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THEORETICAL STUDIES OF SEGREGATED INTERNAL INTERFACES
Y. Ishida, M. Mori
To cite this version:
Y. Ishida, M. Mori. THEORETICAL STUDIES OF SEGREGATED INTERNAL INTERFACES.
Journal de Physique Colloques, 1985, 46 (C4), pp.C4-465-C4-474. �10.1051/jphyscol:1985451�. �jpa- 00224702�
JOURNAL DE PHYSIQUE
Colloque C4, suppl6rnent au n04, Tome 46, avril 1985 page C4-465
THEORETICAL STUDIES OF SEGREGATED INTERNAL INTERFACES
Y. I s h i d a and M. Mori
I n s t i t u t e o f I n d u s t r i a l S c i e n c e , U n i v e r s i t y o f Tokyo, 22-1, Roppongi, 7 Chome, dfinato-ku, Tokyo 106, Japan
Abstract - Molecular dynamic c a l c u l a t i o n s were performed on t h e g r a i n bound- ary s t r u c t u r e of i r o n , e s p e c i a l l y [loo] and [I101 symmetrical tilt coinci- dence-related boundaries segregated with phosphorous atoms with t h e u l t i m a t e purpose t o understand t h e mechanism of g r a i n boundary embrittlement i n s t e e l s by t h e g r a i n boundary s e g r e g a t i o n o f 1% t o VIb elements. The simulated s t r u c t u r e s showed t h e phosphorous segregation a l t e r s t h e g r a i n boundary s t r u c - t u r e e x t e n s i v e l y and consequently,the mechanical behavior under an a p p l i e d s t r e s s d i f f e r s upon impurity atom segregation. The i n t e r a c t i o n o f t h e segre- gated i n t e r f a c e s with nearby l a t t i c e d i s l o c a t i o n and atomic process of crack propagation were simulated a l s o by molecular dynamics techniaue.
I . INTRODUCTION
Impurity s e g r e g a t i o n i s undoubtedly t h e most i n f l u e n t i a l parameter of i n t e r n a l i n t e r f a c e s . I n t e r f a c i a l d i f f u s i o n , migration, s l i d i n g , f r a c t u r e , corrosion and a l l o t h e r chemical and p h y s i c a l p r o p e r t i e s a r e s t r o n g l y a f f e c t e d by t h e segregation.
I t is considered t h a t t h e i n t e r f a c e s t r u c t u r e i t s e l f i s a l t e r e d e x t e n s i v e l y . (1) Large p r o p e r t y changes a r e expected a s t h e n a t u r a l consequence of t h e a l t e r a t i o n . In s p i t e o f t h e c l o s e r e l a t i o n s h i p between t h e i n t e r f a c i a l s t r u c t u r e and impurity s e g r e g a t i o n , t h e two s t u d i e s d i d n o t mix well u n t i l r e c e n t l y . An ~ u g k r m e t a l l u r - g i s t l i k e s t o s t a y with s t r u c t u r e l e s s monolayer o f atoms except i n a few s t u d i e s ( 2 ) , while c o i n c i d e n c e - s i t e l a t t i c e t h e o r e t i c i a n s were r e l u c t a n t t o d e a l with segregated s t r u c t u r e s . I t may be s a i d t h a t l a t t i c e imaging e l e c t r o n microscopy i s going t o b r i d g e t h e two s u b j e c t s experimentally and opening t h e long waited atomic s t r u c t u r e study o f segregated i n t e r f a c e s .
Caiculation of segregated i n t e r f a c e has been l i m i t e d t o c e n t r a l f o r c e type c a l c u l a - t i o n s . (1) (4)-(9) F u r t h e r modification may be needed because s t r o n g s e g r e g a t i o n s e s p e c i a l l y t h a t of IVB toVIB elements i n t h e g r a i n boundary of i r o n i n v a r i a b l y involves i n changes i n bonding c h a r a c t e r i s t i c s . B r i a n t and Messemer(l0) - (12), f o r example, showed a l a r g e charge t r a n s f e r t o t h e impurity and r e s u l t e d decrease i n valence e l e c t r o n charge d e n s i t y between t h e neighboring Fe atoms using f u l l y quantum mechanical c l u s t e r c a l c u l a t i o n s o f S l a t e r ( l 3 ) (SCF-Xa SW method), while covalent type bonding i s suggested by Losch between impurity and h o s t m e t a l ( l 4 ) . Further i n v e s t i g a t i o n i s needed along t h i s l i n e , because t h e boundary s t r u c t u r e assumed by Briant and Messmer i s t o o u n r e a l i s t i c . The l o c a l d e n s i t y of e l e c t r o n s t a t e s (LDOS) c a l c u l a t i o n s (1) have assumed charge n e u t r a l i t y which must be proved.
I t i s t h e purpose of t h e p r e s e n t paper t o review our r e c e n t atomic s t u d i e s of t h e segregated i n t e r f a c e through computer simulations,only t o prepare t h e o r e t i c a l l y t h e b a s i c information t o be used upon observation of t h e segregated i n t e r f a c e by l a t t i c e imaging e l e c t r o n microscopy. Present c a l c u l a t i o n i s l i m i t e d t o coincidence-related t i l t type boundaries, because t h e l a t t i c e imaging observation i s most l i k e l y achieved with tilt t y p e ordered boundaries. The bonding c h a r a c t e r c a l c u l a t i o n of t h e boundaries by SCF-Xa SW method a s w e l l as molecular dynamical simulations of mechanical p r o p e r t i e s such a s g r a i n boundary f r a c t u r e a r e included.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985451
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11. MOLECULAR DYNAMIC CALCULATIONS
I n t e r f a c e s t r u c t u r e s both pure and impurity segregated were simulated by assuming i n d i v i d u a l atoms obeying -the following Newton's law:
where r. . i s t h e d i f f e r e n c e between t h e v e c t o r s o f t h e i t h and j t h atom p o s i t i o n . And v ( r t J ) i s i n t e r a t o m i c p o t e n t i a l . Morse p o t e n t i a l ( l 5 ) w a s used i n t h e p r e s e n t
calcula$ion, whose d e t a i l s a r e given elsewhere(1). The atom p o s i t i o n and v e l o c i - t i e s were c a l c u l a t e d by t h e following approximate equations. The c a l c u l a t i o n s t a r t e d f o r i n d i v i d u a l atoms with zero v e l o c i t y .
ai ( t ) = F. / mi
vi ( x + At/2 ) = vi ( x - At/2 ) + a. ( t ) A t
ri ( t + A t ) = r i ( t ) + v i ( x + A t / Z ) A t
where m. i s t h e atomic mass, v. i s t h e v e l o c i t y l p d ri i s t h e p o s i t i o n o f t h e i t h atom. h e time u n i t A t was taken t o be 2 X 10- sec. Phosphorous atoms were p l a c e d a t l a t t i c e s i t e s s u b s t i t u t i o n a l l y n e a r t h e g r a i n boundary and r e l a x e d by t h e method described elsewhere(1). P e r i o d i c o r d e r i n g c h a r a c t e r i s t i c t o t h e c o i n c i d e n c e - r e l a t e d boundaries were no longer used i n s i m u l a t i n g t h e s t r u c t u r e o f t h e boundary d i s l o c a - t i o n s . A f i n i t e region was s e t f o r t h e molecular dynamic c a l c u l a t i o n as a r e shown i n l a t e r f i g u r e s . The same framework was used t o apply s t r e s s i n f r a c t u r i n g t h e segregated g r a i n boundary. A g r a i n boundary notch was made by making t h e atomic f o r c e s a r t i f i c i a l l y zero across t h e notched s u r f a c e .
111. STRUCTURE OF IMPURITY SEGREGATED GRAIN BOUNDARIES
The molecular dynamical c a l c u l a t i o n showed t h e i n t e r f a c e s t r u c t u r e of lowest energy d i f f e r completely upon impurity atom s e g r e g a t i o n even though t h e p e r i o d i c u n i t s t r u c t u r e i s of t h e same length corresponding t o t h e coincidence system. There a r e many metastable c o n f i g u r a t i o n s b u t i n Fig. 1 lowest energy c o n f i g u r a t i o n s a r e shown.
Reflecting t h e tilt type symmetry, t h e d i s t o r t e d prism type s t r u c t u r e i s c h a r a c t e r - i s t i c o f both C=5 and C=9 boundaries. In case o f phosphorous segregated boundary, however, t h e columnar Fe3P type prism s t r u c t u r e i s t y p i c a l and t h e d i s t r i b u t i o n of t h i s prism s t r u c t u r e i s wide i n zigzag p a t t e r n along t h e boundary. Naturally, t h e l a y e r o f t h e l a t t i c e d i s t o r t i o n along t h e boundary appears r a t h e r wide.
The f l u c t u a t i o n i n t h e h y d r o s t a t i c s t r e s s surrounding i n d i v i d u a l atoms n e a r t h e boundary i s a u s e f u l parameter t o d e f i n e t h e thickness o f phosphorous segregated boundary. Fig. 2 shows h y d r o s t a t i c s t r e s s of each atom n e a r t h e boundary. The r a d i u s corresponds t o e i t h e r excess o r d e f i c i t i n t h e h y d r o s t a t i c s t r e s s o f f t h e matrix value as shown h e r e i n t h e f i g u r e . The d i s t r i b u t i o n of t h e h y d r o s t a t i c s t r e s s g e t s much wider i n phosphorous segregated boundaries. For t h e case of t h e comparison those with h y d r o s t a t i c compression a r e shown i n black c i r c l e s and white c i r c l e s f o r h y d r o s t a t i c tension.
O . O . O . O .
O . O . C . O .
0 0 ' 0 ' 0 . 0
. O * C . O .
" 1 0 .
Fig. 1. Grain boundary s t r u c t u r e s ( a , b ) o f a C=5 and (c,d) of a C=9 coincidence boundary(a,c) without P and (b,d) with P segregation. Double c i r c l e s i n d i c a t e P atom p o s i t i o n s . Black and white c i r c l e s i n d i c a t e atom s i t e s one plane above o r below t h a t o f t h e r e s t .
Fig. 2 . H y d r o s t a t i c s t r e s s of atoms i n coincidence boundaries of Fig. 1. White and black c i r c l e s i n d i c a t e t e n s i l e and compression s i t e s r e s p e c t i v e l y .
IV. OBSERVATION OF IMPURITY SEGREGATED GRAIN BOUNDARIES
A conventional transmission e l e c t r o n micrograph does n o t give d e t a i z s of t h e impu- r i t y segregated atomic s t r u c t u r e . I t only gives some h i n t of t h e e f f e c t of segre- g a t i o n from t h e shape o r t h e remaining image o f t h e d i s l o c a t i o n . L a t t i c e imaging microscopy has a p o t e n t i a l t o observe such an impurity segregated s t r u c t u r e e s p e c i a l -
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Fig. 3 . The s t r u c t u r e of g r a i n boundary d i s l o c a t i o n s ( a , c , e l without P and-(b,d,f) with P segregation. The Burgers v e c t o r s a r e a/5 [021] (a,b) , a/9 [221] (c,d)
and a/9[114](e,f).
Photo li. L a t t i c e imaging e l e c t r o n micrograph of C = l l coincidence boundary o f gold 17
18) Photo 2. L a t t i c e imaging e l e c t r o n micrograph of an e p i t a x i a l [loo] tilt boundary
of i r o n
l y i n coincidence-related pure tilt boundaries. The micrograph l i k e Photo 1 i s with white s p o t s along t h e C = l l coincidence r e l a t e d g r a i n boundaries o f gold. (17) The white image i s d i f f i c u l t t o simulate by changing t h e focusing condition. I t i s thus suspected t h a t t h e image i s r e l a t e d t o t h e impurity atom segregation along t h e boundary. Phosphorous atom segregation i n i r o n boundary i s one o f t h e popular sub- j e c t s of t h e l a t t i c e imaging observation.(l8) Only l i m i t e d success, however, has been achieved i n imaging t h e segregated boundary. Photo 2 i s one example which i s n o t coincidence-related symmetrical tilt boundary. I t i s an e p i t a x i a l boundary p a r a l l e l t o a densely packed l a t t i c e p l a n e which does not contain p e r i o d i c s t r u c t u r e u n i t .
V. BINDING CHARACTERISTICS OF THE SEGREGATED GRAIN BOUNDARY
The c h a r a c t e r i s t i c s of atomic bonding a t t h e impurity segregated boundary can d i f f e r appreciably i n n a t u r e . B r i a n t and Messmer(l0)-(12) showed t h a t e l e c t r o n charge t r a n s f e r t o impurity atoms from t h e surrounding i r o n atoms and t h e r e s u l t e d l o s s of electrofis which have been a s s o c i a t e d with bonding i n between t h e neighboring i r o n atoms. In t h e p r e s e n t research t h e same SCF-Xu method i s adopted t o c a l c u l a t e t h e bonding s t a t e along t h e boundaries. The l o c a l d e n s i t y o f s t a t e s type c a l c u l a t i o n assuming t h e charge n e u t r a l i t y may no more be a p p r o p r i a t e i n accessing t h e charge t r a n s f e r between t h e segregated impurity atom and h o s t metal. (1) (19) (20) As t h e f i r s t c a l c u l a t i o n C=5 coincidence-related g r a i n boundary of i r o n simulated by t h e p r e s e n t molecular dynamical technique was chosen. The c a l c u l a t i o n is performed because s i m i l a r c a l c u l a t i o n s having been r e p o r t e d using SCF-Xu method(l0)-(12) has been on u n r e a l i s t i c atom configuration. I t may be easy t o c a l c u l a t e b u t t o o sche- matic t o be t h e boundary configuration. Fig. 4(a) i s f o r t h e t o t a l e l e c t r o n d e n s i t y and (b) f o r t o t a l d band e l e c t r o n d e n s i t y . The d e n s i t i e s edge o f t h e s t r u c t u r e u n i t i s n a t u r a l l y influenced by t h e f i n i t e c l u s t e r s i z e . Only t h e range marked by broken l i n e s may be taken a s t h e r e p e a t i n g u n i t . The e l e c t r o n d e n s i t y c o n f i g u r a t i o n d i f f e r only s l i g h t l y whether t o t a l o r d band d e n s i t y i s c a l c u l a t e d mainly because t h e con- f i g u r a t i o n i s f o r pure i r o n . Computation of phosphorous segregated boundary i n pro- g r e s s would answer t h e e x t e n t o f charge t r a n s f e r between P and neighboring Fe atoms and t h e r e s u l t e d decrease i n e l e c t r o n charge p a r t i c i p a t i n g i n t h e Fe-Fe bonds.
Fig. 5 shows t h e d e n s i t y s t r u c t u r e constructed from t h e u n i t . Electron d e n s i t y a t t h e boundary plane i s shown i n Fig. 5 ( a ) and t h e e l e c t r o n d e n s i t y a q u a r t e r o f one atomic plane o f f t h e boundary i s shown i n (b).
G B GB
Fig. 4. Contours of (a) t o t a l and (b) 3 d band e l e c t r o n charge d e n s i t y across the C=5 coincidence boundary of Fig. l ( a ) .
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Fig. 5. Contours of 3 d electron density (a) i n t h e C=5 coincidence boundary and (b) a q u a r t e r of {310) plane off t h e boundary plane
VI. INTERACTION OF LATTICE DISLOCATION WI'M IMPURITY SEGREGATED GRAIN BOUNDARIES Lattice d i s l o c a t i o n i s l i k e l y t o i n t e r a c t with impurity segregated grain boundary very differently, because the boundary s t r u c t u r e i t s e l f d i f f e r extensively by the segregation. The i n t e r a c t i o n i s investigated presently a s an example, with P=9 coincidence boundaries of iron. I t would c o n s t i t u t e a b a s i c process connected t o mechanical properties of impurity segregated grain boundaries.
(1) Dislocation i n t e r a c t i o n with pure grain boundaries
In Fig. 6(a) the black points correspond t o atoms i n the atomic plane one above o r below those of white c i r c l e s . A l a t t i c e dislocation marked by b i s seen t o the r i g h t i n t h e upper grain. The Burgers vector of the l a t t i c e d i s h c a t i o n bL is a/2[111] type l a t t i c e dislocation. Molecular dynamical calculation of the s t r u c t u r e a f t e r one h ~ p e d times of calculation with the repeating u n i t time corresponding t o the 2x10 s e c i s shown i n Fig. 6(b). The l a t t i c e dislocation was absorbed t o the pure grain boundary as shown i n the figures. The d i s t o r t i o n of t h e s t r u c t u r e u n i t a t the boundary c l e a r l y show t h a t the dislocation i s nearly a t the center of the figure. Jaswon and Forman(21) calculated t h i s type of i n t e r a c t i o n between the grain boundary and the l a t t i c e dislocation, which i s always repulsive. But i f a p a i r of dislocation core i s placed near, the core relaxation allows absorption of a l a t t i c e dislocation t o even t h i s type of an ordered boundary. The absorbed l a t t i c e dislocation, however, i s no more s t a b l e . I t can d i s s o c i a t e i n t o a number of boundary dislocations with smaller Burgers vector. Dissociation of t h e l a t t i c e d i s l o c a t i o n was a l s o simulated i n t h e f i g u r e ; Fig. 6(c) shows the s t r u c t u r e a f t e r another hundred times of molecular dynamical calculation. The l a t t i c e dislocation d i s i n t e g r a t e d i n t o bl and b boundary dislocations. The dislocations a r e shown i n Fig. 3 and a l s o i n Fig. 6 ( d j where the coincidence p l o t r i g h t below shows the Burgers vector as well as the inherent s t e p height corresponding t o the r e s u l t a n t boundary dislocation. Conservation of t h e Burgers vector as well a s t h e s t e p height characterizes the dissociation process;
After 400 times of repeated calculation as shown i n Fig. 6(d) the boundary disloca- t i o n b l with the Burgers vector p a r a l l e l t o the boundary migrated out from the boundary t o the l e f t and the boundary dislocation b2 remained i n the center as the r e s u l t of the repeated calculation. This type of dissociation and migration of the boundary dislocations must be the cause of t h e image d i s s i p a t i o n commonly observed
i n t h i n f o i l transmission e l e c t r o n microscopy o f absorbed l a t t i c e d i s l o c a t i o n .
Fig. 6. Dislocation i n t e r a c t i o n with a C=a coincidence boundary without P segrega- t i o n . The l a t t i c e d i s l o c a t i o n b=a/2[111] introduced a t (a) was absorbed
(b) , d i s s o c i a t e d ( c ) i n t o two boundary d i s l o c a t i o n s bl=a/9 [221] and b2=a/18[557] and (d) t h e b l moved o u t by g l i d e .
(2) Dislocation i n t e r a c t i o n with segregated g r a i n boundary
The same coincidence-related boundary C=9 i n t e r a c t s d i f f e r e n t l y with a nearby l a t t i - ce d i s l o c a t i o n when t h e boundary i s segregated with phosphorous. Fig. 7 shows an example o f t h e l a t t i c e d i s l o c a t i o n introduced n e a r a segregated coincidence-related boundary of C=9. The l a t t i c e d i s l o c a t i o n i n s t e a d of being absorbed towards t h e c e n t e r o f t h e boundry repuLsed o u t from t h e boundary a f t e r 500 times o f repeated c a l c u l a t i o n u n l i k e i n pure i r o n boundary. A s shown i n Fig. 7(b) t h e l a t t i c e d i s l o - c a t i o n became f u r t h e r away from t h e boundary. I t g l i d e d out towards upper r i g h t corner by t h e back s t r e s s of t h e phosphorous u n i t s t r u c t u r e c h a r a c t e r i z i n g t h e segregated boundary. Apparently, t h e s t r o n g l y bonded prism s t r u c t u r e centered with a phosphorous atom w a s t h e cause o f t h e back s t r e s s . When t h e l a t t i c e d i s l o c a t i o n i s placed i n i t i a l l y a t t h e boundary i n s t e a d , t h e d i s l o c a t i o n d o e s n ' t move n o r d i s - s o c i a t e i n t o p a r t i a l boundary d i s l o c a t i o n e i t h e r .
Fig. 7(C) shows such an example. The d i s l o c a t i o n core d i d n o t extend along t h e g r a i n boundary as was t h e case with t h e nonsegregated g r a i n boundary. Apparently, t h e s t r a i n f i e l d i s l o c a l i z e d by t h e r e s i s t a n c e o f t h i s t i g h t l y bound u n i t prism s t r u c t u r e s . I f t h e d i s l o c a t i o n i s f o r c e d t o move i n t h e segregated boundary, t h e boundary s t r u c t u r e i s no more l i k e l y t o be conserved l i k e i n t h e case o f t h e nonsegregated g r a i n boundary because t h e s t r u c t u r e u n i t i s n o t j u s t one atomic
l a y e r along t h e boundary b u t dispersed n e a r t h e boundary. The nonconservative motion of t h e boundary d i s l o c a t i o n i n v a r i a b l y leaves a high energy metastable boundary t h a t might s e r v e as the n u c l e a t i o n s i t e of t h e g r a i n boundary f r a c t u r e a t
low temperatures.
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Fig. 7 . Dislocation i n t e r a c t i o n with a C=9 coincidence boundary w i t h P s e g r e g a t i o n . L a t t i c e d i s l o c a t i o n introduced a t (a) repulsed away (b) from t h e boundary.
Dislocation introduced a t t h e boundary (c) d i d n o t d i s s o c i a t e .
Fig. 8. Simulation o f f r a c t u r e along C=5 coincidence boundary segregated with P
( i n black circles).
VII. FRACTURE OF IMPURITY SEGREGATED GRAIN BOUNDARIES
A simple example o f p e r f e c t l y b r i t t l e f r a c t u r e o f coincidence-related boundary i s shown i n Fig. 8. A t e n s i l e s t r e s s was a p p l i e d h o r i z o n t a l l y with r e s p e c t t o t h e g r a i n boundary which i s notched a t t h e top. The epeated molecular dynamical calcu- l a t i o n f o r 100 times with each u n i t equals 2X10'1Ssec showed t h e atomic sequence of t h e f r a c t u r e . The f r a c t u r e propagated zigzag s k i r t i n g through t h e phosphorous centered prism s t r u c t u r e . The weak bondings between t h e prism u n i t were broken i n t o t h e f r a c t u r e path. This example e x p l a i n s why t h e reduction i n t h e g r a i n boundary energy by phosphorous s e g r e g a t i o n d o e s n ' t i n c r e a s e t h e f r a c t u r e s t r e n g t h of t h e boundary i n s t e a d . The reduction i n t h e energy of t h e s t r u c t u r e u n i t i s s o l a r g e t h a t t h e high energy due t o t h e weak bonding of t h e s t r u c t u r e u n i t i s p a r t i a l l y compensated and lowered t h e i n c r e a s e i n t h e energy upon f r a c t u r e . For t h i s e x p e r i - ment a s t r e s s of 1-2 GPa was s u f f i c i e n t as f a r a s notch i s introduced a t one corner of t h e boundary. The crack propagation speed d i d n ' t change much by t h e impurity s e g r e g a t i o n . The notch r a d i u s , however, was s m a l l e r with segregated boundaries r a t h e r than with nonsegregated ones. The crack propagation r a t e must be a f f e c t e d by a slow atomic v i b r a t i o n s along t h e advancing crack, which simply r e f l e c t s weak bonding. (2) The crack propagation i n more d u c t i l e m a t e r i a l such a s s t e e l i s n o t
t h i s much simple because matrix deforms a t t h e same time. The i n t e r a c t i o n of t h e l a t t i c e d i s l o c a t i o n a s w a s discussed i n t h e former chapter has t o be incorporated i n t h i s r a t h e r d u c t i l e crack propagation process.
ACKNOWLEDGEMENTS
The authors would l i k e t o thank D r . M. Hashimoto f o r molecular dynamic c a l c u l a t i o n method, D r . H. Ichinose f o r l a t t i c e imaging micrographs. This s t u d y owes t o t h e Research Enhancement Fund o f t h e I r o n and S t e e l I n s t i t u t e o f Japan (Committee on Basic Research o f Surface P r o p e r t i e s of S t e e l )
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JOURNAL DE PHYSIQUE
DISCUSSION
J. Vitek: You showed a simulated f r a c t u r e f o r a segregated boundary. Did you do a s i m i l a r simulation f o r a pure boundary, and how d i d t h e r e s u l t s compare?
Y. I s h i d a : I n t h e nonsegregated boundary, t h e a r t i f i c i a l notch simply widened and no c r a c k l i k e propagation was n o t i c e d .
Y. Vitek: Could you explain how one deduces from t h e change i n t h e l o c a l d e n s i t y o f s t a t e s t h e change i n cohesion?
Y. I s h i d a : The l o c a l d e n s i t y o f e l e c t r o n o r b i t a l s t a t e s o f atom i n c r e a s e s a t e n e r g i e s around t h e atomic l e v e l (&=O). T h i s means t h a t t h e bonding s t a t e s o f atom E a r e decreased, while t h e non-bonding s t a t e s a r e increased.
L. P r i e s t e r : Concerning t h e d i s s o c i a t i o n o f d i s l o c a t i o n s , what do you t h i n k about t h e i n f l u e n c e o f an element which does n o t induce embrittlement a s C o r B i n Fe?
Y. Ishida: Molecular dynamic c a l c u l a t i o n s o f H a s h i m t o e t a l . ( 1 ) on B i n Fe showed t h a t t h e small s i z e B atoms t a k e p o s i t i o n s i n a more f l e x i b l e manner than P atoms do. A more e x t e n s i v e d i s l o c a t i o n c o r e r e l a x a t i o n i n t h e boundary is expected. D i s s o c i a t i o n o f t h e d i s l o c a t i o n occurs a t lower temperatures involving atomic s c a l e s h u f f l i n g t y p e d i f f u s i o n o f C o r B.
(1) H. H a s h i m t o , Y. I s h i d a , R. Yamamoto and M. Doyama: Acta Metall. 12 ( I ) , 1-11 (1984).
