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NUCLEATION AND GROWTH OF APERIODIC CRYSTALS IN ALUMINUM ALLOYS

R. Schaefer, L. Bendersky, F. Biancaniello

To cite this version:

R. Schaefer, L. Bendersky, F. Biancaniello. NUCLEATION AND GROWTH OF APERIODIC CRYS- TALS IN ALUMINUM ALLOYS. Journal de Physique Colloques, 1986, 47 (C3), pp.C3-311-C3-320.

�10.1051/jphyscol:1986332�. �jpa-00225744�

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JOURNAL DE PHYSIQUE

Colloque C3, supplement au n o 7, Tome 47, juillet 1986

NUCLEATION AND GROWTH OF APERIODIC CRYSTALS IN ALUMINUM ALLOYS

R.J. SCHAEFER, L.A. BENDERSKY and F.S. BIANCANIELLO

Metallurgy Division, National Bureau of Standards.

Gaithersburg, MD 20899, U.S.A.

A b s t r a c t

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The i c o s a h e d r a l and decagonal a p e r i o d i c phases dominate t h e m i c r o - s t r u c t u r e s o f r a p i d l y s o l i d i f i e d A1 -Mn a1 l a y s because t h e i r n u c l e a r i o n and growth b e h a v i o r d i f f e r s s u b s t a n t i a l l y from t h a t o f t h e e q u i l i b r i u m phases. E l e c t r o n beam s u r f a c e m e l t i n g can be used t o produce a wide range o f s o l i d i f i c a t i o n c o n d i t i o n s , i n which t h e d i f f e r e n t stages o f t h e n u c l e a t i o n and growth processes can be observed. It i s found t h a t t h e i c o s a h e d r a l phase n u c l e a t e s a b u n d a n t l y i n s u p e r c o o l e d A1-Mn m e l t s , and t h a t t h e decagonal phase i s subsequently n u c l e a t e d by t h e i c o s a h e d r a l phase. A d d i t i o n o f S i t o t h e Al-Mn a l l o y s suppresses f o r m a t i o n o f t h e decagonal phase, b u t i n t h e s e a l l o y s t h e hexagonal p phase can grow r a p i d l y .

I

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INTRODUCTION

I n r a p i d l y s o l i d i f i e d A1-Mn a l l o y s , a p e r i o d i c c r y s t a l s form by a n u c l e a t i o n and growth process and t h e i n d i v i d u a l c r y s t a l s r e a c h d i a m e t e r s o f t y p i c a l l y a few m i c r o m e t e r s b e f o r e t h e i r growth i s stopped. Moreover, t h e y a r e u s u a l l y embedded

i n a m a t r i x o f f.c.c. A l , and f r e q u e n t l y e x i s t as m i x t u r e s o f t h e i c o s a h e d r a l [I]

and decagonal [2] phases. Although much has been l e a r n e d from s t u d y o f such m a t e r i a l s , t h e y a r e f a r from i d e a l f o r many measurements. Thus t h e y may be q u i t e s u i t a b l e f o r c e r t a i n TEM s t u d i e s , somewhat 1 ess s a t i s f a c t o r y f o r e x a m i n a t i o n by x - r a y and n e u t r o n d i f f r a c t i o n and a l m o s t u s e l e s s f o r o t h e r s t u d i e s such as measure- ment o f t r a n s p o r t p r o p e r t i e s .

Looked a t f r o m a more p o s i t i v e v i e w p o i n t , however, s t u d i e s o f t h e m i c r o s t r u c t u r a l d e t a i l s o f t h e s e samples p r o v i d e us w i t h a g r e a t d e a l o f i n f o r m a t i o n a b o u t t h e n u c l e a t i o n and growth b e h a v i o r o f t h e a p e r i o d i c phases. I t i s found t h a t t h e n u c l e a t i o n b e h a v i o r o f t h e a p e r i o d i c phases i s q u i t e d i f f e r e n t f r o m t h a t o f t h e e q u i l i b r i u m phases o f t h e A1-Mn system, and t h i s d i f f e r e n c e can account f o r t h e dominance o f t h e a p e r i o d i c phases i n r a p i d l y s o l i d i f i e d a l l o y s .

I n t h i s paper, t h e m i c r o s t r u c t u r e s o f r a p i d l y s o l i d i f i e d A1 -Mn a l l o y s w i l l be i n t e r p r e t e d i n terms o f c o n v e n t i o n a l metal s o l i d i f i c a t i o n processes such as n u c l e a t i o n and d e n d r i t i c growth, and t h e i m p l i c a t i o n s w i t h r e s p e c t t o c r y s t a l - m e l t i n t e r f a c e p r o p e r t i e s and sample inhomogeneity w i l l be discussed. Most o f t h e c o n c l u s i o n s w i l l n o t depend on t h e f a c t t h a t t h e phases i n q u e s t i o n have unusual c r y s t a l s t r u c t u r e s .

I 1

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COMPOSITION OF THE PHASES

The f i r s t i c o s a h e d r a l c r y s t a l s r e p o r t e d were observed i n m e l t - s p u n r i b b o n s o f an a l l o y w i t h o v e r a l l c o m p o s i t i o n c l o s e t o t h a t o f AlgMn, i.e., 14 a t % Mn. At t h i s c o m p o s i t i o n , TEM shows t h a t t h e i c o s a h e d r a l c r y s t a l s have grown w i t h a d e n d r i t i c s t r u c t u r e , c h a r a c t e r i s t i c o f a s o l u t e r e d i s t r i b u t i o n process. The i c o s a h e d r a l c r y s t a l s a r e embedded i n f.c.c. A1 which stands o u t s t r o n g l y i n x - r a y d i f f r a c t i o n p a t t e r n s . These f a c t s a l o n e i n d i c a t e t h a t t h e i c o s a h e d r a l phase c r y s t a l s have a Mn c o n t e n t g r e a t e r t h a n 14 at%.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986332

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C3-312 JOURNAL DE PHYSIQUE

In an attempt t o e l i m i n a t e t h e residual A1 , a s e r i e s of melt-spun samples was prepared w i t h higher Mn c o n t e n t s [3]. X-ray d i f f r a c t i o n indicated t h a t t h e A1 peaks disappeared when t h e Mn content reached s l i g h t l y g r e a t e r than 20 a t % . However, a t about 16 a t % an a d d i t i o n a l phase, subsequently i d e n t i f i e d a s decagonal, s t a r t e d t o appear i n t h e x-ray d i f f r a c t i o n p a t t e r n s . The amount of t h i s phase was s t r o n g l y dependent on t h e ribbon t h i c k n e s s , being g r e a t e r i n t h i c k e r ribbons.

I t was found t h a t t h e t h i n n e s t p a r t s of 20 a t % Mn ribbons were almost s i n g l e phase icosahedral but t h e t h i c k e r p a r t s of ribbons with t h e same composition were approximately half icosahedral and half decagonal phase. The composition of t h e icosahedral phase was thus concluded t o be approximately 20 a t % Mn, b u t t h e production of s i n g l e phase material required t h e highest possible cooling r a t e s t o avoid formation of t h e decagonal phase. Other authors [4,5,6] have reached a s i m i l a r conclusion with r e s p e c t t o t h e composition of t h e icosahedral phase.

Ribbons o f 21 t o 22 a t % Mn were s i n g l e phase decagonal except i n t h e i r t h i n n e s t p a r t s where some icosahedral phase was s t i l l present. One might note t h a t none of t h e Al-rich i n t e r m e t a l l i c phases of t h e A1-Mn equilibrium system can be formed a s s i n g l e phase m a t e r i a l s by e i t h e r slow o r rapid cooling an a l l o y having t h e composition of t h e d e s i r e d phase: they a l l form by p e r i t e c t i c r e a c t i o n s and t h e equilibrium d i s t r i b u t i o n of phases would be expected only a f t e r long anneals.

Measurements of t h e position of t h e s t r o n g e s t x-ray d i f f r a c t i o n peak from t h e icosahedral phase showed t h a t a s t h e overall a l l o y composition changed from 8 t o 26 a t % Mn, t h e spacing of t h e planes responsible f o r t h i s peak decreased by approximately 1%. This change i n d i c a t e s t h a t t h e icosahedral phase i s not a

" l i n e compound" with a fixed composition, but forms with a range of compositions.

Not only does t h i s imply t h a t in a l l o y s of d i f f e r i n g i n i t i a l compositions t h e icosahedral c r y s t a l s w i l l have d i f f e r e n t average compositions, i t a l s o implies t h a t by t h e normal processes of a l l o y s o l i d i f i c a t i o n a range of compositions w i l l occur i n d i f f e r e n t p a r t s of an individual icosahedral c r y s t a l [7]. Although t h e x-ray data by themselves a r e not s u f f i c i e n t t o a c c u r a t e l y determine what t h e range of composition of t h e icosahedral phase i s , they c l e a r l y show t h a t caution must be used i n a s s i g n i n g any fixed composition o r l a t t i c e parameter t o t h e icosahedral phase.

Although a composition (20 a t % Mn) was thus i d e n t i f i e d a t which almost s i n g l e - phase icosahedral material can be formed, t h e s o l i d i f i c a t i o n r a t e s required t o avoid formation of t h e decagonal phase a t t h i s composition were d i f f i c u l t t o obtain f o r l a r g e amounts of m a t e r i a l . I t was t h e r e f o r e attempted t o s t a b i l i z e t h e icosahedral phase by replacing a f r a c t i o n of t h e A1 atoms with smaller atoms, thereby s l i g h t l y reducing t h e average s i z e of t h e atoms a t t h e A1 s i t e s . Addition o f Si t o t h e a l l o y was found t o be e f f e c t i v e , with an a l l o y containing 20 a t % Mn and 6 a t % Si being s i n g l e phase icosahedral with no formation of decagonal phase.

Other authors [8] have found t h e same r e s u l t and have a t t r i b u t e d t h e easy

icosahedral phase formation a t t h i s composition t o t h e s i m i l a r i t y i n s t r u c t u r e of t h e icosahedral phase t o t h e cubic t e r n a r y a phase of Al-Mn-Si. There i s nothing c o n t r a d i c t o r y about t h e two explanations f o r t h e beneficial e f f e c t s of adding Si

.

I11 - NUCLEATION OF ICOSAHEDRAL PHASE

To o b t a i n more d e t a i l e d information on t h e c o n d i t i o n s under which t h e a p e r i o d i c phases form, a s e r i e s of e l e c t r o n beam s u r f a c e melting experiments was c a r r i e d out. In t h i s work, a 25 kV e l e c t r o n beam, focussed t o a diameter.of approximately 1 mm, i s scanned across t h e s u r f a c e of a bulk a l l o y sample a t v e l o c i t i e s ranging from 0.1 t o 500 cmlsec. A moving melt zone i s thus c r e a t e d , and t h e m i c r o s t r u c t u r e s of t h e r e s o l i d i f i e d material a r e examined by x-ray d i f f r a c t i o n , and o p t i c a l and e l e c t r o n microscopy. Such experiments have been c a r r i e d o u t i n a l l o y s containing 1.5, 3.6, 4.6, 7.4, 9.7, 12.2, 14.1, and 17.4 a t % Mn. A t low scan r a t e s ,

d e n d r i t e s o r p l a t e s o f t h e equilibrium phases propagate from t h e s u b s t r a t e i n t o t h e me1 t zone. Above a c r i t i c a l v e l o c i t y , which i n c r e a s e s with i n c r e a s i n g Mn c o n t e n t , t h e i n t e r m e t a l l i c phases cannot propagate f a s t enough t o keep up with t h e melt zone. In a l l o y s containing < 7.4 a t % Mn, A1 6Mn d e n d r i t e s a r e replaced by a

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e u t e c t i c between A16Mn and A l , and t h e n a t s l i g h t l y h i g h e r v e l o c i t y by c e l l u l a r aluminum. I n t h e a l l o y s c o n t a i n i n g a 9.7 a t % Mn, t h e growth o f t h e e q u i l i b r i u m

i n t e r m e t a l l i c s s t o p s when t h e a p e r i o d i c phases n c u l e a t e ahead o f them i n t h e m e l t zone. The e f f e c t i s seen c l e a r l y i n t h e 9.7 a t % Mn a l l o y , where t h e i c o s a h e d r a l c r y s t a l s a r e e a s i l y r e c o g i z e d by t h e i r pentagonal shape ( d i s c u s s e d below).

F i g . 1 shows an example o f how t h e p r o g r e s s o f a f a m i l y o f A16Mn d e n d r i t e s has been b l o c k e d by i c o s a h e d r a l c r y s t a l s n u c l e a t e d i n t h e m e l t , i n an e l e c t r o n beam m e l t scanned a t 2.5 cm/sec.

U s i n g an e x t e n s i o n o f an e a r l i e r a n a l y s i s by Eady e t a l . [ 9 ] o f t h e growth k i n e t i c s o f A16Mn d e n d r i t e s , we can e s t i m a t e [ 3 ] t h a t t h e t i p s o f t h e d e n d r i t e s shown i n Fig. 1 were a t a temperature about 16°C below t h e e q u i l i b r i u m e u t e c t i c t e m p e r a t u r e o f 658°C. The i c o s a h e d r a l g r a i n s n u c l e a t e d ahead o f t h e d e n d r i t e s , where t h e t e m p e r a t u r e must have been s l i g h t l y h i g h e r . The e q u i l i b r i u m l i q u i d u s t e m p e r a t u r e a t t h i s c o m p o s i t i o n i s a b o u t 850°C, so i t i s c l e a r t h a t a r a t h e r h i g h s u p e r c o o l i n g (150-200°C) i s r e q u i r e d t o produce t h e i c o s a h e d r a l phase n u c l e a t i o n . Once t h i s s u p e r c o o l i n g i s reached, however, 1 a r g e numbers of. i c o s a h e d r a l c r y s t a l s n u c l e a t e i n t h e m e l t . From o b s e r v a t i o n o f t h e s i z e a t t a i n e d by t h e i c o s a h e d r a l c r y s t a l s f o l l o w i n g n u c l e a t i o n , i t was e s t i m a t e d t h a t a t t h i s c o m p o s i t i o n t h e i c o s a h e d r a l c r y s t a l s grew w i t h v e l o c i t i e s between 1 and 2.5 cm/sec, a t t h e i r n u c l e a t i o n temperature. T h e i r a b i l i t y t o r e p l a c e t h e e q u i l i b r i u m phases i s t h u s due n o t t o a p a r t i c u l a r l y h i g h growth r a t e , b u t t o a h i g h n u c l e a t i o n r a t e i n t h e undercooled l i q u i d .

The h i g h n u c l e a t i o n r a t e o f t h e i c o s a h e d r a l c r y s t a l s w i t h i n t h e supercooled m e l t , and i t s r a t h e r sudden o n s e t when a s p e c i f i c l e v e l o f s u p e r c o o l i n g i s reached, suggest t h a t t h e n u c l e a t i o n o f t h i s phase may be homogeneous, i.e., n o t c a t a l y z e d by a s u b s t r a t e o f some s o l i d i m p u r i t y m i c r o p a r t i c l e s w i t h i n t h e me1 t. The e x p e r i m e n t s o f Bendersky and R i d d e r [ l o ] , i n which e x t r e m e l y h i g h r a t e s o f i c o s a h e d r a l phase n u c l e a t i o n were seen i n r a p i d l y c o o l e d submicron d r o p l e t s o f A1-Mn, p r o v i d e f u r t h e r s u p p o r t f o r homogeneous n u c l e a t i o n o f t h e i c o s a h e d r a l phase. T h i s i s i n c o n t r a s t t o t h e b e h a v i o r o f t h e e q u i l i b r i u m phases, w h i c h n u c l e a t e o n l y on a few heterogeneous s i t e s . A t s l o w c o o l i n g r a t e s , t h e e q u i l i b r i u m phases can t h e n grow t h r o u g h o u t t h e e n t i r e sample and p r e v e n t f o r m a t i o n o f t h e i c o s a h e d r a l phase, whereas a t h i g h c o o l i n g r a t e s t h e e q u i l i b r i u m phases do n o t become l a r g e enough t o be d e t e c t e d b e f o r e t h e o n s e t o f r a p i d n u c l e a t i o n o f t h e i cosahedral phase.

I V

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GROWTH FORM OF THE ICOSAHEDRAL PHASE

Although i n most m e l t - s p u n r i b b o n s i c o s a h e d r a l c r y s t a l s do n o t have an e x t e r n a l shape which r e v e a l s t h e symmetry o f t h e i r s t r u c t u r e , i n t h e a l l o y s c o n t a i n i n g 9.7 a t % Mn t h e e l e c t r o n beam m e l t i n g experiments produced c r y s t a l s w i t h d r a m a t i c i c o s a h e d r a l symmetry i n t h e i r e x t e r n a l shape ( F i g . 2). The c r y s t a l s grow f r o m t h e i r p o i n t o f n u c l e a t i o n as d e n d r i t e s which p r o p a g a t e a l o n g t h e i r t h r e e - f o l d axes, w i t h s i d e branches f r o m each main stem g r o w i n g i n t h e d i r e c t i o n o f a d j a c e n t t h r e e - f o l d axes. The r e s u l t i n g f i g u r e i s a h o l l o w - f a c e d pentagonal dodecahedron.

How i s t h e p r e f e r r e d d e n d r i t e growth d i r e c t i o n s e l e c t e d ? D e n d r i t i c growth has been t h e s u b j e c t o f much t h e o r e t i c a l a n a l y s i s r e c e n t l y [Ill, b u t many d e t a i l s o f t h e process a r e s t i l l p o o r l y understood. D e n d r i t e growth o c c u r s under c o n d i t i o n s where d i f f u s i o n o f h e a t and/or s o l u t e a r e l i m i t i n g f a c t o r s i n t h e r a t e o f c r y s t a l growth. D i f f u s i o n i n t h e l i q u i d and i n t h e i c o s a h e d r a l phase must be i s o t r o p i c and t h u s cannot c o n t r i b u t e t o s e l e c t i n g t h e d e n d r i t e growth d i r e c t i o n . However, t h e p r o p e r t i e s o f t h e s o l i d - l i q u i d i n t e r f a c e , s p e c i f i c a l l y t h e s o l i d - l i q u i d s u r f a c e energy and t h e k i n e t i c f u n c t i o n r e l a t i n g t h e growth r a t e o f t h e c r y s t a l t o t h e l o c a l t e m p e r a t u r e and c o m p o s i t i o n a t t h e i n t e r f a c e , can be a n i s o t r o p i c and c a n account f o r t h e growth d i r e c t i o n .

When c u b i c m a t e r i a l s grow d e n d r i t i c a l l y , t h e y a l m o s t i n v a r i a b l y grow a l o n g t h e cube axes. The d i f f u s i o n a l e f f e c t s which d r i v e d e n d r i t i c growth can t r a n s f o r m a s l i g h t a n i s o t r o p y o f s u r f a c e energy o r k i n e t i c s i n t o a s t r o n g a n i s o t r o p y o f growth d i r e c t i o n .

For example, i n d e n d r i t i c growth o f t h e b.c.c. o r g a n i c m a t e r i a l s u c c i n o n i t r i l e i n t o

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C3-314 JOURNAL DE PHYSIQUE

Fig. 1. Icosahedral c r y s t a l s nucleated Fig. 2. Icosahedral d e n d r i t e s i n ahead of advancing AlsMn d e n d r i t e s i n an an A1-9.7 a t % Mn a l l o y .

Al-9.7 a t % Mn a l l o y

a pure supercooled m e l t , t h e growth d i r e c t i o n i s believed t o be c o n t r o l l e d by anisotropy of s u r f a c e energy, with i n t e r f a c i a l k i n e t i c e f f e c t s being r e l a t i v e l y unimportant. Although t h e s u r f a c e energy a n i s o t r o p y i s measured t o be only about 2% [12], t h e d e n d r i t e s grow with a - s t r o n g preference f o r t h e [ l O O ] d i r e c t i o n s . The s o l i d - l i q u i d s u r f a c e energy i s g r e a t e s t on t h e planes normal t o t h e cube axes with t h e r e s u l t t h a t a small c r y s t a l i n equilibrium with t h e melt would have t h e shape o f a sphere with s l i g h t bulges i n t h e d i r e c t i o n s of t h e cube axes. These s l i g h t bulges a r e s u f f i c i e n t , however, t o give t h e cube a x i s d i r e c t i o n s an advantage i n t h e d i f f u s i o n a l growth process, with t h e r e s u l t t h a t t h e bulges become progressively f u r t h e r developed and grow outward a s t h e t i p s of d e n d r i t e s . Fig. 3 shows such a c r y s t a l s t a r t i n g t o develop i n t o a d e n d r i t e w i t h [ l O O ] branches. Note t h a t no c r y s t a l l o g r a p h i c f a c e t s a r e present on t h e c r y s t a l .

When i n t e r f a c i a l k i n e t i c s play an important r o l e i n t h e c r y s t a l l i z a t i o n process, t h e growth form f r e q u e n t l y becomes bounded by f a c e t s of t h e slowly growing p l a n e s , which i n t h e c a s e of most f.c.c. metals a r e t h e

{ill\

planes. Such f a c e t s may be seen i n cases where t h e composition d i f f e r s g r e a t l y from t h a t of t h e m e l t , o r i n e l e c t r o l y t i c a l l y formed d e n d r i t e s . C r y s t a l s with complex c r y s t a l s t r u c t u r e s a r e almost always f a c e t t e d . Fig. 4 shows a c r y s t a l of white phosphorus (Pq, complex cubic a-Mn s t r u c t u r e ) which had a rounded form when e q u i l i b r a t e d f o r several hours w i t h i t s melt a t t h e equilibrium temperature, but developed (1101 f a c e t s when growth was s t a r t e d by lowering t h e temperature 8 m i l l i d e g r e e s . The (1101 f a c e t s of t h e Pq c r y s t a l lead t o sharp p o i n t s i n t h e [ I 0 0 d i r e c t i o n s , and a t higher supercooling [ l O O ] d e n d r i t e s a r e formed. For {1111 f a c e t s , t h e c r y s t a l assumes an octahedral morphology and t h e d e n d r i t e s grow from t h e points of t h e octahedron, i n t h e [I001 d i r e c t i o n s .

The observation t h a t icosahedral c r y s t a l s grow along t h e t h r e e - f o l d axes can thus imply an a n i s o t r o p y of e i t h e r s u r f a c e energy o r i n t e r f a c i a l k i n e t i c s . Either o f t h e s e mechanisms would be c o n s i s t e n t with t h e idea t h a t planes normal t o t h e f i v e - f o l d axes of t h e icosahedral phase a r e t h e most densely packed: t h e s e planes would then have t h e lowest s u r f a c e energy and would a l s o be t h e most d i f f i c u l t planes on which t o s t a r t new l a y e r s of c r y s t a l .

While d e n d r i t e growth can r e s u l t from d i f f u s i o n of e i t h e r heat o r s o l u t e , t h e l a t t e r m u s t have been t h e dominant . e f f e c t f o r t h e d e n d r i t e s shown i n Fig. 2. A region d e p l e t e d i n Mn can be seen surrounding t h e icosahedral d e n d r i t e s but

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Fig. 3 . Dendrite of Fig. 4. A c r y s t a l of white phosphorus ( a ) a t succinoni t r i l e. equilibrium w i t h i t s melt and ( b ) growing a t a

supercool ing of 0.008 K.

extending only about a micrometer from t h e d e n d r i t e s , which grew t o a r a d i u s of about 10 pm i n about 1 msec. Estimating d i f f u s i o n d i s t a n c e s a s x =dm, where t i s t h e time and D i s thermal o r s o l u t e d i f f u s i v i t y , we obtain f o r 1 msec about 300 pm f o r thermal d i f f u s i o n ( D 2 1 cm2/sec) and 1 pm f o r s o l u t e d i f f u s i o n ( D = 10-5 cmz/sec). The l a t t e r i s comparable t o t h e s i z e of t h e d e n d r i t e arms, whereas t h e former i s more than an order of magnitude l a r g e r than t h e e n t i r e d e n d r i t i c c r y s t a l , i n d i c a t i n g t h a t t h e thermal f i e l d must have been almost uniform within an individual c r y s t a l .

V

-

NUCLEATION OF DECAGONAL PHASE

The decagonal phase i s favored i n a l l o y s having a higher Mn content and a lower s o l i d i f i c a t i o n r a t e . Many a l l o y s contain both phases and i n t h e s e cases i t i s found t h a t t h e decagonal phase has grown a s an o r i e n t e d overgrowth on t h e icosahedral phase, with t h e ten-fold a x i s of t h e decagonal phase coinciding with one of t h e f i v e - f o l d axes of t h e icosahedral phase [13]. Thus each icosahedral phase c r y s t a l generates s i x d i f f e r e n t o r i e n t a t i o n s of decagonal c r y s t a l s , one f o r each of t h e f i v e - f o l d axes.

In e l e c t r o n beam s u r f a c e melts of A1-14 a t % Mn a l l o y s , 100 cm/sec scans were found t o contain icosahedral c r y s t a l s with morphologies s i m i l a r t o those found i n t h e 18 w t % Mn a l l o y s , but t h e s t r u c t u r e i s f i l l e d i n with a higher volume f r a c t i o n o f t h e icosahedral phase. Scans a t 25 cm/sec show a mixture of icosahedral and decagonal, while scans a t 10 cm/sec o r l e s s show only t h e decagonal phase o r ( i n o t h e r p a r t s of t h e me1 t ) Al6Mn.

The decagonal phase regions i n t h e slower melts show a d i s t i n c t i v e morphology.

Individual decagonal c r y s t a l s have t h e form of blocky c y l i n d e r s , 1-2 pm long and about 1 pm i n diameter (Fig. 5 ) . Although t h e s e c r y s t a l s do not in general appear t o be connected t o one a n o t h e r , many of them a r e arranged i n long rows which a t low magnification give t h e impression of a d e n d r i t i c s t r u c t u r e . The symmetry of t h i s d e n d r i t i c s t r u c t u r e suggests icosahedral d e n d r i t e s such a s those shown i n Fig. 2. Moreover, within any one such d e n d r i t e - l i k e colony, t h e decagonal c r y s t a l s have a l i m i t e d n u m b e r of o r i e n t a t i o n s , namely those which would r e s u l t i f they had a l l been nucleated by an individual icosahedral c r y s t a l .

This morphology i n d i c a t e s t h a t t h e decagonal c r y s t a l s i n t h e slower melts did not r e s u l t from homogeneous nucleation e v e n t s , but t h a t they formed by heterogeneous e p i t a x i a l nucleation on l a r g e icosahedral c r y s t a l s . The l a t t e r , however, have completely disappeared by t h e time s o l i d i f i c a t i o n i s complete, having been replaced by t h e more s t a b l e decagonal phase. I t i s s t i l l p o s s i b l e , however, t o f i n d

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C3-316 JOURNAL DE PHYSIQUE

Fig. 5. Decagonal c r y s t a l s i n an Fig. 6. Remnant o f an i c o s a h e d r a l A1-14 a t % Mn a l l o y scanned a t 2.5 cm/sec. s t r u c t u r e among decagonal c r y s t a l s ,

f r o m t h e same sample as Fig. 5.

r e g i o n s where t h e i c o s a h e d r a l c r y s t a l s have l e f t a " g h o s t " image, p r o b a b l y due t o l o c a l l y h i g h e r Mn c o n t e n t i n t h e A1 m a t r i x ( F i g . 6).

The processes found i n t h e s e a l l o y s r e n d e r i t e x t r e m e l y d i f f i c u l t t o grow l a r g e c r y s t a l s o f t h e i c o s a h e d r a l phase i n b i n a r y A1-Mn a l l o y s . The i c o s a h e d r a l phase s e r v e s as a h i g h l y e f f e c t i v e n u c l e a n t f o r t h e more s t a b l e decagonal phase, and f o r m a t i o n o f t h e decagonal phase can be suppressed o n l y b y v e r y r a p i d c o o l i n g f o l l o w i n g g r o w t h o f t h e i c o s a h e d r a l phase. T h i s r a p i d c o o l i n g , however, r e s u l t s i n a h i g h r a t e o f n u c l e a t i o n o f t h e i c o s a h e d r a l phase, w i t h subsequent s m a l l g r a i n s i z e .

The o r d e r o f magnitude o f t h e c o o l i n g r a t e s r e q u i r e d t o suppress f o r m a t i o n o f t h e decagonal phase i n t h e A1-14 a t % Mn a l l o y can be e s t i m a t e d f r o m t h e e l e c t r o n beam m e l t i n g experiments. Numerical models [ I 4 1 o f s u r f a c e m e l t i n g by a moving h e a t s o u r c e show t h a t so l o n g as t h e parameter U a / 2 a i s c 1, t h e shape o f t h e m e l t pool i s n o t g r e a t l y d i s t o r t e d from t h e shape o f a s t a t i o n a r y m e l t . Here U i s t h e v e l o c i t y , a t h e r a d i u s o f t h e h e a t source, and a t h e thermal d i f f u s i v i t y . I n t h i s case a = 5 x 10-2 cm and a = 1 cm2/sec, and t h e m e l t pool d i s t o r t i o n i s s m a l l f o r U c 40 cm/sec. O b s e r v a t i o n o f r i p p l e s l e f t b e h i n d on t h e sample s u r f a c e a l s o i n d i c a t e t h a t i n m e l t s o f t h i s v e l o c i t y t h e m e l t pool remains f a i r l y round, w h i l e a t h i g h e r speeds (200 o r 500 cm/sec) t h e t r a i l i n g edge o f t h e me1 t pool becomes p o i n t e d .

For m e l t s i n t h e s l o w e r v e l o c i t y range, t h e t e m p e r a t u r e g r a d i e n t a t t h e s o l i d - l i q u i d i n t e r f a c e i s t h u s s i m i l a r t o t h a t i n a s t a t i o n a r y m e l t , which i s (Tm

-

To)/R, i n t h e case where a l l o f t h e heat i s c o n c e n t r a t e d a t one p o i n t and o n l y s l i g h t l y s m a l l e r when t h e h e a t i s u n i f o r m l y d i s t r i b u t e d o v e r t h e s u r f a c e o f t h e m e l t . Here T, i s t h e m e l t i n g t e m p e r a t u r e , To i s t h e s u b s t r a t e t e m p e r a t u r e f a r f r o m t h e m e l t , and R i s t h e r a d i u s o f t h e m e l t . I n t h e p r e s e n t case t h e g r a d i e n t a t t h e s o l i d l i q u i d i n t e r f a c e i s t h u s a p p r o x i m a t e l y 104 K/cm, and t h e c o o l i n g r a t e i s equal t o t h e g r a d i e n t a t t h e i n t e r f a c e t i m e s t h e e l e c t r o n beam scan v e l o c i t y . I n t h e 1 0 cm/sec scans, t h i s g i v e s 105 K/sec, and a t t h i s c o o l i n g r a t e t h e i c o s a h e d r a l phase was t o t a l l y t r a n s f o r m e d i n t o decagonal phase.. For 100 cm/sec scans t h e same c a l c u l a t i o n would g i v e 106 K/sec, b u t due t o m e l t pool d i s t o r t i o n a t t h i s scan speed t h e a c t u a l c o o l i n g r a t e was p r o b a b l y somewhat s m a l l e r . A t t h i s scan speed, t h e i c o s a h e d r a l c r y s t a l s a r e preserved. We can t h u s conclude t h a t f o r t h e 1 4 a t % Mn a l l o y t h e c r i t i c a l c o o l i n g r a t e needed f o l l o w i n g n u c l e a t i o n t o p r e v e n t t r a n s f o r m a t i o n o f t h e i c o s a h e d r a l c r y s t a l s i n t o t h e decagonal phase i s between 105 and 106 K / s ~ c .

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V I

-

ALLOYS CONTAINING SILICON

The a d d i t i o n o f S i t o A1-Mn a l l o y s e l i m i n a t e s t h e f o r m a t i o n o f t h e decagonal phase and t h u s makes i t p o s s i b l e t o o b t a i n s i n g l e - p h a s e i c o s a h e d r a l m a t e r i a l i n me1 t - s p u n r i b b o n s o f A1-20 a t % Mn-6 a t % S i . Chen and Chen [15] have d i s c u s s e d d i f f e r e n c e s between t h e e l e c t r o n d i f f r a c t i o n p a t t e r n s o f a l l o y s w i t h and w i t h o u t S i , b u t t h e d i f f e r e n c e i n s t r u c t u r e i s a p p a r e n t l y s u b t l e and t h e x - r a y d i f f r a c t i o n p a t t e r n s a r e v e r y s i m i l a r .

E l e c t r o n beam s u r f a c e m e l t i n g o f a l l o y s c o n t a i n i n g S i demonstrated t h a t t h e hex- agonal P phase o f Al-Mn-Si grows r a p i d l y so t h a t h i g h scan r a t e s were necessary t o e l i m i n a t e it. T a b l e I shows t h e phases d e t e c t e d by x - r a y d i f f r a c t i o n i n e l e c t r o n beam m e l t s o f t h r e e s i l i c o n - c o n t a i n i n g a l l o y s . It i s seen t h a t t h e 0 phase p e r s i s t s t o r e l a t i v e l y h i g h v e l o c i t i e s and t h e a phase t o even h i g h e r v e l o c i t i e s . M e t a l l o g r a p h i c e x a m i n a t i o n shows a m i c r o s t r u c t u r e ( F i g . 7 ) c o n t a i n i n g p l a t e s w h i c h a r e i d e n t i f i e d as t h e B phase on t h e b a s i s o f t h e i r r e a c t i o n t o p o l a r i z e d l i g h t . I n t h e r e g i o n s between t h e P p l a t e s i s a g r a n u l a r s t r u c t u r e which does n o t r e a c t t o p o l a r i z e d l i g h t and which i s i d e n t i f i e d as a m i x t u r e o f i c o s a h e d r a l and t h e c u b i c a phase. Koskenmaki e t a l . 1161 have demonstrated t h a t i n t h e s e a l l o y s t h e m phase forms as an o r i e n t e d o v e r g r o w t h on t h e i c o s a h e d r a l phase, somewhat s i m i l a r t o t h e o v e r g r o w t h o f decagonal phase on i c o s a h e d r a l i n b i n a r y A1-Mn a l l o y s . The n u c l e a t i o n and growth o f t h e a phase on t h e i c o s a h e d r a l phase

Fig. 7. M i c r o s t r u c t u r e o f Al-15 a t % Mn-6 a t % S i a l l o y scanned a t 5 cm/sec.

TABLE I

I n t e r m e t a l l i c phases p r e s e n t i n a l l o y s w i t h 6 a t % Si. The dominant phase i s l i s t e d f i r s t .

Mn Content ( a t %)

Scan V e l o c i t y 10 15.5 20

(cm/sec)

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C3-3 18 JOURNAL DE PHYSIQUE

i n Al-Mn-Si i s apparently not so rapid a s t h e nucleation and growth of t h e decagonal phase on icosahedral i n Al-Mn, making i t e a s i e r t o o b t a i n single-phase icosahedral material i n t h e former system.

VII

-

DECAGONAL PHASE NUCLEATION A N D GROWTH IN A1-Pd ALLOYS

There have been no r e p o r t s of icosahedral phase formation i n t h e A1-Pd system, but Bancel e t a l . 1171 have found t h a t t h e decagonal phase i s present. E a r l i e r s t u d i e s by S a s t r y e t a l . [ I 8 1 i n d i c a t e d t h a t t h i s phase formed by p r e c i p i t a t i o n , and t h e data suggest t h a t t h e decagonal phase may be an equilibrium phase i n t h i s system.

We have found t h a t i n melt-spun ribbons of A1-14 a t % Pd d e n d r i t e s of t h e decagonal phase a r e formed, followed by a e u t e c t i c between t h e decagonal phase and aluminum (Fig. 8 ) . In e l e c t r o n beam melt t r a i l s , examination of t h e sample s u r f a c e reveals d e n d r i t i c c r y s t a l s with ten-fold branching ( F i g . 9 ) . In n e i t h e r melt spun ribbons nor e l e c t r o n beam melts was any i n d i c a t i o n seen of decagonal c r y s t a l s which had multiple o r i e n t a t i o n s due t o e p i t a x i a l nuclation on an underlying icosahedral c r y s t a l . I t i s t h e r e f o r e concluded t h a t t h e decagonal phase c r y s t a l s which we observed i n A1-Pd were not nucleated by an icosahedral precursor, but t h e r e i s not y e t enough evidence t o i n d i c a t e whether t h e nucleation was homogeneous.

Fig. 8 Decagonal c r y s t a l s i n a melt-spun A1-Pd a l l o y , showing ( a ) d e n d r i t e s surrounded by e u t e c t i c and ( b ) d e t a i l of e u t e c t i c s t r u c t u r e .

Fig. 9 Decagonal d e n d r i t e i n an e l e c t r o n beam melted A1-14 a t % Pd a l l o y .

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VII - CONCLUSIONS

( 1 ) The icosahedral phase s o l i d i f i e s from t h e melt of binary A1 -Mn a l l o y s with a composition c l o s e t o 20 a t % M n , but i t i s not a l i n e compound.

( 2 ) A supercooling of 150-200" below t h e equilibrium 1 iquidus temperature i s required t o produce nucleation o f t h e icosahedral phase: t h e r e s u l t i n g nucleation r a t e i s extremely high, suggesting homogeneous nucleation.

( 3 ) The decagonal phase i s nucleated heterogeneously on t h e s u r f a c e of t h e icosahedral phase.

( 4 ) A t a cooling r a t e of 105 K/sec o r l e s s , t h e decagonal phase r e p l a c e s t h e icosahedral phase, whereas a t a cooling r a t e of 106 U s e c o r more t h e icosahedral phase i s preserved.

( 5 ) The a d d i t i o n of 5% Si t o A1-Mn a l l o y s prevents t h e formation of t h e decagonal phase, but t h e t e r n a r y fl phase can grow r a p i d l y i n such a l l o y s .

( 6 ) In A1-Pd a l l o y s , a decagonal phase n u c l e a t e s and grows without an icosahedral phase precursor.

V I I I

-

ACKNOWLEDGEMENTS

The authors thank DARPA f o r f i n a n c i a l support of t h i s work.

REFERENCES

Shechtman, D., Blech, I . , G r a t i a s , D., and Cahn, J. W . , Phys. Rev. Lett.

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(1984) 1951.

Bendersky, L., Phys. Rev. Lett. 55 (1985) 1461.

Schaefer, R. J., t o be p u b l i s h e d y n S c r i p t a Met.

Guyot, P. and Audier, M., Phil. Mag. B 52 (1985) L15.

Krishnan, K. M . , Gronsky, R . , and Tanner, L. E., S c r i p t a Met.

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(1986) 239.

Kimura, K., Hashimoto, T., Suzuki, K., Nagayama, K., Ino, H., and Takeuchi, S., J. Phys. Soc. Japan 54 (1985) 3217.

Bower, T. F., Brody, H.

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and Flemings, M. C . , Trans. AIME

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COMMENTS AFTER THE SCHAEFER TALK :

J. ZAK.- Your slides show beautiful pentagonal shape structures in the icosahedral phase. I didn't seem to see any clear decagons in the decagonal phases. Do they at all appear ?

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JOURNAL DE PHYSIQUE

Response by R. SCHAEFER :

The decagonal phase of A1-Mn grows in the form of blocky cylinders which sometimes show a slight indication of 10-fold faceting. However, we have not seen them with a dendritic growth form, and the dentritic form greatly emphasises crystal symmetry because the dentrites graw in specific crystallographic directions. Preliminary experiments with A1-Pd, which solidifies with a decagonal structure, show some dendrites with branches having decagonal symmetry.

1. Is there an epitaxial relationship between I-A1Mn and e-g., orthorhombic A16 Mn ?

2. Is the e-beam power a useful control parameter ?

Response by R. SCHAEFER

1. None has been identified

2. The beam power can be used to vary the depth and shape of the melt zone within a limited range, but it is best to adjust the power to give a good melt and vary the scan rate. Somewhat higher powers are required to give an equivalent melt at high scan rates.

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