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The use of TEM in the study of ordering alloys
Abstracts of The Netherlands Society of Electron Microscopy Annual Meeting
of t w i s t b o u n d a r i e s w e r e p r e p a r e d in such a w a y t h a t i n i t i a l l y the b o u n d a r y r e g i o n was free f r o m d i s l o c a t i o n s . This e n a b l e d us to s t u d y the g e n e r a t i o n of d e n s e disl o c a t i o n n e t w o r k s in the c o u r s e of annealing. The i m a g i n g in T E M and the f o r m of the d i s l o c a t i o n n e t w o r k s w i l l be disc u s s e d in s o m e d e t a i l for a l o w - a n g l e (iii) t w i s t b o u n d a r y a n d a n e a r - c o h e r e n t twin boundary. F o r a l o w - a n g l e (iii) twist boundary a hexagonal dislocation n e t w o r k of a/2 < 1 1 0 > s c r e w d i s l o c a t i o n s is e x p e c t e d to o c c u r . However, TEM imaging of s u c h a n e t w o r k is c o m p l i c a t e d by the s u p e r p o s i t i o n of s t r a i n c o n t r a s t f r o m the d i s l o c a t i o n s a n d the i n t e r f e r e n c e eff e c t s ( m o i r ~ f r i n g e s ) a s s o c i a t e d w i t h the t w i s t at the b o u n d a r y . This m a y r e s u l t in d i f f e r e n t t y p e s of i m a g e s , d e p e n d i n g on the d i f f r a c t i o n c o n d i t i o n s e m p l o y e d . We f o u n d t h a t a h e x a g o n a l n e t w o r k can be i m a g e d as a t r i a n g u l a r b l a c k - w h i t e cont r a s t p a t t e r n , as if the h e x a g o n a l n e t w o r k is d i s s o c i a t e d into a t r i a n g u l a r n e t w o r k of p a r t i a l s c r e w d i s l o c a t i o n s . F o r a n e a r - c o h e r e n t twin b o u n d a r y a d i s l o c a t i o n n e t w o r k c o n s i s t s of a / 6 < i 1 2 > screw dislocations. In this case a s t e p in the b o u n d a r y is a s s o c i a t e d w i t h e a c h dislocation. It can be s h o w n that in o r d e r to p r e s e r v e the a v e r a g e b o u n d a r y n o r m a l , a n e t w o r k in the f o r m of a regu l a r s i x - s t a r p a t t e r n s h o u l d o c c u r , as observed. I A l s o in this case the n e t w o r k i m a g e d e p e n d s on the d i f f r a c t i o n c o n d i tions employed. F i n a l l y s o m e o b s e r v a t i o n s on l o w - a n g l e p h a s e b o u n d a r i e s in (iii) A u / A u P d b i c r y s tals w i l l be d i s c u s s e d . H e r e the b o u n d ary c o n s i s t s of a r o t a t i o n as w e l l as a d i l a t a t i o n c o m p o n e n t , and it is f o u n d t h a t the l a t t e r c o m p o n e n t d i s a p p e a r s f i r s t on a n n e a l i n g the b i c r y s t a l . i.
R. F. Scott, P. J. Goodhew, Phil. Mag. A44 (1981) 373.
THE U S E OF T E M IN THE S T U D Y OF ORDERING ALLOYS G. J. L. V a n Der W e g e n , P. M. a n d J. Th. M. De H o s s o n
Bronsveld
D e p a r t m e n t of A p p l i e d P h y s i c s , M a t e r ials S c i e n c e C e n t r e , U n i v e r s i t y of G r o n i n g e n , G r o n i n g e n , The N e t h e r l a n d s
At high temperature C u 2 N i Z n possesses an f . c . c , d i s o r d e r e d s t r u c t u r e . Below the f i r s t c r i t i c a l t e m p e r a t u r e , T C I = 7 7 4 K , a m o d i f i e d L12 s t r u c t u r e e x i s t s , in w h i c h the Zn a t o m s o c c u p y one of the four interpenetrating simple cubic sublattices, w h i l e the Cu a n d Ni a t o m s are s t i l l rand o m l y d i s t r i b u t e d o v e r the r e m a i n i n g three sublattices. Below a second criti-
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cal t e m p e r a t u r e , T c 2 = 5 9 8 K , a m o d i f i e d L10 s t r u c t u r e e x i s t s , in w h i c h the Cu a n d Ni a t o m s e a c h o c c u p y t h e i r own sublattice. D e f o r m a t i o n of f . c . c , m e t a l s and diso r d e r e d s o l i d s o l u t i o n s o c c u r s by the m o v e m e n t of unit d i s l o c a t i o n s , c o n s i s t i n g of two S h o c k l e y p a r t i a l s , s e p a r a t e d from e a c h o t h e r by a s t a c k i n g f a u l t . S l i p of such a unit d i s l o c a t i o n in an o r d e r e d alloy w o u l d c r e a t e a s h e e t of a n t i p h a s e b o u n d a r y (APB) b e h i n d it. The d i s o r d e r a s s o c i a t e d w i t h this A P B can be r e s t o r e d by s l i p of a s e c o n d unit d i s l o c a t i o n , w i t h the same B u r g e r s v e c t o r , b e h i n d the first dislocation. Such a d i s l o c a t i o n p a i r , c o n s i s t i n g of two u n i t d i s l o c a t i o n s s e p a r a t e d by an A P B is c a l l e d a s u p e r lattice dislocation. Each of t h e s e unit d i s l o c a t i o n s is s p l i t up i n t o two S h o c k ley p a r t i a l s , s e p a r a t e d by an A P B and a s t a c k i n g fault. I A n i s o t r o p i c elasticity t h e o r y p r o v i d e s the r e l a t i o n b e t w e e n the s e p a r a t i o n of the S h o c k l e y p a r t i a l s and the A P B e n e r g y , s t a c k i n g f a u l t e n e r g y and the c h a r a c t e r of the d i s l o c a t i o n . S e v e r a l s u p e r l a t t i c e d i s l o c a t i o n s in C u 2 N i Z n h a v e b e e n c h a r a c t e r i z e d a n d i m a g e d in the w e a k beam mode. The s e p a r a t i o n of the u n i t d i s l o c a t i o n s has b e e n d e t e r m i n e d from the e l e c t r o n m i c r o g r a p h s by m e a n s of a d e n s i tometer. A c o r r e s p o n d i n g A P B e n e r g y of 89 ± 9 m J / m 2 has b e e n c a l c u l a t e d . H o w e v e r the s p l i t t i n g of e a c h unit d i s l o c a t i o n i n t o two S h o c k l e y p a r t i a l s c o u l d not be o b s e r v e d , w h i c h m e a n s that t h e i r s e p a r a tion is less than a b o u t 0.5 nm, i n d i c a t ing a h i g h s t a c k i n g f a u l t e n e r g y c o m p a r e d to Cu (40 m J / m 2 ) . An e x t e n d e d d i s l o c a t i o n node is the p r o d u c t of a r e a c t i o n b e t w e e n two unit dislocations h a v i n g different Burgers v e c t o r s and s l i p p i n g on d i f f e r e n t type {iii} planes. 2 S e v e r a l theoretical t r e a t m e n t s of the node p r o b l e m h a v e b e e n p u b l i s h e d in the l i t e r a t u r e , 2 r e s u l t i n g in a s i m i l a r p r e d i c t i o n for an e x t e n d e d d i s l o c a t i o n node of s c r e w c h a r a c t e r . The p r e d i c t e d d i m e n s i o n s for e d g e c h a r a c t e r n o d e s d i f f e r a g r e a t deal, but t h e s e n o d e s are o n l y s e l d o m o b s e r v e d . A b o u t 17 e x t e n d e d dislocation n o d e s have been c h a r a c t e r i z e d in d i s o r d e r e d C u 2 N i Z n , 3 f r o m w h i c h it can be c o n c l u d e d that the m o d e l s of S i e m s , J @ s s a n g et al. and S c a t t e r g o o d et al. p r o v i d e g o o d a g r e e m e n t , w h e r e a s the m o d e l s of B r o w n et al. and M i l o n et al. e x h i b i t i n t r i n s i c discrepancy. A s t a c k i n g f a u l t e n e r g y of 32 ± 7 m J / m 2 is o b t a i n e d for C u 2 N i Z n . The f a u l t e n e r g i e s of C u 2 N i Z n as det e r m i n e d by T E M are of i m p o r t a n c e , as w i l l be e x p l a i n e d w i t h two e x a m p l e s . F i r s t l y , the v a l u e of the A P B e n e r g y is calculated using effective interatomic p o t e n t i a l s of the t r a n s i t i o n m e t a l s w h i c h are c o m p u t e d w i t h i n the f r a m e w o r k
Abstracts of The Netherlands Society of Electron Microscopy Annual Meeting
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of the p s e u d o p o t e n t i a l a p p r o x i m a t i o n b a s e d on a t r a n s i t i o n - m e t a l - m o d e l - p o t e n tial of the H e i n e - A b a r e n k o v - S h a w t y p e . 4 The e x p e r i m e n t a l l y d e t e r m i n e d A P B e n e r g y o f 89 m J / m 2 a g r e e s v e r y w e l l w i t h t h e ab i n i t i o c a l c u l a t e d v a l u e of 94 m J / m 2. Secondly, with t h e s e interatomic p o t e n t i a l s l o n g - r a n g e and s h o r t - r a n g e o r d e r p a r a m e t e r s of C u 2 N i Z n h a v e b e e n c a l c u l a t e d S as a f u n c t i o n o f t e m p e r a t u r e , u s i n g the C l u s t e r V a r i a t i o n M e t h o d . The s t r e n g t h e n i n g t h e o r i e s o f M o i n e e t al. and R u d m a n for o r d e r i n g a l l o y s h a v e b e e n u s e d to p r e d i c t the y i e l d s t r e n g t h of C u 2 N i Z n as a f u n c t i o n o f t h e q u e n c h t e m p e r a t u r e , u s i n g the e x p e r i m e n t a l l y d e t e r m i n e d f a u l t e n e r g i e s a n d the c a l c u l a t e d order parameters. The a g r e e m e n t w i t h the experimentally determined y i e l d strength o f t h e a l l o y is r e a s o n a b l e . M e a s u r e m e n t s of p h y s i c a l p r o p e r t i e s (resistivity, lattice parameter, m i c r o h a r d n e s s , e t c . ) r e s u l t in a d e t e r m i n a t i o n of the c r i t i c a l t e m p e r a t u r e for o r d e r i n g , b u t they c a n n o t r e v e a l the e x t e n t of a t w o - p h a s e r e g i o n , w h i c h is p r e s e n t in t h e case of a f i r s t - o r d e r t r a n s i t i o n . TEM, h o w e v e r , can s u p p l y d i r e c t e v i d e n c e of a t w o - p h a s e r e g i o n by i m a g i n g the t h e r m a l APB domains with t h r e e independent superlattice reflections. A m o d i f i c a t i o n of t h i s m e t h o d is a p p l i e d 6 t o o b t a i n t h e quasi-binary cross-section Cu50Ni50_xZnx, b e c a u s e o n l y two i n d e p e n d e n t s u p e r lattice reflections with e n o u g h intensity c o u l d be u s e d . The u p p e r b o u n d a r y of t h i s t w o - p h a s e r e g i o n as d e t e r m i n e d b y TEM 6 agrees well with resistivity measurements. 1. 2. 3.
4. 5.
6.
M. J. Marcinkowski, N. Brown, R. M. Fisher, Aeta Met. 9 (1961) 129. A. W. Ruff, Met. Trans. 1 (1970) 2391. G. J. L. Van Der Wegen, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 14 (1980) 285. J. De Groot, P. M. Bronsveld, J. Th. M. De Hosson, Phys. Stat. Sol. (a) 52 (1979)635. A. De Rooy, G. J. L. Van Der Wegen, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 15, no. 12, 1981. G. J. L. Van Der Wegen, A. De Rooy, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 15, no. 12, 1981.
F I T A N D M I S F I T IN I M A G I N G O F CRYSTALS J.
"UNFIT"
Van L a n d u y t C e n t r u m voor H o o g s p a n n i n g s e l e k t r o n e n m i k r o s k o p i e , U n i v e r s i t y of A n t w e r p , R U C A , G r o e n e n b o r g e r l a a n 171, B-2020 Antwerp, Belgium
A f t e r a s h o r t r e v i e w of the v a r i o u s t y p e s of t w o - d i m e n s i o n a l d e f e c t s , the
i m a g i n g p r o p e r t i e s a r e s u m m a r i z e d as d e r i v e d for t w o - b e a m c o n d i t i o n s f r o m d y n a m ical d i f f r a c t i o n t h e o r y i n c l u d i n g a n o m a l ous a b s o r p t i o n . 1 D e f e c t s to be d i s c u s s e d are a n t i p h a s e b o u n d a r i e s s t a c k i n g f a u l t s , d o m a i n - and twin b o u n d a r i e s , i n v e r s i o n b o u n d a r i e s and g r a i n b o u n d a r i e s . It w i l l b e s h o w n h o w t h e s e a r e u s u a l l y i m a g e d as f r i n g e p a t t e r n s , a n d h o w the i m a g e c h a r a c t e r i s tics can be u t i l i z e d to d e r i v e c r y s t a l l o g r a p h i c c h a r a c t e r i s t i c s of the d e f e c t s . T h r e e m a i n t y p e s of p a t t e r n s can be distinguished: s-type fringes, 6-type f r i n g e s , and t h i c k n e s s or w e d g e f r i n g e patterns. The a v a i l a b i l i t y of h i g h r e s o l u t i o n m i c r o s c o p e s now a l s o e n a b l e s t h e s e def e c t s t o b e o b s e r v e d in t h e s t r u c t u r e projection imaging mode. The b r i g h t and d a r k - f i e l d m e d i u m m a g n i f i c a t i o n i m a g e s can now be s u i t a b l y c o m p l e m e n t e d by m u l t i p l e b e a m i m a g i n g at w o r k i n g magn i f i c a t i o n s of m i l l i o n t i m e s . In s p e c i m e n s of a p p r o p r i a t e o r i e n t a t i o n , t h i c k n e s s and s t r u c t u r e type the a b o v e m e n t i o n e d d e f e c t s are d i r e c t l y r e v e a l e d . The effect of o b j e c t i v e a p e r t u r e size a n d s p e c i m e n t h i c k n e s s w i l l be i l l u s t r a t e d on the i m a g i n g of s t r u c t u r e s and s t r u c t u r a l d e f e c t s s u c h as a n t i p h a s e b o u n d a r ies, e t c . in o r d e r e d a l l o y s . Gold m a n g a n e s e a n d y - b r a s s w i l l b e u s e d as e x a m p l e s 2,3 f o r i l l u s t r a t i n g t h e i m a g i n g o f s t r u c t u r e s , m i c r o t e x t u r e s and d e f e c t s . I.
2. 3.
S. Amelinckx, J. Van Landuyt, in "Diffraction and Imaging Techniques in Materials Science" vol. i, pp. 107-152 (North-Holland Publishing Company, Amsterdam, 1978). G. Van Tendeloo, S. Amelinckx, Phys. Stat. Sol. (a) 49 (1978) 337. M. Van Sande, J. Van Landuyt, S. Amelinckx, Phys. Stat. Sol. (a) 55 (1979) 41.
THE V O L T A G E D I L E M M A : A P P L I C A T I O N TO MATERIALS SCIENCE G.
Van T e n d e l o o L a b o r a t o r y for High V o l t a g e E l e c t r o n M i c r o s c o p y , U n i v e r s i t y of A n t w e r p , R U C A , G r o e n e n b o r g ~ r l a a n 171, B - 2 0 2 0 Antwerp, Belgium
The f i r s t e l e c t r o n m i c r o s c o p e was b u i l t in 1 9 3 1 b y E. R u s k a a n d M. K n o l l . A t t h a t t i m e t h e y w e r e d e l i g h t e d to d e m o n s t r a t e a r e s o l u t i o n b e t t e r than any o p t i c a l m i c r o s c o p e c o u l d do. Now almost e x a c t l y f i f t y y e a r s l a t e r we h a v e m o d i f i e d t h e i r p r o t o t y p e to s u c h an e x t e n t t h a t a t o m i c r e s o l u t i o n is n o l o n g e r an idle d r e a m , 3 M e V e l e c t r o n m i c r o s c o p e s h a v e b e e n c o n s t r u c t e d and EELS or X - r a y analysis have b e c o m e generally accepted. C o m m e r c i a l f i r m s are o f f e r i n g a w i d e
of t w i s t b o u n d a r i e s w e r e p r e p a r e d in such a w a y t h a t i n i t i a l l y the b o u n d a r y r e g i o n was free f r o m d i s l o c a t i o n s . This e n a b l e d us to s t u d y the g e n e r a t i o n of d e n s e disl o c a t i o n n e t w o r k s in the c o u r s e of annealing. The i m a g i n g in T E M and the f o r m of the d i s l o c a t i o n n e t w o r k s w i l l be disc u s s e d in s o m e d e t a i l for a l o w - a n g l e (iii) t w i s t b o u n d a r y a n d a n e a r - c o h e r e n t twin boundary. F o r a l o w - a n g l e (iii) twist boundary a hexagonal dislocation n e t w o r k of a/2 < 1 1 0 > s c r e w d i s l o c a t i o n s is e x p e c t e d to o c c u r . However, TEM imaging of s u c h a n e t w o r k is c o m p l i c a t e d by the s u p e r p o s i t i o n of s t r a i n c o n t r a s t f r o m the d i s l o c a t i o n s a n d the i n t e r f e r e n c e eff e c t s ( m o i r ~ f r i n g e s ) a s s o c i a t e d w i t h the t w i s t at the b o u n d a r y . This m a y r e s u l t in d i f f e r e n t t y p e s of i m a g e s , d e p e n d i n g on the d i f f r a c t i o n c o n d i t i o n s e m p l o y e d . We f o u n d t h a t a h e x a g o n a l n e t w o r k can be i m a g e d as a t r i a n g u l a r b l a c k - w h i t e cont r a s t p a t t e r n , as if the h e x a g o n a l n e t w o r k is d i s s o c i a t e d into a t r i a n g u l a r n e t w o r k of p a r t i a l s c r e w d i s l o c a t i o n s . F o r a n e a r - c o h e r e n t twin b o u n d a r y a d i s l o c a t i o n n e t w o r k c o n s i s t s of a / 6 < i 1 2 > screw dislocations. In this case a s t e p in the b o u n d a r y is a s s o c i a t e d w i t h e a c h dislocation. It can be s h o w n that in o r d e r to p r e s e r v e the a v e r a g e b o u n d a r y n o r m a l , a n e t w o r k in the f o r m of a regu l a r s i x - s t a r p a t t e r n s h o u l d o c c u r , as observed. I A l s o in this case the n e t w o r k i m a g e d e p e n d s on the d i f f r a c t i o n c o n d i tions employed. F i n a l l y s o m e o b s e r v a t i o n s on l o w - a n g l e p h a s e b o u n d a r i e s in (iii) A u / A u P d b i c r y s tals w i l l be d i s c u s s e d . H e r e the b o u n d ary c o n s i s t s of a r o t a t i o n as w e l l as a d i l a t a t i o n c o m p o n e n t , and it is f o u n d t h a t the l a t t e r c o m p o n e n t d i s a p p e a r s f i r s t on a n n e a l i n g the b i c r y s t a l . i.
R. F. Scott, P. J. Goodhew, Phil. Mag. A44 (1981) 373.
THE U S E OF T E M IN THE S T U D Y OF ORDERING ALLOYS G. J. L. V a n Der W e g e n , P. M. a n d J. Th. M. De H o s s o n
Bronsveld
D e p a r t m e n t of A p p l i e d P h y s i c s , M a t e r ials S c i e n c e C e n t r e , U n i v e r s i t y of G r o n i n g e n , G r o n i n g e n , The N e t h e r l a n d s
At high temperature C u 2 N i Z n possesses an f . c . c , d i s o r d e r e d s t r u c t u r e . Below the f i r s t c r i t i c a l t e m p e r a t u r e , T C I = 7 7 4 K , a m o d i f i e d L12 s t r u c t u r e e x i s t s , in w h i c h the Zn a t o m s o c c u p y one of the four interpenetrating simple cubic sublattices, w h i l e the Cu a n d Ni a t o m s are s t i l l rand o m l y d i s t r i b u t e d o v e r the r e m a i n i n g three sublattices. Below a second criti-
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cal t e m p e r a t u r e , T c 2 = 5 9 8 K , a m o d i f i e d L10 s t r u c t u r e e x i s t s , in w h i c h the Cu a n d Ni a t o m s e a c h o c c u p y t h e i r own sublattice. D e f o r m a t i o n of f . c . c , m e t a l s and diso r d e r e d s o l i d s o l u t i o n s o c c u r s by the m o v e m e n t of unit d i s l o c a t i o n s , c o n s i s t i n g of two S h o c k l e y p a r t i a l s , s e p a r a t e d from e a c h o t h e r by a s t a c k i n g f a u l t . S l i p of such a unit d i s l o c a t i o n in an o r d e r e d alloy w o u l d c r e a t e a s h e e t of a n t i p h a s e b o u n d a r y (APB) b e h i n d it. The d i s o r d e r a s s o c i a t e d w i t h this A P B can be r e s t o r e d by s l i p of a s e c o n d unit d i s l o c a t i o n , w i t h the same B u r g e r s v e c t o r , b e h i n d the first dislocation. Such a d i s l o c a t i o n p a i r , c o n s i s t i n g of two u n i t d i s l o c a t i o n s s e p a r a t e d by an A P B is c a l l e d a s u p e r lattice dislocation. Each of t h e s e unit d i s l o c a t i o n s is s p l i t up i n t o two S h o c k ley p a r t i a l s , s e p a r a t e d by an A P B and a s t a c k i n g fault. I A n i s o t r o p i c elasticity t h e o r y p r o v i d e s the r e l a t i o n b e t w e e n the s e p a r a t i o n of the S h o c k l e y p a r t i a l s and the A P B e n e r g y , s t a c k i n g f a u l t e n e r g y and the c h a r a c t e r of the d i s l o c a t i o n . S e v e r a l s u p e r l a t t i c e d i s l o c a t i o n s in C u 2 N i Z n h a v e b e e n c h a r a c t e r i z e d a n d i m a g e d in the w e a k beam mode. The s e p a r a t i o n of the u n i t d i s l o c a t i o n s has b e e n d e t e r m i n e d from the e l e c t r o n m i c r o g r a p h s by m e a n s of a d e n s i tometer. A c o r r e s p o n d i n g A P B e n e r g y of 89 ± 9 m J / m 2 has b e e n c a l c u l a t e d . H o w e v e r the s p l i t t i n g of e a c h unit d i s l o c a t i o n i n t o two S h o c k l e y p a r t i a l s c o u l d not be o b s e r v e d , w h i c h m e a n s that t h e i r s e p a r a tion is less than a b o u t 0.5 nm, i n d i c a t ing a h i g h s t a c k i n g f a u l t e n e r g y c o m p a r e d to Cu (40 m J / m 2 ) . An e x t e n d e d d i s l o c a t i o n node is the p r o d u c t of a r e a c t i o n b e t w e e n two unit dislocations h a v i n g different Burgers v e c t o r s and s l i p p i n g on d i f f e r e n t type {iii} planes. 2 S e v e r a l theoretical t r e a t m e n t s of the node p r o b l e m h a v e b e e n p u b l i s h e d in the l i t e r a t u r e , 2 r e s u l t i n g in a s i m i l a r p r e d i c t i o n for an e x t e n d e d d i s l o c a t i o n node of s c r e w c h a r a c t e r . The p r e d i c t e d d i m e n s i o n s for e d g e c h a r a c t e r n o d e s d i f f e r a g r e a t deal, but t h e s e n o d e s are o n l y s e l d o m o b s e r v e d . A b o u t 17 e x t e n d e d dislocation n o d e s have been c h a r a c t e r i z e d in d i s o r d e r e d C u 2 N i Z n , 3 f r o m w h i c h it can be c o n c l u d e d that the m o d e l s of S i e m s , J @ s s a n g et al. and S c a t t e r g o o d et al. p r o v i d e g o o d a g r e e m e n t , w h e r e a s the m o d e l s of B r o w n et al. and M i l o n et al. e x h i b i t i n t r i n s i c discrepancy. A s t a c k i n g f a u l t e n e r g y of 32 ± 7 m J / m 2 is o b t a i n e d for C u 2 N i Z n . The f a u l t e n e r g i e s of C u 2 N i Z n as det e r m i n e d by T E M are of i m p o r t a n c e , as w i l l be e x p l a i n e d w i t h two e x a m p l e s . F i r s t l y , the v a l u e of the A P B e n e r g y is calculated using effective interatomic p o t e n t i a l s of the t r a n s i t i o n m e t a l s w h i c h are c o m p u t e d w i t h i n the f r a m e w o r k
Abstracts of The Netherlands Society of Electron Microscopy Annual Meeting
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of the p s e u d o p o t e n t i a l a p p r o x i m a t i o n b a s e d on a t r a n s i t i o n - m e t a l - m o d e l - p o t e n tial of the H e i n e - A b a r e n k o v - S h a w t y p e . 4 The e x p e r i m e n t a l l y d e t e r m i n e d A P B e n e r g y o f 89 m J / m 2 a g r e e s v e r y w e l l w i t h t h e ab i n i t i o c a l c u l a t e d v a l u e of 94 m J / m 2. Secondly, with t h e s e interatomic p o t e n t i a l s l o n g - r a n g e and s h o r t - r a n g e o r d e r p a r a m e t e r s of C u 2 N i Z n h a v e b e e n c a l c u l a t e d S as a f u n c t i o n o f t e m p e r a t u r e , u s i n g the C l u s t e r V a r i a t i o n M e t h o d . The s t r e n g t h e n i n g t h e o r i e s o f M o i n e e t al. and R u d m a n for o r d e r i n g a l l o y s h a v e b e e n u s e d to p r e d i c t the y i e l d s t r e n g t h of C u 2 N i Z n as a f u n c t i o n o f t h e q u e n c h t e m p e r a t u r e , u s i n g the e x p e r i m e n t a l l y d e t e r m i n e d f a u l t e n e r g i e s a n d the c a l c u l a t e d order parameters. The a g r e e m e n t w i t h the experimentally determined y i e l d strength o f t h e a l l o y is r e a s o n a b l e . M e a s u r e m e n t s of p h y s i c a l p r o p e r t i e s (resistivity, lattice parameter, m i c r o h a r d n e s s , e t c . ) r e s u l t in a d e t e r m i n a t i o n of the c r i t i c a l t e m p e r a t u r e for o r d e r i n g , b u t they c a n n o t r e v e a l the e x t e n t of a t w o - p h a s e r e g i o n , w h i c h is p r e s e n t in t h e case of a f i r s t - o r d e r t r a n s i t i o n . TEM, h o w e v e r , can s u p p l y d i r e c t e v i d e n c e of a t w o - p h a s e r e g i o n by i m a g i n g the t h e r m a l APB domains with t h r e e independent superlattice reflections. A m o d i f i c a t i o n of t h i s m e t h o d is a p p l i e d 6 t o o b t a i n t h e quasi-binary cross-section Cu50Ni50_xZnx, b e c a u s e o n l y two i n d e p e n d e n t s u p e r lattice reflections with e n o u g h intensity c o u l d be u s e d . The u p p e r b o u n d a r y of t h i s t w o - p h a s e r e g i o n as d e t e r m i n e d b y TEM 6 agrees well with resistivity measurements. 1. 2. 3.
4. 5.
6.
M. J. Marcinkowski, N. Brown, R. M. Fisher, Aeta Met. 9 (1961) 129. A. W. Ruff, Met. Trans. 1 (1970) 2391. G. J. L. Van Der Wegen, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 14 (1980) 285. J. De Groot, P. M. Bronsveld, J. Th. M. De Hosson, Phys. Stat. Sol. (a) 52 (1979)635. A. De Rooy, G. J. L. Van Der Wegen, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 15, no. 12, 1981. G. J. L. Van Der Wegen, A. De Rooy, P. M. Bronsveld, J. Th. M. De Hosson, Scripta Met. 15, no. 12, 1981.
F I T A N D M I S F I T IN I M A G I N G O F CRYSTALS J.
"UNFIT"
Van L a n d u y t C e n t r u m voor H o o g s p a n n i n g s e l e k t r o n e n m i k r o s k o p i e , U n i v e r s i t y of A n t w e r p , R U C A , G r o e n e n b o r g e r l a a n 171, B-2020 Antwerp, Belgium
A f t e r a s h o r t r e v i e w of the v a r i o u s t y p e s of t w o - d i m e n s i o n a l d e f e c t s , the
i m a g i n g p r o p e r t i e s a r e s u m m a r i z e d as d e r i v e d for t w o - b e a m c o n d i t i o n s f r o m d y n a m ical d i f f r a c t i o n t h e o r y i n c l u d i n g a n o m a l ous a b s o r p t i o n . 1 D e f e c t s to be d i s c u s s e d are a n t i p h a s e b o u n d a r i e s s t a c k i n g f a u l t s , d o m a i n - and twin b o u n d a r i e s , i n v e r s i o n b o u n d a r i e s and g r a i n b o u n d a r i e s . It w i l l b e s h o w n h o w t h e s e a r e u s u a l l y i m a g e d as f r i n g e p a t t e r n s , a n d h o w the i m a g e c h a r a c t e r i s tics can be u t i l i z e d to d e r i v e c r y s t a l l o g r a p h i c c h a r a c t e r i s t i c s of the d e f e c t s . T h r e e m a i n t y p e s of p a t t e r n s can be distinguished: s-type fringes, 6-type f r i n g e s , and t h i c k n e s s or w e d g e f r i n g e patterns. The a v a i l a b i l i t y of h i g h r e s o l u t i o n m i c r o s c o p e s now a l s o e n a b l e s t h e s e def e c t s t o b e o b s e r v e d in t h e s t r u c t u r e projection imaging mode. The b r i g h t and d a r k - f i e l d m e d i u m m a g n i f i c a t i o n i m a g e s can now be s u i t a b l y c o m p l e m e n t e d by m u l t i p l e b e a m i m a g i n g at w o r k i n g magn i f i c a t i o n s of m i l l i o n t i m e s . In s p e c i m e n s of a p p r o p r i a t e o r i e n t a t i o n , t h i c k n e s s and s t r u c t u r e type the a b o v e m e n t i o n e d d e f e c t s are d i r e c t l y r e v e a l e d . The effect of o b j e c t i v e a p e r t u r e size a n d s p e c i m e n t h i c k n e s s w i l l be i l l u s t r a t e d on the i m a g i n g of s t r u c t u r e s and s t r u c t u r a l d e f e c t s s u c h as a n t i p h a s e b o u n d a r ies, e t c . in o r d e r e d a l l o y s . Gold m a n g a n e s e a n d y - b r a s s w i l l b e u s e d as e x a m p l e s 2,3 f o r i l l u s t r a t i n g t h e i m a g i n g o f s t r u c t u r e s , m i c r o t e x t u r e s and d e f e c t s . I.
2. 3.
S. Amelinckx, J. Van Landuyt, in "Diffraction and Imaging Techniques in Materials Science" vol. i, pp. 107-152 (North-Holland Publishing Company, Amsterdam, 1978). G. Van Tendeloo, S. Amelinckx, Phys. Stat. Sol. (a) 49 (1978) 337. M. Van Sande, J. Van Landuyt, S. Amelinckx, Phys. Stat. Sol. (a) 55 (1979) 41.
THE V O L T A G E D I L E M M A : A P P L I C A T I O N TO MATERIALS SCIENCE G.
Van T e n d e l o o L a b o r a t o r y for High V o l t a g e E l e c t r o n M i c r o s c o p y , U n i v e r s i t y of A n t w e r p , R U C A , G r o e n e n b o r g ~ r l a a n 171, B - 2 0 2 0 Antwerp, Belgium
The f i r s t e l e c t r o n m i c r o s c o p e was b u i l t in 1 9 3 1 b y E. R u s k a a n d M. K n o l l . A t t h a t t i m e t h e y w e r e d e l i g h t e d to d e m o n s t r a t e a r e s o l u t i o n b e t t e r than any o p t i c a l m i c r o s c o p e c o u l d do. Now almost e x a c t l y f i f t y y e a r s l a t e r we h a v e m o d i f i e d t h e i r p r o t o t y p e to s u c h an e x t e n t t h a t a t o m i c r e s o l u t i o n is n o l o n g e r an idle d r e a m , 3 M e V e l e c t r o n m i c r o s c o p e s h a v e b e e n c o n s t r u c t e d and EELS or X - r a y analysis have b e c o m e generally accepted. C o m m e r c i a l f i r m s are o f f e r i n g a w i d e