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AND Al-V ALLOYS BY MEANS OF TRANSMISSION ELECTRON MICROSCOPY
K. Urban, J. Mayer, M. Rapp, M. Wilkens, A. Csanady, J. Fidler
To cite this version:
K. Urban, J. Mayer, M. Rapp, M. Wilkens, A. Csanady, et al.. STUDIES ON APERI- ODIC CRYSTALS IN Al-Mn AND Al-V ALLOYS BY MEANS OF TRANSMISSION ELEC- TRON MICROSCOPY. Journal de Physique Colloques, 1986, 47 (C3), pp.C3-465-C3-474.
�10.1051/jphyscol:1986347�. �jpa-00225759�
JOURNAL DE PHYSIQUE
C o l l o q u e
C3,
s u p p l e m e n ta u n 0 7 ,
T o m e 4 7 ,j u i l l e t 1986
STUDIES O N APERIODIC CRYSTALS I N A1-Mn AND A1-V ALLOYS BY MEANS
OFTRANSMISSION ELECTRON MICROSCOPY
K. URBAN,
J.MAYER,
M.RAPP, M. WILKENS, A. CSANADY' and J. FIDLER"
M a x - P l a n c k - I n s t i t u t fiir M e t a l l f o r s c h u n g , 0-7000
S t u t t g a r t 80,
F . R . G . ,' ~ u n g a r i a n
A l u m i n i u m C o r p o r a t i o n , H - 1 3 8 9 B u d a p e s t , H u n g a r y"~echnische
U n l v e r s i t d tWien,
A - 1 0 4 0 W i e n , A u s t r i aA b s t r a c t
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E l e c t r o n m i c r o s c o p y h a s been used t o s t u d y q u a s i c r y s t a l s and T phase i n r a p i d l y c o o l e d Al-14atXMn and N-16atXV. The T p h a s e c a n a l s o b e produced by i n t e r d i f f u s i o n i n e v a p o r a t e d Al/Mn l a y e r s . A l s o i n t h e s e e x p e r i - ments we h a v e o b s e r v e d a new c r y s t a l l i n e phase w i t h s t r u c t u r a l e l e m e n t s r e l a t e d t o t h e a p e r i o d i c c r y s t a l s by a s h e a r p a r a l l e l t o a p a r t i c u l a r symmetry d i r e c t i o n . The phase t r a n s i t i o n s between t h e q u a s i c r y s t a l l i n e o r t h e T p h a s e t o t h e c r y s t a l l i n e o r amorphous p h a s e s were s t u d i e d i n s i t u i n a h i g h - v o l t a g e e l e c t r o n m i c r o s c o p e .1. INTRODUCTION
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More t h a n 30 y e a r s a g o i t was s u g g e s t e d t h a t m e t a l l i c m e l t s s h o u l d e x h i b i t s h o r t - r a n g e o r d e r i n which a g i v e n atom i s s u r r o u n d e d by a n i c o s a h e d r a l s h e l l o f o t h e r atoms / I / . T h e r e a r e two k i n d s of p h a s e s i n which i c o s a h e d r a l n e a r e s t - n e i g h b o u r c o o r d i n a t i o n may be p r e s e r v e d , a t l e a s t t o a l a r g e e x t e n t , i n t h e s o l i d s t a t e ; t h e amorphous p h a s e s / 2 , 3 / and t h e Frank-Kasper p h a s e s 1 4 1 . A t h i r d phase i n which i c o s a h e d r a l symmetry i s p r e s e r v e d i s t h e q u a s i c r y s t a l l i n e p h a s e . It h a s a n a p e r i o d i c s t r u c t u r e s i n c e i t s 6 f i v e f o l d symmetry a x e s a r e i n c o n s i s t e n t w i t h l a t t i c e t r a n s l a t i o n a l i n v a r a n c e . The q u a s i c r y s t a l l i n e phase h a s b e e n found i n Al w i t h 1 0
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20 a t % Mn, R e , C r , Ru, V , Mo, and W 15-71, i n A1-Si-Mn 1 8 1 , Al-Mg-Zn-Cu 1 9 1 , U-Pd-Si 1 1 0 1 , and Ni-Ti a l l o y s /11/. A r e l a t e d p h a s e , t h e T p h a s e , i s o b s e r v e d i n A1-Mn 1 1 2 , 1 3 1 . I t e x h i b i t s a t e n f o l d symmetry a x i s and a p e r i o d i c atom a r r a n g e m e n t i n two d i m e n s i o n s b u t p e r i o d i c i t y i n t h e t h i r d d i m e n s i o n .I n t h i s p a p e r we r e p o r t o n a s t u d y by means o f e l e c t r o n m i c r o s c o p y of t h e s t r u c t u r e o f A1-Mn q u a s i c r y s t a l s . I n o r d e r t o e l u c i d a t e t h e r e l a t i v e s t a b i l i t y o f t h e q u a s i c r y s t a l l i n e phase o r t h e T p h a s e we have s t u d i e d t h e p h a s e t r a n s i t i o n s t o and from t h e c r y s t a l l i n e o r t h e amorphous p h a s e s i n Al-14atXMn and Al-l6atXV. F u r t h e r m o r e we have c a r r i e d o u t t h e r m a l i n t e r d i f f u s i o n s t u d i e s i n Al/Mn c o u p l e s which p r o d u c e t h e T p h a s e and a new c r y s t a l l i n e phase w i t h s t r u c t u r a l e l e m e n t s r e l a t e d t o t h e a p e r i o d i c c r y s t a l s by a s h e a r p a r a l l e l t o a p a r t i c u l a r symmetry d i r e c t i o n .
2. OBSERVATIONS
IN
Al-14atXMn -Q u a s i c r y s t a l s i n spin-quenched s a m p l e s were s t u d i e d a t medium r e s o l u t i o n i n a SIEMENS 102 and a PHILIPS 400T e l e c t r o n m i c r o s c o p e . They o c c u r a s flower-shaped den- d r i t i c p r e c i p i t a t e s o f up t o 2 pm i n d i a m e t e r embedded i n A l - r i c h s o l i d s o l u t i o n o f f c c s t r u c t u r e . F i g . 1 shows a d a r k - f i e l d p i c t u r e t a k e n w i t h a (110000) r e f l e c t i o n 1 6 1 a l o n g a n a x i s o f f i v e f o l d symmetry ( h e n c e f o r t h d e n o t e d by "A5"). We r e c o g n i z e a c o a r s e - g r a i n e d s t r u c t u r e i n d i c a t i n g t h a t d i f f e r e n t a r e a s o f t h e same p r e c i p i t a t e con-
...
x Now a t U n i v e r s i t l t Erlangen-Niirnberg
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I n s t i t u t f i r ~ e r k s t o f f v i s s ~ n s c h a f t e n I , M a r t e n s s t r . 5 , 8520 E r l a n g e n , GermanyArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986347
t r i b u t e d i f f e r e n t l y t o t h e r e f l e c t i o n i n t e n s i t i e s . This i s a l s o e v i d e n t i n F i g . 2 where a d i f f r a c t i o n p a t t e r n along A5 i s shown recorded with t h e i n t e r m e d i a t e l e n s d e l i b e r a t e l y defocussed. This produces a complete s e t of s m a l l d a r k - f i e l d p i c t u r e s of t h e imaged p r e c i p i t a t e . We recognize t h e o u t e r shape of t h e p r e c i p i t a t e . However, i t i s v i r t u a l l y i m p o s s i b l e t o i d e n t i f y c e r t a i n i n t e r n a l f e a t u r e s i n two o r more of t h e s e p i c t u r e s simultaneously. T i l t i n g t h e specimen by a few t e n t h s of a degree produces a n e n t i r e l y d i f f e r e n t f i n e s t r u c t u r e i n the d a r k - f i e l d p i c t u r e s .
The r e f l e c t i o n i n t e n s i t i e s respond much more s e n s i t i v e l y t o t i l t i n g t h a n i n o r d i n a r y c r y s t a l s . This i n d i c a t e s t h a t e l e c t r o n t r a n s m i s s i o n i s e s s e n t i a l l y based on chan- n e l l i n g modes which a r e only e x c i t e d i f t h e incident-beam d i r e c t i o n c o i n c i d e s with a high-symmetry zone a x i s . T i l t i n g by a few t e n t h s of a degree t h e r e f l e c t i o n s v a n i s h and a d i f f u s e background appears. On t h i s b a s i s we e x p l a j n t h e g r a i n y c o n t r a s t i n t h e d a r k - f i e l d p i c t u r e s a s r e l a t e d t o i n t e r n a l s t r a i n s i n r e g i o n s with dimensions down t o some t e n nanometers. These s t r a i n s l e a d t o l a t t i c e m i s o r i e n t a t i o n s which l o c a l l y ex- t i n g u i s h s c a t t e r i n g i n t o c e r t a i n r e f l e c t i o n s . This i n d i c a t e s t h a t t h e q u a s i c r y s t a l s t r u c t u r e i s such t h a t i t does n o t permit t o b u i l d a m i s o r i e n t a t i o n - f r e e l a r g e r s i n g l e q u a s i c r y s t a l . N e v e r t h e l e s s , i n o r d i n a r y d i f f r a c t i o n experiments i n t h e e l e c - t r o n microscope a p e r f e c t p a t t e r n can be observed i f , due t o t h e l a r g e s i z e of t h e d i f f r a c t i o n a p e r t u r e , a s u f f i c i e n t number of small s u i t a b l y o r i e n t e d regions con- t r i b u t e s t o each r e f l e c t i o n . It i s , however, a p r e r e q u i s i t e t h a t t h e l a t t i c e m i s o r i e n t a t i o n s a r e n o t random but s y s t e m a t i c a l l y r e s t r i c t e d t o c e r t a i n t y p e s of atom displacements.
Fig. 1 A1-Mn: (110000) dark f i e l d F i g . 2 A1-Mn: D i f f r a c t i o n p a t t e r n a l o n g A5 with t h e i n t e r m e d i a t e l e n s defocussed
F i g . 3 A1-Mn: A5 d i f f r a c t i o n p a t t e r n F i g . 4 C o n s t r u c t e d p a t t e r n ( s e e t e x t )
F i g . 5 Al-Mn: S t r u c t u r e lmage alonq fivefold a x i s ( l e f t ) ; Penrose l a t t i c e ( r i g h t )
Fig. 6 A1-Mn: S t r u c t u r e image a l o n g t h r e e f o l d a x i s ( l e f t ) ; Penrose l a t t i c e ( r i g h t )
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/ 1 - : St ruc t uic .linage dl.onc{ t w o l I o l d a x i s ( l e f t ) ; Penrose l a t t i c e ( r i g h t )F i g . 3 shows a d i f f r a c t i o n p a t t e r n a l o n g A5 t a k e n w i t h f o c u s s e d i n t e r m e d i a t e l e n s . We d e n o t e t h e r e f l e c t i o n d e c a g o n s a c c o r d i n g t o i n c r e a s i n g l e n g t h o f t h e d i f f r a c t i o n v e c t o r by D D and D
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The D3 r e f l e c t i o n s a r e o f t h e t y p e ( 1 1 0 0 0 0 ) i n t h e n o t a t i o n ib;r:$iced i n111.
Many r e f l e c t i o n s a r e t r i a n g u l a r i n s h a p e . I n p a r t i c u l a r t h i s c o n c e r n s t h e i n n e r m o s t D and t h e r e f l e c t i o n s forming t h e p e n t a g o n s between and D.
T r i a n g u l a r r e f l e c k i o n s a r e o n l y o b s e r v e d i n t h i c k e r s ? e c i m e n s (more t h a n 3 a b o u t f 0 0 w i n t h i c k n e s s ) .An e x p l a n a t i o n f o r t h i s may b e found i n t h e p r e s e n c e o f domains w i t h T p h a s e s t r u c - t u r e . E v i d e n c e f o r T p h a s e c o a t i n g o f q u a s i c r y s t a l s u r f a c e s h a s been r e p o r t e d 1 1 3 , 1 4 1 . The t e n f o l d symmetry a x i s (A10) o f t h e T phase i s p a r a l l e l t o one o f t h e f i v e f o l d a x e s o f t h e q u a s i c r y s t a l . I f t h e f o i l normal of o u r sgecimen i s p a r a l l e l t o a f i v e f o l d a x i s t h e r e a r e 10 o t h e r A5 a x e s making a n g l e s o f 63 o r 117' w i t h t h e f o i l normal. The T p h a s e e x h i b i t s , i n a d d i t i o n t o i t s t e n f o l d a x i s , 20 t w o f o l d a x e s which a r e a l s o c a l l e d p s e u d o - f i v e f o l d a x e s (M5) s i n c e t h e r e s p e c t i v e d i f f r a c t i o n p a t t e r n s c o n t a i n c o n c e n t r i c s l i g h t l y d i s t o r t e d d e c a g o n s ( c . f . F i g . 9 ) . I f t h e s e a x e s make a n a n g l e o f 63O o r 117' w i t h A10 t h e r e a r e 10 o r i e n t a t i o n v a r i a n t s o f T p h a s e domains p o s s i b l e w i t h a n AP5 p a r a l l e l t o t h e f o i l normal. Assuming t h a t a l l v a r i a n t s o c c u r a t e q u a l p r o b a b i l i t y we o b t a i n by s u p e r p o s i t i o n of t h e r e s p e c t i v e d i f f r a c t i o n p a t t e r n s t h e d i a g r a m shown i n F i g . 4. I t h a s b e e n c o n s t r u c t e d on t h e b a s i s of t h e AP5 p a t t e r n o f F i g . 9 by r o t a t i n g i t t e n times i n 36' s t e p s around t h e i n c i d e n t beam d i r e c t i o n . We r e a l i z e good agreement w i t h F i g . 2. I n p a r t i c u l a r , t h e r e l a t i v e s i z e and o r i e n t a t i o n o f t h e t r i a n g l e s i s w e l l d e s c r i b e d . The T phase domains may b e l o c a t e d a t t h e s u r f a c e o f t h e d e n d r i t e s . They may a l s o b e c o n t a i n e d i n t h e b u l k where growing d e n d r i t e b r a n c h e s may h a v e e n c l o s e d T p h a s e . I n t h e same framework o f o r i e n t a t i o n r e l a t i o n s h i p s between T phase and i c o s a h e d r a l phase a n a l t e r n a t i v e ex- p l a n a t i o n i s t h a t t h e l a t t i c e m i s o r i e n t a t i o n s mentioned above l e a d t o l o c a l atom r e l a x a t i o n s t o w a r d s t h e T p h a s e s t r u c t u r e . T h i s would e x p l a i n t h e o b s e r v a t i o n t h a t pronounced t r i a n g u l a r s h a p e of r e f l e c t i o n s I s o n l y o b s e r v e d i n t h i c k e r s p e c i m e n s . H i g h - r e s o l u t i o n p i c t u r e s o f q u a s i c r y s t a l s were t a k e n i n a JEOL 200CX e l e c t r o n m i c r o s c o p e . We h a v e c o n s t r u c t e d t h e t h r e e - d i m e n s i o n a l P e n r o s e l a t t i c e d e c o r a t e d by atoms a t t h e v e r t i c e s b y means o f p r o j e c t i o n of t h e s i x - d i m e n s i o n a l c u b i c l a t t i c e 1 1 5 , 1 6 1 . F i g s . 5-7 p e r m i t a c o m p a r i s o n between m i c r o g r a p h s and P e n r o s e l a t t i c e . Taking i n t o a c c o u n t t h e m i c r o s c o p e r e s o l u t i o n o f a b o u t 0.25 w we f i n d v e r y good a g r e e m e n t . S i m i l a r r e s u l t s were r e p o r t e d i n 1171.
Fig. 8 T p h a s e p r e c i p i t a t e s i n c a s t Al-Mn Fig. 9 A P 5 d i f f r a c t i o n p a t t e r n
F i g . 1 0 A1-V q u a s i c r y s t a l s F i g . 1 1 A1-V: A 5 d i f f r a c t i o n p a t t e r n
The e l e c t r o n m i c r o g r a p h s , i n p a r t i c u l a r when n o t a c c o m p a n i e d by i m a g e s i m u l a t i o n d o n o t p e r m i t d i r e c t c o n c l u s i o n s o n t h e c o r r e l a t i o n b e t w e e n t h e w h i t e d o t s i n t h e p i c - t u r e s and t h e p o s i t i o n o f t h e a l l o y a t o m s . On g r o u n d s o f s i m i l a r i t i e s b e t w e e n t h e d i f f r a c t i o n p a t t e r n s o f c r y s t a l l i n e a(AlMnSi)
,
a n d A1-Mn-Si q u a s i c r y s t a l s i t h a s b e e n s u g g e s t e d t h a t Mn a t o m s a r e l o c a t e d a t t h e v e r t i c e s o f t h e P e n r o s e rhombohedra /la/.3. OBSERVATION OF T PHASE I N A1-Mn
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The T p h a s e o c c u r s i n A1-Mn e i t h e r a s c o a t i n g o f q u a s i c r y s t a l s o r i n t h e f o r m o f p r e c i p i t a t e s 1 1 3 1 . S l o w e r c o o l i n g r a t e s f a v o u r T p h a s e f o r m a t i o n o v e r t h a t o f q u a s i c r y s t a l s . W e found t h a t r o d s 1 0 cm l o n g and 1 an i n d i a m e t e r c a n b e p r o d u c e d c o n t a i n i n g a d e n s e p a c k i n g o f T p r e c i p i t a t e s w i t h mean d i m e n s i o n s a r o u n d 0 . 2 mm ( F i g . 8 ) b y p o u r i n g Al-14atXMn m e l t i n t o a c o p p e r mould. F i g . 9 shows t h e AP5 d i f - f r a c t i o n p a t t e r n and F i g . 14a t h e A10 p a t t e r n . I n F i g . 8 t h e flower-shaped p r e c i p i t a t e s h a v e A10 r o u g h l y p a r a l l e l t o t h e image p l a n e n o r m a l w h i l e i t m a k e s a l a r g e a n g l e t o i t i n t h e r o d - s h a p e d p r e c i p t a t e s .
4. OBSERVATIONS OF QUASICRYSTALS IN A1-V
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S p i n - q u e n c h i n g Al-16atS.V m e l t p r o d u c e s t h i n p e r f o r a t e d f l a k e s a b o u t 1 mm w i d e and 1 an l o n g . Along t h e h o l e e d g e s t h e f l a k e s a r e t r a n s p a r e n t t o t h e e l e c t r o n s s o t h a t t h e y c a n b e i n v e s t i g a t e d i n t h e a s c a s t c o n d i t i o n . The q u a s i c r y s t a l s o c c u r a s p l a s t e r s t o n e s h a p e d p r e c i p i t a t e s . By STEM a n a l y s i s i n a JEOL 200CX e l e c t r o n m i c r o s c o p e t h e c o m p o s i t i o n was d e t e r m i n e d t o b e n e a r s t o i c h i o m e t r i c Alg4V1?. The p r e c i p i t a t e s a r e s e p a r a t e d b y n a r r o w w a l l s c o n s i s t i n g o f Al oV p a r t i c l e s n a n u n i d e n t i f i e d amorphous m a t r i x . P r e s u m a b l y d u e t o a g r a d i e n t f n c o o l i n g r a t e d u r i n g q u e n c h i n g , t h e s i z e o f t h e q u a s i c r y s t a l s r a n g e s from some 10 w c l o s e t o t h e f o i l e d g e ( h i g h c o o l i n g r a t e ) t o a b o u t 2 pm i n t h i c k e r p a r t s o f t h e s p e c i m e n ( l o w c o o l i n g r a t e ) ( F i g . 1 0 ) . F i g . 11 shows a d i f f r a c t i o n p a t t e r n a l o n g A5 o f a q u a s i c r y s t a l a b o u t 300 m t h i c k . A r e m a r k a b l e f e a t u r e o f t h i s p a t t e r n is t h a t t h e i n t e n s i t y o f t h e z e r o o r d e r s p o t i s t h e same a s t h a t o f o t h e r m a j o r r e f l e c t i o n s . T h i s i s n e v e r o b s e r v e d i n o r d i n a r y c r y s t a l d i f f r a c t i o n . However, s u c h i n t e n s i t y d i s t r i b u t i o n s a r e f o u n d i n d i f f r a c t i o n p a t t e r n s o f c r y s t a l s c o n t a i n i n g a h i g h d e n s i t y o f l a t t i c e d e f e c t s , i . e . d i s l o c a t i o n s i n t r o d u c e d b y p l a s t i c d e f o r m a t i o n o r r a d i a t i o n damage / 1 9 / . The r e a s o n f o r t h i s i s m u l t i p l e e l e c t r o n s c a t t e r i n g a t s t r a i n c e n t r e s . I n d e e d b o t h A1-Mn and Al-V q u a s i c r y s t a l s a r e h e a v i l y s t r a i n e d . D e t a i l s , h o w e v e r , h a v e t o a w a i t a q u a n t i t a t i v e d e s c r i p t i o n o f dynamic s c a t t e r i n g i n a p e r i o d i c s t r u c t u r e s . 5. IN-SITU OBSERVATION4 OF PHASE TRANSITIONS IN A1-Mn AND A1-V
-
H e a t t r e a t m e n t s w e r e c a r r i e d o u t i n s i t u and u n d e r d i r e c t o b s e r v a t i o n i n a high-
Fig. 12 A1-V: Quasicrystalline to amorphous transition (a-c); 1 MeV e-irradiation
Fiq. 12c Fig. 13a
Fig. 1 3 Al-Mn: Transition from nearly amorphous to quasicrystalline at 520 K ( a - C )
v o l t a g e e l e c t r o n m i c r o s c o p e (HVEM) equipped w i t h a n u l t r a - h i g h vacuum specimen chamber. Also s a m p l e s c o u l d be i r r a d i a t e d i n s i t u by 1 MeV e l e c t r o n s .
We found t h a t t h e Al-Mn and A l - V q u a s i c r y s t a l s ( h e n c e f o r t h d e n o t e d by Q%pfn
and Q ~ l x a s w e l l a s t h e T phase (TllMn) belong t o t h e few a l l o y p h a s e s which c a n e r e n d e r e amorphous by e l e c t r o n r r a d i a t i o n . The o t h e r s a r e w i t h o u t e x c e p t i o n i n t e r m e t a l l i c compounds / 2 0 , 2 1 / w i t h a n a r r o w c o m p o s i t i o n r a n g e . However, i t was r e p o r t e d / 1 3 / t h a t Q e x i s t s o v e r a s i g n i f i c a n t b u t n o t y e t q u a n t i f i e d r a n g e o f c o m p o s i t i o n . A d e t a i l e 8 1 % u d y o f t h e phase t r a n s i t i o n s between t h e q u a s i c r y s t a l l i n e , c r y s t a l l i n e and amorphous p h a s e s i n N-14at%Mn h a s been p u b l i s h e d e l s e w h e r e / 2 2 / .
The t e m p e r a t u r e s a t which i r r a d i a t i o n p r o d u c e s t h e amorphous s t a t e a r e 130 K , 60 K , and 30 K f o r Q A I M n , and T r e s p e c t i v e l y . F i g . 12 shows t h e d i f f r a c t i o n p a t t e r n of QAly alo&x; d u r i n k l B ' e t r a n s i t i o n a t 60 K. A r e m a r k a b l e f e a t u r e o f t h i s p i c t u r e s e r i e s s t h a t t h e r i n g s o f d i f f r a c t i o n i n t e n s i t y i n t h e amorphous s t a t e oc- c u r a t t h e p o s i t i o n o f t h e r e f l e c t i o n d e c a g o n s i n t h e q u a s i c r y s t a l l i n e s t a t e . T h i s i n d i c a t e s t h a t t h e e l e m e n t a r y d e n s i t y - w a v e v e c t o r s i n t h e q u a s i c r y s t a l l i n e and t h e amorphous s t a t e s a r e v i r t u a l l y o f t h e same l e n g t h .
I f i n QA t h e t r a n s i t i o n t o t h e amorphous s t a t e i s d e l i b e r a t e l y l e f t i n c o m p l e t e w i t h weak [ r ~ 0 0 0 0 ) i n t e n s i t y r e m a i n i n g t h e r m a l a n n e a l i n g ( w i t h o u t i r r a d i a t i o n ) a t
Fig. 14a A10 pattern before irradiation Fig. 14b After e-irradiation at 30 K
Fig. 14c At 600 K (with irradiation) Fig. 14d At 700 K (with irrad~ation)
F i g .
-
i n t e r d i f f u s i o n a t
Fig. 15c 820 K Fig. lid 900 K
T.2 470 K r e p r o d u c e s t h e p e r f e c t q u a s i c r y s t a l l i n e s t a t e ( F i g . 1 3 ) . A n n e a l i n g f u l l y amorphous m a t e r i a l ( u n d e r weak i r r a d i a t i o n t o i n d u c e atom m o b i l i t y ) l e a d s a t T ? 400 K t o a m i c r o q u a s i c r y s t a l l i n e s t a t e . T h i s r e v e r s i b i l t y o f t h e q u a s i c r y s t a l - a m o r p h o u s t r a n s i t i o n i n d i c a t e s t h a t i t is n o t a l a c k o f a t o m i c m o b i l i t y which s t a b i l i z e s t h e q u a s i c r y s t a l l i n e s t a t e b u t a g e n u i n e ( m e t a s t a b l e ) f r e e e n e r g y minimum. I n A1-V an- n e a l i n g i n t h e n e a r l y amorphous s t a t e a l s o l e a d s back t o t h e q u a s i c r y s t a l l i n e s t a t e b u t c r y s t a l l i z a t i o n s e t s i n b e f o r e a p e r f e c t q u a s i c r y s t a l l i n e s t r u c t u r e i s a c h i e v e d . An amorphous t o q u a s i c r y s t a l l i n e t r a n s f o r m a t i o n h a s a l s o been o b s e r v e d u n d e r i o n ir- r a d i a t i o n o f e v a p o r a t e d Al/Mn l a y e r s by o t h e r a u t h o r s 1231. The s a m p l e s became a m o r phous d u r i n g ion-beam mixing a t room t e m p e r a t u r e . Subsequent i r r a d i a t i o n a t 420 K produced a m i c r o q u a s i c r y s t a l l i n e s t a t e . T h i s s t a t e i s produced d i r e c t l y i f t h e ion- beam mixing i s performed a t t e m p e r a t u r e s a b o v e 360 K 1241.
I n a l l t h r e e m a t e r i a l s we f i n d t h a t t h e c r y s t a l l i z a t i o n t e m p e r a t u r e of t h e amorphous s t a t e i s l o w e r t h a n t h a t of t h e q u a s i c r y s t a l l i n e s t a t e . Q c r y s t a l l i z e s a t a b o u t 700 K w h i l e t h e amorphous phase c r y s t a l l i z e s a t 560 K , i n
!dq
we have 720 K and660 K , r e s p e c t i v e l y . F i g . 14 shows t h e A10 d i f f r a c t i o n p a t t e r n o f TAIM d u r i n g i r r a d i a t i o n and d u r i n g s u b s e q u e n t a n n e a l i n g . I n F i g . 14b t h e sample i s n e a r l y com- p l e t e l y amorphous w i t h v e r y l i t t l e i n t e n s i t y l e f t a t t h e p o s i t i o n o f t h e i n i t i a l l y most i n t e n s i v e decagon. We f i n d t h a t a l t h o u g h t h e i n t e n s i t y o f t h e s e s p o t s i n c r e a s e s
d u r i n g a n n e a l i n g a complex t e n - f o l d p a t t e r n d e v e l o p s . I n F i g . 14d t h e sample h a s c r y s t a l l i z e d i n t h e i r r a d i a t e d a r e a ( b u t n o t o u t s i d e i n d i c a t i n g t h e h i g h e r c r y s t a l - l i z a t i o n t e m p e r a t u r e i n p e r f e c t TAIMn). The p a t t e r n c a n b e i n t e r p r e t e d a s t o r e s u l t from f i v e d i f f e r e n t t w i n - r e l a t e d c r y s t a l o r i e n t a t i o n s .
C r y s t a l l i z a t i o n o f QA Mn j u s t a b o v e 700 K l e a d s t o c r y s t a l s w i t h a c o m p o s i t i o n n o t much d i f f e r e n t from t h a $ of t h e o r i g i n a l q u a s i c r y s t a l ( a b o u t 22 atXMn / 6 / ) . These c r y s t a l s r e a d i l y become amorphous d u r i n g i r r a d i a t i o n a t T S 300 K. T h i s i n d i c a t e s a much l o w e r r e s i s t a n c e a g a i n s t t h e t r a n s i t i o n compared t o Q
.
T h i s i s c o r r o b o r a t e d by t h e f a c t t h a t t h e e l e c t r o n d o s e r e q u i r e d f o r t h e tr%!?tion i s o n l y 40% of t h e d o s e r e q u i r e d t o r e n d e r , a t 130 K , q u a s i c r y s t a l l i n e m a t e r i a l amorphous. From t h e amorphous s t a t e , by a n n e a l i n g , we a g a i n o b t a i n t h e q u a s i c r y s t a l l i n e s t a t e . A d i r e c t t r a n s i t i o n from t h e c r y s t a l l i n e t o t h e q u a s i c r y s t a l l i n e s t a t e u n d e r i r r a d i a t i o n c o u l d n o t b e o b s e r v e d . C r y s t a l l i z a t i o n o f Q l e a d s t o A13V f o r m a t i o n , a n i n t e r m e t a l l i c compound w i t h DOz2 s t r u c t u r e . Under i r r a 8 i X t i o n t h i s d i s o r d e r s b u t d o e s n o t show t h e t r a n s i t i o n t o t h e amorphous s t a t e .6 . PRODUCTION O F T PHASE BY THERMAL INTERDIFFUSION
-
- 7
D i f f u s i o n c o u p l e s were produced by vapour d e p o s i t i o n i n vacuum (5x10 T o r r ) on a c a r b o n f o i l suspended on a Mo g r i d . The A1 l a y e r was 36 w t h i c k and t h e M n l a y e r ( o n t o p ) 6 w . The s p e c i m e n s were h e a t t r e a t e d i n t h e HVEM which was o p e r a t e d a t 400 kV i n o r d e r t o p r e v e n t r a d i a t i o n damage f o r m a t i o n .
I n t e r d i f f u s i o n s t a r t s a t a b o u t 530 K where t h e f o r m a t i o n o f a n o r d e r e d p h a s e , presumably AlbMn c a n b e o b s e r v e d ( F i g . 1 5 ) . C o a r s e n i n g i s o b s e r v e d up t o a b o u t 820
K.
At h i g h e r t e m p e r a t u r e s ( 8 2 0 4 T 900 K ) T p h a s e p r e c i p i t a t e s grow r a p i d l y r e p l a c i n g t h e c r y s t a l s grown e a r l i e r . The p r e c i p t a t e s r e a c h s i z e s o f a b o u t 2 pm i n d i a m e t e r . The i n t e r d i f f u s i o n e x p e r i m e n t p r o d u c e s T phase i n a l l o r i e n t a t i o n v a r i a n t s b u t a l s o a n o t h e r phase whose d i f f r a c t i o n p a t t e r n is shown i n F i g . 16. Superimposed on a p e r i o d i c l a t t i c e of r e f l e c t i o n s we s e e a p a t t e r n o f s t r o n g r e f l e c t i o n s a r r a n g e d i n two c o n c e n t r i c e l o n g a t e d d e c a g o n s . T h e r e i s e v e n a n i n d i c a t i o n o f a t h i r d decagon.
D i s t o r t e d p e n t a g o n s c a n a l s o be s e e n a t l a r g e r s c a t t e r i n g a n g l e s . The l o n g a x i s o f t h e d e c a g o n s makes a n a n g l e o f a b o u t 48' t o t h e r e f l e c t i o n rows w i t h t h e l a r g e r s e p a r a t i o n . T h i s i s i n c o n t r a s t t o t h e f e a t u r e s o f t h e AP5 p a t t e r n o f t h e T p h a s e . T h e r e t h e d e f o r m a t i o n o f t h e d e c a g o n s i s much weaker and p a r a l l e l t o t h e rows o f weak r e f l e c t i o n s on which t h e more i n t e n s e s p o t s a r e superimposed ( c f . F i g . 9 ) .
F i g . 17 shows a h i n h - r e s o l u t i o n s t r u c t u r e image. It shows l o z e n g e s h a p e d u n i t s c o w t a i n i n g e l y p t i c a l l y d i s t o r t e d decag o n s a r r
- -
anged i n p a i r s and i n t e r r u p t e d by a
F i g . 16 A1-Mn: C r y s t a l l i n e p h a s e
Fig.
p e r i c
n i f i c .on. we-presume thHt t h i s phase i s r e l a t e d t o t h e a p e r i o d i c p h a s e s I n AI-Mn b u t a n a p p r o p E i a t e s h e a r h a s e l i m i n a t e d t h e f i v e f o l d o r t e n f o l d symmetry a x e s which p e r m i t s t h e f o r m a t i o n of a n o r d i n a r y c r y s t a l l a t t i c e . The s p a c i n g of t h e a n t i p h a s e b o u n d a r i e s i s 2.4 nm and t h e e d g e l e n g t h o f t h e l o z e n g e s i s 1.2 nm which c o r r e s p o n d s t o t h e r e f l e c t i o n p a r a l l e l o g r a m i n F i g . 16.
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