MICRO-SEGREGATION OF CHROMIUM IN Fe-Cr ALLOY

HAL Id: jpa-00224455

https://hal.archives-ouvertes.fr/jpa-00224455

Submitted on 1 Jan 1984

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.

MICRO-SEGREGATION OF CHROMIUM IN Fe-Cr ALLOY

N. Igata, S. Sato, T. Ando, H. Doi, O. Nishikawa, M. Shibata

To cite this version:

N. Igata, S. Sato, T. Ando, H. Doi, O. Nishikawa, et al.. MICRO-SEGREGATION OF CHROMIUM IN Fe-Cr ALLOY. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-403-C9-408.

�10.1051/jphyscol:1984967�. �jpa-00224455�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n ° 1 2 , Tome 45, d e c e m b r e 1984 page C9-403

MICRO-SEGREGATION OF CHROMIUM IN Fe~Cr ALLOY

N. I g a t a , S. S a t o , T. Ando, H. D o i , 0 . Nishikawa* and M. S h i b a t a *

Department of Materials Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

*Materiale Soienee and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan

Résumé - L ' o b j e c t i f de c e t t e i n v e s t i g a t i o n e s t l e comportement des atomes de Chrome dans un a l l i a g e de f e r avec 5at.% Cr. Après une trempe s o l u b i l i s a n t e a 1150°C pendant une heure ( l e s spécimens ' é t a n t trempés dans l ' e a u ) , l'image FIM ne m o n t r a i t qu'une région c l a i r e , mais après détrempe e n t r e 450 à 650°C on p o u v a i t o b s e r v e r des r é g i o n s c l a i r e s et sombres. Dans ces spécimens trempés i l y a v a i t de la m i c r o - s é g r é g a t i o n des atomes de Chromé : Dans l e s r é g i o n s c l a i r e s l a c o n c e n t r a t i o n de Chrome é t a i t moins grande que l a moyenne, t a n d i s que dans l e s r é g i o n s sombres e l l e é t a i t p l u s grande que l a moyenne. La d i f f é r e n c e des concentrations de Chrome augmentait avec l a température du r e c u i t . Cependant i l n'y a pas de m i c r o s é g r é g a t i o n d'atomes de Chrome dans des spécimens de f e r r i t e en s o l u t i o n s o l i d e r e f r o i d i s lentement. Le r ô l e des d i s l o c a t i o n s dans la microségrégation d'atomes de Chrome e s t d i s c u t é e e n f i n .

A b s t r a c t - The o b j e c t i v e o f t h i s i n v e s t i g a t i o n i s t o c l a r i f y t h e b e h a v i o r o f chromium atoms i n i r o n - 5 a t . % c h r o m i u r n a l l o y . When t h e specimens were quenched a f t e r s o l u t i o n annealing at 1150°C f o r I h r , FIM image was only a b r i g h t area, but when they were tempered from 450°C t o 650 C, both b r i g h t areas and dark areas were observed i n the FIM image.

In t h e s e quenched specimens t h e r e was m i c r o s e g r e g a t i o n o f chromium atoms: In b r i g h t areas the chromium concentration was lower, and i n dark areas i t was higher than the average c o n c e n t r a t i o n . The d i f f e r e n c e of chromium c o n c e n t r a t i o n s became l a r g e r w i t h i n c r e a s i n g t e m p e r i n g temperature. However, there i s no microsegregation o f chromi-um atoms i n s l o w l y cooled s o l i d s l o l u t i o n f e r r i t e specimen. The r o l e of d i s l o c a t i o n s i n the microsegregation of chromium atoms was discussed then.

1 - INTRODUCTION

The m e c h a n i c a l p r o p e r t i e s o f a l l o y s t e e l s a r e v e r y s e n s i t i v e t o t h e a l l o y i n g elements i n s t e e l s , among which chromium i s a fundamentel element. And t h e r e f o r e i t i s one o f the most i m p o r t a n t problems t o c l a r i f y the behavior of chromium atoms i n s t e e l s . In t h i s i n v e s t i g a t i o n , the o b j e c t i v e i s t o study on microsegregation i n Fe - Cr a l l o y system f o r the fundamental understanding of chromium s t e e l s .

H i t h e r t o , i n Fe - Cr system, 14-75at.%Cr a l l o y s have been i n v e s t i g a t e d and spinodal decomposition was o b s e r v e d [ l - 3 ] , but i n t h i s study 0-10at.%Cr a l l o y s are of main i n t e r e s t because the phase diagram is s t i l l not c l e a r . The a l l o y was treat t r e a t e d and observed by the atom-probe (AP).

2 - EXPERIMENTAL PROCEDURE

The m a t e r i a l _ used i s i r o n - c h r o m i u m a l l o y w h i c h was p r e p a r e d from h i g h p u r i t y m a t e r i a l s by an a r c - m e l t i n g o f c o n s t i t u e n t s i n a p u r i f i e d Ar atomosphere. The c h e m i c a l c o m p o s i t i o n i s 5.59at.%Cr, 0.014at.%0, 0.028at.%C and i r o n i s b a l a n c e d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984967

C9-404 JOURNAL DE PHYSIQUE

A f t e r d r a w n 0.3mmb w i r e s , t h e specimens were s o l u t i o n t r e a t e d a t 1150°C f o r I h r , t h e n quenched i n t o water. The t e m p e r i n g was p e r f o r m e d a t 150-650°C f o r I h r and t h e w i r e s w e r e w a t e r quenched. T h e o t h e r t r e a t m e n t was w a t e r q u e n c h i n g f r o m 6 5 0 ' ~ a f t e r s l o w c o o l i n g f r o m 1 150°C ( k e e p i n g f o r 1 hr). The f i e l d i o n m i c r o s c o p y ( F I M ) specimens were made b y e l e c t r o l y t i c p o l i s h i n g . F I N o b s e r v a t i o n was p e r f o r m e d a t b o t h 80K and 20K, and Ne gas was used f o r imaging. The atom-probe m i c r o a n a l y s i s was p e r f o r m e d a t 20K under Ne gas atornosphere.

3 - EXPERILIENTAL RESULTS AND DISCUSSION

I n Fig.1, t h e t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s show m i c r o s t r u c t u r e s o f t h e s p e c i m e n s , ( a ) s o l u t i o n a n n e a l e d a t 1150°C f o r I h r a n d quenched, ( b ) t e m p e r e d a t 450°C f o r I h r a n d q u e n c h e d a n d ( c ) t e m p e r e d a t 650°C f o r 1 h r and quenched. T h o s e s t r u c t u r e s a r e a 1 1 m a r t e n s i t e and a f t e r h e a t i n g a t 6 5 0 ' ~ m i c r o - p r e c i p i t a t e s were o b s e r v e d a l o n g t h e d i s l o c a t i o n l i n e . F i g . 2 shows F I M i m a g e s o f t h e s p e c i m e n s c o r r e s p o n d i n g t o F i g . 1 (a), ( b ) a n d (c), r e s p e c t i v e l y . I n F i g . 2 ( a ) o n l y b r i g h t a r e a was o b s e r v e d , b u t i n F i g . 2 ( b ) a n d ( c ) b o t h b r i g h t a n d d a r k a r e a s w e r e observed. F i g u r e 3 shows t h e r e s u l t s o f AP c h e m i c a l a n a l y s i s o f specimen s o l u t i o n t r e a t e d . T h e n u m b e r o f c h r o m i u m i o n s d e t e c t e d a r e p l o t t e d v e r s u s t o t a l i o n s e v a p o r a t e d i n (a), and t h e c o n c e n t r a t i o n o f c h r o m i u m i o n s p e r e v e r y 40 e v a p o r a t e d i o n s a r e p l o t t e d v e r s u s n u m b e r o f t o t a l d e t e c t e d i o n s i n ( b ) a n d t h e a v e r a g e c o n c e n t r a t i o n o f chromium was 5.37 .at.Z w h i c h was t h e same as c h e m i c a l a n a l y s i s . F i g u r e 4 shows t h e same measurements as Fig.3 f o r t h e specimens t e m p e r e d a t 450°C.

The b r i g h t a r e a s a n d d a r k a r e a s h a d t h e d i f f e r e n t c h r o m i u m c o n c e n t r a t i o n s . The c h r o m i u m c o n c e n t r a t i o n was 5.50at.Z f o r d a r k areas and 4.52at.Z f o r b r i g h t areas.

F i g u r e 5 s h o w s t h e r e s u l t s o f AP a n a l y s i s f o r t h e s p e c i m e n t e m p e r e d a t 650°C f o r I h r . I n t h i s case t h e d i f f e r e n c e . o f chromium c o n c e n t r a t i o n i n c r e a s e d between b r i g h t and d a r k areas. F i g u r e 6 shows t h e chromium c o n c e n t r a t i o n s o f b o t h d a r k and b r i z h t a r e a s as t h e f u n c t i o n o f t e m p e r i n g t e m p e r a t u r e .

To c l a r i f y t h e m e c h a n i s m o f m i c r o s e g r e g a t i o n s , t w o k i n d s o f e x p e r i m e n t s w e r e p e r f o r m e d . The f i r s t e x p e r i m e n t i s s i m u l t a n e o u s o b s e r v a t i o n s o f a t i p b y b o t h t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y and F I b l o b s e r v a t i o n . The second e x p e r i m e n t i s t h e c o m p a r i s o n o f t h e d i s t r i b u t i o n o f c h r o m i u m a t o m s b e t w e e n q u e n c h e d a n d t e m p e r e d specimens and s l o v r l y c o o l e d specimens. I n t h e f i r s t e x p e r i m e n t , as shown i n Fig.7, t h e d a r k a r e a s i n an FIM image seem t o be c o r e s p o n d e n t t o t h e i r r e g u l a r i t y a t t h e s u r f a c e . On t h e f o r m a t i o n o f s u c h i r r e g u l a r i t y a t t h e s u r f a c e , t h e r e a r e t w o

Fig. 1 - T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s o f iron-5at.Zchromium a l l o y (a) s o l u t i o n

annealed a t 1150°C f o r I h r and quenched, (b) tempered a t 4 5 0 ' ~ f o r I h r and quenched

and (c) tempered a t 6 5 0 ' ~ f o r I h r and quenched.

Fig. 2 - F i e l d i o n m i c r o g r a p h s of iron-5at.%chromium a l l o y (a) s o l u t i o n annealed a t 1 1 5 0 ~ ~ f o r I h r a n d quenched, ( b ) t e m p e r e d a t 4 5 0 ' ~ f o r I h r a n d quenched, ( c ) tempered a t 6 5 0 ' ~ f o r l h r and quenched.

1150°C x Ihr W.Q.

NUMBER OF DETECTED Fe AND C r IONS

F i g . 3 - C o m p o s i t i o n p l o f i l e s o f c h r o m i u m f o r iron-5at.%chromium a l l o y s o l u t i o n a n n e a l e d a t 1 150°C f o r I h r a n d quenched. ( a ) Number o f d e t e c t e d c h r o m i u m i s n s v e r s u s number o f d e t e c t e d ions. (b) C o n c e n t r a t i o n o f chromium p e r 40 e v a p o r a t e d i o n s v e r s u s number o f d e t e c t e d ions.

p o s s i b i l i t i e s . The one i s p r e f e r e n t i a l f i e l d e v a p o r a t i o n o f chromium atoms.since

e v a p o r a t i o n v o l t a g e i s 3.OV/A f o r Cr+' a n d 3.4V/A f o r ~ e + ~ . The o t h e r i s

p r e f e r e n t i a l e v a p o r a t i o n o f h i g h s t r e s s f i e l d o f d i s l o c a t i o n . I f we c o n s i d e r t h e

d i f f u s i o n d i s t a n c e o f c h r o m i u m a t o m s , a b s o r p t i o n o f q u e n c h e d - i n v a c a n c i e s b y

d i s l o c a t i o n w o u l d p r o d u c e t h e m i c r o s e g r e g a t i o n o f c h r o m i um atoms. O r o t h e r w i s e

d i s l o c a t i o n p i p e d i f f u s i o n w o u l d a s s i s t t o f o r m chrolnium r i c h areas. Thus t h e r o l e

o f d i s l o c a t i o n s i n m a r t e n s i t e s t r u c t u r e s i s c o n s i d e r e d t o be v e r y e f f e c t i v e i n

f o r m i n g c h r o m i u m r i c h a r e a s . I n t h e s e c o n d e x p e r i m e n t i t was r e l e a v e d t h a t i n

s l o w l y c o o l e d s o l i d s o l u t i o n f e r r i t e , a s s h o w n i n F i g . 8 a n d F i g . 9 t h e r e i s no

JOURNAL DE PHYSIQUE

-

- Fe - ^5at.%Cr - 1150°C'x l h r W.Q.

+450°C x l h r W.Q. Dark a r e a

I Dark area

It B c i g h t a r e a

NUMBER OF DETECTED F e AND Cr IONS

Fig. 4 - C o m p o s i t i o n p l o f i l e s o f c h r o m i u m f o r b r i g h t and d a r k a r e a s i n i r o n - 5at.%chromium a l l o y tempered a t 450°C f o r I h r and quenched. (a) Number o f d e t e c t e d chromium i o n s v e r s u s number o f d e t e c t e d ions. (b) C o n c e n t r a t i o n o f chromium p e r 40 e v p o r a t e d i o n s v e r s u s number o f d e t e c t e d ions.

Fe - ^5at.%Cr

1150QC x l h r W.Q.

+ ^650°C x l h r W.Q.

Dark area

0 200 400 600 800 1000

NUMBER OF DETECTED Fe AND Cr IONS

Fig. 5 - C o m p o s i t i o n p l o f i l e s o f c h r o m i u m f o r b r i g h t a n d d a r k a r e a s i n i r o n -

5at.%chrornium a l l o y tempered a t 650°C f o r l h r and quenched.

h z

³

W

s ²

0

* 1

"

0

0 200 400 600

RE-HEATING TEMPERATURE

C9-407

Dark area

CL--- --. --- /

- \

B r i g h t area

, , I t

F i g . 6 - T e m p e r i n g t e m p e r a t u r e dependence o f s e g r e g a t i o n o f chromium.

Fig. 7 - R e l a t i o n between t i p o f iron-5at.%chrorniurn tempered a t 45Q0c f o r I h r and quenched and i t s f i e l e d i o n image.

Fig. 8 - ~ i e l d i o n m i c r o g r a p h o f i r o n - 5 a t . Z c h r o r n i u m a l l o y s o l u t i o n a n n e a l e d a t

1 1 5 0 ~ ~ f o r I hr, s l o w l y cooled t o 650'~. k e p t f o r I h r and quenched.

JOURNAL DE _PHYSIQUE

Fe - ^5at.%Cr

1150°C x l h r + 6 5 O 0 C F.C.

+ 650°C x l h r W.Q.

NUMBER OF DETECTED Fe AND C r IONS

Fig. 9 - C o m p o s i t i o n p r o f i l e s o f c h r o m i um f o r iron-5at.%chromium a l l o y s o l u t i o n annealed a t 1150°C f o r I h r , s l o w l y c o o l e d t o 650°C, k e p t f o r I h r and quenched.

s e g r e g a t i o n as t h e specimen quenched-in f r o m 1150°C. T h e r e f o r e t h e m i c r o s e g r e g a t i o n o c c u r s o n l y when t h e s t r u c t u r e i s t e m p e r e d m a r t e n s i t e i n w h i c h h i g h d i s l o c a t i o n d e n s i t y i s i n c l u d e d and a n n i h i l a t i o n o f excess v a c a n c i e s occurs. T h i s r e s u l t s a l s o suggest t h e r o l e o f d i s l o c a t i o n i s v e r y i m p o r t a n t i n m i c r o s e g r e g a t i o n o f chromium atoms. F u r t h e r m o r e , i n phase d i a g r a m a t 400-600°C t h e s o l i d s o l u t i o n i s c o n s i d e r e d t o be s t a b l e v h o w e v e r t h e m i c r o s e g r e g a t i o n c o u l d become a p p a r e n t r y p o s s i b l e when t h e s o l i d s o l u t i o n i n c l u d e v e r y h i g h i n t e r n a l e n e r g y due t o h i g h d i s l o c a t i o n d e n s i t y .

4 - SUMMARY

( 1 ) A t l o w c o n c e n t r a t i o n o f chromium (5at.%), i n t h e specimens s o l u t i o n t r e a t e d and tempered, d a r k and b r i g h t a r e a s were o b s e r v e d i n F I M images and m i c r o s e g r e g a t i o n of chromium atoms was d e t e c t e d b y t h e AP.

(2) B o t h a f t e r s o l u t i o n t r e a t m e n t a n d a f t e r s l o w c o o l i n g , o n l y b r i g h t a r e a was o b s e r v e d i n FIP4 images and no m i c r o s e g r e g a t i o n was d e t e c t e d b y t h e AP.

( 3 ) T h e c o n t r i b u t i o n o f d i s l o c a t i o n s t o m i c r o s e g r e g a t i o n o f c h r o m i u m a t o m s was d i s c u s s e d .

REFERENCES

[I] Denys, T. and G i e l e n , M. P ~ i e t a l l u r g i c a l T r a n s a c t i o n s 2(1971) 1423

[ 2 ] B r e n n e r , S. S., P l i l l e r , P.1. K. and S o f f a , W. A. S c r i p t a M e t a l l u r g i c a 1 6 ( 1 9 8 2 ) 871 --.

[ 3 ] M i l l e r , M. K., B r e n n e r , S. S., Cammus, P. P., P i l l e r , J. a n d S o f f a , W. A. Proc.

2 9 t h IFES, Sweden. (1 982) 489