FIM OBSERVATION OF GP[2] ZONE IN Al-Cu ALLOY

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FIM OBSERVATION OF GP[2] ZONE IN Al-Cu ALLOY

M. Wada, H. Kita, T. Mori, O. Nishikawa

To cite this version:

M. Wada, H. Kita, T. Mori, O. Nishikawa. FIM OBSERVATION OF GP[2] ZONE IN Al-Cu ALLOY.

Journal de Physique Colloques, 1984, 45 (C9), pp.C9-251-C6-255. �10.1051/jphyscol:1984942�. �jpa-

00224422�

JOURNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome *5, décembre 198* page C9-251

FIM OBSERVATION OF G P [ 2 ] ZONE IN A l ~ C u ALLOY

M. Wada, H. K i t a , T. Mori and 0 . Nishikawa

Department of Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute of Technology, 42S9 Nagatsuta, Midori-ku, Yokohama 227, Japan

Résumé - Les zones de Guinier-Preston GP[2] dans un monocristal Al-4% pds Cu ont été examinées en microscopie ionique de champ.

Abstract - GP[2] zones in an aged Al - 4wt% Cu single crystal were examined by a field ion microscope.

In a previous study / l / , it was demonstrated that by the FIM, individ- ual Cu atoms of a GP[1] zone in an aged Al-4wt% Cu could be resolved into isolated spots in regions where matrix image was observed. The observed image indicated that GP[1] was a single layer of Cu atoms.

In this study, the structure of GP[2] zones was examined by the FIM.

Al - 4wt% Cu single crystals which had been homogenized for 200 h at 793 K, solution-treated for 2 h at 793 K and quenched into ice water were used. They were then aged for 17 h at 403 K and subsequently for

5 h at 44 3 K. Single crystal rods of about 0.6 mm in diameter and 20 mm in length were prepared by mechanical cutting and polishing in such a way that the long axis was along [001]. To prepare FIM specimens, they were electropolished in a solution of HC1(1), HN03(1) and water

(2) at ~5 V dc. Imaging gas was 10~3pa of Ne and the background pressure was ~10~

Pa. The specimen temperature at the observation was 20 K to 30 K.

All the images shown here except Fig. 6 were obtained from four areas around the (022), (202), (022) and (202) planes on the tip surface.

For the sake of simplicity, however, we do not discriminate them and simply consider that they are for the (022)area. First, we consider why GP zones are imaged. In Fig. 1(a), a GP[1] parallel to the tip axis is seen on the (022) plane. Although individual Cu atoms are not resolved on this plane because of a small spacing between Cu atoms, the bright line is quite likely the image of a row of Cu atoms of a GP[1] zone formed on the (200) plane. It is possible that these Cu atoms are protruded from the (022) surface of Al matrix. Fig. 1(b) is an image right after the row of Cu atoms on the (022) plane has been field evaporated. After these protruded atoms are removed, the top

(022) atomic plane is flat and no Cu atoms is on the top layer. This is shown in Fig. 2. Although one may expect a uniform contrast on the (022) plane as shown in Fig. 2(b), Fig. 1(b) shows a dark line on the position of the zone. Thus the presence of the Cu atoms seems to reduce the ionization probability of the imaging gas, and therefore the bright line on the zone in Fig. 1(a) should be due to the geomet- rical protrusion of Cu atoms on the (022) surface.

For the imaging of a GP[2] zone which is formed on (200) and observed

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984942

C9-252 JOURNAL DE PHYSIQUE

F i g . 1 - ( a ) ; A G P [ l l formed on t h e (200) and o b s e r v e d on t h e (022) s u r f a c e i n t h e [001] o r i e n t e d t i p . ( b ) ; A f t e r t h e imaged Cu atoms i n

( a ) a r e f i e l d e v a p o r a t e d .

F i g . 2 - V e r t i c a l - s e c t i o n of t h e (022) p l a n e . The p l a n e on t h e paper i s ( 0 2 2 ) . Atoms marked by a h o r i z o n t a l b a r a r e l o c a t e d above and below t h e p l a n e o f t h e p a p e r . Shaded c i r c l e s a r e Cu atoms o f a G P [ l ]

zone on t h e (200) p l a n e .

Ffg. 3 - V e r t i c a l - s e c t i o n o f t h e (022) p l a n e . C r y s t a l l o g r a p h i c

situation i s i d e n t i c a l t o t h a t i n F i g . 2.

Fig. 4 - An FIM image o f a GP[2]

formed on t h e (200) p l a n e and ob- s e r v e d on t h e (022) s u r f a c e o f t h e

[001] o r i e n t e d t i p . The e s t i m a t e d s e p a r a t i o n o f t h e two rows o f s p o t s

s a b o u t 1.1 nm.

on t h e (022) p l a n e , two c a s e s must b e c o n s i d e r e d and t h e y a r e i l l u s - t r a t e d i n F i g . 3. F i g . 3 ( a ) i s t h e c a s e when an odd number o f t h e

(200) A 1 l a y e r s a r e p r e s e n t between t h e Cu l a y e r s . Under t h i s s i t u a - t i o n , we e x p e c t two b r i g h t l i n e s on t h e (0221 p l a n e . When an even number o f t h e (200) A 1 l a y e r s a r e between t h e Cu l a y e r s , a geometry shown i n F i g . 3 ( b ) can be e x p e c t e d and we may o b s e r v e a s i n g l e l i n e due t o t h e Cu l a y e r i n t h e l e f t . When t h e t o p l a y e r o f t h e (022) p l a n e i s f i e l d e v a p o r a t e d , t h e n t h e o t h e r Cu l a y e r i n t h e r i g h t would b e o b s e r v e d a s shown i n F i g . 3 ( c ) . ^{F i g . 4 i s} an image o b s e r v e d on t h e

(022) p l a n e . Two b r i g h t l i n e s a r e s e e n , a l t h o u g h t h e y a r e n o t con- t i n u o u s . Judging from F i g . 3 , it i s p o s s i b l e t h a t an odd number o f t h e (200) a t o m i c l a y e r s a r e p r e s e n t between t h e Cu l a y e r s . S i n c e m a t r i x images i n t h e l e f t and t h e r i g h t s i d e s o f t h e (022) s u r f a c e i n F i g . 4 are n o t v i s i b l e , an a c c u r a t e d e t e r m i n a t i o n o f t h e m a g n i f i c a t i o n of t h e o b s e r v e d GP[21 image i s n o t p o s s i b l e . By g e o m e t r i c a l c o n s i d e r - a t i o n and c o u n t i n g t h e r i n g numbers between two p o l e s o f t h e i d e n t i - f i e d p l a n e s , however, a r a d i u s o f c u r v a t u r e o f t h e r e g i o n whose m a t r i x image i s c l e a r l y o b s e r v e d can b e e s t i m a t e d . I t i s a b o u t 33 nm and t h i s i n d i c a t e s a b o u t 1.1 nm s p a c i n g between t h e two b r i g h t l i n e s . I f t h r e e ( 2 0 0 ) atomic l a y e r s a r e p r e s e n t between two Cu l a y e r s , we e x p e c t a s p a c i n g o f 0.8 nm and i f f i v e l a y e r s a r e between t h e Cu l a y e r s , t h e s p a c i n g w i l l b e a b o u t 1 . 2 nm, s i n c e t h e (200) s p a c i n g o f m a t r i x i s 0.2 nm. T h e r e f o r e t h e l a t t e r would be t h e c a s e f o r t h e o b s e r v e d image i n F i g . 4 . C e r t a i n l y , t h i s i s n o t t h e r e p r e s e n t a t i v e GP[21 zone, s i n c e t h i s i s t h e o n l y image o f a good q u a l i t y we have o b t a i n e d s o f a r . I n f a c t a c c o r d i n g t o a TEM s t u d y /2/, GP[2] zones w i t h 3 and 4 a t o m i c l a y e r s between Cu l a y e r s a r e f r e q u e n t l y o b s e r v e d . Major problem o f t h e p r e s e n t method i s t h a t a p r o b a b i l i t y o f f i n d i n g a GP[2] zone on t h e c e n t e r o f t h e I2201 p l a n e s on FIM specimens i s q u i t e s m a l l b e c a u s e o f s m a l l s i z e s of t h e t o p C2201 p l a n e s on t h e s p h e r i c a l FIM t i p .

F u r t h e r m o r e , FIM images of GP[2] zones a r e q u i t e u n s t a b l e and a c l e a r m a t r i x image c a n b e o b t a i n e d o n l y on l i m i t e d s m a l l a r e a s such a s t h a t

shown i n F i g . 4 . Although t h e r e was an i n d i c a t i o n o f t h e s i t u a t i o n shown i n F i g s . 3 ( b ) and 3 ( c ) d u r i n g t h e o b s e r v a t i o n , we were n o t s u c c e s s f u l t o r e c o r d them on t h e photographs.

F i g . 5 shows images o f GP[21 zones formed on t h e (020) p l a n e . Unlike t h e GP[2] i n F i g . 4 , t h e p l a n e o f t h e zones a r e n o t p a r a l l e l t o t h e s u r f a c e normal i n t h i s c a s e . Because o f a poor q u a l i t y o f t h e m a t r i x image, o n l y rough e s t i m a t e s o f t h e w i d t h s o f t h e zones can b e made.

The r a d i u s of c u r v a t u r e f o r F i g . 5 ( a ) i s e s t i m a t e d t o b e i n a r a n g e

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Fig. 5 - FIM images of GP[2] zones formed on the (020) plane of the [001] oriented tip. The plane of the zone is not parallel to the surface normal.

Fig. 6 - (a); Concentric rings of bright spots observed on the (002) surface. (b); Image of a pure A1 in the same area as that in (a).

from 80 to 95 nm. For Fig. 5(b), the ring counting is difficult and it is assumed that the radius is proportional to the applied voltage.

The separation of the two bright lines on the left of the (046) plane in Fig. 5(a) is in a range from 1.2 to 1.4 nm, and for those in Fig.

5(b), they are between 1.0 and 1.2 nm. These values are somewhat larger than the TEM estimate of the spacing (0.8 nm and 1.0 nm)/2/.

It is possible that the present method overestimates the width of the zones, because a local magnification on the protruded Cu layers may be higher than the average magnification in the area.

In Fig. 6(a), an image observed near the (002) surface plane is shown.

Two concentric rings of bright spots, although not quite clear, can be

noticed. This is probably due to two Cu planes Cormed perpendicular

to the [001] tip axis. On the FIM image of the A1-Cu alloy, matrix

of t h i s r e g i o n around t h e (002) s u r f a c e i s r a t h e r d a r k and o n l y t h e s p o t s p o s s i b l y due t o Cu atoms a r e imaged. I n F i g . 6 ( b ) , an image of t h e same a r e a of a pure A 1 i s shown. The t i p r a d i i o f t h e two

speimens a r e c o n s i d e r e d t o be s i m i l a r b e c a u s e of t h e s i m i l a r a p p l i e d v o l t a g e s . B r i g h t n e s s o f t h e c e n t r a l r e g i o n i n F i g . 6 ( b ) i s photo- g r a p h i c a l l y i n c r e a s e d . , Comparison o f two images i n d i c a t e s t h a t t h e r e a r e p o s s i b l y t h r e e (002) atomic l a y e r s between t h e two b r i g h t r i n g s . T h i s s t r u c t u r e c o i n c i d e s w i t h one o f t h o s e f r e q u e n t l y observed GP[2]

zones by TEM.

P r e s e n t s t u d y can be summarized a s f o l l o w s . GP zones i n an aged Al-4wt% Cu a l l o y can be imaged by FIM b e c a u s e o f t h e p r o t r u s i o n o f Cu l a y e r s on t h e t i p s u r f a c e . A GP 121 zone formed on t h e (200) p l a n e and observed on t h e (022) s u r f a c e o f t h e [001] o r i e n t e d t i p c a n b e o b s e r v e d a s two rows o f b r i g h t s p o t s i f an odd number o f t h e (200) a t o m i c l a y e r s a r e p r e s e n t between t h e Cu l a y e r s . I t i s a l s o i n d i c a t e d t h a t t h e imaging c o n d i t i o n on t h e (022) s u r f a c e would be d i f f e r e n t and more c o m p l i c a t e d i f an even number o f t h e (200) l a y e r s a r e between t h e Cu l a y e r s . The e s t i m a t e d s p a c i n g s o f t h e p a r a l l e l b r i g h t l i n e s due t o GP[2] zones which a r e p a r a l l e l t o t h e t i p a x i s a r e i n t h e r a n g e from 1.1 nm t o 1 . 4 nm. T h i s i s l a r g e r t h a n t h e r a n g e o f t h e s e p a r a t i o n s e s t i m a t e d by a TEM s t u d y and i s p o s s i b l y due t o a l o c a l l y h i g h e r m a g n i f i c a t i o n on t h e p r o t r u d e d Cu l a y e r s on t h e t i p s u r f a c e comapared w i t h t h e a v e r a g e m a g n i f i c a t i o n determined by t h e m a t r i x image. A GP[2] zone p e r p e n d i c u l a r t o t h e [001] t i p a x i s was observed a s concen- t r i c r i n g s o f b r i g h t s p o t s on t h e (002) s u r f a c e o f t h e t i p . I t i s i n d i c a t e d t h a t t h r e e (002) atomic l a y e r s a r e p r e s e n t between t h e Cu l a y e r s .

R e f e r e n c e s

f l ] Mori,T., Wada,M., K i t a , H . , Uemori,R., H o r i e , S . , Sato,A., and Nishikawa,O., J a p a n e s e J . Appl. Phys. 22 (1983) L203.

[ 2 ] P h i l i p s , V . A . , Acta M e t a l l . 2 (19%) 7 5 1 .