SEGREGATION AND DIFFUSION OF SULPHUR IN SOME POLYCRYSTALS AND BICRYSTALS : INFLUENCE OF CRYSTALLOGRAPHIC FACTORS

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SEGREGATION AND DIFFUSION OF SULPHUR IN SOME POLYCRYSTALS AND BICRYSTALS :

INFLUENCE OF CRYSTALLOGRAPHIC FACTORS

M. Pierantoni, B. Aufray, F. Cabane

To cite this version:

M. Pierantoni, B. Aufray, F. Cabane. SEGREGATION AND DIFFUSION OF SULPHUR IN SOME POLYCRYSTALS AND BICRYSTALS : INFLUENCE OF CRYSTALLOGRAPHIC FAC- TORS. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-517-C4-522. �10.1051/jphyscol:1985457�.

�jpa-00224708�

JOURNAL DE PHYSIQUE

Colloque C£, supplement au n ° 4 , Tome 46, avril 1985 page C4-517

SEGREGATION AND DIFFUSION OF SULPHUR IN SOME POLYCRYSTALS AND BICRYSTALS : INFLUENCE OF CRYSTALLOGRAPHIC FACTORS

M. Pierantoni, B. Aufray and F. Cabane

Laboratoire de Métallurgie, Equipe de Recherche , Faculté des Sciences et Techniques, Rue Henri Poincarê, 13397 Marseille Cedex 13, France

RESUME - Nous déduisons de données expérimentales, le rôle essentiel de la structure intergranulaire sur la diffusion et la ségrégation dans des bi- cristaux et des conséquences sur le sens des mesures dans les polycristaux.

ABSTRACT - From experimental data, we show the main role of intergranular structure on diffusion and segregation in bicrystals and some consequences on the data obtained in polycrystals.

Most experimental studies relative to grain boundaries (G.B.) are performed with polycrystalline materials ; the data are then interpreted on the assumption that a set of grain boundaries can be represented by a single grain boundary which is a homogeneous slab of matter, about 6 = 5.10 cm thick, between two adjacent crystals

(1). In this way, we have studied G.B. diffusion and segregation of sulphur in some f.c.c. metals(2,3,4). Actually, the various grain boundaries of a polycrystalline sample are likely very different with respect to segregation and diffusion. Conse- quently, it is difficult to give a precise meaning, at the atomic scale, to quantities measured in polycrystals.

In order to study the influence of crystallographic factors, we have performed expe- riments on bicrystals ; we have carried out a new technique to study G.B. segregation and diffusion ; for the first time,all these data could be obtained from only one experiment on a bicrystal (5). Presently, we describe on the one hand, the results we have obtained in Cu, Ag and Ni bicrystals and on the other hand, measurements made in polycrystalline alloys with different grain size and G.B. appearance. The experimental procedures have been described elsewhere in details so that we are going to only recall some points of particular interest for this study.

I - MATERIALS

High purity metals (99,999%) have always been used ; bicrystals were prepared through melting and solidification under appropriate conditions ; the crystallographic cha- racteristics of their grain boundaries are given in Table I ; note that the twin boundary Cu(tw), was the only grain boundary to be strictly straight ; the others were slightly bent or sinuous.

Polycrystalline samples were made from CuNi solid solutions of various nickel concen- tration Cj,(Ni) ; they have been elaborated with three different methods :

- method N° 1 ( C U N D T : melting of CuIJi mixture in a HF furnace, solidification, rolling and annealing at high temperature, under pure hydrogen.

- method N°2 (CuNi)j-j- : the last anneal was made in hydrogen containing very small amounts of hydrogen sulphide.

- methode S° 3 (CuNi)j-|--]- : nickel was diffused in pure copper previously annealed in pure hydrogen.

As shown in table II, the grain size depends on the preparation and in some cases on the Ni concentration.

+ Associee au C.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985457

JOURNAL DE PHYSIQUE

I1

-

R a d i o a c t i v e s u l p h u r ( 3 5 ~ ) , was i n t r o d u c e d by means of a (E2S-H2) m i x t u r e t h e compo- s i t i o n of which was s o t h a t n o s u l p h i d e c o u l d form on t h e sample (6). Then t h e d i s - t r i b u t i o n of s u l p h u r i n t h e sample was d e t e r m i n e d from r a d i o a c t i v i t y measurements A, e a s i l y r e l a t e d t o s u l p h u r c o n c e n t r a t i o n C (7), b7hithin t h i s frame, t h r e e t e c h n i q u e s have been used.

The Secfinning and Cauvtting Technique (S.C.T.) was t h e more c l a s s i c a l one t o s t u d y G.B. d i f f u s i o n ; i t allowed t o d e t e r m i n e t h e G.B. d i f f u s i o n p a r a m e t e r ( 3 ) P= a6D gb i n g r a i n b o u n d a r i e s p e r p e n d i c u l a r t o t h e s u r f a c e . As shown i n f i g . 1, t h e superficial a c t i v i t y (A) was measured a s s l i c e s were removed p a r a l l e l t o t h e s u r f a c e . The para- m e t e r P was deduced from t h e a c t i v i t y v a r i a t i o n w i t h d e p t h .

REMOVAL OF SLICES

-

PP,RALLCL TO THE SURFACE

TIG. 1 : S e c t i o n i n g and Counting Technique (S.C.T.) f o r d i f f u s i o n

s t u d y w i t h r a d i o t r a c e r s such a s 3 5 s .

The FLUX Boundmy Technique (F.B.T.) (8) e n a b l e d u s t o a n a l y s e g r a i n b o u n d a r i e s pa- r a l l e l t o t h e s u r f a c e ; i n p r a c t i c e , a sample c o n t a i n i n g a f l a t boundary was made by s i n t e r i n g a p l a t e and a p l a t e l e t of t h e same m e t a l and s u b m i t t i n g t h e sample t o s e g r e g a t i o n t r e a t m e n t s . The b u l k c o n c e n t r a t i o n Cb of s u l p h u r n e a r t h e f l a t boundary, t h e g r a i n boundary C and t h e s e g r e g a t i o n c o e f f i c i e n t were deduced from t h e a c t i v i t y v a r i a t i o n s w i t h deptR ( f i g . 2 ) .

FIG. 2 : The F l a t Boun- d a r y Technique (F.B.T.) : a = (Amax-%)

/%.

( f i i s t h e a b s o r p t i o n c o e f f i c i e n t f o r 35~-L3 r a y s by t h e m e t a l ) '

The Scanning Radiochexkcd E/lictroanuLynD (S.R.M.) r e c e n t l y developped i n o u r l a b o r a - t o r y ( 9 , 5 ) , e n a b l e d u s t o s t u d y g r a i n b o u n d a r i e s ~ e r p e n d i c u l a r t o t h e s u r f a c e i n b i - c r y s t a l s ; t h e r a d i o c h e m i c a l a n a l y s i s was performed t h r o u g h a narrow s l i t (45 um) p a r a l l e l t o t h e g r a i n boundary, moving a l o n g t h e s u r f a c e ( f i g . 3 ) . I n a g e n e r a l man- n e r , t h e a r e a s under t h e A/A = f ( x ) c u r v e s r e p r e s e n t e d t h e amount of s u l p h u r con- c e r n e d by s e g r e g a t i o n (S )Ofor a n a l y s i s performed on t h e s u r f a c e o r v e r y n e a r t o i t , by s e g r e g a t i o n and

G%!

(Y > 0 ) ; A, was t h e b u l k a c t i v i t y f o r y = 0. &e f i r s t a n a l y s i s ( Y % 0) was s u f f i -

c i e n t t o o b t a i n a and t h e s e g r e g a t e d amount ; w i t h t h e second a n a l y s i s , we o b t a i n e d i n a d d i t i o n Db and Dgb. Thus,we were a b l e t o o b t a i n a l l t h e d a t a r e l a t i v e t o G.B.

d i f f u s i o n and s e g r e g a t i o n i n a s i n g l e :rain boundary,from one e x p e r i m e n t i n a b i c r y s t a l

.

FIG. 3 : The Scanning Radiochemical M i c r o a n a l y s i s . I11

-

111-1- Sulphur s e g r e g a t i o n and d i f f u s i o n i n b i c r y s t a l s

The e x p e r i m e n t a l c o n d i t i o n s and d a t a r e l a t i v e t o b i c r y s t a l s of Ag, Cu and N i a r e c o l l e c t e d i n T a b l e ( I ) ( 2 , 3 , 5 ) ; t h e same q u a n t i t i e s , r e l a t i v e t o p o l y c r y s t a l s under t h e same c o n d i t i o n s a r e added f o r comparison. The e x p e r i m e n t a l t e c h n i q u e s a r e b r i e f l y i n d i c a t e d i n t h e p a r e n t h e s e s . R e c a l l t h a t t h e d a t a r e l a t i v e t o Cu ( 9 ) and Ag ( 1 0 ) were t h e f i r s t o n e s o b t a i n e d w i t h t h e S.R.M., when we had n o t y e t d e v e l o p e d a l l t h e p o s s i b i l i t i e s of t h e t e c h n i q u e . Thus t h e Cu t w i n b i c r y s t a l was o n l y a n a l y s e d v e r y n e a r t o t h e s u r f a c e where s e g r e g a t i o n c o u l d o n l y b e i n v o l v e d . The s i l v e r G.B. Ag(Rd) was r a t h e r s i n u o u s and t h e a n a l y s i s c o u l d n o t b e performed n e a r t h e s u r f a c e s o t h a t t h e r e s p e c t i v e c o n t r i b u t i o n of d i f f u s i o n and s e g r e g a t i o n was r o u g h l y known ; f o r t h e s e r e a s o n , w e h a v e g o t o n l y a n a p p r o x i m a t e v a l u e f o r a . N e v e r t h e l e s s i t i s c l e a r t h a t ct can be much more g r e a t e r o r s m a l l e r i n some b i c r y s t a l s t h a n i n p o l y c r y s t a l s of t h e same m e t a l .

A comparison of t h e f i r s t d a t a o b t a i n e d f o r tJi(2Oland NiftO) l e d t o admit l a r g e r s e g r e - g a t i o n i n t h e f i r s t one (19) ; i n d e e d , t h i s r e s u l t f a i l e d when more e x p e r i m e n t a l d a t a were a v a i l a b l e f o r b o t h t h e s e b i c r y s t a l s : t h e p r e s e n t r e s u l t s have been o b t a i n e d re- c e n t l y and h a v e p r o f i t e d by v a r i o u s improvementsof t h e Scanning Radiochemical Xicro- a n a l y s i s : t a k i n g i n t o a c c o u n t p a r a m e t e r s , s u c h a s t h e s l i t w i d t h and t h e a c t u a l s h a p e of t h e g r a i n boundary, which g r e a t l y i n f l u e n c e t h e shape of t h e c u r v e

A/Ao - f ( ~ ) ~ , t r e a t m e n t of e x p e r i m e n t a l d a t a i n o r d e r t o h a v e a smooth a n a l y s i s c u r - v e , u s e of s e v e r a l a n a l y s e s a t d i f f e r e n t d e p t h s ( y ) t o improve t h e r e s u l t s ( 5 ) . It a p p e a r s c l e a r l y ( t a b l e I ) t h a t s e g r e g a t i o n i s g r e a t e r and t h e d i f f u s i o n s l o w e r i n N i ( 4 0 ) t h a n i n Mi(2P) ; t h e amount of s e g r e g a t e d s u l p h u r remains always l a r g e i f com- p a r e ? t o t h e monolayer Z e f i n e d on t h e (100) s u r f a c e ( 1 1 ) . I t may b e s e e n t h a t , what- e v e r t h e t e c h n i q u e used, t h e G.B. d i f f u s i o n c o e f f i c i e n t s and t h e i r v a r i a t i o n s w i t h m i s o r i e n t a t i o n a r e w i t h i n t h e same r a n g e , which a c c o u n t s f o r t h e v a l i d i t y of t h e S.R.M. ; i n d e e d t h e c o e f f i c i e n t s f o u n d by S.C.T. a r e always s l i g h t l y s m a l l e r t h a n t h o s e found by S.R.M. ; t h i s r e s u l t i s w e l l e x p l a i n e d by some d i f f e r e n c e s i n t h e d i f - f u s i o n t r e a t m e n t b e f o r e S.C.T. o r S.R.M. i s u s e d ( 1 2 ) .

111-2- Sulphur s e g r e g a t i o n and d i f f u s i o n i n CuNi p o l y c r y s t a l s

A l l t h e i n f o r m a t i o n s a r e c o l l e c t e d i n t a b l e 11. S e g r e g a t i o n c o e f f i c i e n t s and d i f f u - s i o n p a r a m e t e r s have b e e n d e t e r m i n e d u n d e r t h e same e x p e r i m e n t a l c o n d i t i o n s , r e s p e c - t i v e l y by t h e F l a t Boundary Technique (F.B.T.) and by t h e S.C.T.

It f o l l o w s from r e s u l t s o b t a i n e d i n ( C U N ~ ) ~ , t h a t t h e s u l p h u r s e g r e g a t i o n c o e f f i c i e n t ct d e c r e a s e s when t h e a d d i t i o n of n i c k e l i s acconpaned w i t h a c o a r s e n i n g of t h e g r a i n s . From r e s u l t s o b t a i n e d i n ( C U P J ~ ) ~ ~ and ( C U N ~ ) ~ ~ ~ i t may b e s e e n t h a t on t h e c o n t r a r y , ct i n c r e a s e s w i t h t h e n i c k e l c o n c e n t r a t i o n .

C4-520 JOURNAL DE PHYSIQUE

TABLE I : C a t a r e l a t i v e t o G.B. s e g r e g a t i o n and d i f f u s i o n e x p e r i m e n t s i n Ag,Cu,Ni S i c r v s t a l s and ~ o l v c r v s t a l s .

1

I

I

I

Bulk c o n c e n t r a t i o n

C b ( a t / a t ) ~ 1 0 6

I

1

BICRYSTALS :

g r a i n boundary random" t w i n boundary a s s y m e t r i c

13

d e f i n i t i o n

1 1

1

0 = 36"57' 6= 1O0C+1O030' 1 0 = 20°+19"3

O f s u r f a c e m o n o 1 % 3 0

1

l a y e r i n t h e G.B.

---

--- --- --- --- ---

P a . 6 (cm3xs-l) gb

(s.R.H.) 7.4 1 0 - l ~ 1.7 I S - ' ~

(s.c.T.) 1.6 r 10-l3 3 . 7 1 9 - l ~

(S.C.T) 3.5 x 10-I

POLYCRYSTALS :

a (P.B.T.)

TABLE I1 : D a t a r e l a t i v e t o s e g r e g a t i o n e x p e r i m e n t s i n ( C u ~ i ) p o l y c r y s t a l s . ANNEALING

CONDITIONS

P r e p a r a t i o n Cb(Ni) ( a t l a t ) Cb ( S )

( a t / a t ) x l O 6 a (F.B.T.)

G r a i n s i z e C ( a t / a t ) x l o 6

---

p(H2S)/p(H2) = 1 - 7 10 -4 ; T = 600°C

...

Mkthode I

,

M6th.111 (CuNi)LII

7 x 1.28

21000

-

s m a l l

-- -

26900

-- -

14.6:

---

1.1 x 1 o 2 . 7 8

5100 l a r g e r 4000

---

2.2%

0 .7

10500 sma 11 7350

---

4 z

Pt6thoZe I1 , ( C U N ~ ) ~ ~ .5 x 1 c 2

.74

8100 l a r g e 6000

---

3.3R

0

- 7

10000 s m a l l 7000

4%

.5 x l o 2 -74

s m a l l

1.1 x 1 c 2 - 7 8

15000 s m a l l 11700

6.4%

I V - DISCUSSION

It f o l l o w s form r e s u l t s g i v e n h e r e , t h a t c r y s t a l l o g r a p h i c f a c t o r s i n f l u e n c e G.B. se- g r e g a t i o n and d i f f u s i o n of s u l p h u r i n b i c r y s t a l s and p o l y c r y s t a l s of m e t a l s .

IV-I-Origin of t h e d i f f e r e n c e s o b s e r v e d i n b i c r y s t a l s ^'

There a r e s i m i l a r mean amounts of s e g r e g a t e d s u l p h u r i n Cu and Ag p o l y c r y s t a 1 s ; t h e comparison of t h e d a t a o b t a i n e d r e s p e c t i v e l y f o r Cu(tw) and A g ( ~ d ) w i t h t h o s e o b t a i - ned i n p o l y c r y s t a l s l e a d s t o t h e c o n c l u s i o n t h a t t h e amount of s u l p h u r i s s m a l l e r i n a twinG.B.and l a r g e r i n a random G.B. t h a n t h e mean amount i n a s e t of G.B..This d i f f e r e n c e c a n b e a s s o c i a t e d w i t h a n o b s e r v e d i n c r e a s e i n G.B. energy found i n aG.B.

of random m i s o r i e n t a t i o n i n comparison t o t h e G.B. e n e r g y found i n a t w i n G.B. ; t h i s i s e a s i l y e x p l a i n e d by t h e f a c t t h a t t h e d i f f e r e n c e between G.B. s i t e s and b u l k s i t e s i s g e n e r a l l y more marked i n t h e c a s e of a"randomt' G.B.than i n t h a t of a t w i n G.B..

It i s n o t s o e a s y t o e x p l a i n t h e d i f f e r e n c e o b s e r v e d between Mi(20) and N i ( 4 0 ) f o r t h e G.B. e n e r g i e s a r e q u i t e t h e same f o r t h e s e two G.B. i n f . c . c . m e t a l s ( 1 3 ) , and wehave t e s t e d i t was s o i n n i c k e l . Hence, G.B. e n e r g y i s n o t a s u f f i c i e n t p a r a m e t e r t o e x p l a i n a l a r g e r s e g r e g a t i o n i n ~ i ( 4 0 ) t h a n i n N i ( 2 0 ) . A c t u a l l y , G.B. energy i s a sum, e x t e n d e d t o t h e v a r i o u s G.B. s i t e s , of t e r m s (Ni/N) x E i , p r o d u c t o f t h e pro- p e r e n e r g y E i of e v e r y s i t e by t h e p r o p o r t i o n of t h e s e s i t e s i n t h e G.B.,Rence seve- r a l G.B. w i t h e q u i v a l e n t e n e r g i e s , c a n b e d e s c r i b e d w i t h d i f f e r e n t s t r u c t u r e s and s e v e r a l s t r u c t u r e s a r e o f t e n p o s s i b l e f o r t h e same G.B. ( 1 4 ) .

A c r i t e r i o n c o n n e c t e d t o G.B. s t r u c t u r e c a n t h e r e f o r e a c c o u n t f o r d i f f e r e n t beha- v i o u r s of G.B. s u c h a s N i ( 2 0 ) an2 N i ( 4 0 ) . I h i t h i n t h i s frame, c a l c u l a t i o n s made f o r Bi s e g r e g a t i o n i n Cu ( 1 5 ) , showed t h a t t h e energy a s s o c i a t e d t o s e g r e g a t i o n v a r i e s from one s i t e t o a n o t h e r , f o r low G.B. c o n c e n t r a t i o n s ; s e g r e g a t i o n might t h e n e l i - m i n a t e some of t h e p o s s i b l e s t r u c t u r e s and f a v o u r o t h e r s . As t h e s e g r e g a t e d amount i n c r e a s e s , i n t e r a c t i o n s between s e g r e g a t e d atoms o c c u r and change t h e p r o p e n s i t y t o s e g r e g a t i o n of unoccupied s i t e s , G.B. atoms move a s i d e t h e i r i n i t i a l p o s i t i o n s and f i n a l l y a two d i m e n s i o n a l o r d e r t a k e s p l a c e s o t h a t t h e mean d i s t a n c e s Bi-Bi and Cu-Bi come t o agreement w i t h t h e i n t e r a c t i o n s between t h e s e atoms. It i s o b v i o u s t h a t t h i s s t r u c t u r a l a s p e c t i s much more i m p o r t a n t f o r s u l p h u r t h a n f o r a m e t a l i n a n o t h e r m e t a l ; i t was shown, f o r i n s t a n c e , t h a t G.B. e m b r i t t l e m e n t of n i c k e l by s u l p h u r , i s n o t s t r i c t l y a f u n c t i o n of c o n c e n t r a t i o n b u t of t h e s t r u c t u r e a s w e l l ( 1 6 ) . Indeed t h e marked c o v a l e n t c h a r a c t e r of t h e Ni-S bond on t h e s u r f a c e (1 7 ) l e a d s t o e x p e c t some d i r e c t i v i t y f o r Ni-S bond i n t h e G . B . , t h a t d e t e r m i n e s t o a g r e a t ex- t e n t , a s t r u c t u r e f o r t h e s e g r e g a t e d g r a i n boundary which i s n o t e q u a l l y p o s s i b l e f o r a l l t h e G.B. and depends on s t r u c t u r a l f a c t o r s more t h a n on e n e r g e t i c a l ones. F i n a l - l y , s t r u c t u r e q u i t e d i f f e r e n t f o r N i ( 2 0 ) and N i ( 4 0 ) i n p u r e M i ( l a ) , c o u l d b e t h e o r i g i n of t h e d i f f e r e n c e i n a . Note moreover t h a t f o r such h i g h c o v e r a g e s ( 0 . 3 a n 3 0 . 9 rnonolayer),atwo i i m e n s i o n a l Xi-S phase p r o b a b l y forms, t h e s t r u c t u r e of which i s c o m p a t i b l e w i t h t h e Ni-S c h e m i c a l bond and w i t h t h e G . B . c r y s t a l l o g r a p h i c f a c t o r s . The t r a n s f e r of m a t t e r a l o n g a g r a i n boundary r e s u l t s from a c o m p e t i t i o n between d i f - f u s i o n i n t h e G.B. and b u l k d i f f u s i o n from t h e G.B..For t h i s r e a s o n , i t i s g e n e r a l l y a d m i t t e d t h a t s e g r e g a t i o n , which i m p l i e s a t t r a c t i o n of s o l u t e atoms towards G.B., f a v o u r s G.B. d i f f u s i o n . T h i s enhancement a p p e a r s t h r o u g h t h e e x p r e s s i o n P = a&?

where P v a r i e s p r o p o r t i o n a l t o a ; t h i s e f f e c t , o f t e n o b s e r v e d , i s o n l y u n d e r s t a n d a - gb b l e i f t h e G.B. : ' i f f u s i o n c o e f f i c i e n t v a r i e s o n l y a l i t t l e w i t h t h e c h e m i c a l G.B.

c o m p c s i t i o n . F o r t h e N i b i c r y s t a l s we h a v e s t u d i e d , P i s s m a l l e r f o r N i ( 4 0 ) where s e g r e g a t i o n i s g r e a t e r

,

C4-522 JOURNAL DE PHYSIQUE

IV-2- C r y s t a l l o g r a p h i c e f f e c t s i n p o l y c r y s t a l s

I n Cu-Ni s o l i d s o l u t i o n ( t a b l e 1 1 ) , o p p o s i t e r e s u l t s have been a p p a r e n t l y o b t a i n e d ; inc'eed, i t can b e c o n n e c t e d t o t h e e f f e c t of c r y s t a l l o g r a p h i c f a c t o r s i f t h e f o l l o - wing o b s e r v a t i o n s a r e talceri i n t o a c c o u n t :

- i n t h e p r e s e n t work, a i s a n a v e r a g e v a l u e on a l l t h e g r a i n b o u n d a r i e s ;

-

- Cu(tw) and Ag(Rd) b e h a v i o u r s l e a d t o t h e c o n c l u s i o n t h a t s e g r e g a t i o n o c c u r s g e n e r a l l y i n h i g h e s t energy G.B.

Thus, t h e v a r i o u s e x p e r i m e n t s performed i n ( C U N ~ ) ~ s e r i e s cannot be compared b e c a u s e t h e y a r e performed i n d i f f e r e n t s e t s of G.B.

.

C r y s t a l l o g r a p h i c f a c t o r s p l a y a l s o a r o l e when s e g r e g a t i o n i s connected t o G.B. d i f - f u s i o n i n t h e same p o l y c r y s t a l . Thus, under t h e same e x p e r i m e n t a l c o n d i t i o n s , t h e d i f f u s i o n p a r a m e t e r P i n c r e a s e s from Ni(40) t o Ni(20) and t o Ni(Po1y) ( t a b l e I ) ; t h a t i s t o s a y , t h a t i n a p o l y c r y s t a l , G.B. t r a n s f e r of m a t t e r c h i e f l y o c c u r s t h r o u g h G.B. where s e g r e g a t i o n i s s m a l l . I n o t h e r words, P i s measured i n G.B. which l i t t l e c o n t r i b u t e t o a . It f o l l o w s from t h i s s i t u a t i o n t h a t s i m u l t a n e o u s d e t e r m i n a t i o n of

a and P i n p o l y c r y s t a l s o n l y l e a d t o a n a p p a r e n t Dgb v a l u e c a l c u l a t e d by P =aGDgb ; no p h y s i c a l meaning c a n be a t t r i b u t e d t o Dgb a t t h e atomic s c a l e , p r i n c i p a l l y when s e g r e g a t i o n and d i f f u s i o n g r e a t l y v a r y from one G.B. t o a n o t h e r , a s f o r s u l p h u r i n m e t a l s .

COPJCLUSION

Experiments of G.B. s e g r e g a t i o n and d i f f u s i o n of s u l p h u r i n f . c . c . m e t a l s e n a b l e d us t o l a y some emphasis on t h e r o l e of c r y s t a l l o g r a p h i c f a c t o r s . I n b i c r y s t a l s t h e main a s p e c t s concern t h e p o s s i b i l i t y f o r a G.B. t o p r o v i d e a s t r u c t u r e i n agreement w i t h t h e s t r o n g somewhat c o v a l e n t Ni-S bond, and t h e l a r g e d e c r e a s e of t h e s u l p h u r m o b i l i t y when s e g r e g a t i o n o c c u r s . Both t h e s e e f f e c t s may i n f l u e n c e t h e r e s u l t s ob- t a i n e d i n p o l y c r y s t a l s and must b e t a k e n i n t o a c c o u n t t o compare d i f f u s i o n and s e g r e - g a t i o n d a t a .

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