t h e s t r u c t u r e f a c t o r s of t h e s e a l l o y s have t h e c h a r a c t e r i s t i c form, with an i n t e n s e f i r s t peak and w e l l d e f i n e d second peak shoulder. With n e u t r o n s S(Q) h a s t h e r a t h e r unusual form with an

1

i n t e n s e pre-peak a t Q = 1.9

8-

,

a s r e p o r t e d by us p r e v i o u s l y ( 3 ) . The pre-peak i s a l s o p r e s e n t i n t h e x-ray s(Q), a s seen i n f i g u r e 1 , but a s expected from Table 1 , it i s very weak.

%

and SCc a r e o b t a i n e d f o r each c o m p o s i t ~ o n by a s o l u t i o n of t h e two equations

( 2 ) , f o r t h e x-ray and neutron c a s e s , and t h e s e s t r u c t i i r e f a c t o r s a r e shown i n f l g u r e 3, f o r t h e

Q

In

A"

Figure 3 S m ( ~ ) (heavy I l n e ) and S C C ( ~ ) (dashed l i n e ) f o r Cu 58T142, o b t a i c e d from x-ray and n e u t r o n d a t a .

(4)

C8-192

JOURNAL DE PHYSIQUE

F l g u r e

4

The r a d l a l c i l s t r i b u t i o n f u n c t l o n

2

4 x r

p N N

f o r Cu TI

58

42'

For a ( c h e m ~ c a l l ~ ) disordered a l l o y , SCC must be e q u a l t o t h e product cAcg a t every value of Q. The CSRO parameter

a,

of t h e f l r s t cc-ordlnatlon s h e l l can be d e f l n e d a s

where <nBA> 1s mean number of n e a r e s t n e ~ g h b o u r A-atoms around a %at om, and < n > 1s t h e

c c - o r d l n a t ~ o n number. Then 3: may be d e r i v e d from t h e F o u r i e r t r a n s f o r m of SCC, namely t h e so-called r a d i a l c o n c e n t r a t i o n c o r r e l a t i o n f u n c t l o n (RCF)

2

4 ~

p

rcC, a s shown i n e q u a t l o n

6

below

r,

i s t h e most probable f i r s t neighbour d i s t a n c e and t h e I n t e g r a l c o v e r s t h e f i r s t co-ordination

s h e l l . The co-Ordination number < n > can be o b t a i n e d from t h e RDF ( ~ a d i a l D l s t r i b u t i o n F'unction) whlch i s t h e F o u r i e r t r a n s f o r m of S

_NN'

The RDF and t h e RCF f o r Cu58T142 a r e shown i n f i g u r e s 4 and

5

and t h e v a l u e s of

a

and <n> a r e l i s t e d I n Table 2 t o g e t h e r with v a l u e s of o t h e r R o d ~ a l d~stance r In

A

F l g u r e

5

The r a d i a l c o n c e n t r a t i o n c o r r e l a t i o n 2 f u n c t i o n 4 x r p C C f o r Cu Ti

58

42' r e l e v a n t parameters.

Table 2 The o r d e r parameter C, c o - o r d i n a t i o n number < n > , and f i r s t and second neighbour d i s t a n c e r l and r2 i n Cu-Ti amorphous a l l o y s .

From Table 2, it can be s e e n t h a t t h e average c o - o r d i n a t ~ o n number i s v e r y c l o s e t o 12 f o r Cu Ti and s l i g h t l y d e c r e a s e s with t h e

66

34 i n c r e a s e of Ti c o n c e n t r a t i o n . rl does n o t depend v e r y s t r o n g l y on t h e c o n c e n t r a t i o n , which s u g g e s t s t h a t t h e assumption t h a t CuTi a l l o y s r e p r e s e n t a s u b s t i t u t i o n a l system i s a r e a s o n a b l e approximation. r, i s somewhat s m a l l e r t h a n t h e weighted i n t e r a t o m i c d i s t a n c e of n e a r e s t nelghbours i n t h e pure c r y s t a l l i n e m e t a l s , which

(5)

not unexpected, s i n c e t h e deep e n t e c t i c s t h a t c h a r a c t e r i s e many m e t a l l l c g l a s s forming systems, a r l s e from s t r o n g chemical interactions between u n l i k e a t oms.

The v a l u e of t h e o r d e r parameter observed f o r a l l t h r e e a l l o y s 1 s negative,

indicating a p r e f e r e n c e f o r u n l l k e nelghbours. The v a l u e s of

a

appear t o vary r e g u l a r l y with compositlon, although t h e CSRO observed 1s i n f a c t f a r from i t s theoretical maxlmum v a l u e In each case a s glven by

cx

=

-

cg/cA,and typically one atom i n twelve of t h e f l r s t c o - o r d ~ n a t l o n s h e l l 1 s "misplaced". T h l s 1s I n c o n t r a s t wlth t h e c a s e of a molten

Mg

B1 a l l o y ( ' ) whlch showed

70 30

maximum CSRO. Another distinction 1s I n t h e p o s l t l o n of t h e pre-peak. T n l s pre-peak a r l s e s from t h e p s e u d o p e r i o d i c i t y due t o t h e CSRO, and

j t can be thought of a s analogous t o a super-

l a t t l c e peak i n an ordered c r y s t a l l i n e a l l o y . Therefore t h e r a t l o of t h e pre-peak and f i r s t peak p o s i t l o n i n Q, y i e l d s u s e f u l lnformatlon about t h e c o n f i g u r a t i o n . I n t h e p r e s e n t c a s e , t h i s r a t l o

1s about 0.61 which 1s I n extremely good agreement wlth t h e geometrical packlng i n t h e s l m u l a t l o n by B l e t r y ( 4 ) , while t h e r a t l o observed f o r molten Mg70Bi30 1s

-

0.7.

Slnce Ti and Zr belong t o t h e same group I n t h e p e r i o d i c t a b l e i t 1s appropriate t o compare CuTi and CuZr a l l o y s . Amorphous CuZr a l l o y s have been widely s t u d i e d ( 5 ) ( 6 ) . On t h e b a s i s of p a r t i a l s t r u c t u r e f a c t o r s o b t a i n e d by n e u t r o n

diffraction u s l n g Gu i s o t o p e s C U ~ ~ , C U ~ ~ , i t h a s

been concluded t h a t Cu57Zr43 i s completely d i s o r d e r e d . However l t must be n o t e d t h a t t h e c o n t r i b u t i o n of ScC t o S(Q) i s very small (only 4% i n t h e most f a v o u r a b l e c a s e cu6?zr

),

because t h e

57

4 3

s c a t t e r i n g l e n g t h s of Cu, C U ~ ~ , ~u~~ and Zr a r e q u i t e c l o s e t o each o t h e r . It i s t h u s u n l l k e l y

t h a t i n f o r m a t i o n on CSRO can be obtained from t h e s e a l l o y s . We a r e c u r r e n t l y extending our measurements t o a l l o y s of both h l g h e r and lower Cu content., l n t h l s CuTi system, which t h u s o f f e r s a unique o p p o r t ~ u l i t y t o s t u d y chemlcal o r d e ~ i n g over a wlde compositlon range. Measurements a r e a l s o i n p r o g r e s s on annealed

samples, t o study p o s s i b l e e f f e c t s of thermal r e l a x a t i o n on chemical order. S t u d i e s of stability, c r y s t a l l l s a t l o n and mechanical

( 7 ) p r o p e r t i e s w i l l be r e p o r t e d elsewhere

.

Acknowledgements.- F i n a n c i a l support from t h e s c i e n c e Research Council, and t h e a s s i s t a n c e of t h e U n i v e r s i t y Support Group, AERE,Rarwellare g r a t e - f u l l y acknowledged. The X-ray d a t a were a n a l y s e d u s i n g a modified v e r s i o n of a computer program supplied by C.N.J. Wagner

.

References

(1

)

Boos A. m d Steeb S.,

Phys. L e t t e r s , &A,

333

(1977). ( 2 ) B h a t i a A. B. and Thornton D. E.,

Phys. Rev.

2,

3004-12 (1970). ( 3 ) Sakata

M.,

Cowlam N. and Davies H. A.,

J. Phys.

F2,

L735 (1979). ( 4 ) B l e t r y J.,

2. Naturforsch.

a,

327 (1978). ( 5 ) Mlzoguchi T., Kudo T., I r i s a w a T.,

Watanabe N.

,

Niimura A.

,

Misawa M. and Suzuki K . ,

Proc. 3rd I n t . Conf. Rapidly Quenched M e t a l s Brighton 2, 384 (1978).

( 6 )

Waseda Y. and Masumoto T.,

Z. Physik

E ,

235, (1975).

( 7 )

Davies H. A.,