STRUCTURAL RELAXATION AND ATOMIC MOBILITY BY MAGNETIC ANISOTROPY MEASUREMENTS IN SOME METALLIC GLASSES

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STRUCTURAL RELAXATION AND ATOMIC

MOBILITY BY MAGNETIC ANISOTROPY

MEASUREMENTS IN SOME METALLIC GLASSES

W. Chambron, A. Chamberod

To cite this version:

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CoZZoque

C S ,

suppZ&ment au nO1O, Tome

4 2 ,

octobre 1981

page

C5-5 1 1

STRUCTURAL R E L A X A T I O N AND ATOMIC M O B I L I T Y B Y MAGNETIC ANISOTROPY MEASUREMENTS I N SOME M E T A L L I C GLASSES

W.

Chambron and A. Chamberod

Centre d

'Etudes

NucZdaires de GrenobZe, Dgpmtement de Recherche FondamentaZe

Section de Physique du SoZide, 85

X

-

38041 GrenobZe Cedex, France

Abstract. - 1Je compare the magnetic anisotropy induced by thermomagnetic treatments in some amorphous metallic alloys, considered in three different states : as-received, annealed, annealed and subsequently quenched. The atomic mobility is shown to decrease during an anneal, and to be partially res- tored by a quench. It is concluded that some processes involved in what is called "structural relaxation" are reversible, at least in part.

1. Introduction.- Internal friction (IF) measurements have been used for a long time to study the atomic mobility of crystalline alloys [l]. By such a method, anelastic relaxation produced by the short range directional order, indltced by a stress, is investigated. The Snoek peaks, observed in insertion solid solutions, allow to deter mine interstitial atoms mobility, while Zener peaks, insubstitution solid solutions, characterize constituent atoms mobility.

In ferromagnetic crystalline alloys, a directional order (DO) can also be obtained by thermal treatment under magnetic field, called thermomagnetic treatment (T'fT). This DO is detected by measuring the associated induced magnetic anisotropy (I'IA). As in the case of stress-induced DO, the phenomenon is observed both in insertion and substitution solid solutions 121.

In amorphous metallic alloys, an exponential increase of IF is observed at high temperature [31. It is due both to an anelastic,and aviscoplastic deformation

[ & l .

The latter prevails at high temperature 151, but it is often difficult to se-

parate the two contributions.

Let us now perform a TFIT on a ferromagnetic amorphous alloy : one observes an IHA which is

-

contrary to the deformation

-

entirely reversible as a function of temperature, and direction of the applied magnetic field 161. This PfA is probably due to a short range DO [ 7 ] . Analogy with crystalline materials leads to think that

IMA,

and anelastic relaxation, arise from atomic displacements. However, when a quantitative comparison has been attempted, restrictions have been expressed [6], because of differences between activation energies.

In this paper, we discuss the influence of structural state (as--received, annealed, annealed and subsequently quenched) on the kinetics of establishment of

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C5-5 12 JOURNAL DE PHYSIQUE

I Y A , and t h e r e f o r e on t h e atomic m o b i l i t y .

2. Experiments.- The amorphous a l l o y s used a r e t h i n ribbons (50um t h i c k ) from ALLIED

CHEaIICAL COSPOMTION (Fe4o N i 4 0 PI B6), VACUP4SCBIELZE (Cogg N i 1 0 Fe5 S i l l B, 6 )

,

and GENERAL ELECTRIC B 1 4 , 5 S i 4 ) . Samples a r e d i s k s of 7 mm d i a m e t e r , c u t i n

t h e s e r i b b o n s . The IYA i s measured w i t h an automatic t o r q u e magnetometer, under a

magnetic f i e l d of 0.34 T , with an accuracy of 2 0.3 .T.mb3. TMT a r e performed " i n

s i t u " i n t h e t o r q u e magnetometer. The II!A energy d e n s i t y i s denoted KU.

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t h e time (10 min) of each TMT i s s h o r t , compared t o t h e time c o n s t a n t of t h e IE?A es-

m

t a b l i s h m e n t . Then, one o b t a i n s only a f r a c t i o n

(a

= AK

/K,

) of t h e asymptotic value,

m u

KU

,

and t h i s f r a c t i o n i n c r e a s e s w i t h temperature, because t h e process i s t h e r m a l l y a c t i v a t e d . On t h e d e c r e a s i n g s i d e , t h e time c o n s t a n t of t h e

IMA

i s s h o r t enough s o

m

t h a t 0 i s c l o s e t o 1 ; t h e n

IAK,~

v a r i e s a s KU

,

and one observes t h e d e c r e a s e of m

KU

,

a s a f u n c t i o n of T , a s i t does t o b e zero a t t h e Curie temperature.

a ) "as-received" s t a t e . Curves "a", i n f i g . 1-3, a r e o b t a i n e d on samples i n as- r e c e i v e d s t a t e . They a r e n o t r e p e a t i d l y r e p r o d u c i b l e on a same sample : i f t h e s e r i e s of i s o c h r o n a l TMT i s stopped a t a g i v e n temperature, then taken a g a i n a t a lower temperature, t h e second curve i s l o c a t e d below t h e f i r s t one, and o v e r t a k e s t h i s one p r a c t i c a l l y a t t h e s t o p temperature. So, an e v o l u t i o n , g e n e r a l l y considered a s i r r e v e r s i b l e , t a k e s p l a c e a l l a l o n g t h e thermal sequence of curves a . I t corresponds t o a p r o g r e s s i v e d e c r e a s e of

(AK

I,

and of t h e atomic m o b i l i t y . This e v o l u t i o n i s c a l l e d " s t r u c t u r a l r e l a x a t i o n " .

b ) "annealed" s t a t e . Curves "b", i n f i g . 1-3, correspond t o samples " s t a b i l i - zed", s a y annealed a t a temperature a l i t t l e lower t h a n t h e c r y s t a l l i z a t i o n o n s e t , and cooled slowly. They can be r e p e a t i d l y reproduced, w i t h t h e same sample, because t h e IMA i s h e n c e f o r t h r e v e r s i b l e : t h e s t r u c t u r a l r e l a x a t i o n h a s been achieved du- r i n g t h e s t a b i l i z a t i o n a n n e a l , and does no more i n t e r f e r e t o modify t h e phenomena. Such an experiment i s o f t e n c a l l e d " i s o c o n f i g u r a t i o n a l " .

F i g . 4. Isothermal TMT on Fe40Ni40P14B6 a l - F i g . 5.. Arrhenius p l o t of t h e time--

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C5-514 JOURNAL

DE PHYSIQUE

On fig. 4 are plotted the isothermal kinetics of IYA establishment for the alloy Fe40 Ni40 P14 B6, annealed 30 min at 360°C. These curves can be deduced one from each other by a shift along the log t axis. They are well described by an exponential~ sum, with a gaussian distribution of the time constants [ g ] , whose the width

P

is about

4

[10]. The mean time constants of the curves on fig.

4

are drawn on fig. 5, in the shape of an Arrhenius plot. They can be observed to obey very well an Arrhenius law, with an activation energy of 1.74 ?!

0.04

eV, and a preexponential term of about 10-15s ; this value is close to that characterizing atomic jumps in metallic crystals.

A similar study on an alloy C O ~ ~ N ~ gives analogous results ~ ~ F ~ ~ S ~ : ~ ~ B ~ ~ 1.85 eV and 5 x 10-l6 S ,

c) "annealed and subsequently quenched" state. Curves "c"

,

in fig. 1-3, correspond to samples annealed as in "b", but quenched, instead of slowly cooled. For each alloy one observes an enhancement

of

the atomic mobility, induced by quench, and this enhancement disappears gradually during the following TFIT. For a given sample, the effect is larger the higher the quench temperature, and is a reversible function of this temperature. Let us note that it is specially outstanding for the alloy Ni10 Fe5 S i l l B16; the reason is that it has been possible to quench this alloy from a higher temperature than the others, thanks to a higher crystalli- zation temperature (500°C).

Fig. 6. Some isothermal TMT on Fe40Ni40P14B6 Fig. 7. Arrhenius plot of the time alloy, at a fixed temperature, TA = 160'~. required to obtain a fractional va- The initial state is obtained by

2

hours an- lue, C S = 0.03, of the

IPfA

energy. nea1,at T2, followed by a quench.

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r e c i p r o c a l quench t e m p e r a t u r e , TQ. P o i n t s a r e observed t o b e p r a c t i c a l l y a l i g n e d , e x c e p t f o r low v a l u e s of TQ. T h a t c o u l d b e d u e t o a time of a n n e a l a t Tq t o o s h o r t , s o t h a t t h e a l l o y does n o t r e a c h i t s c o n f i g u r a t i o n a l e q u i l i b r i u m [ I l l . Then, we ought t o c o n s i d e r o n l y t h e h i g h t e m p e r a t u r e p a r t of t h e diagram, which g i v e s a n a p p a r e n t a c t i v a t i o n e n e r g y of 0.26 eV.

a ) O r i g i n of IHA. I n c r y s t a l l i n e a l l o y s , t h e m o b i l i t y of s m a l l s i z e atoms, i n i n t e r s t i t i a l p o s i t i o n , d i f f e r s markedly from t h a t o f s u b s t i t u f i o n a l a t o m s . For i n s - t a n c e , i n Fe-Ni-C a l l o y s , t h e DO due t o c a r b o n atoms a p p e a r s between 100 and 200°C [12], w h i l e t h e one due t o Fe and N i atoms o c c u r s between 400 and 5 0 0 " ~ [13].

I n amorphous a l l o y s , a s shown on f i g . 1-3, o n l y one s t a g e i n o b s e r v e d , v e r y broad i n t e m p e r a t u r e , and w i t h o u t s u b s t r u c t u r e . We n e i t h e r o b s e r v e any fundamental d i f f e r e n c e between a l l o y s c o n s t i t u t e d o f two o r t h r e e m e t a l s ( f i g . 1-2), o r o n l y one ( f i g . 3 ) . So, i t seems t h a t , i n amorphous a l l o y s , i t s h o u l d b e c o n s i d e r e d a s i m u l t a n e o u s r e a r r a n g e m e n t of a l l atoms, m e t a l s and metalloYds, r a t h e r t h a n a sepa- r a t e d c o n t r i b u t i o n of 2 t y p e s of DO, a s proposed by Luborsky 171.

b) S t r u c t u r a l r e l a x a t i o n and q u e n c h i n g - e f f e c t . The d i f f e r e n c e between c u r v e s a and b on f i g . 1-3 shows t h a t t h e s t r u c t u r a l r e l a x a t i o n a c c o u n t s f o r a r e d u c t i o n of a t o m i c m o b i l i t y , a s i n d i f f u s i t i v i t y [ l 4 1 o r c r e e p [ l 5 1 e x p e r i m e n t s . On t h e o t h e r hand, t h e quenching e x p e r i m e n t s show t h a t t h e a t o m i c m o b i l i t y i s p a r t i a l l y r e s t o r e d by a quench from T4. Such a quench r e s u l t s i n f r e e z i n g t h e s t r u c t u r a l s t a t e e s t a - b l i s h e d a t T On t h e o t h e r hand, t h e a s - r e c e i v e d a l l o y , o b t a i n e d by t h e quench from

Q'

t h e m e l t , i s i n a s t r u c t u r a l s t a t e which c a n b e c o n s i d e r e d a s f r o z e n from a f i c t i v e t e m p e r a t u r e , Tf > TQ ; t h e n , t h e a t o m i c m o b i l i t y i s g r e a t e r . As a c o n c l u s i o n , t h e s e e x p e r i m e n t s show t h a t t h e e v o l u t i o n o c c u r i n g d u r i n g s t r u c t u r a l r e l a x a t i o n i s , a t l e a s t p a r t i a l l y , r e v e r s i b l e . The p r o c e s s can i n v o l v e l o c a l i z e d h o l e s [161, o r " d i s - t r i b u t e d f r e e volume" [ l 7

1,

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