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MORPHOLOGICAL INTERPRETATION OF MODULATED MICROSTRUCTURES
M. Miller, M. Burke, S. Brenner
To cite this version:
M. Miller, M. Burke, S. Brenner. MORPHOLOGICAL INTERPRETATION OF MODU- LATED MICROSTRUCTURES. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-239-C9-244.
�10.1051/jphyscol:1984940�. �jpa-00224420�
MORPHOLOGICAL INTERPRETATION OF MODULATED MICROSTRUCTURES M.K. Miller, M.G. Burke* and S.S. Brenner*
Oak Ridge national Laboratory, Oak Ridge, TN 3?831, U.S.A..
^University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.
Résumé - Des analyses de microstructures modulées par microscopie ionique de champ et microscopie électronique en transmission ont montré que les deux techniques peuvent introduire des erreurs graves dans l'interprétation de la morphologie et les mesures quantitatives de la microstrùcture des alliages contenant de larges proportions volumétriques de seconde phase. Il a été démontré que l'emploi des deux techniques en parallèle améliore la précision et la sûreté des résultats.
Abstract - Atom probe field-ion microscopy and transmission electron microscopy analyses of modulated microstructures have shown that both techniques can introduce serious errors in the interpretation of the morphology and in the quantitative measurements of the microstructure of alloys containing high volume fractions of second phase. It has been demonstrated that the combined use of the two techniques improves the accuracy and reliability of the results.
I - INTRODUCTION
The aging or coarsening behavior of a metallurgical alloy system depends intimately on the morphology of its microstructure. Both the atom probe field-ion microscope and the transmission electron microscope have proven to be capable of directly following the microstructura1 changes of a variety of materials. Sometimes, however, the microstructures produced in alloys can be complex and extremely fine-scaled, pushing both techniques to the limits of resolution. These types of microstructure are typical in alloys that decompose within low temperature miscibility gaps.
In this paper, such an alloy is examined to compare the morphological interpretations resulting from the two techniques. The discussion is restricted to aligned modulated microstructures with a wavelength of less than approximately 20 nm and containing a high volume fraction of second phase. However, it is equally valid for isotropica1ly decomposed systems, where there is no crystallographical alignment, and other systems with a high density of second phase precipitation.
The iron-beryllium system was selected to compare the results obtained by the two analytical techniques. The system has a low temperature miscibility gap within which alloys decompose to form a
<100> modulated two-phase microstructure. The two phases that constitute this triaxially modulated microstructure are an iron-rich ferrite phase and a B2-ordered beryllium-enriched phase. The microstructure exhibits excellent contrast in both instruments. In a previous atom probe field-ion microscopy and transmission electron microscopy study, the change in morphology of an Fe- 25 at.% Be alloy aged at 350 and 375°C was determined [1].
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984940
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I1 - DISCUSSION
Image i n t e r p r e t a t i o n i n m a t e r i a l s w h e r e t h e r e i s a h i g h v o l u m e f r a c t i o n o f a s e c o n d p h a s e i s v e r y c o m p l e x b e c a u s e o f t h e many f a c t o r s t h a t c o n t r i b u t e t o i m a g e f o r m a t i o n . An e x a m p l e o f t h e c o m p l e x i t y o f i m a g e i n t e r p r e t a t i o n i s shown b y t h e b r i g h t f i e l d 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 i n f i g u r e 1. T h e m i c r o g r a p h was o b t a i n e d w i t h a JEOL 200CX 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 e o p e r a t e d a t 200kV. The n a t u r e o f t h e m i c r o s t r u c t u r e c a n n o t b e i n f e r r e d f r o m t h i s m i c r o g r a p h a l t h o u g h some p e r i o d i c i t y i n s e v e r a l d i r e c t i o n s i s e v i d e n t .
F i g u r e 1. B r i 350 8 h t f i e l d TEM m i c r o g r a p h o f Fe 25 a t % Be a g e d 8 h a t C showing some e v i d e n c e o f t h e t r i a x i a l l y m o d u l a t e d m i c r o s t r u c t u r e .
The m i c r o s t r u c t u r a l i n t e r p r e t a t i o n may b e s i m p l i f i e d by e x a m i n i n g a d a r k f i e l d m i c r o g r a p h formed u s i n g a s u p e r l a t t i c e r e f l e c t i o n . Such a d a r k f i e l d 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 o f t h i s s e c o n d p h a s e may o n l y b e o b t a i n e d u s i n g s u p e r l a t t l c e r e f l e c t i o n s which d e r i v e from t h e o r d e r i n g r e a c t i o n . O r d e r i n g , h o w e v e r , d o e s n o t a l w a y s a c c o m p a n y s p i n o d a l d e c o m p o s i t i o n .
I n some c a s e s , t h e c o m p l e x i t y may b e f u r t h e r r e d u c e d by a l i g n i n g t h e s p e c i m e n w i t h a d i r e c t i o n p a r a l l e l t o t h e t r i a x i a l l y a l i g n e d m o d u l a t i o n s . An e x a m p l e o f a d a r k f i e l d m i c r o g r a p h o f a s p e c i m e n a l i g n e d w i t h t h e [ 0 0 1 ] d i r e c t i o n o b t a i n e d w i t h t h e u s e of a ( 1 0 0 ) B2 s u p e r l a t t i c e r e f l e c t i o n i s shown i n f i g u r e 2. The m o d u l a t e d n a t u r e
beam d i r e c t i o n i s a l o n g [ 0 0 1 ] and t h e t r i a x i a l l y m o d u l a t e d m i c r o s t r u c t u r e i s c l e a r l y e v i d e n t .
o f t h e m i c r o s t r u c t u r e b e c o m e s c l e a r l y e v i d e n t . I n t h i s m i c r o g r a p h o n l y two o f t h e t h r e e s e t s of m o d u l a t i o n s a r e v i s i b l e . The t h i r d s e t i s p a r a l l e l t o t h e f o i l and t h u s d o e s n o t c o n t r i b u t e s i g n i f i c a n t l y t o t h e c o n t r a s t . F r o m t h i s m i c r o g r a p h , t h e m o r p h o l o g y c o u l d b e i n t e r p r e t e d a s c o n s i s t i n g of a ( 1 0 0 ) a l i g n e d m a c r o l a t t i c e of c u b o i d a l p a r t i c l e s o f t h e b r i g h t l y - i m a g i n g B 2 - o r d e r e d p h a s e embedded i n a c o n t i n u o u s m a t r i x o f t h e n o n - o r d e r e d f e r r i t e p h a s e , w h e n , i n a c t u a l i t y , t h e s i t u a t i o n i s t h e r e v e r s e , a s d i s c u s s e d below.
A n o t h e r method of s i m p l i f y i n g t h e image of t h i s s t r u c t u r e i s t o r e d u c e t h e number o f m o d u l a t i o n s w i t h w h i c h t h e e l e c t r o n beam i n t e r a c t s b y u s i n g u l t r a - t h i n f o i l s o r t h o s e r e g i o n s i n t h e f o i l t h i n n e r t h a n u s u a l . A d a r k f l e l d 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 formed w i t h a B2 s u p e r l a t t i c e r e f l e c t i o n w h ~ c h i s n o t d i r e c t l y r e l a t e d t o t h e d i r e c t i o n o f t h e modulations i s shown i n f i g u r e 3. A t t h e l e f t o f t h i s m i c r o g r a p h , i n t h e t h i c k e r r e g i o n o f t h e f o i l , n o i n f o r m a t i o n c a n b e d e d u c e d b e c a u s e o f t h e c o n f u s i o n c a u s e d b y i m a g e o v e r l a p . I n t h e t h i n n e s t r e g i o n s a t t h e r i g h t o f t h e m i c r o g r a p h t h e b r i g h t l y - i m a g i n g B 2 - o r d e r e d r e g i o n s a p p e a r c o n t i n u o u s w i t h d a r k l y - i m a g i n g f e r r i t e p a r t i c l e s e m b e d d e d i n t h e m . T h i s e l e c t r o n m e t a l l o g r a p h i c i n t e r p r e t a t i o n i s i s i n d i r e c t c o n t r a d i c t i o n t o t h a t o b t a i n e d f r o m a n a n a l y s i s o f f i g u r e 2.
A f i e l d - i o n m i c r o g r a p h o f a s p e c i m e n a g e d a s t h e o n e e x a m i n e d b y 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 ( f i g u r e 2 ) i s shown i n f i g u r e 4.
Each f i e l d - i o n m i c r o g r a p h i s a h i g h r e s o l u t i o n two d i m e n s i o n a l v i e w of t h e s u r f a c e of t h e specimen. The t r u e morphology i s o n l y r e v e a l e d i n t h r e e d i m e n s i o n s a f t e r c a r e f u l r e m o v a l o f t h i n l a y e r s of m a t e r i a l from
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T h i c k T h i n
F i g u r e 3. Dark f i e l d TEM m i c r o g r a p h of Fe -25 a t % Be a g e d 8 h a t 3 5 0 ~ ~ u s i n g a B 2 s u p e r l a t t i c e r e f l e c t i o n . The b r i g h t B 2 p h a s e a p p e a r s t o b e c o n t i n u o u s w i t h d a r k d i s c r e t e p a r t i c l e s of f e r r i t e .
t h e s p e c i m e n by f i e l d e v a p o r a t i o n . U s i n g t h i s f i e l d e v a p o r a t i o n t e c h n i q u e t h e m i c r o s t r u c t u r e was found t o c o n s i s t of s m a l l b r i g h t l y - i m a g i n g c u b o i d a l p a r t i c l e s embedded i n a d a r k l y - i m a g i n g m a t r i x . The p h a s e s c a n n o t b e i m m e d i a t e l y i d e n t i f i e d a s i n e l e c t r o n m i c r o s o p y w i t h o u t f u r t h e r a n a l y s i s s u c h a s e s t i m a t i n g t h e r e l a t i v e f i e l d - e v a p o r a t i o n c h a r a c t e r i s t i c s o f t h e two p h a s e s , o r , more d e f i n i t i v e l y , by atom p r o b e c h e m i c a l m i c r o a n a l y s i s .
The i d e n t i t y o f p h a s e s o b s e r v e d i n t h e FIM may a l s o b e i n f e r r e d from t h e c o n t r a s t e x h i b i t e d and t h e v o l u m e f r a c t i o n o r number d e n s i t y o f e a c h p h a s e . W h i l e t h e v o l u m e f r a c t i o n may b e e s t i m a t e d f r o m t h e a r e a 1 f r a c t i o n of e a c h p h a s e i n t h e m i c r o g r a p h , t h e e f f e c t s of l o c a l m a g n i f i c a t i o n o f e a c h p h a s e m u s t b e t a k e n i n t o a c c o u n t f o r a n a c c u r a t e d e t e r m i n a t i o n 121. A s i t i s d i f f i c u l t t o d e t e r m i n e t h e l o c a l m a g n i f i c a t i o n o f t h e s e f i n e m i c r o s t r u c t u r e s a c c u r a t e l y , a n d s i n c e t h e volume f r a c t i o n of e a c h p h a s e i s a p p r o x i m a t e l y e q u a l , i t i s b e t t e r t o o b s e r v e t h e c h a n g e i n v o l u m e f r a c t i o n o f t h e two p h a s e s i n a l l o y s of d i f f e r e n t c o n c e n t r a t i o n s t h a t a r e h e a t - t r e a t e d i n t h e same manner.
I t s h o u l d b e n o t e d t h a t t h e c o n t r a s t i n a field-ionmicrograph is n o t s i m p l y d e p e n d e n t on t h e c o m p o s i t i o n of t h e p h a s e s , b u t a l s o d e p e n d s on c r y s t a l l o g r a p h y and n e a r e s t n e i g h b o r c o n f i g u r a t i o n . F o r e x a m p l e , i n t h e i r o n - b e r y l l i u m s y s t e m t h e b e r y l l i u m - e n r i c h e d B2-ordered p h a s e ( - 4 0 a t % B e ) i m a g e s d a r k l y w h i l e t h e B 3 2 - o r d e r e d e q u i a t o m i c FeBe p h a s e images b r i g h t l y w i t h r e s p e c t t o t h e f e r r i t e ( < 10 a t % Be).
More p o s i t i v e p h a s e i d e n t i f i c a t i o n may b e o b t a i n e d from a n atom p r o b e c h e m i c a l a n a l y s i s . Even when u s i n g t h e atom p r o b e , good c o n t r a s t i s r e q u i r e d i n t h e f i e l d - i o n images s o t h a t t h e s e l e c t e d a r e a o f a n a l y s i s may b e a c c u r a t e l y a l i g n e d w i t h t h e e n t r a n c e a p e r t u r e t o t h e m a s s
p a r t i c l e s embedded i n t h e d a r k f e r r i t e m a t r i x .
s p e c t r o m e t e r . The a p e r t u r e m u s t b e s m a l l e r t h a n t h e p r o j e c t e d s i z e o f t h e p h a s e t o p r e v e n t t h e a t o m s o f t h e n e i g h b o r i n g p h a s e f r o m e n t e r i n g t h e s p e c t r o m e t e r . T h e l i m i t a t i o n s c a u s e d b y t h e e f f e c t s o f l o c a l m a g n i f i c a t i o n , t r a j e c t o r y aberrations, s m a l l composition differences b e t w e e n t h e p h a s e s , c o m p o s i t i o n gradients a n d variations, a n d t h e a c c u r a c y o f t h e s p e c l m e n a l i g n m e n t w i t h t h e a p e r t u r e t h r o u g h o u t t h e a n a l y s i s m u s t b e t a k e n i n t o a c c o u n t i n a s s e s s i n g t h e s t a t i s t i c a l v a l i d i t y o f t h e d a t a .
When t h e s c a l e o f t h e m i c r o s t r u c t u r e i s s e v e r a l n a n o m e t e r s a n d t h e r e i s a c l e a r c o n t r a s t b e t w e e n t h e p h a s e s , a t o m p r o b e a n a l y s i s becomes much s i m p l e r . T h u s , t h e d a r k l y i m a g i n g p h a s e i n t h e f i e l d - i o n m i c r o g r a p h o f f i g u r e 4 c o u l d e a s i l y b e i d e n t i f i e d a s t h e B e - e n r i c h e d ( B2 ) p h a s e . A c o m p a r i s o n o f f i e l d - l o n m i c r o g r a p h s o f a 1 l o y s c o n t a i n i n g 1 7 a n d 2 5 a t % b e r y l l i u m , a g e d u n d e r s i m i l a r c o n d i t i o n s , r e v e a l e d t h a t t h e r e was a d e c r e a s e i n v o l u m e f r a c t i o n o f t h i s d a r k l y i m a g i n g p h a s e when t h e b e r y l l i u m c o n t e n t o f t h e a 1 l o y , w a s r e d u c e d , t h u s c o n f i r m i n g t h i s c o n c l u s i o n . O n c e t h e p h a s e s i n t h e f i e l d - i o n m i c r o g r a p h s w e r e i d e n t i f i e d , t h e i n f o r m a t i o n c o u l d b e u s e d t o c o r r e c t l y i n t e r p r e t t h e d a r k f i e l d e l e c t r o n m i c r o g r a p h s i n f i g u r e s 2 a n d 3 w i t h r e s p e c t t o t h e i d e n t i t y o f t h e c o n t i n u o u s m a t r i x p h a s e a n d t h e c u b o i d a l p r e c i p i t a t e s .
111 - CONCLUSIONS
B o t h a t o m p r o b e f i e l d - i o n m i c r o s c o p y a n d 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 may b e u s e d t o e x a m i n e c o m p l e x m i c r o s t r u c t u r e s c o n t a i n i n g h i g h v o l u m e f r a c t i o n s o f s e c o n d p h a s e . However, c a r e m u s t b e t a k e n i n b o t h t h e i d e n t i f i c a t i o n o f p h a s e s a n d i n t h e m o r p h o l o g i c a l i n t e r p r e t a t i o n o f t h e m i c r o s t r u c t u r e . T h e c o m b i n e d u s e o f t h e t w o
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t e c h n i q u e s r e d u c e s t h e r i s k o f m a k i n g m i s l e a d i n g c o n c l u s i o n s a n d i m p r o v e s t h e a c c u r a c y a n d r e 1 i a b i l i t y o f t h e r e s u l t s .
Acknowledgments
R e s e a r c h s p o n s o r e d b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n u n d e r a U n i v e r s i t y / I n d u s t r y c o o p e r a t i v e g r a n t , DMR-8022225 a n d b y t h e D i v i s i o n o f M a t e r i a l s S c i e n c e s , U.S. D e p a r t m e n t o f E n e r g y , u n d e r c o n t r a c t DE-AC05-840R21400 w i t h M a r t i n M a r i e t t a Energy S y s t e m s , I n c .
R e f e r e n c e s
[ l ] M. K. M i l l e r , S.S. B r e n n e r , M.G. B u r k e a n d W.A. S o f f a , S c r i p t a Met. 1 8 , . ( 1 9 8 4 ) 111.
[ 2 ] M. K . M i l l e r , C . J. M i l l e r , and S.S. B r e n n e r , P r o c 2 8 t h I n t F i e l d E m i s s i o n Symposium, Oregon 1 9 8 1 , p195.
