COMPARISON OF SITE OCCUPATION DETERMINATIONS BY APFIM AND AEM

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COMPARISON OF SITE OCCUPATION DETERMINATIONS BY APFIM AND AEM

M. Miller, J. Bentley

To cite this version:

M. Miller, J. Bentley. COMPARISON OF SITE OCCUPATION DETERMINATIONS BY APFIM AND AEM. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-463-C7-468.

�10.1051/jphyscol:1986778�. �jpa-00225973�

Colloque C7, supplement au n o 11, Tome 47, Novembre 1 9 8 6

COMPARISON OF SITE OCCUPATION DETERMINATIONS BY APFIM AND AEM

M.K. MILLER and J. BENTLEY

Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

A b s t r a c t - ^A c o m p a r i s o n o f t h e s i t e o c c u p a t i o n o f a s u b s t i t u t i o n a l element determined from atom probe f i e l d - i o n microscopy and from zone a x i s e l e c t r o n channel- ing m i c r o a n a l y s i s i n t h e a n a l y t i c a l e l e c t r o n m i c r o s c o p e i s p r e s e n t e d . T h e s e t e c h n i q u e s permit t h e s i t e occupation f o r m u l t i p l e elements t o be determined i n p o l y c r y s t a l l i n e specimens. The m a t e r i a l used i n t h e s e experiments was a n L12 o r d e r e d (A3B) N i - 24 at.% A 1 - 0.24 a t . % B s e r i e s o f a l l o y s c o n t a i n i n g a d d i t i o n s o f hafnium, c o b a l t , o r i r o n . The r e s u l t s o f t h e s e two t e c h n i q u e s i n d i c a t e d t h a t , f o r t h e p a r t i c u l a r a l l o y compositions s t u d i e d , hafnium h a s a s t r o n g p r e f e r e n c e f o r t h e aluminum s i t e s , c o b a l t has a s t r o n g p r e f e r e n c e f a r t h e n i c k e l s i t e s , and i r o n has a weak p r e f e r e n c e f o r t h e aluminum s i t e s . These r e s u l t s a r e i n agreement w i t h t h e s i t e p r e f e r e n c e s u g g e s t e d from t h e d i r e c t i o n o f t h e s o l u b i l i t y l o b e s i n e x p e r i m e n t a l l y determined t e r n a r y phase diagrams.

I INTRODUCTION

The l o c a t i o n o f an a l l o y i n g a d d i t i o n i n a n ordered l a t t i c e is an important parameter i n a l l o y development. The s i t e l o c a t i o n of t h e s e a d d i t i o n s can be determined by e i t h e r atom probe f i e l d - i o n microscopy (APFIM) [ I ] o r i n t h e a n a l y t i c a l e l e c t r o n microscope (AEM) by zone a x i s e l e c t r o n channeling m i c r o a n a l y s i s

which is a n e x t e n s i o n o f t h e ALCHEMI method [3]. Both t h e s e t e c h n i q u e s have t h e advantage t h a t p o l y c r y s t a l l i n e specimens can be used and s i n c e t h e ordered matrix can be s e l e c t e d f o r a n a l y s i s t h e y a r e n o t a f f e c t e d by any o t h e r phases t h a t might be prese'nt i n t h e m i c r o s t r u c t u r e . This is i n c o n t r a s t t o t h e s i n g l e c r y s t a l specimens r e q u i r e d by t h e more c o n v e n t i o n a l X-ray d i f f r a c t i o n and ion channeling techniques. The APFIM and AEM t e c h n i q u e s a r e a l s o capable o f q u a n t i f y i n g t h e s i t e occupation f o r m u l t i p l e e l e m e n t s s i m u l t a n e o u s l y . The t e c h n i q u e s r e q u i r e n o s p e c i a l e q u i p m e n t o r m o d i f i c a t ions t o e x i s t i n g i n s t r u m e n t s . The atom probe has t h e add it i o n a l advantage o f being a b l e t o examine m a t e r i a l s t h a t are f a r from t h e s t o i c h i o m e t r i c composition.

I n t h i s paper a comparison o f t h e s i t e p r e f e r e n c e o f s u b s t i t u t i o n a l e l e m e n t s measured by t h e s e two t e c h n i q u e s is presented. A s e r i e s o f n i c k e l aluminides was s e l e c t e d f o r t h e comparison s i n c e many d i f f e r e n t elements a r e s o l u b l e i n t h e L12 o r d e r e d Ni3A1 phase and t h e degree o f long r a n g e o r d e r e x h i b i t e d by t h i s phase is high. S e v e r a l s u b s t i t u t i o n a l elements have been added t o t h e Ni3A1 system t o r e f i n e its good p r o p e r t i e s f o r h i g h temperature a p p l i c a t i o n s C41. Three s u b s t i t u t i o n a l a d d i t i o n s were s e l e c t e d f o r t h i s c o n t r o l l e d s t u d y because o f t h e i r a n t i c i p a t e d d i f f e r e n t s i t e p r e f e r e n c e .

I1 EXPERIMENTAL

The compositions o f t h e a l l o y s chosen f o r t h i s i n v e s t i g a t i o n a r e summarized i n Table 1 . The a l l o y s were based on a composition of N i - 24.0 a t . % A 1 - 0.24 a t . % B with a d d i t i o n s o f hafnium, c o b a l t and i r o n . The aluminum and n i c k e l l e v e l s i n t h e a l l o y s were a d j u s t e d assuming t h a t t h e s u b s t i t u t i o n a l element a d d i t i o n s would p r e f e r a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986778

C7-464 JOURNAL DE PHYSIQUE

s p e c i f i c s i t e i n t h e ordered l a t t i c e . The a l l o y s were a l l w i t h i n t h e s o l u b i l i t y l i m i t s o f t h e L12 phase f i e l d i n t h e i r r e s p e c t i v e t e r n a r y phase diagrams.

Table 1. Nominal Compositions o f Alloys

atomic 5 ).

S u b s t i t u t i o n a l Element 0.5 % Hf

1 - 8 Hf 22.9 75.8 0;24

3 % Hf 20.9 75.8 0.24

6 % Co 69.8 0.24

6 % Fe 20;9 72 ;8 0.24

The APFIM a n a l y s e s were conducted on t h e ORNL energy-compensated atom probe [5].

The e l e c t r o n channeling m i c r o a n a l y s i s was performed on P h i l i p s EM400T/FEG and EM430T a n a l y t i c a l e l e c t r o n microscopes equipped w i t h EDAX 9100/70 EDS systems.

I11 ATOM PROBE MICROANALYSIS

The s i t e occupation o f a s u b s t i t u t i o n a l element is c a l c u l a t e d i n t h e atom probe from t h e p r e f e r e n c e o f t h a t element between t h e d i f f e r e n t atomic l a y e r s i n t h e o r d e r e d s t r u c t u r e . The optimum p l a n e s t o a n a l y z e a r e t h o s e with t h e l a r g e s t composition d i f f e r e n c e i n s u c c e s s i v e l a y e r s , i d e a l l y pure A v e r s u s pure B, and which have t h e maximum p l a n a r d e n s i t y o f atoms. Unfortunately t h e L12 (A3B) s t r u c t u r e does n o t e x h i b i t any p l a n e s w i t h a n A:B s t a c k i n g sequence. The 001 p l a n e s , which a l t e r n a t e between pure A , and mixed A+B, f u l f i l l t h e above c r i t e r i a i n t h e L12 s t r u c t u r e and were t h e r e f o r e used f o r a l l a n a l y s e s .

The p o s i t i o n of t h e probe a p e r t u r e must be c a r e f u l l y chosen t o p r e v e n t a r t i f a c t s . Three p o s s i b l e p o s i t i o n s a r e shown s c h e m a t i c a l l y i n f i g u r e 1. The p o s i t i o n o f t h e probe a p e r t u r e is chosen t o e n s u r e t h a t a l l atoms from t h e f i r s t l a y e r a r e c o l l e c t e d b e f o r e any atoms a r e c o l l e c t e d from t h e n e x t plane. P o s i t i o n X f u l f i l l s t h e s e r e q u i r e m e n t s . P o s i t i o n Y is n o t s u i t a b l e a s two p l a n e s w i l l be s a m p l e d s i m l t a n e o u s l y . P o s i t i o n Z should n o t be used because o f t h e t r a j e c t o r y a b e r r a t i o n s t h a t have been-observed a t t h e c e n t e r o f p o l e s C61.

Fig. 1. Schematic diagram o f t h e e f f e c t i v e p o s i t i o n o f t h e probe a p e r t u r e w i t h

r e s p e c t t o t h e pole. I n p o s i t i o n X , a l l atoms from l a y e r 1 a r e c o l l e c t e d b e f o r e any

atoms a r e c o l l e c t e d . f r o m l a y e r 2. P o s i t i o n Y i s n o t used a s two l a y e r s a r e sampled

simultaneously. P o s i t i o n Z is n o t used because o f t r a j e c t o r y a b e r r a t i o n s .

than those normally demanded i n atom probe composition determinations a s shown i n f i g u r e 2. I n t h i s ladder diagram a comparison is made between an a n a l y s i s from a high index r e g i o n and an a n a l y s i s on a 001 pole using i d e n t i c a l e x p e r i m e n t a l conditions. I n t h i s t y p e of diagram t h e a r r i v a l o f each n i c k e l atom is p l o t t e d a s one h o r i z o n t a l s t e p and t h e a r r i v a l o f each aluminum a s one v e r t i c a l s t e p . It is e v i d e n t from t h e o v e r a l l s l o p e of t h e curve and from t h e number of atoms c o l l e c t e d i n b o t h t h e mixed and t h e p u r e n i c k e l l a y e r s t h a t n i c k e l atoms a r e b e i n g p r e f e r e n t i a l l y evaporated when t h e a n a l y s i s is performed on t h e pole, whereas t h e a n a l y s i s i n t h e high index region produces a r e l i a b l e composition. The f a c i l i t y t o view t h e d a t a a s a ladder diagram during t h e experiment was found t o be invaluable i n s e l e c t i n g s u i t a b l e experimental c o n d i t i o n s [5]. The pulse f r a c t i o n , s t a n d i n g v o l t a g e , and specimen t e m p e r a t u r e were s e l e c t e d t o ensure t h a t t h e r e was no p r e f e r e n t i a l evaporation o r r e t e n t i o n o f any element [7]. P r e f e r e n t i a l r e t e n t i o n can be d e t e c t e d from t h e f i e l d - i o n micrograph a s shown i n t h e 1% hafnium a l l o y , f i g u r e 3. P r e f e r e n t i a l r e t e n t i o n was a l s o observed by t h e non-uniform evaporation o r pinning of p l a n e s when i n a p p r o p r i a t e c o n d i t i o n s were employed. I t should a l s o be noted t h a t extremely slow evaporation r a t e s (0.1 t o 0.001 atoms per p u l s e ) were found t o be necessary t o c o l l e c t a l l atoms, e s p e c i a l l y a t t h e end of plane where t h e instantaneous evaporation r a t e is much higher.

A s h o r t s e c t i o n o f t h e atom probe d a t a f o r each s u b s t i t u t i o n a l element is shown i n f i g u r e 4. The p o s i t i o n i n t h e evaporation sequence of t h e s u b s t i t u t i o n a l elements is superimposed on t h e s e l a d d e r diagrams. I f t h e s o l u t e has a preference f o r t h e aluminum s i t e s , which a r e only p r e s e n t i n t h e mixed l a y e r s , then t h a t element w i l l be observed i n t h e s e c t i o n s with t h e 45O slope. This is t h e case f o r hafnium. When t h e s u b s t i t u t i o n a l element does not have a s t r o n g preference f o r e i t h e r s i t e then t h e element w i l l be observed on both t y p e s of l a y e r s . This is t h e case f o r iron.

I f t h e s u b s t i t u t i o n a l element has a preference f o r t h e n i c k e l s i t e s then twice a s much of t h e element w i l l be found on t h e h o r i z o n t a l pure n i c k e l l a y e r s a s on t h e mixed l a y e r s . This is t h e c a s e f o r c o b a l t . The s i t e preference is c a l c u l a t e d from t h e number o f s u b s t i t u t i o n a l atoms d e t e c t e d on each type o f plane t a k i n g i n t o account t h e number of n i c k e l and aluminum s i t e s t h a t a r e p r e s e n t i n each plane and t h e a l l o y composition.

Nickel

Fig. 2. Comparison of ladder diagrams o f t h e 0.5% Hf a l l o y taken on t h e 001 pole

and from a high index r e g i o n under i d e n t i c a l experimental conditions. The numbers

i n t h e i n s e r t i n d i c a t e t h e d i f f e r e n c e i n t h e number of atoms c o l l e c t e d i n each type

of l a y e r . Note t h e r e t e n t i o n of Hf t o t h e end of t h e layer.

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F i g . 3. F i e l d - i o n m i c r o g r a p h showing p r e f e r e n t i a l r e t e n t i o n on 001 pole.

Fig. 4. Ladder diagrams o f t h e 001 p l a n e s i n t h e . l % H f , 6% Fe and 6% Co a l l o y s . The h o r i z o n t a l s e c t i o n s a r e t h e n i c k e l l a y e r s and t h e s e c t i o n s with t h e 4 5 O s l o p e a r e t h e mixed n i c k e l p l u s aluminum l a y e r s . Arrows i n d i c a t e p o s i t i o n o f s u b s t i t u t i o n a l elements.

200

N i c k e l A t o m s

N i c k e l A t o m s -

- - -

-

- - 6 % C O B A L T

-

-

-.

- - - -

- -

6 % I R O N

4 4 4 4

0 100 2 0 0 3 0 0 400

N i c k e l A t o m s

p

^{4 4}

Electron channeling microanalysis is a l s o complicated by t h e absence o f any planes i n t h e L12 s t r u c t u r e containing only B atoms. However, c e r t a i n zone axes, including

<001> and < I l l > , c o n t a i n columns of A atoms separated from columns of B atoms. The c o n c e n t r a t i o n , CX, of element X on t h e ^B s i t e s can be shown t o be given by

zone

where R ( X / A )

( N X / N A ) / ( N X / N A ) random , e t c . and N x , N A , and N B a r e t h e c h a r a c t e r i s t i c X-ray i n t e n s i t i e s of e l e m e n t s X. A , and B. The p r o c e d u r e t o determine t h e s i t e occupation is t h e r e f o r e t o record energy d i s p e r s i v e X-ray s p e c t r a a t two o r i e n t a t i o n s - a n a p p r o p r i a t e c h a n n e l i n g zone a x i s and a "randomn non-channeling condition.

An example of t h e zone a x i s e l e c t r o n channeling microanalysis is shown i n f i g u r e 5 f o r t h e 3% hafnium a l l o y . The r a t i o s of Hf/Ni and A l / N i were both observed t o i n c r e a s e from t h e on-axis condition t o the l o _and

d e v i a t i o n s from zone a x i s i n d i c a t i n g t h a t hafnium follows t h e aluminum behavior. Unfortunately, c o r r e c t i o n s f o r i o n i z a t i o n d e l o c a l i z a t i o n , which a r e normally ignored [2], were found t o be necessary. The A l K / N i K r a t i o s were corrected by comparisons with t h e N i L / N i K and t h e HfM/HfL r a t i o s which were found t o change d r a m a t i c a l l y f o r small d e v i a t i o n s from t h e zone a x i s , f i g u r e 5.

V DISCUSSION

The q d a n t i f i c a t i o n of t h e r e s u l t s f o r a l l t h r e e s u b s t i t u t i o n a l elements from t h e two techniques is given i n Table

Fig. 5. Zone a x i s e l e c t r o n channeling microanalysis of t h e 3% Hf a l l o y taken near

t h e 111 zone. ( a , b , and c ) a r e energy d i s p e r s i v e X-ray s p e c t r a , a t 100kV, and ( d , e ,

and f ) a r e t h e corresponding d i f f r a c t i o n conditions. Note t h e change i n i n t e n s i t i e s

r e l a t i v e t o N i Kg.

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Table 2. S i t e occupation o f s u b s t i t u t i o n a l element on aluminum s i t e s ( X ) .

I f t h e s o l u t e h a s no p r e f e r e n c e between t h e two t y p e s o f s i t e s i n t h e p e r f e c t L12 (A3B) s t r u c t u r e , t h e n 25% o f t h e s u b s t i t u t i o n a l element should be on t h e B (aluminum) s i t e s and t h e remainder on t h e A ( n i c k e l ) s i t e s . The zone a x i s e l e c t r o n channeling m i c r o a n a l y s i s r e s u l t s c o r r e c t e d f o r i o n i z a t i o n d e l o c a l i z a t i o n e f f e c t s and t h e atom probe r e s u l t s both i n d i c a t e t h e same p r e f e r e n c e f o r a l l t h r e e elements;

h a f n i u m h a s a s t r o n g p r e f e r e n c e f o r t h e aluminum s i t e s , c o b a l t h a s a s t r o n g p r e f e r e n c e f o r t h e n i c k e l sites, and i r o n h a s a weak p r e f e r e n c e f o r t h e aluminum s i t e s . I n a d d i t i o n t o t h e s e v e r e e f f e c t s o f i o n i z a t i o n d e l o c a l i z a t i o n , t h e r e l i a b l l i t y o f t h e channeling can be compromised by t h e presence o f s u r f a c e f i l m s , a n t i s i t e d e f e c t s , a n t i p h a s e boundaries and d i s l o c a t i o n s . The atom probe r e s u l t s are not a f f e c t e d by t h e s e f e a t u r e s . The s m a l l d i f f e r e n c e s i n magnitude between t h e APFIM and AEM r e s u l t s could be e x p l a i n e d by s u c h e f f e c t s .

Both sets o f r e s u l t s a r e i n agreement w i t h t h e s i t e p r e f e r e n c e suggested by O c h i a i , Oya, and Suzuki C83, based on t h e d i r e c t i o n o f t h e s o l u b i l i t y l o b e s i n t h e t e r n a r y phase diagrams determined from t h e experimental d a t a o f Schramm [9], and Bradley [lo]. This agreement would seem t o s u g g e s t t h a t t h e p o s i t i o n o f t h e s o l u b i l i t y l o b e s is a r e l i a b l e i n d i c a t i o n o f s i t e p r e f e r e n c e .

V I CONCLUSIONS

The r e s u l t s of t h e s e two techniques were i n agreement f o r a l l t h e s u b s t i t u t i o n a l elements s t u d i e d . The r e s u l t s i n d i c a t e d t h a t , f o r t h e p a r t i c u l a r a l l o y compositions s t u d i e d , hafnium h a s a s t r o n g p r e f e r e n c e f o r t h e aluminum s i t e s , c o b a l t h a s a s t r o n g p r e f e r e n c e f o r t h e n i c k e l s i t e s , and i r o n has a weak p r e f e r e n c e f o r t h e aluminum s i t e s . These r e s u l t s were a l s o i n agreement w i t h t h e s i t e p r e f e r e n c e suggested from t h e d l r e c t i o n of t h e s o l u b i l i t y l o b e s i n e x p e r i m e n t a l l y determined t e r n a r y phase d i a g r a m s . T h i s s t u d y has a l s o i n d i c a t e d t h a t c a r e should be e x e r c i s e d i n t h e c o r r e c t implementation o f t h e s e experimental techniques.

Acknowledgments

Research sponsored by 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. Department o f Energy, under c o n t r a c t DE-AC05-840R21400 w i t h Martin M a r i e t t a Energy Systems, Inc. The a u t h o r s would l i k e t o thank J.A. Horton f o r h i s a s s i s t a n c e .

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[9] J. Schramm, 2. Metallk, 33, 403 (1941

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