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Submitted on 1 Jan 1979
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TWO-PHASE SEPARATION IN ALNICO 5 ALLOY
R. Tahara, Y. Nakamura
To cite this version:
R. Tahara, Y. Nakamura. TWO-PHASE SEPARATION IN ALNICO 5 ALLOY. Journal de Physique
Colloques, 1979, 40 (C2), pp.C2-592-C2-593. �10.1051/jphyscol:19792206�. �jpa-00218585�
JOURNAL DE PHYSIQUE Colloque C2, supplbment au n o 3 , Tome 40, mars 1979, page C2-592
TWO-WASE SEPARATION I N A L N I C O 5 ALLOY
R. Tahara* and Y. Nakamura
Department of MetaZ Science and Technology, Kyoto University, Kyoto 606, Japan
RBsum6.- On e t u d i e p a r e f f e t ~ g s s b a u e r du 5 7 ~ e l e p r o c e s s u s d e s B p a r a t i o n d e p h a s e s d a n s un a l l i a g e magnBtique A l n i c o 5. AprSs trempe 1 p a r t i r d ' u n e r e g i o n monophasee, l e s B c h a n t i l l o n s minces s o n t r e c u i t s isothermiquement e t l e s s p e c t r e s Mzssbauer s o n t e n r e g i s t r s s 1 l a t e m p e r a t u r e ambiante.
L ' a n a l y s e p a r o r d i n a t e u r d e s s p e c t r e s Xgssbauer montre que l ' a l l i a g e s u b i t une dBcomposition s p i n o - d a l e v e r s 8 0 0 ' ~ ; l a p h a s e d ' b q u i l i b r e r i c h e e n FeCo commence 1 s e f o r m e r au b o u t d e 5 s e c o n d e s e n v i - ron. P a r c o n t r e , pour un r e c u i t 2 650°c, l a s e p a r a t i o n de p h a s e s e p r o d u i t p a r un mBcanisme d e nuclea- n u c l e a t i o n e t c r o i s s a n c e .
A b s t r a c t . - The p r o c e s s of phase s e p a r a t i o n i n t h e A l n i c o 5 magnet a l l o y i s i n v e s t i g a t e d by u s i n g t h e MGssbauer e f f e c t o f " ~ e . A f t e r b e i n g quenched from a one-phase r e g i o n , t h e t h i n samples a r e aged f o r v a r i o u s p e r i o d s a t c o n s t a n t t e m p e r a t u r e s , and t h e Xzssbauer s p e c t r a a r e t a k e n a t room t e m p e r a t u r e . The computer a n a l y s i s of MGssbauer s p e c t r a shows t h a t f o r a g i n g around 8 0 0 ° c , t h i s a l l o y decomposes by a s p i n o d a l mechanism and t h e e q u i l i b r i u m FeCo-rich p h a s e b e g i n s t o b e formed w i t h i n a b o u t 5 s e d s , b u t f o r a g i n g below 650°c, t h e p h a s e s e p a r a t i o n p r o c e e d s by a n u c l e a t i o n and growth mechanism.
The d e s i r a b l e m a g n e t i c p r o p e r t i e s f o r a per- manent magnet of t h e A l n i c o 5 a l l o y (Fe-21.2atXCo -12.6atmi-15.3at%A1-2.5atZCu) r e s u l t from a f i n e l y d i s p e r s e d p r e c i p i t a t e s t r u c t u r e where s t r o n g l y f e r n - m a g n e t i c a l p a r t i c l e s a r e embedded i n a weakly f e r r o m a g n e t i c o r nonmagnetic
ci2m a t r i x . The p h a s e d i a g r a m and t h e p e r i o d i c a l l y modulated s t r u c t u r e of p r e c i p i t a t e s i n t h i s a l l o y s u g g e s t t h a t t h e decom- p o s i t i o n of h i g h - t e m p e r a t u r e p h a s e i n t o two low- t e m p e r a t u r e p h a s e s , an FeCo-rich p h a s e and a NiA1- r i c h p h a s e , i s of s p i n o d a l t y p e / I / . However, p r e c i s e i n f o r m a t i o n a b o u t t h e mechanism of d e c o m p o s i t i o n i n t h i s a l l o y h a s n o t been o b t a i n e d s o f a r .
I n t h e p r e s e n t s t u d y , t h e p r o c e s s of p h a s e
a r e t h e same t o t h o s e i n t h e A l n i c o 5 a l l o y , t h a t i s , 0.69 : 0.31 and 0.45 : 0 . 5 5 , r e s p e c t i v e l y .
T h e o r e t i c a l a b s o r p t i o n s p e c t r a of t h e aged A l n i c o 5 a l l o y s w i t h t h e c o m p o s i t i o n f l u c t u a t i o n s were s y n t h e - s i z e d a s a s u p e r p o s i t i o n of t h e M k s b a u e r s p e c t r a of t h e p s e u d o b i n a r y a l l o y s . The a n a l y s i s method of spec- t r a i s t h e same u s e d i n a n a l y z i n g t h e s p e c t r a of FeCrCo a l l o y s 1 2 1 .
YGssbauer s p e c t r a of as-quenched and aged specimens a r e shown i n f i g u r e 1 f o r 7 5 0 ' ~ a g i n g .
*lnico aged at 7 5 0 ' ~ s e p a r a t i o n i n t h e A l n i c o 5 a l l o y h a s been i n v e s t i g a -
t e d u s i n g t h e Mzssbauer e f f e c t of " ~ e .
A f t e r s o l u t i o n t r e a t m e n t a t 1250°c, t h e b l o c k samples were quenched i n t o w a t e r . The t h i n p l a t e s p r e p a r e d by wire-sawing were i s o t h e r m a l l y aged a t s e v e r a l t e m p e r a t u r e s between 8 0 0 ' ~ and 5 0 0 ' ~ by d i p - p i n g t h e samples i n t o a s a l t b a t h and t h e n t h e y w e r e ground i n t o powders. The ~ Z s s b a u e r s p e c t r a were taken a t room t e m p e r a t u r e and t h e d o p p l e r v e l o c i t y a x i s was r e f e r r e d t o p u r e a i r o n a t room t e m p e r a t u r e .
As t h e two-phase s e p a r a t i o n i s e x p e c t e d t o o c c u r i n t h e p s e u d o b i n a r y FeCo-NiAl s y s t e m , a s e r i e s o f random s o l i d s o l u t i o n a l l o y s w i t h d i f f e r e n t NiAl c o n t e n t s were p r e p a r e d b y s p l a t - q u e n c h i n g and t h e d s s b a u e r s p e c t r a were measured. The a t o m i c r a t i o s
e f Fe t o Co and N i t o A 1 i n t h e p s e u d o b i n a r y a l l o y s
- 6 -4VELOCITY (mrnls
- 2 0 2 ) 4 6 - - - -*permanent a d d r e s s : Research and Development L a b o r i r F i g . I : ~ z s s b a u e r s p e c t r a of t h e A l n i c o 5 a l l o y t o r y , M a t s u s h i t a E l e c t r i c Works, L t d . , Kadoma,Osaka quenched from 1250°C and aged a t 750°c.
5 7 1 , Japan.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792206
The s o l i d c u r v e s a r e s y n t h e s i z e d s p e c t r a f o r t h e s p i n o d a l d e c o m p o s i t i o n under t h e assumption t h a t t h e s i n u s o i d a l c o m p o s i t i o n waves w i t h a c o n s t a n t wave- l e n g t h a r e developed i n d e p e n d e n t l y a l o n g t h e t h r e e
< l o o > d i r e c t i o n s . S i n c e t h e t h r e e - d i m e n s i o n a l compo- s i t i o n f l u c t u a t i o n can b e w r i t t e n a s t h e sum of t h e t h r e e waves, t h e maximum f l u c t u a t i o n becomes f3Ac, where Ac means t h e one-dimensional a m p l i t u d e of t h e NiAl c o m p o s i t i o n . The agreement between o b s e r v e d s p e c t r a and c a l c u l a t e d c u r v e s i s v e r y good, i n d i c a -
t i n g t h a t t h e p h a s e s e p a r a t i o n of t h i s a l l o y i n i t s e a r l y s t a g e i s a t t r i b u t a b l e t o a s p i n o d a l decomposi- t i o n . The s p e c t r u m f o r 5 s a g i n g c o r r e s p o n d s t o t h a t f o r Ac w i t h 10 a t % NiA1. T h i s means t h a t , s i n c e t h e o r i g i n a l c o m p o s i t i o n of NiAl i n t h i s a l l o y i s 29 a t % , t h e maximum f l u c t u a t i o n r e a c h e s t h e phase boundary of a, by a b o u t 5 s aging.
I n c o n t r a s t t o h i g h e r t e m p e r a t u r e a g i n g , wide b r o a d e n i n g o f a b s o r p t i o n peaks i s n o t observed f o r a g i n g below 700°c, a l t h o u g h t h e h y p e r f i n e f i e l d o f 5 7 ~ e i n t h e FeCo-rich r e g i o n i n c r e a s e s c o n t i n u o u s l y w i t h i n c r e a s i n g a g i n g t i m e a s shown, f o r example,
i n f i g u r e 2 f o r 650°C a g i n g .
F i g . 2 : M6ssbauer s p e c t r a of t h e A l n i c o 5 a l l o y quenched from 1250°C and aged a t 6 5 0 ' ~ .
The s o l i d c u r v e s i n f i g u r e 2 a r e s y n t h e s i z e d s p e c t r a f o r a n u c l e a t i o n and growth mechanism i n which t h e e q u i l i b r i u m NiAl-rich a2 p h a s e p r e c i p i t a t e s from t h e b e g i n n i n g of a g i n g . The p e r c e n t a g e s i n d i c a t e t h e amount o f a* p r e c i p i t a t e s . The good agreement between o b s e r v e d s p e c t r a and s y n t h e s i z e d c u r v e s sug- g e s t s t h a t t h e two-phase s e p a r a t i o n a t lower temper-
a t u r e s i s a s c r i b e d t o a n u c l e a t i o n and growth mecha- nism.
We have a l s o made t h e MEssbauer measurements f o r s e v e r a l p s e u d o b i n a r y a l l o y s w i t h d i f f e r e n t NiAl.
c o n t e n t s . The r e s u l t s of t h e s p e c t r a a n a l y s e s f o r t h e s e a l l o y s a r e shown i n f i g u r e 3 , where t h e s y r b o l s , s and
n,i n d i c a t e whether t h e two-phase s e p a - r a t i o n of t h e quenched a l l o y s o c c u r s by a s p i n o d a l mechanism o r a n u c l e a t i o n and growth mechanism.
FQ~CO, 20 40 60 80 NiA5AIs5
at% NiAl
