HAL Id: jpa-00225230
https://hal.archives-ouvertes.fr/jpa-00225230
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
LOCAL STRUCTURE OF AN AMORPHOUS Cu24 Zr76 BY EXAFS : FROM THE AS-PREPARED
ALLOY TO THE FIRST STEPS OF
CRYSTALLIZATION EXISTENCE OF FRUSTATION ?
A. Sadoc, J. Lasjaunias
To cite this version:
A. Sadoc, J. Lasjaunias. LOCAL STRUCTURE OF AN AMORPHOUS Cu24 Zr76 BY EXAFS : FROM THE AS-PREPARED ALLOY TO THE FIRST STEPS OF CRYSTALLIZATION EXIS- TENCE OF FRUSTATION ?. Journal de Physique Colloques, 1985, 46 (C8), pp.C8-505-C8-509.
�10.1051/jphyscol:1985879�. �jpa-00225230�
LOCAL STRUCTURE OF AN AMORPHOUS Cu Zr BY EXAFS : FROM THE
AS-PREPARED ALLOY TO THE FIRST STEPS OF CRYSTALLIZATION EXISTENCE OF FRUSTATION ?
A. Sadoc and J . C . L a s j a u n i a s *
DURE, Bât. 209 c, Université Paris-Sud, 91405 Orsay Cedex, France
*CRTBT, CNRS, 166 X, 38042 Grenoble Cedex, France
Résumé - Nous r a p p o r t o n s une é t u d e EXAFS de l ' a l l i a g e Cu.j, Zr , depuis l ' é t a t amorphe j u s q u ' à l ' é t a t cristallisé, où a p p a r a î t la phase u-Zr.
Abstract - We r e p o r t an EXAFS study of t h e C u . ^ Z r , allô y from t h e amorphous s t a t e t o t h e crystallized s t a t e , when t h e u r z r phase a p p e a r s .
1 - Introduction
In r e c e n t years, a l o t of e x p e r i m e n t a l and t h e o r e t i c a l work has been done on amorphous alloys in order t o understand t h e i r physical and mechanical properties. A Cu-Zr amorphous alloy is one example of t y p i c a l m e t a l - m e t a l amorphous s y s t e m . Previous EXAFS s t u d i e s have been c a r r i e d out on
Cu -Z f f°r x = 0.60, 0.46, 0.33 / 1 , 2 / . From t h i s work, a picture of t h e l o c a l order in t h e s e very disordered materials can be drawn and also a subshell modelling t o i n t e r p r e t i t . Moreover, i t has become evident t h a t t h e neighbourhood of a Cu atom is c o n c e n t r a t i o n independent in this range of composition (0.60 i x i 0.33).
The p r e s e n t EXAFS study above t h e Cu and Zr K absorption edges extends t o include a zirconium r i c h e r composition, Cu?^Zr , . For this composition, we r e p o r t also EXAFS m e a s u r e m e n t s of t h e amorphous - t o - crystalline t r a n s i t i o n . In C u . ^ Z r , , t h e ordering associated with crystallization does not occur homogeneously but begins with h e t e r o g e n e o u s nucleation. Two s t a g e s in t h e annealing process a r e of p a r t i c u l a r i n t e r e s t . The first s t e p , or s t r u c t u r a l r e l a x a t i o n , consists of a h e a t t r e a t m e n t a t 2 0 0 " C . Then, s t r o n g modifications occur for s e v e r a l macroscopic p r o p e r t i e s such as t h e r m a l conductivity, specific h e a t / 3 / . However, t h e sample is s t i l l fully amorphous. For t h e second s t e p , a t 240-250°C, s t a n d a r d X-ray diffraction shows t h e p r e s e n c e of a f r a c t i o n of w-Zr phase surimposed on t h e p a t t e r n of t h e amorphous matrix. This
metastable phase is s y s t e m a t i c a l l y obtained as t h e first crystallization s t a g e for t h e s e high Zr c o n c e n t r a t i o n s / 4 / . Full crystallization in t h e form ct Zr + Cu Zr_ is achieved a t 500°C.
For t h e a s - p r e p a r e d and r e l a x e d s a m p l e s , t h e d e t a i l s of t h e study a r e given elsewhere / 5 / and will be only briefly s k e t c h e d h e r e .
2 - Experimental
Amorphous C u? 4Z r7 6 samples have been prepared as thin films by DC magnetron s p u t t e r i n g and t h e amorphous n a t u r e of t h e samples was ensured by X-ray scanning. Thermal t r e a t m e n t s were done first a t 200° C in argon for 1h t h e n in ultra high vacuum a t 21)0°C for 24 h.
EXAFS data have been collected above t h e Cu and Zr K absorption edges a t LURE using t h e D CI r i n g . The e x p e r i m e n t s were carried out a t 77K so t h a t disorder e f f e c t s due t o t h e r m a l Debye Waller f a c t q r could be ignored. Then the s p e c t r a have been analyzed using s t a n d a r d procedure / 5 / . Fourier t r a n s f o r m s (FT) of t h e k 3 , . , r v» n < - <- v. i_-
x0<) EXAFS s p e c t r a have been achieved using one k-window for all t h e Cu EXAFS s p e c t r a and a n o t h e r one for t h e Zr EXAFS s p e c t r a .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985879
JOURNAL DE PHYSIQUE
3
-
ResultsFT a r e compared i n Figure 1 f o r t h e as-prepared and annealed samples. On t h e Cu edge, t h e FT show a single peak a t a b o u t 2.25 A ( u n c o r r e c t e d f o r t h e phase shifts). This f i r s t peak r e p r e s e n t s t h e f i r s t s h e l l of Cu and Zr a t o m s around a Cu atom. It does n o t change much with h e a t t r e a t m e n t s , e v e n when t h e w-Zr phase a p p e a r s (c).
On t h e Z r edge, t h e FT display t w o peaks f o r t h e as-prepared s a m p l e (a) and f o r t h e s a m p l e annealed a t 200°C (b). A s h a s been shown previously /2, 5/
t h e f i r s t peak corresponds t o t h e contribution of Cu a t o m s whereas t h e second peak corresponds t o t h e contribution of Zr atoms. A t h i r d peak a p p e a r s a f t e r h e a t i n g a t 240°C (c). Therefore, t h e first-shell s t r u c t u r e around a Z r atom changes drastically with annealing t r e a t m e n t s .
The n e x t s t a g e i n t h e analysis is t o back-Fourier transform t h e f i r s t s h e l l of a t o m s in o r d e r t o f i t t h e inverse FT using t h e EXAFS formula. First a narrow R-window was used (such a s one covering only one peak) i n a n e f f o r t t o find t h e t y p e of t h e back s c a t t e r i n g atom and a rough e s t i m a t i o n of t h e s t r u c t u r a l parameters. But, a s t h e peaks a r e n o t well resolved, it is n o t possible t o e x t r a c t , with a c c u r a c y , t h e s e p a r a t e contributions of t h e t w o a t o m i c c o n s t i t u a n t s of t h e amorphous alloy t o t h e Z r EX A FS, despite t h e i r g r e a t l y d i f f e r e n t radii. For t h i s reason, a wide window was t h e n applied t o t h e transform i n performing t h e band-pass f i l t e r a n d f i t s were made t o t h e resulting back transform.
Figures 2a and 2b display t h e quality of t h e f i t s obtained f o r t h e s a m p l e h e a t e d a t 240°C. The r e f i n e d s t r u c t u r a l p a r a m e t e r s yielded from t h e f i t s a r e l i s t e d in t a b l e 1. N is t h e number of neighbour a t o m s a t t h e d i s t a n c e R from t h e c e n t r a l atom and t h e disorder p a r a m e t e r o r e p r e s e n t s t h e f l u c t u a t i o n of t h e distances due t o t h e s t r u c t u r a l disorder.
4
-
Discussiona ) As-prepared s a m ple
A s s e e n i n t a b l e 1 , t h e environment of a Cu atom consists of t w o subshells of Cu a t o m s a t 2.50 A and 3.10 A and a single s h e l l of Z r a t o m s a t 2.70-2.74 A. This neighbourhood is very s i m i l a r t o t h o s e found f o r Cu-rich amorphous alloys /1, 2 / and h a s s o m e r e s e m b l a n c e t o t h e crystalline Cu Z r On t h e opposite, t h e s h o r t Cu-Cu distance (2.50 8 ) does n o t ex&?
1;
crystalline C u Z r i , where t h e r e f o r e , t h e r e a r e no Cu-Cu n e x t n e a r e s t neighbours. CuZr2 ~s our r e f e r e n c e phase, s i n c e t h e Cu Z r system does n o t c r y s t a l l i z e i n t o a single phase in t h e c o n c e n t r a t i o n rzngk-%.66 < l - x < l (pure zirconium), b u t i n t o a mixture of CuZr and Zr.
Around a Zr a t o m , t h e r e is one subshe& of Cu a t o m s a t 2.70-2.74 A and t w o subshells of Zr a t o m s a t 3.12 A and 3.45 A. This t y p e of Zr subshells e x i s t i n t h e CuZr c r y s t a l and was found a l s o f o r t h e corresponding amorphous alloy,
c u Z r 2/2/.
It 3& %70teworthy t h a t t h e s e r e s u l t s a r e i n good a g r e e m e n t with X-ray d i f f r a c t i o n analysis 5 Indeed, t h e t w o t y p e s of e x p e r i m e n t s a r e com plementary. EX A FS is m o r e sensitive t o sharp f e a t u r e s in t h e r a d i a l distribution function, i.e. t o t h e Cu-Cu pairs and t o Zr-Cu ones, t h e s h o r t e s t i n t e r a t o m i c d i s t a n c e s being r e l a t e d t o t h e s m a l l e s t o disorder p a r a m e t e r ( t a b l e 1). On t h e opposite, X-ray s c a t t e r i n g a p p r e c i a t e s b e t t e r t h e broad f e a t u r e s of t h e r a d i a l distribution, t h e r e b y t h e Zr-Zr pairs. This is because X-ray s c a t t e r i n g h a s l o w e r s p a t i a l resolution, t h e wave number e n t e r i n g t h e product q r i n s c a t t e r i n g and 2kr i n EX A FS s o t h a t t h e t r a n s f e r t momentum is respectively q and 2k. Since q m a x is typically 15 A-I f o r X-ray, t h i s is equivalent t o kma, = 7.5
a'.
A s our EXAFS s p e c t r a e n t e n d t o k =12-13m a x
A-l ,
EX A FS h a s significantly higher s p a t i a l resolution.TABLE 1 : Structural parameters for Cu24 Zr76
(Atomic diameter : Cu-Cu = 2.56
A ,
Cu-Zr = 2.88% ,
Zr-Zr = 3.20
A )
3.3 Cu 2.50 .I16 5 Zr 2.74 .I22 1.5 Cu 3.10 .I16 1.5-2 Cu 2.70 .I22 5 Zr 3.12 '1.5-2 Zr 3.45 I
0 0
0 1 2 3 4 5 0 1 2 3 4 5
~ ( l i )
~ ( i )
Cu edge Zr edge
Fig. 1 : Fourier transforms of the Cu K-edge EXAFS of ( a ) as-prepared Cu24 Zr76, (b) relaxed Cu24Zr76, ( c ) Cu24Zr76
annealed at 240°C.
.---.---
3.5 Cu 2.52 .I16 6fl Zr 2.76 -120 1.5-2 Cu 3.10 .I16
.---.---
2 Cu 2.69 .I27 5.5 Zr 3.13
1.132 2 Zr3.45
3.3 Cu 2.52 .I16 4 Zr 2.74 .I22 2 Cu 3.15 -116 1.5-2 Cu 2.70 ,125 5.5 Zr 3.28 .I24 2 Zr3.50
1.132
JOURNAL DE PHYSIQUE
Turning back t o t h e f i r s t shell s t r u c t u r e , it is interesting t o r e m a r k t h a t t h e f i r s t of t h e t w o distances found f o r t h e Cu-Cu pairs and f o r t h e Zr-Zr pairs corresponds t o t h e f i r s t neighbour distance in t h e pure m e t a l (2.56 A f o r copper and 3.20 A f o r zirconium a t room temperature). This could be a more general f e a t u r e i n metallic amorphous alloys s i n c e t h i s was found also by EXAFS f o r o t h e r concentrations of t h e CuxZr system /2/ a s well as f o r o t h e r s y s t e m s such a s N i Y / 1 / o r Mg znl-' /6/. Such a result has also been obtained r e c e n t l y b t 6 d z o g u c h i e t
A? /a9 using neutron diffraction in N i Z r g 4 . For t h i s alloy, t h e f i r s t N i - N i peak points a t 2.45 A and exhibits a sh&!?lder on i t s high R side. Therefore t h e N i - N i contribution clearly appears doubly composed in t w o subshells, t h e f i r s t distance corresponding t o t h e f i r s t neighbour distance in pure metal.
b) Annealed samples
After t h e annealing t r e a t m e n t a t 200°C t h e l o c a l s t r u c t u r e of t h e relaxed sample does not change much. A slight increase of t h e t o t a l number of Zr-Zr pairs, from about 6.75 t o 7.5, is however noticeable (Table 1).
On t h e opposite, t h e h e a t t r e a t m e n t a t 240°C, which ensures t h e crystallization of t h e u-Zr phase, drastically modif&% t h e l o c a l s t r u c t u r e around a Z r atom. Above t h e Zr edge, t h e new 3 peak in t h e FT is identified a s being due t o Zr-Zr contributions a t 3.90 A. Moreover, t h e f i r s t Z r - Z r subshell is also strongly a l t e r e d : t h e Z r Z r distance is s h i f t e d by AR=0.15 A and t h e disorder p a r a m e t e r is reduced from Ao = -.008 A. Indeed, all t h e s e r e s u l t s indicate an ordering of t h e environment of t h e Z r a t o m s a t higher distances.
On t h e contrary, f o r t h e Cu neighbourhood, t h e s t r u c t u r a l p a r a m e t e r s a r e a l m o s t unchanged. Nevertheless, a slight decrease of t h e N u-Zr coordination number c a n be pointed out.
Therefore highlights of t h e a t o m i c rearrange m e n t which t a k e s place during t h e f i r s t s t e p s of crystallization, when t h e u-Zr phase h a s crystallized, a r e :
( 1 ) The neighbourhood ;of a Cu atom becomes poorer in Zr atoms, while clustering of t h e Zr a t o m s appears. This tendency f o r segregation already exists in t h e relaxed sample.
(2) The l o c a l a r c h i t e c t u r e around t h e Zr atom builds up in higher coordination s u b s h a .
(3) The organization of t h e Cu-Cu pairs does n o t change. This is surely t h e most surprising point since t h e r e a r e no Cu-C u pairs in close c o n t a c t in CuZr2. If we r e p r e s e n t t h e double Cu-Cu subshells by a double-w e l l potential, a supplementary amount of energy is necessary f o r t h e Cu atom t o become mobile and t o jump from t h e f i r s t minimum of configurational energy (2.52 A) t o t h e second one (3.15 A). This second distance corresponds t o t h e t r u e Cu-Cu f i r s t distance in CuZr If t h e second minimum h a s a 'lower energy t h a n t h e f i r s t one, s o m e kin$' of frustration could occur involving strong constraints in t h e alloy which induce t h e formation of t h e w-Zr phase. In f a c t , t h e zirconium undergoes under high pressure a phase transition from a t o w /8/, t h e l o w e s t pressure a t which t h e u phase was observed t o form from t h e a phase is 3 9 , kbar a t room t e m p e r a t u r e and around 20 kbar a t 200°C.
These r e s u l t s could be correlated with r e c e n t calculations of Ashby e t Gyorfly (to be published). These authors have studied t h e "freeze inv of concentrations fluctuations, in metallic glasses, induced by s o m e kind of frustration due t o t h e simultaneous presence of both ordering and clustering tendencies.
SOL
50,
331-3119./2/ Sadoc A., Calvayrac Y., Quivy A., Harmelin M . and Flank A.M., (19841, J.
non Cryst. SOL
5,
109-1 29./3/ Ravex A., Lasjaunias J.C. and ~ Q t h o u x O., (1984), J. Phys. F. : Met.
Phys.
2,
329./4/ Goudeau P., Lasjaunias J.C., Naudon A., Ravex A. and Bethoux O., (19841, Rapidly Quenched Metals, Vth Int. Conf., t o be published.
/5/ Sadoc A., Lasjaunias J.C., J. Phys. F : Met. Phys.
2,
(1985), 1021./6/ Sadoc A., Krishnan R., Rougier P., J. Phys. F : Met. Phys.
2,
(1985), 241./7/ Mizoguchi T., Yoda S., Akutsu N., Yamada S., Nishioka J., Suemasa T.
and Watanabe N., Rapidly Quenched Metals, Vth Int. Conf., t o be published.
/8/ Olinger B., J a m i e s o n J.C., High t e m p e r a t u r e s . High pressures,
5,
(1 973) 123.k(i-l)
Fig. 2 : F i l t e r e d E XAFS s p e c t r a (dots) and itslo ' 1 l 2 s i m u l a t i o n (full curve) f o r
Cu Z r h e a t e d a t 240°C (a) abc??e X'Ee Cu edge, (b) above t h e Zr edge.
