Phonon Density of States in fcc-Fe Precipitates in Cu | SPring-8/SACLA Research Frontiers

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Fig. 1. Schematic illustration of coherent precipitates. P h o n o n D e n s i t y o f S t a t e s i n P r e c i p i t a t e s i n P h o n o n D e n s i t y o f S t a t e s i n fcc - Fe P r e c i p i t a t e s i n Cu S i n c e a n u c l e a r r e s o n a n t s c a t t e r i n g m e t h o d gives us information only from the resonant nuclei, i f w e a p p l y t h i s m e t h o d t o t h e p r e c i p i t a t e s o f a r e s o n a n t n u c l e u s , e . g . 5 7 F e , w e c a n o b t a i n information for t he Fe-precipitates alone, apart from the host metal. Accordingly , we can study how the host metal affects and how far the effect extends to t h e p r e c i p i t a t e s t h r o u g h t h e i n t e r f a c e . N a n o - p a r t i c l e p r e c i p i t a t e s h a v e a t t r a c t e d w i d e s p r e a d a t t e n t i o n d u e t o t h e i r m a n y t e c h n o l o g i c a l a p p l i c a t i o n s , s u c h a s i m p r o v i n g t h e m e c h a n i c a l p r o p e r t i e s o f m a t e r i a l s . T h e r e f o r e t h e d a t a o b t a i n e d p r o v i d e i m p o r t a n t k n o w l e d g e a n d c a n contribute to the development of new materials. In this article, we report the phonon density of states ( D O S ) o f f c c - 5 7 F e p r e c i p i t a t e s i n a C u m a t r i x , i n v e s t i g a t e d u s i n g t h e n u c l e a r r e s o n a n t i n e l a s t i c scattering method at beamline BL09XU [1]. Even below room temperature, Fe precipitates i n C u r e t a i n t h e f c c s t r u c t u r e , w h i c h i s s t a b l e f o r bulk crystal of Fe above 1185 K under n ormal a t m o s p h e r i c c o n d i t i o n s . T h e p r e c i p i t a t e s a r e kn ow n to be sp he ri ca l a nd ar e co he re nt wi th th e C u m a t r i x , a s s h o w n i n F i g . 1 . T h e s i z e s o f t h e p r e c i p i t a t e s c a n b e e a s i l y c o n t r o l l e d b y t h e r m a l treatment [2]. The particle sizes of the precipitates with diameters of 3, 8, 15, 30, 50 and 80 nm were studied at room temperature. Experimental phonon D O S s d e t e r m i n e d f r o m t h e p r e s e n t e x p e r i m e n t f o r the fcc -Fe precipitates at various particle sizes are given in Fig. 2 . The experimental data on the i s o l a t e d F e i m p u r i t i e s i n C u p r e v i o u s l y r e p o r t e d b y M . S e t o e t a l . a r e a l s o g i v e n a s z e r o - n m p r e c i p i t a t e s f o r c o m p a r i s o n [ 3 ] . W i t h i n c r e a s i n g pa rt ic le si ze s, ph on on DO S cu rv es sh ow co nt in uo us change up to 15 nm , but no appreciable change is f o u n d a b o v e 3 0 n m , i n d i c a t i n g t h a t t h e i n f l u e n c e o f the Cu host extends to about 30 nm. Since all p r e c i p i t a t e s h a v e a c o m m o n a t o m i c m a s s , t h e s e changes in the phonon DOS should be attributed to the inter-atomic forces f Fe-Cu and f Fe-Fe . If the f Fe-Cu is greatly different from the f Fe-Fe , the phonon DOS cur ves for sma ll pre cip ita tes sho uld be com pos ed of two parts, one from the interface area ( f Fe-Cu ) and 43 C u F e Fi g. 2. Ex pe ri me nt al ph on on de ns it y of st at es (D OS ) fo r va ri ou s si ze s of fc c- Fe pr ec ip it at es in Cu me as ur ed at ro om te mp er at ur e. Data for 0 nm indicate those for the isolated Fe impurities in Cu. Fig. 3 the other from i nside t he precipitates ( f Fe-Fe ). The e xperimental phonon DOS changes gradually with increasing particle siz e . Thus, the influence of the C u h o s t i s n o t c o n f i n e d t o t h e i n t e r f a c e a r e a b u t u n i f o r m l y e x t e n d s o v e r e n t i r e p a r t i c l e s , a n d t h e effect should be attributed to the inter-atomic force f Fe-Fe its elf . One pos sib le exp lan ati on wou ld be a c h a n g e o f t h e l a t t i c e p a r a m e t e r . T h e l a t t i c e p a r a m e t e r of the fcc -Fe precipitates is slightly smaller (0.7%) t h a n t h a t o f C u m a t r i x a n d t h e p r e c i p i t a t e s a r e ex po se d to a st ro ng ex pa ns iv e for ce . Co ns eq ue nt ly , t h e s m a l l p r e c i p i t a t e s h a v e l a r g e r l a t t i c e s p a c i n g than the large ones [4], resulting in a change in the inter-atomic force f Fe-Fe of small precipitates. Fcc -Fe precipitates in Cu undergo a structural p h a s e t r a n s i t i o n a t l o w t e m p e r a t u r e ( T C ≈ 6 5 K ) , b e l o w w h i c h a t o m i c p o s i t i o n s a r e d e s c r i b e d b y a periodic displacement wave propagating along the [ 1 1 0 ] d i r e c t i o n w i t h t h e < 1 – 1 0 > p o l a r i z a t i o n v e c t o r . D r a s t i c s o f t e n i n g o f t h e s o u n d v e l o c i t y t o w a r d T C h a s b e e n r e p o r t e d [ 5 ] . T h u s , t h e t e m p e r a t u r e v a r i a t i o n o f t h e p h o n o n D O S f o r t h e fcc - F e p r e c i p i t a t e s w a s a l s o s t u d i e d . D a t a o b t a i n e d a t v a r i o u s t e m p e r a t u r e s u s i n g t h e precipitates with a 50 - nm diameter are given in . Phonon DOS curves show drastic change with d e c r e a s i n g t e m p e r a t u r e e v e n t h o u g h t h e d i s p l a c e m e n t o f l a t t i c e s b e l o w T C i s v e r y s m a l l . The pea k int ens ity aro und 30 meV inc rea ses and t h e p e a k a r o u n d 2 2 m e V s h i f t s t o w a r d t h e h i g h - e n e r g y s i d e w i t h d e c r e a s i n g i n t e n s i t y . T h e st ru ct ur al ph as e tr an si ti on is th e fi rs t or de r, wh il e 44 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 P h o n o n E n e r g y ( m e V ) 0 n m 3 n m 8 n m 1 5 n m 3 0 n m 5 0 n m 8 0 n m N o r m a l i z e d P h o n o n D e n s i t y o f S t a t e s References [1] Y. Tsunoda, Y. Kurimoto, M. Seto, S. Kitao and Y. Yoda, Phys. Rev. B 66 (2002) 214304. [2] R. Borrelly et al. , Scr. Metall. 15 (1975) 747. [3] M. Seto et al. , Phys. Rev. B 61 (2000) 11425. [ 4 ] Y . T s u n o d a e t a l . , J . P h y s . F : M e t . P h y s . 1 8 (1988) 1421. [ 5 ] Y . T s u n o d a a n d N . K u n i t o m i , S o l i d S t a t e Commun. 54 (1985) 547. [6] Y. Tsunoda, J. Phys. Soc. Jpn. 58 (1989) 1648. Fig. 3. Temperature variation of phonon density of states (DOS) of fcc-Fe precipitates. A specimen with precipitates of 50 nm in diameter was used. Yorihiko Tsunoda Waseda University E-mail: tsunoda@waseda.jp t h e c h a n g e i n p h o n o n D O S a p p e a r s t o b e c o n t i n u o u s . T h e s h a p e o f t h e p h o n o n D O S c u r v e a t 1 5 0 K a l r e a d y d i f f e r s f r o m t h a t a t r o o m t e m p e r a t u r e , su gg es ti ng th at th e ph on on di sp er si on cu rv e, i. e. , inter-atomic potential s t a r t s t o c h a n g e f a r a b o v e t h e transition temperature. R e c a l l i n g t h a t fcc - Fe bu lk c r y s t a l i s s t a b l e o n l y a t m u c h h i g h t e m p e r a t u r e under n ormal atmos pheri c condi tion , w e belie ve that the structural phase transition is the fcc - bcc martensitic transformation of the Fe metal. However, volume e x p a n s i o n a n d e x t e r n a l s h a p e c h a n g e o f t h e precipitates are tightly restricted by the host metal, an d th e ma rt en si ti c tr an sf or ma ti on st op pe d at th e halfway point of the transformation [6]. 45 0 1 0 2 0 3 0 4 0 5 0 E n e r g y ( m e V ) 4 5 K 6 2 K 9 0 K 1 5 0 K 3 0 0 K N o r m a l i z e d P h o n o n D e n s i t y o f S t a t e s