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THE MICROSTRUCTURE OF THE PRECIPITATES IN A Ni-Be ALLOY, AS STUDIED WITH
ATOM-PROBE FIM
Z. Liu, Y. Cao, D. Feng
To cite this version:
Z. Liu, Y. Cao, D. Feng. THE MICROSTRUCTURE OF THE PRECIPITATES IN A Ni-Be ALLOY,
AS STUDIED WITH ATOM-PROBE FIM. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-
343-C6-348. �10.1051/jphyscol:1987656�. �jpa-00226861�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6ment au nO1 1, Tome 48, novembre 1987
THE MICROSTRUCTURE OF THE PRECIPITATES I N A Ni-Be ALLOY, AS STUDIED WITH ATOM-PROBE FIM
Z.G. Liu, Y.N. Cao and D. Feng
Institute of Solid State Physics, Nanjing University, Nanjing, P.R. China
A b s t r a c t
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The m i c r o s t r u c t u r e , c h e m i s t r y and m o r p h o l o g y o f a decomposed Ni-12atXBe a l l o y were i n v e s t i g a t e d w i t h atom-probe FIM. The 8 - p r e c i p i t a t e s were r e v e a l e d t o be d i s c shaped. The o r i e n t a t i o n r e l a t i o n s h i p between t h e 4 - m a t r i x ( A l ) and t h e 8- p r e c i p i t a t e s (82) was d e t e r m i n e d . Atom-probe a n a l y s e s showed t h a t t h e i n t e r p h a s e b o u n d a r i e s a r e v e r y s h a r p . On t h e i n t e r f a c e s some l e d g e were r e v e a l e d . The r e s u l t s were i n t e r p r e t e d i n t e r m s o f G i b b s - W u l f f theorem and g r o w t h l e d g e mechanism.I
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INTRODUCTIONBelow. 1200l< t h e e q u i l i b r i u m m i c r o s t r u c t u r e o f Ni-12at%Be c o m p r i s e s f .c.c. @-phase and 82 o r d e r e d NiBe B-phase /I/. The sequence and m o r p h o l o g y o f t h e p r e c i p i t a t i o n i n t h i s a l l o y have n o t been e x t e n - s i v e l y s t u d i e d , e s p e c i a l l y i n t h e t e m p e r a t u r e r a n g e b e l o w 1000K.
T h e r e a r e s t i l l some d i s c r e p a n c i e s i n t h e commonly u s e d phase d i a - grams /1-2/.
When t h e p r e c i p i t a t e s and t h e m a t r i x have d i f f e r e n t c r y s t a l s t r u c - t u r e s , such as t h e 0(-phase and 8-phase i n t h i s a l l o y , i t i s i n v a r i - a b l y f o u n d t h a t t h e r e i s d e f i n i t i v e o r i e n t a t i o n r e l a t i o n s h i p between t h e phases. The m i c r o s t r u c t u r e o f t h e i n t e r p h a s e b o u n d a r i e s and t h e a n i s o t r o p y o f t h e i n t e r f a c i a l e n e r g y w i l l c o n t r o l t h e e q u i l i b r i u m shape o f t h e p r e c i p i t a t e s /3/. I t seems v e r y i n t e r e s t i n g t o i n v e s t i - g a t e t h e m o r p h o l o g y o f t h e p r e c i p i t a t e s and t h e s t r u c t u r e o f t h e i n t e r p h a s e b o u n d a r i e s i n t h i s a l l o y .
I t has r e s e n t l y been d e m o n s t r a t e d t h a t f i e l d i o n m i c r o s c o p y , i n con- j u n c t i o n w i t h atom-probe m i c r o a n a l y s i s , i s a p o w e r f u l t o o l f o r t h e i n v e s t i g a t i o n o f t h e e a r l y s t a g e s o f t h e d e c o m p o s i t i o n p r o c e s s e s /4/.
I n t h e work p r e s e n t e d h e r e , atom-probe FIM was u s e d t o s t u d y t h e B- p r e c i p i t a t e s i n a N i - 1 2 a t Z B e a l l o y .
II
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INSTRUMENTATIONA new F I M i n c o n j u n c t i o n w i t h ToF atom-probe was r e s e n t l y c o n s t r u c t e d i n The I n s t i t u t e o f S o l i d S t a t e P h y s i c s , N a n j i n g U n i v e r s i t y . The m a i n f e a t u r e s o f t h e i n s t r u m e n t a r e shown i n f i g u r e 1 and b r i e f l y summa-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987656
JOURNAL DE PHYSIQUE
r i z e d below.
The vacuum system o f t h e FIM c o n s i s t s o f two chambers. The specimens a r e l o a d e d i n t o t h e s t o r a g e chamber where up t o 1 0 can be k e p t . A specimen can t h e n be t r a n s f e r r e d t o t h e a n a l y s i s chamber, which has l i q u i d n i t r o g e n c o o l i n g and m a n i p u l a t i o n f a c i l i t i e s and a l l o ~ ~ a n a l y - s i s by atom-probeTOThe main chamber vacuum i s t y p i c a l l y 3x10 T o r r and reaches 5x10 T o r r when t h e s u b l i m a t i o n pump i s o p e r a t e d . The i n s t r u m e n t i s equipped w i t h a 70 mm s i n g l e m i c r o c h a n n e l p l a t e w i t h a 2 mm d i a m e t e r a p e r t u r e as image a m p l i f i e r , and has a 0-30kV h i g h v o l t a g e s u p p l y f o r f i e l d i o n i z a t i o n . The FIM has h e l i u m , neon and hydrogen c o n t r o l l e d by t h e l e a k v a l v e s as image gas. The d i s t a n c e o f t h e FIM c h a n n e l p l a t e t o t h e specimen may be adjustQd from 40 mm t o 160 mm t o p r o v i d e a v a r i a b l e e f f e c t i v e probe a p e r t u r e f o r c h a n g i n g t h e a r e a o f a n a l y s i s . The ToF mass s p e c t r o m e t e r c o n s i s t s o f a s t r a i - g h t f l i g h t tube w i t h a f l i g h t p a t h o f 180 cm, an E i n z e l l e n s , a 25 mm double c h a n n e l p l a t e s i n g l e atom d e t e c t o r and a computer c o n t r o l l e d 4 c h a n n e l t i m i n g system w i t h t h e t i m e r e s o l u t i o n o f 5 ns. The atom- p r o b e i s m o n i t o r e d by an IBM PC-XT microcomputer equipped w i t h a m o n i t o r , a g r a p h i c s p r i n t e r and a p l o t t e r . Under t y p i c a l c o n d i t i o n s
( l a r g e s t acceptance a n g l e , 15% p u l s e f r a c t i o n ) , t h i s y i e l d s a mass r e s o l u t i o n o f m/Am=lOO.
"dc 0-30kV
1 OOM
D e t e c t o r S t o r a g e
Chamber
M a n i p u l a t o r
F i g u r e 1. Schematic diagram o f t h e atom-probe FIM a t N a n j i n g U n i v e r s i t y ,
I11
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EXPERIMENTAL METHODSThe a l l o y used i n t h i s work 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 (99.9%8e and 99.99XNi) by i n e r t gas e l e c t r i c a r c me t i n g I n a copper c r u c i b l e . A f t e r a n n e a l i n g a t 1273K f o r 5 hours, t h e i n g o t was drawn i n t o w i r e 0.2 ma i n d i a m e t e r . The w i r e was t h e n s o l u t i o n t r e a t e d f o r an h o u r a t 1273K i n an argon atmosphere and quenched i n t o water.
Atom-probe a n a l y s i s showed t h a t t h e a l l o y i s homogenized. The w i r e used were t h e n i s o t h e r m a l l y aged a t 843K f o r 30 h o u r s . The atom-probe specimens were s u b s e q u e n t l y p r e p a r e d by s t a n d a r d e l e c t r o p o l i s h i n g t e c h n i q u e s . To determine t h e m i c r o s t r u c t u r e and c o m p o s i t i o n o f d i f f e - r e n t phase zones i n t h e decomposed a l l o y , t h e F I M p e r s i s t e n c e s i z e t e c h n i q u e , t h e atom-probe s e l e c t e d area a n a l y s i s and t h e r e c o r d i n g o f t h e c u m u l a t i v e p r o f i l e were employed. The specimens were imaged w i t h neon a t 90K. The atom-probe a n a l y s e s were p e r f o r m e d i n t h e absence o f t h e imaging gas.
I V
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RESULTS AND DISCUSSIONA f i e l d i o n m i c r o g r a p h o f t h e homogenized a l l o y i s shown i n f i g u r e 2.
The a l l o y c o m p o s i t i o n as determined by atom-probe a n a l y s i s i s 11.62 0.6atXBe. I t i s i n good agreement w i t h t h e n o m i n a l c o m p o s i t i o n . No phase s e p e r a t i o n o r Be atom c l u s t e r s was d e t e c t e d .
F i g u r e 2.
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homogenized Neon image, about 90K.
F I M image o f t h e Ni-12atXBe a l l o y , t i p t e m p e r a t u r e
A n n e a l i n g t h e s u p e r s a t u r a t e d a l l o y a t 843K, l e a d s t o p r e c i p i t a t i o n . Some b r i g h t e r imaged d i s c s embedded i n t h e d a r k l y imaged m a t r i x were r e v e a l e d . The micrographs o f two p r e c i p i t a t e s a r e shown i n f i g u r e 3.
To determine t h e c h e m i c a l c o m p o s i t i o n o f t h e p r e c i p i t a t e s and t h e m a t r i x , t h e p r e b e h e l e was p o s i t i o n e d a t b r i g h t o r d a r k area and f o l l o w e d by atom-probe a n a l y s i s . The mass spectrum o f t h e d i s c shaped p r e c i p i t a t e s i s shown i n f i g u r e 4 a ) . The atom-probe s e l e c t e d a r e a a n a l y s e s a v e r a g i n g o v e r more t h a n 5000 i o n s r e v e a l e d t h a t t h e Be- c o n c e n t r a t i o n i n t h e b r i g h t e r a r e a i s 49+3at%. A c c o r d i n g t o t h e Ni-Be phase diagram, t h e e q u i l i b r i u m p r e c i p i t a t e s i n t h i s a l l o y a r e 8-phase w i t h 82 (CsC1) s t r u c t u r e . Our r e s u l t c o n f i r m e d t h a t t h e b r i g h t e r
i m a g i n g d i s c s a r e e q u i l i b r i u m R - p r e c i p i t a t e s . Because t h e Be atoms a r e commonly n o t d e t e c t a b l e by means o f energy d i s p e r s i v e X-ray ana- l y s i s , t h e s o l u b i l i t y l i m i t o f Be i n N i below 1000K has n o t been accurate3.y determined. The atom-probe s e l e c t e d a r e a a n a l y s e s avera- g i n g o v e r more t h a n 10000 i o n s r e v e a l e d t h a t t h e s o l u b i l i t y l i m i t o f Be i n N i a t 843K i s 7.29.4at%. T h i s v a l u e i s l a r g e r t h a n t h e v a l u e ( 3 . l a t X ) i n d i c a t e d i n Hansen's phase diagram /2/. The mass spectrum o f t h e d - m a t r i x i s shown i n f i g u r e 4 b ) .
I n o r d e r t o v i s u a l i z e t h e v a r i a t i o n o f t h e c h e m i c a l c o m p o s i t i o n w i t h d e p t h a c r o s s t h e i n t e r p h a s e boundary, succeeding mass a n a l y s e s were
C6-346 JOURNAL DE PHYSIQUE
F i g u r e 3.
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a ) Neon f i e l d i o n image o f a Ni-12atXBe a l l o y aged a t 843K f o r 30 hours. Two b r i g h t e r imaged 1 3 - p r e c i p i t a t e s embedded i n t o t h e m a t r i x can be seen.b ) Same a r e a 30 atomic l a y e r s deeper. A l e d g e i s i n d i c a t e d
.
w n ( A M v ) W n l A H W
a 1 b)
F i g u r e 4.
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a ) Mass spectrum o f t h e B-phase i n a Ni-12atXBe a l l o y aged a t 843K f o r 30 hours. 800 i o n s were c o l l e c t e d .b ) Mbss spectrum o f t h e OC-phase i n t h e same specimen, 800 i o n s .
F i g u r e 5.
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Atom-probe l a d d e r diagram t a k e n a c r o s s t h e i n t e r f a c e between oc- and 6- phase. The Ni-12atXBe a l l o y was aged a t 843K f o r 30 h o u r s / 5 / .V)
E
2 co- 4
I 0 1 d
(21 30-20-
10-
I I I I I I
0 20 L O 60 80 100 120
p e r f o r m e d w i t h t h e atom-probe
.
The concent r a t io n l a d d e r diagram r e c o r d e d i s shown i n f i g u r e 5. Because t h e a n a l y s e d c y l i n d e r i s n o t e x a c t l y p e r p e n d i c u l a r t o t h e boundary, t o reduce t h e number o f mixed l a y e r , t h e e f f e c t i v e p r o b e h o l e was m i n i m i z e d t o about 1 nm i n d i a - meter. 5-8 i o n s c o u l d be d e t e c t e d d u r i n g t h e e v a p o r a t i o n of each a t o m i c l a y e r . I t can be e s t i m a t e d t h a t t h e i n t e r p h a s e boundary i s v e r y sharp.D u r i n g r e p e a t e d f i e l d e v a p o r a t i o n , more t h a n 1 5 B - p r e c i p i t a t e s were observed. W i t h t h e h e l p o f t h e F I M p e r s i s t e n c e s i z e t e c h n i q u e , t h e morphology and s i z e o f t h e p r e c i p i t a t e s were determined. The average t h i c k n e s s a n d . d i a m e t e r o f t h e , b - d i s c s were e s t i m a t e d t o be 3.89.5 nm and 4027 nm r e s p e c t i v e l y . T h e i r aspect r a t i o i s about 10. T h e i r h a b i t p l a n e s were d e t e r m i n e d t o be on (100) p l a n e o f t h e m a t r i x . By c a r e - f u l l y a n a l y s i n g t h e p o l e s i n d i c a t e d by r i n g s t e p s on t h e m a t r i x and d i f f e r e n t p r e c i p i t a t e s , t h e o r i e n t a t i o n r e l a t i o n s h i p between t h e c ( - m a t r i x ( f .c.c. ) and t h e B - p r e c i p i t a t e s ( C s C 1 - s t r u c t u r e ) c o u l d be d e t e r m i n e d t o be
( o l O ) ( j l / (OlO),
, <lol>a// (ool)O(
/5/.F r i e d e r i c h e t a l . r e p o r t e d t h a t t h e l a t t i c e parameters o f d- and B- phases i n t h i s a l l o y a r e 0.351 nm and 0.264 nm r e s p e c t i v e l y . Accor- d i n g t o t h e observed o r i e n t a t i o n r e l a t i o n s h i p , t h e m i s f i t between t h e two phases on t h e ( l o o ) & h a b i t p l a n e a l o n g (101) i . e . ( 0 0 1 ) H d i r e c t i o n i s about 6%. A c c o r d i n g t o t h e t h e o r y o f m i s f i t d i s l o c a t i o n s /6-7/, t h e broad i n t e r f a c e s which a r e p a r a l l e l t o t h e ( l o o ) * p l a n e s , a r e e x p e c t e d t o be semicoherent i n t e r f a c e s . On t h e o t h e r hand, t h e a t o m i c f i t on t h e r i m s o f t h e d i s c s i s much p o o r e r . Along (010)o( i . e . ( 0 1 0 ) ~ d i r e c t i o n t h e m i s f i t can be e s t i m a t e d t o be 25%. The r i m s must be
i n c o h e r e n t i n t e r f a c e s w i t h much h i g h e r i n t e r f a c i a l energy. The o r i e n - t a t i o n r e l a t i o n s h i p appears t o a l l o w a good f i t t o develop between t h e c r y s t a l s e i t h e r a l o n g a p a r t i c u l a r p l a n e o r o c c a s i o n a l l y a l o n g a p a r t i c u l a r d i r e c t i o n . P r e c i p i t a t e s a r e however, t h r e e - d i m e n s i o n a l and t h e i n t e r f a c e must l i e a t a l l p o s s i b l e o r i e n t a t i o n s i n space around t h e c r y s t a l . The i n t e r f a c i a l energy w i l l v a r y r a p i d l y around t h e p r e c i p i t a t e s . T h i s a n i s o t r o p i c i n t e r f a c i a l e n e r g y w i l l c o n t r o l t h e e q u i l i b r i u m shape o f t h e p r e c i p i t a t e s . The schematic d r a w i n g o f t h e atomic f i t between t h e m a t r i x and t h e p r e c i p i t a t e s and t h e c o r - r e s p o n d i n g w - p l o t i s shown i n f i g u r e 6.
F i g u r e 6.
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Schematic diagram i l l u s t r a t i n g t h e a t o m i c f i t between t h e o ( - m a t r i x and t h e B - p r e c i p i t a t e s and t h e c o r r e s p o n d i n g 0 . - p l o t , i n w h i c h t h e p r e d i c t e d e q u i l i b r i u m shape o f t h e p r e c i p i t a t e i s shown as a hatched a r e a .C6-348 JOURNAL DE PHYSIQUE
The G i b b s - w u l f f theorem /8/ p r e d i c t s t h a t t h e e q u i l i b r i u m shape o f t h e p r e c i p i t a t e i s t h e i n n e r envelope o f s o - c a l l e d W u l f f p l a n e s t h a t a r e p l a n e s drawn p e r p e n d i c u l a r t o t h e v e c t o r from t h e o r i g i n a t t h e i n t e r s e c t i o n w i t h t h e c r - p l o t . The W u l f f c o n s t r u c t i o n i s shown i n f i g u r e 6. The p r e d i c t i o n i s a t l e a s t q u a l i t a t i v e l y i n agreement w i t h o u r o b s e r v a t i o n s .
I t i s p o s t u l a t e d t h a t t h e displacement o f a smooth g o o d - f i t i n t e r f a c e t a k e s p l a c e by means o f l e d g e mechanism /9/. The f i e l d i o n images r e v e a l e d t h a t on t h e b r o a d f a c e s o f t h e R - p r e c i p i t a t e s t h e l e d g e s do e x i s t . I n f i g u r e 3 b ) , a t y p i c a l l e d g e as h i g h as about 1.5 nm, i n d i - c a t e d by an a r r o w , can be seen.
ACKNOWLEDGMENTS
The a u t h o r a r e g r a t e f u l t o p r o f . P. Haasen, Dr. R. Wagner, M r . L.
A l v e n s l e b e n and M r . R . Grune f o r t h e i r support o f t h i s work. Some o f t h e micrographs were t a k e n w i t h t h e e x c e l l e n t FIM i n G o t t i n g e n
U n i v e r s i t y . We thank P r o f . S. Nakamura f o r h i s e f f o r t s t o g e t u s t o t h i s symposium. The work was s u p p o r t e d by t h e Chinese N a t i o n a l N a t u r a l S c i e n t i f i c Foundat i o n under g r a n t number 1860752.
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