GP-ZONE OF MOLYBDENUM IN STEEL REVEALED BY FIM-AP

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GP-ZONE OF MOLYBDENUM IN STEEL REVEALED BY FIM-AP

S. Stato, O. Nishikawa, N. Igata

To cite this version:

S. Stato, O. Nishikawa, N. Igata. GP-ZONE OF MOLYBDENUM IN STEEL REVEALED BY FIM- AP. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-441-C6-446. �10.1051/jphyscol:1987672�.

�jpa-00226880�

JOURNAL DE PHYSIQUE

Colloque C6, supplbment au n O 1 l , Tome 48, novembre 1987

GP-ZONE OF MOLYBDENUM IN STEEL REVEALED BY FIM-AP

Department of Materials Science, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

a ate rials

**Department of Materials Science and Technology, Science University of Tokyo a t present, 2641 Yamazaki, Noda-City,Chiba-Prefecture 278, Japan

ABSTRACT: T h e r e are many k i n d s o f a t o m i s t i c s t r u c t u r e t o b e c l a r i f i e d i n c o m m e r c i a l a l l o y s , e s p e c i a l l y i n s t e e l s . I n t h i s i n v e s t i g a t i o n t h e o b j e c t i v e i s t o c l a r i f y t h e b e h a v i o r o f molybdenum a t o m s i n 19Cr- l M o ( a t . % ) s t e e l , w h i c h w a s s o l u t i o n a n n e a l e d a t llOO°C f o r l h r a n d a g e d a t 6000C f o r l h r . The r e s u l t showed t h a t t h e r e w e r e t w o g r o u p s o f Mo GP z o n e s w i t h d i a m e t e r s o f a b o u t 70A a n d 130A. T h e t o t a l d e n s i t y a s 5 . 1 7 ~ 1 0 ~ ~ / c m 3 a n d t h e z o n e s w e r e f o r m e d o n ( 1 0 0 ) . T h e a t o m i c s i t e s o f a GP z o n e w a s n o t f u l l y o c c u p i e d by molybdenum a t o m s . T h e c o n t r i b u 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 w a s b r i e f l y d i s c u s s e d .

INTRODUCTION

S i n c e 1961, t h e r e h a s b e e n a r e p o r t o n t r a n s m i s s i o n e l e c t r o n microscopic(TEM) o b s e r v a t i o n o f GP z o n e s o f molybdenum i n s t e e l s [ l ] . B u t h i t h e r t o , t h e r e h a s b e e n n o r e p o r t o n o b s e r v a t i o n o f GP z o n e i n steel by f i e l d i o n microscopy(F1M) a n d atom probe(AP), a l t h o u g h GP z o n e s o f c o p p e r i n aluminum[2-41 a n d c o p p e r i n t i t a n i u m [ 5 ] w e r e o b s e r v e d .

I n t h i s s t u d y , t h e o b j e c t i v e i s t o i d e n t i f y m o l y b d e n u m GP z o n e i n h e a t t r e a t e d 19Cr-lMo(at.%) s t e e l by b o t h FIM-AP a n d TEM a n d t o c l a r i f y t h e a t o m i s t i c s t r u c t u r e o f t h e zone.

EXPERIMENTAL METHOD

The m a t e r i a l u s e d w a s 19Cr-lMo(at.%) steel a n d i t s c h e m i c a l c o m p o s i t i o n i s s h o w n i n T a b l e 1. S m a l l s p e c i m e n s o f s i z e 0.5x0.5x20mm3 w e r e c u t o u t by a e l e c t r i c a l d i s c h a r g e method. The s p e c i m e n s w e r e s o l u t i o n t r e a t e d a t llOO°C f o r l h r a n d a g e d a t 6000C f o r l h r . FIM t i p s w e r e made f r o m t h e s e s p e c i m e n s . FIM t i p s w e r e o b s e r v e d f i r s t by TEM a n d t h e n by FIM. T h e FIM o b s e r v a t i o n w a s p e r f o r m e d a t 30-78K a n d t h e i m a g i n g g a s w a s He o f 2 ~ 1 0 - ~ t o r r . T h e c h e m i c a l a n a l y s i s w a s p e r f o r m e d by AP.

T a b l e 1 C h e m i c a l c o m p o s i t i o n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987672

C6-442 JOURNAL

DE

RESULTS AND DISCUSSION

A TEM m i c r o g r a p h o f h e a t t r e a t e d s p e c i m e n i s shown i n Fig.1. The s t r u c t u r e i s f e r r i t e a n d a l m o s t n o d i s l o c a t i o n i s o b s e r v e d , h o w e v e r s m a l l l o o p c o n t r a - s t ( > 1 0 0 A ) a n d b l a c k d o t s ( < l O O A ) a r e o b s e r v e d o n (100) a s r e p o r t e d p r e v i o u s l y [ l ] . The d e n s i t y o f b o t h t h e l o o p s a n d t h e b l a c k d o t s a n d t h e i r a v e r a g e d i s t a n c e s are s h o w n i n T a b l e 2. T h e n u m b e r d e n s i t y o f l o o p s w i t h d i a m e t e r o f a b o v e lOOA w a s t h r e e times h i g h e r t h a n t h a t of b l a c k d o t s w i t h d i a m e t e r below 100A.

A FIM m i c r o g r a p h o f t h e s a m e s p e c i m e n i s shown i n Fig.2. The c e n t r a l r i n g i n t h e i m a g e i s a (110) p o l e . When t h e a p p l i e d v o l t a g e was 9.4kV a n i m a g e c o n s i s t i n g o f b r i g h t s p o t s w a s o b s e r v e d b u t w h e n i t w a s b e l o w 6.5kV many b r i g h t s p o t s d i s a p p e a r e d a n d b r i g h t s p o t a r r a y s a n d o t h e r l o c a l b r i g h t s p o t s w e r e o b s e r v e d . T h e s e a r r a y s s e e m t o c o r r e s p o n d t o s m a l l l o o p s o r b l a c k d o t s o n ( 1 0 0 ) i n TEM m i c r o g r a p h s , The b r i g h t s p o t a r r a y s c o r r e s p o n d t o a r r a y s o f p r o t r u d i n g a t o m s a n d t h e r e f o r e t h e p r e s e n c e o f l a r g e r a t o m s t h a n i r o n o r chromium. By f i e l d e v a p o r a - t i o n o f s e v e r a l t e n s o f a t o m i c l a y e r s , i t w a s p r o v e d t h a t t h e b r i g h t s p o t s e x t e n d e d i n t w o d i m e n s i o n s a n d f o r m e d d i s c s a s shown i n Fig.3. Those b r i g h t s p o t a r r a y s w e r e d e t e c t e d 4 5 times a n d are t h u s c o n f i r m e d t o e x i s t .

F i g . 1 TEM m i c r o g r a p h o f h e a t t r e a t e d specimen.

T a b l e 2 The d e n s i t y a n d a v e r a g e d i s t a n c e o f l o o p s a n d b l a c k d o t s , as d e t e r m i n e d by TEM.

Loop S i z e (A)

>

<

D e n s i t y 1 . 3 2 x lo15 3 . 8 5 x 1015 5 . 1 7 x 1015

Average D i s t a n c e (A) 912

6 3 8 5 7 8

Fig. 2 FIM i m a g e s u n d e r t h e a p p l i e d v o l t a g e o f 9.4kV(a) a n d 6.5kV(b).

evaporate l a y e r s

F i g . 3 T h e c h a n g e o f b r i g h t s p o t s a r r a y s t h r o u g h e v a p o r a t i o n o f a t o m i c p l a n e s .

C6-4 44 JOURNAL DE PHYSIQUE

An example of a c h e m i c a l a n a l y s i s of a b r i g h t s p o t a r r a y by TOF-AP is shown i n Fig.4. The number o f a t o m s o f e a c h e l e m e n t i s p l o t t e d a g a i n s t t h e t o t a l number o f a t o m s d e t e c t e d t h r o u g h t h e p r o b e h o l e f r o m a b r i g h t s p o t a r r a y i n c l u d i n g t h e s u r r o u n d i n g a r e a . ( a ) s h o w s t h e r e s u l t f o r i r o n a n d c h r o m i u m a t o m s , ( b ) s h o w s t h a t f o r t h e o t h e r e l e m e n t a t o m s . A b o u t 3 0 a t o m s e v a p o r a t e d c o r r e s p o n d t o o n e a t o m i c p l a n e . The r e s u l t s s h o w t h a t i r o n a n d c h r o m i u m a t o m s a r e d i s t r i b u t e d u n i f o r m l y , b u t molybdenum a t o m s a r e h i g h l y c o n c e n t r a t e d i n t h e b r i g h t s p o t a r r a y . I n t h i s a n a l y s i s t h e number o f aluminum, s u l f u r and niobium a t o m s was n e g l i g i b l e . I n t h e b r i g h t s p o t a r r a y r e g i o n t h e t o t a l number of d e t e c t e d Fe,Cr,Mo a t o m s w e r e 167,38 and 16 r e s p e c t i v e l y . S i n c e t h e a t o m i c s i z e of molybdenum atom is 2.728 and a b o u t 10% l a r g e r t h a n i r o n o r chromium a t o m , i t i s e a s i l y understood t h a t b r i g h t s p o t s a r r a y i s f o r m e d by p r o t r u d i n g s u r f a c e a t o m s s u c h a s molybdenum a t o m s . T h e r e f o r e i f we assume t h e b r i g h t s p o t a r r a y comes from t h e c l u s t e r o f molybdenum a t o m s i n one a t o m i c p l a n e , t h e c o n c e n t r a t i o n of molybdenum a t o m s c a n b e e s t i m a t e d t o be a b o u t 50% f r o m t h e number o f a t o m s d e t e c t e d t h r o u g h probe hole. Molybdenum b e a r i n g s t e e l i s c o n s i d e r e d t o c o n t a i n molybdenum r i c h GP z o n e s o r Fe2Mo L a v e s p h a s e s , depending on h e a t t r e a t m e n t . J.Higgins d e t e c t e d Fe2Mo Laves p h a s e by h e a t t r e a t m e n t a t 5000C f o r 5 0 0 h r s [ 6 ] . I t i s n e c e s s a r y , a t f i r s t , t o d e t e r m i n e t h e p l a n e o f t h e b r i g h t s p o t a r r a y t o d i s t i n g u i s h between GP zone and Laves phase.

200.

- ^(A) Fe-1 gat. %Cr-1 at. %Mo 11000Cxlhr(W.Q. 1

-

120 -

" 0 50 100 150 200 250 300

T O T A L NUMBER O F ATOMS

Fig. 4 An example o f c h e m i c a l a n a l y s i s of b r i g h t s p o t a r r a y .

By p l o t t i n g [ l o o ) s u r f a c e t r a c e s , i n t h e FIM i m a g e , it c a n b e c h e c k e d w h e t h e r t h e b r i g h t s p o t p l a n e s l i e o n { l o o ) o r n o t . A s s h o w n i n F i g . 5 i t w a s r e r e a l e d t h a t a l l b r i g h t s p o t a r r a y s a r e on { l o o ] s u r f a c e t r a c e s . I n t h e c a s e o f Fe2Mo t h e f o l l o w i n g l a t t i c e r e l a t i o n s h o u l d e x i s t [ 6 ] :

B u t t h e r e s u l t i s d i f f e r e n t f r o m t h e a b o v e r e l a t i o n , a n d a l s o t h e b r i g h t s p o t a r r a y a r e o f m o n o l a y e r t h i c k n e s s . T h e r e f o r e i t i s c e r t a i n t h a t t h e b r i g h t s p o t a r r a y s a r e GP z o n e s o f molybdenum.

The s i z e d i s t r i b u t i o n o f GP z o n e s o b t a i n e d f r o m 4 5 m e a s u r e m e n t s i s shown i n Fig.6. The number d e n s i t y o f GP z o n e s i s p l o t t e d f o r e a c h 10A d i a m e t e r i n t e r v a l . A s shown i n t h i s f i g u r e t h e r e a r e t w o p e a k s a t a b o u t 70A a n d a b o u t 130A. The same r e s u l t s w e r e o b t a i n e d i n TEM o b s e r v a t i o n s . I f we d e n o t e t h e number d e n s i t y o f z o n e s a b o v e lOOA i n FIM o r l o o p s i n TEM, a n d z o n e s below lOOA i n FIM o r b l a c k d o t s i n TEM, a s NL a n d Ns, r e s p e c t i v e l y , t h e r a t i o o f t h o s e t w o v a l u e s c a n b e compared f o r b o t h FIM a n d TEM:

T h i s r e s u l t s h o w s t h a t l o o p s a n d b l a c k d o t s o b s e r v e d i n TEM c o r r e s p o n d t o GP z o n e s o f l a r g e r s i z e a n d s m a l l e r s i z e , r e s p e c t i v e l y , o b t a i n e d i n FIM.

A s f o r t h e a t o m i c s t r u c t u r e o f t h e GP z o n e s , a s s h o w n i n F i g . 7 t h e b r i g h t s p o t s a r e n o t c o n t i n u o u s . T h i s means t h a t t h e c o n t i n u o u s d i s t r i b u t i o n o f molybde- num a t o m s i s n o t e n e r g i c a l l y s t a b l e b e c a u s e o f t h e 10% d i f f e r e n c e o f a t o m i c s i z e b e t w e e n iron(2.48A), chromium(2.50A) a n d molybdenum(2.72A). A t r i a l a t o m i c model o f GP z o n e s o f m o l y b d e n u m i s s h o w n i n F i g . 3 . I t i s c o n s i d e r e d t h a t i r o n o r c h r o m i u m a t o m s a r e m i x e d by r e p l a c i n g w i t h v a c a n c i e s o r t h a t m o l y b d e n u m a t o m s s e g r e g a t e w i t h v a c a n c i e s .

smaller

Size l a r g e r

7

S I Z E O F G P - Z O N E ( A )

F i g . 5 The t r a c e o f { l o o } a t o m i c p l a n e F i g . 6 S i z e d i s t r i b u t i o n o f GP z o n e s

i n FIM image. f r o m 4 5 measurement.

C6-446 JOURNAL DE PHYSIQUE

I t i s e s t i m a t e d t h a t t h e c o n c e n t r a 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 a f t e r q u e n c h i n g f r o m llOOOC i s 3 . 1 3 ~ 1 0 - 5 i f we a s s u m e t h a t t h e e n e r g y o f v a c a n c y f o r m a t i o n of t h i s a l l o y i s 1.4eV. T h i s v a l u e i s v e r y c l o s e t o t h e c o n c e n t r a t i o n o f GP z o n e s i t e s , 4.5x10-~. The r o l e o f v a c a n c i e s i n f o r m i n g GP z o n e i s t h u s c o n s i d e r e d t o be i m p o r t a n t .

Fig. 7 A t o m i s t i c o b s e r v a t i o n o f molybdenum atoms i n GP zone.

SUMMARY

The m i c r o s t r u c t u r e of 19Cr-lMo(at.%) s t e e l , which was s o l u t i o n annealed and aged, was i n v e s t i g a t e d by TEM, FIM and AP and t h e f o l l o w i n g r e s u l t s were o b t a i n e d . (1) I n TEM image, l o o p s and b l a c k d o t s w e r e observed on {loo} and t h e a v e r a g e s i z e

of l o o p s was 135A and t h a t o f b l a c k d o t s was 70A.

( 2 I n FIM i m a g e b r i g h t s p o t a r r a y s w e r e f o u n d a l o n g { l o o } a n d t h e i r s i z e d i s t r i b u t i o n s showed two peaks i n 70A and 135A. These r e s u l t s corresponded t o t h o s e o b t a i n e d by TEM.

(3) By AP measurement, i t was r e v e a l e d t h a t molybdenum a t o m s w e r e c o n c e n t r a t e d t o b r i g h t s p o t a r r a y s .

( 4 ) The a b o v e r e s u l t s show t h a t b r i g h t s p o t a r r a y s a r e GP z o n e s o f molybdenum and t h e a c t u a l arrangement of molybdenum a t o m s i n GP zone i s shown.

(5) S i n c e t h e c o n c e n t r a 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 i s n e a r l y e q u a l w i t h t h a t o f GP zone s i t e s , t h e r o l e of vacancy i n t h e f o r m a t i o n of GP zones seems t o be l a r g e .

REFERENCE

[ l ] E.Hornbogen : J o u r n a l o f Applied P h y s i c s , 32,135(1961)

[ 2 ] T.Mori, M.Wada, H.Kita, R.Uemori, S.Horie, A.Sato and 0.Nishikawa : J a p a n e s e J o u r n a l of Applied P h y s i c s , 22,L203(1983)

[ 3 ] T.Abe, K.Miyazaki and K.Hirano : Acta M e t a l l . , 30, 357(1982)

[4] K.Hono, T.Satoh and K.Hirano : P h i l o s o p h i c a l Magazine A , 53, 495(1986) [ 5 ] L.Hadjadj, A.Menand and D.Blavette : J o u r n a l de Physique : C7-281(1986) [6] J . H i g g i n s and P.Wilkes : P h i l . Mag., 25, 599(1972)