A NEW RELAXATION PEAK IN SOLUTION TREATED, QUENCHED AND ANNEALED Al-Mg-Si ALLOYS

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A NEW RELAXATION PEAK IN SOLUTION TREATED, QUENCHED AND ANNEALED Al-Mg-Si

ALLOYS

S. Urreta de Pereyra, H. Bertorello, A.A. Ghilarducci de Salva

To cite this version:

S. Urreta de Pereyra, H. Bertorello, A.A. Ghilarducci de Salva. A NEW RELAXATION PEAK IN

SOLUTION TREATED, QUENCHED AND ANNEALED Al-Mg-Si ALLOYS. Journal de Physique

Colloques, 1987, 48 (C8), pp.C8-457-C8-462. �10.1051/jphyscol:1987870�. �jpa-00227174�

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A NEW RELAXATION PEAK IN SOLUTION TREATED, QUENCHED AND ANNEALED A1-Mg-Si ALLOYS

S . URRETA DE PEREYRA, H. BERTORELLO and

A.A. GHILARDUCCI DE SALVA*

Pacul t a d de ~ a t e m s t i c a , ~ s t r o n o m i a

y

~ i s i c a , Uni v e r s i d a d Nacional de c z r d o b a , Laprida 854, RA-5000 ~ 6 r d o b a , Argentina ' ~ e n t r o ~ t 6 m i c o Bariloche and I n s t i t u t o B a l s e i r o , ~ o m i s i 6 n Nacional de ~ n e r g i a ~ t c m i c a , RA-8400

S . C .

d e B a r i l o c h e , R.N., Argentina

A p r s s t r a i t e m e n t t h e r m i q u e d e s o l u b i l i s a t i o n , trempe e t r e c u i t s a' d e s t e m p e r a t u r e s p r o c h e s d e l ' a m b i a n t e , q u e l q u e s a l l i a g e s r i c h e s e n A l o n t d e s p i c s d e r e l a x a t i o n non a t t r i b u a b l e s a' un m6canisme d e Zener d a n s l a m a t r i c e . I1 a d t d s u g g 6 r e que c e s p i c s s o n t d u s aux s a u t s a t o m i q u e s i n d u i t s p a r t e n s i o n d a n s l e s amas d e s o l u t e formes pendant l e s p r e m i e r s s t a d e s d e l a p r 6 c i p i t a t i o n . Ces p i c s d i s p a r a i s s e n t quand l e s j e u n e s amas ou l e s z o n e s s e d e v e l o p p e n t . L 1 6 n e r g i e d ' a c t i v a t i o n a p p a r e n t e gdn6ralement donnee e s t p l u s p e t i t e que l l B n e r g i e d e d i f f u s i o n e n volume d e s a t o n e s d e s o l u t e q u i e n t r e n t d a n s l a f o r m a t i o n d e s zones.

Dans l ' a l l i a g e d ' ~ 1 - 1 % Ms2Si, a v e c 0,2% S i e n e x c s s , un p i c d e r e l a x a t i o n a S t 6 o b s e r v e a' 360K (a' 1 Hz), a v e c l e s c a r a c t 6 r i s t l q u e s s u i v a n t e s :

a ) l ' d n e r g i e d ' a c t i v a t i o n p a r d placement e n f r e q u e n c e e s t d e 79 kJ/m, e t l e

- lf

temps d e r e l a x a t i o n d e = 10 s;

b) l a h a u t e u r du p i c d e ' c r o i t a v e c l ' a m p l i t u d e d e

; lom4;

c ) l e p i c d i s p a r a f t pendant d e s r e c u i t s a ' e n v i r o n 400k;

d ) il e s t p l u s grand pour d e s r e f r o i d i s s e m e n t s l e n t s aprOs s o l u b i l i s a t i o n ; e ) l a h a u t e u r du p i c e s t s e n s i b l e a' l a d e f o r m a t i o n p l a s t i q u e apre's trempe;

f ) l a t e m p e r a t u r e du p i c ne change p a s a v e c l ' a m p l i t u d e ou l a pr6-deformation, m a i s depend d e l a t e m p e r a t u r e e t du temps d e v i e i l l i s s e m e n t .

Un modZle q u a l i t a t i f e s t proposd pour c e mecanisme d e r e l a x a t i o n , d a n s l e q u e l l e s j e u n e s amas ( p r o b a b l e m e n t d e S i ) e t l e s d i s l o c a t i o n s s o n t concern6s.

ABSTRACT:

A f t e r s o l u t i o n t r e a t m e n t , quenching and s u b s e q u e n t a n n e a l i n g n e a r room t e m p e r a t u r e , many age-hardenable A l - r i c h a l l o y s show r e l a x a t i o n p e a k s n o t d e s c r i b e d by a Zener mechanism a r i s i n g i n t h e m a t r i x . It was s u g g e s t e d t h a t t h e y may be due t o stress induced a t o m i c jumps w i t h i n s o l u t e c l u s t e r s formed i n t h e e a r l y s t a g e s o f p r e c i p i t a t i o n . These p e a k s d i s s a p p e a r when young c l u s t e r s o r z o n e s develop. The a p p a r e n t a c t i v a t i o n energy r e p o r t e d i s i n g e n e r a l s m a l l e r t h a n t h e e n e r g y f o r volume d i f f u s i o n of s o l u t e atoms i n v o l v e d i n zone f o r m a t i o n . I n a A1-l%MgZSi a l l o y w i t h 0 , 2 % S i i n e x c e s s , a r e l a x a t i o n peak h a s been d e t e c t e d a t 360K ( f o r 1 Hz), w i t h t h e f o l l o w i n g c h a r a c t e r i s t i c s :

a ) a c t i v a t i o n energy by f r e q u e n c y s h i f t o f 79 kJ/mol and r e l a x a t i o n time of 10-l2 s;

h e i g h t d e c r e a s e s f o r i n c r e a s i n g m e a s u r e m e n t a m p l i t u d e from t o c ) t h e peak v a n i s h e s d u r i n g a n n e a l i n g a t a b o u t 400K.

d) i t i s h i g h e r f o r slow c o o l i n g r a t e s a f t e r s o l u t i o n t r e a t m e n t ;

e ) i t s h e i g h t was s t r o n g l y s e n s i t i v e t o p r i o r d e f o r m a t i o n a f t e r w a t e r quench;

f ) i t s peak t e m p e r a t u r e d o e s n o t change w i t h a m p l i t u d e , o r p r i o r d e f o r m a t i o n , b u t depends on a g e i n g t e m p e r a t u r e and t i m e .

A q u a l i t a t i v e model i s proposed f o r t h i s r e l a x a t i o n mechanism i n which young c l u s t e r s ( p r o b a b l y S i o n e s ) and d i s l o c a t i o n s a r e i n v o l v e d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987870

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JOURNAL

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PHYSIQUE

INTRODUCTION

I n t e r n a l f r i c t i o n spectrum of many a g e h a r d e n a b l e A l - r i c h a l l o y s i n t h e r a n g e between room t e m p e r a t u r e and 420 K shows r e l a x a t i o n e f f e c t s t h a t have been a s s o c i a t e d t o t h e (GP) z o n e s o r s o l u t e c l u s t e r s a p p e a r i n g i n t h e e a r l y s t a g e s o f p r e c i p i t a t i o n a f t e r quench.

E n t w i s t l e e t a l . (1) r e p o r t i n t h e b i n a r y Al-0.86X w t X S i a l l o y a r e l a x a t i o n peak a b o u t 370K (1Hz) t h e h e i g h t of which d e c r e a s e s d r a s t i c a l l d u r i n g t h e f i r s t h e a t i n g a f t e r q u e n c h The p e a k l e v e l was found t o b e

-

²^x ^10-'for maximum s t r a i n a m p l i t u d e o f 4 x 1oL6

.

The p e a k i s a b s e n t when q u e n c h i n g i s made from l o w e r t e m p e r a t u r e s o r i n t e r r u p t e d a t a b o u t 393K. They show t h a t t h e peak c a n n o t a r i s e from Zener e f f e c t i n t h e m a t r i x and t h e y c o n c l u d e t h a t t h e r e l a x a t i o n i n v o l v e s stress i n d u c e d c h a n g e s i n zone shape o r s i z e and t h a t t h e mechanism o p e r a t e s d u r i n g a d e f i n i t e s t a g e o f zone development. They p o i n t o u t t h e r o l e p l a y e d by v a c a n c i e s bounded t o them.

I n a Al-1 wt% Mg2 S i a l l o y w i t h 0,2wt% S i i n e x c e s s , a s r e p o r t e d i n a p r e v i o u s work ( 2 ) . an i n t e r n a l f r i c t i o n peak (Po) a p p e a r s d u r i n g t h e f i r s t h e a t i n g a f t e r quench from s o l u t i o n t e m p e r a t u r e . It i s l o c a t e d a t 366 (1Hz) and shows s i m i l a r b e h a v i o u r a s t h o s e d e s c r i b e d above.

I t i s g e n e r a l l y a c c e p t e d , t h a t t h e p r e c i p i t a t i o n sequence showed by t h i s a l l o y f o r a r t i f i c i a l a g e i n g below 400 K a f t e r quench i s t h e following:(3-7)

SSS Sf c l u s t e r s n e e d l e shaped o r d e r i n g n e e d l e s B " n e e d l e s

+ -

w i t h t r a p p e d --r z o n e s

-

( v a c a n c i e s l o o s e ) -60 A d i a m e t e r

vacancy v a c a n c i e s (Si+Mg+vacancies) i d e n t i t y

c l u s t e r s

The two f i r s t s t a g e s a r e completed i n a few s e c o n d s even a t room t e m p e r a t u r e ( 7 , 1 0 1 -

As t h e f i r s t s t a g e s of p r e c i p i t a t i o n mostly d e t e r m i n e t h e f u r t h e r p r e c i p i t a t i o n m i c r o s t r u c t u r e (and m e c h a n i c a l p r o p e r t i e s c o n n e c t e d w i t h i t ) , t h e aim of t h e p r e s e n t work i s t o i n v e s t i g a t e t h e r e l a t i o n e x i s t i n g between zone f o r m a t i o n and t h e o b s e r v e d r e l a x a t i o n e f f e c t .

EXPERIMENTAL PROCEDURE

I n t e r n a l f r i c t i o n measurements were made i n t o r s i o n , between 1 and 50 H z , keeping c o n s t a n t a m p l i t u d e between 2 x and 8 x

Spectrum a n a l y s i s was t h e same a s d e s c r i b e d b e f o r e (2).

Samples were s o l u t i o n t r e a t e d f o r 18 h a t 813 K. G r a i n s i z e s o b t a i n e d were a b o u t 0.8 -1.5 mm. I n some c a s e s t h e y were deformed i n t e n s i o n a t room t e m p e r a t u r e . RESULTS

F i g u r e 1 shows t h e e f f e c t o f p r i o r t e n s i l e d e f o r m a t i o n , a f t e r quench, on peak h e i g h t d u r i n g t h e f i r s t h e a t i n g . It c a n be s e e n t h a t i t i n c r e a s e s f o r e l o n g a t i o n from 4% t o 18%. A s p e c t r u m w i t h o u t p r i o r d e f o r m a t i o n i s g i v e n a s r e f e r e n c e .

F i g u r e 2 shows t h e e f f e c t of s u c c e s s i v e h e a t i n g s on peak c h a r a c t e r i s t i c s . The sample was deformed p r e v i o u s l y 18% i n e l o n g a t i o n . As c a n be s e e n , peak h e i g h t d e c r e a s e s from 22x104 i n t h e f i r s t h e a t i n g t o 8x104 i n t h e second immediate h e a t i n g . The peak a l s o s h i f t s t o lower t e m p e r a t u r e s w i t h o u t change i n i t s a p p a r e n t w i d t h . F i g u r e 3 shows t h e anomalous a m p l i t u d e e f f e c t on peak h e i g h t . The peak i s shown (background s u b s t r a c t e d ) f o r t h r e e d i f f e r e n t a m p l i t u d e s . A s a m p l i t u d e i n c r e a s e s peak h e i g h t d e c r e a s e s . Samples i n t h i s e x p e r i m e n t w r e n o t deformed.

F i g u r e 4 i l l u s t r a t e s t h e e f f e c t of d i f f e r e n t p r e v i o u s h e a t t r e a t m e n t s on t h e i n t e r n a l f r i c t i o n peak o b s e r v e d (background s u b s t r a c t e d ) . Curve 1 shows t h e peak f o r a sample s o l u b i l i z e d , w a t e r quenched, deformed and s t o r e d 1 day a t room t e m p e r a t u r e .

Curve 2 , on t h e o t h e r hand, shows t h e f i r s t h e a t i n g f o r a s o l u b i l i z e d , a i r quenched and deformed sample. For t h i s s l o w e r quenching r a t e , t h e peak i s v e r y s i m i l a r t o t h a t shown i n c u r v e 1 e x c e p t t h a t i t i s a l i t t l e h i g h e r and i t i s s h i f t e d t o h i g h e r t e m p e r a t u r e .

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I t c a n b e s e e n t h a t even though i n c u r v e 1 t h e sample h a s been deformed t h e peak i s l a r g e r f o r low t e m p e r a t u r e s t o r a g e . Peak t e m p e r a t u r e i s a l s o h i g h e r .

Curve 4 c o r r e s p o n d s t o a sample w i t h lower S i i n e x c e s s (0,05w%) t h a t h a s been s o l u b i l i z e d and w a t e r quenched. The peak a p p e a r s h i g h e r t h a n i n samples w i t h h i g h e r S i c o n t e n t and deformed.

I n F i g u r e 5, a TEM micrography o f a n a s quenched specimen i s shown.

Even though no d e f o r m a t i o n h a s been i n t r o d u c e d a f t e r h i g h t e m p e r a t u r e a n n e a l i n g and quenching, many f r e e d i s l o c a t i o n s a r e found.

The A r r h e n i u s p l o t i n f i g u r e 6 must be t a k e n c a r e f u l l y b e c a u s e peak t e m p e r a t u r e i s dependent on a g e i n g t e m p e r a t u r e and time. A c t i v a t i o n energy was roughly e s t i m a t e d t a k i n g p o i n t s from n o t v e r y d i f f e r e n t f r e q y ~ i n c i e s . Peak p a r a m e t e r s v a l u e s were e s t i m a t e d t o be: H = 79 and T O = 1 x 10- S. The. peak i s a b o u t 1.8 t i m e s wider t h a n a Debye one.

DISCUSSION

I t seems d i f f i c u l t t o admit t h a t t h e i n t e r n a l f r i c t i o n peak observed c o u l d o r i g i n a t e i n a Zener mechanism i n t h e m a t r i x . T h i s i s s u p p o r t e d by t h e f o l l o w i n g o b s e r v a t i o n s :

c o n c e r n i n g S i , t h e peak i s h i g h e r f o r samples w i t h a lower S i c o n t e n t .

t h e peak s h i f t t o lower t e m p e r a t u r e s w i t h a n n e a l i n g s . T h i s b e h a v i o u r i s o p p o s i t e t o t h a t o f t h e t r a n s i e n t Zener peak c a u s e d by quenching i n A1-Mg(8). Loss o f v a c a n c i e s moves t h e peak t o h i g h e r t e m p e r a t u r e b e c a u s e o f a l a r g e r T O .

t h e o b s e r v e d dependence on p r i o r d e f o r m a t i o n and a m p l i t u d e e f f e c t s .

On t h e o t h e r hand, a c l a s s i c Zener mechanism i n v o l v i n g s o l u t e s w i t h i n young z o n e s would l e a d t o a peak h e i g h t depending on v o l u m e t r i c c o n c e n t r a t i o n of zones.

T h i s m a n s t h a t t h e peak must be h i g h e r f o r f a s t e r quenched s a m p l e s than f o r t h e a i r quenched o n e s , i n which n u c l e a t i o n on vacancy c l u s t e r s i n volume i s e x t r e m e l y d i f f i c u l t , c o n t r a r y t o t h e observed e f f e c t . For lower S i c o n t e n t , volume n u c l e a t i o n

( f i n e d i s p e r s e d ) i s l e s s e f f e c t i v e b u t t h e peak i s a l s o h i g h e r .

At l a s t , T O i s two o r d e r s o f m a g n i t u d e h i g h e r t h a n t h a t e x p e c t e d f o r Zener r e l a x a t i o n ( 1 0 - 1 4 s ) i n d i c a t i n g t h a t a b o u t 100 jumps a r e needed f o r r e l a x a t i o n t o t a k e p l a c e . T h i s seems t o o much f ~ r such s m a l l zones. A s t h e peak a p p e a r s when t h e i n t e r m e d i a t e s t a g e s o f t h e p r e c i p i t a t i o n sequence mentioned above t a k e p l a c e , we c o n c l u d e t h a t young zones o r i g i n a t e d i n t h e s e s t a g e s a r e i n v o l v e d i n t h e mechanism.

The r e l a t i v e l y low s e n s i b i l i t y shown by t h e peak t o a p p r e c i a b l e changes i n t h e amount of zones n u c l e a t e d i n volume s u g g e s t s t h a t t h o s e z o n e s i n v o l v e d i n r e l a x a t i o n must n u c l e a t e on o t h e r s i t e s whose e f f i c i e n c y i s l e s s d e p e n d e n t on vacancy s u p e r s a t u r a t i o n .

On t h e o t h e r hand, t h e s t r o n g e f f e c t of p r i o r d e f o r m a t i o n s u g g e s t s t h a t t h o s e n u c l e a t i o n s i t e s c o u l d be d i s l o c a t i o n s and t h a t they could a l s o be i n v o l v e d i n t h e r e l a x a t i o n mechanism.

E x p e r i m e n t a l r e s u l t s a l s o i n d i c a t e t h a t peak h e i g h t i s l a r g e r i n samples w i t h l o w e r S i c o n t e n t and i n t h o s e a i r quenched. I n b o t h c a s e s , t h e r e i s a slower growing r a t e o f z o n e s ( 6 ) . Then, we c o n c l u d e t h a t peak h e i g h t i s l a r g e r when zone growth i s s l o w e r .

It h a s been r e p o r t e d ( 3 , 4 ) t h a t d u r i n g e a r l y s t a g e s of n e e d l e development they have a h i g h vacancy c o n c e n t r a t i o n . V a c a n c i e s l o o s e t h e i r i d e n t i t y d u r i n g t h e improvement o f zone i n t e r n a l s t r u c t u r e . O r d e r i n g s t a b i l i z e s t h e zones.

Beaven e t a l . ( 9 ) have d e m o n s t r a t e d t h a t n e e d l e s , when n u c l e a t e d on d i s l o c a t i o n s ( p r e f e r e n t i a l l y edge o n e s ) , t h e y form c o l o n i e s i n s u c h a way t h a t they d e v e l o p a l m o s t p a r a l l e l t o e a c h o t h e r w i t h one t i p r e m a i n i n g i n c o n t a c t w i t h t h e d i s l o c a t i o n l i n e . These a u t h o r s show how n e e d l e s grow l o n g i t u d i n a l l y accompanied by c l i m b o f t h e d i s l o c a t i o n i n c o n t a c t w i t h them.

T h i s p u t s i n e v i d e n c e t h e s t r o n g i n t e r a c t i o n between d e v e l o p i n g zones and d i s l o c a t i o n s where they a r e n u c l e a t e d on. Moreover, t h e s e f a c t s s u g g e s t t h a t zone growing and d i s l o c a t i o n s may be c o u p l e d , t h a t i s : d i s l o c a t i o n c l i m b may i n d u c e growing and v i c e v e r s a .

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JOURNAL

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PHYSIQUE

Based on t h e s e t a c t s , w e p r o p o s e f o r t h e p r e s e n t peak a r e l a x a t i o n e f f e c t o r i g i n a t i n g i n d i s l o c a t i o n movement, i n d u c e d by e x t e r n a l a p p l i e d s t r e s s , and c o n t r o l l e d by young z o n e s c h a n g i n g s i z e , a c t i n g i n t h i s way a s m o b i l e p i n s .

T h i s i s b a s i c a l l y a mechanism o f d i s l o c a t i o n d r a g g i n g o f l o c a l i z e d d e t e c t s o n i t .

Then, we a r e l e d t o t h e scheme shown i n f i g u r e 7. We i d e n t i f y 8 w i t h t h e mean l e n g h t o f d i s l o c a t i o n l i n e between m o b i l e p i n s (young zones) and L that between f i x e d p i n s (grown up zones). Young zones n u c l e a t e d on a d i s l o c a t i o n l i n e a c t a s m o b i l e p i n s when t h e y a r e a b l e t o f o l l o w i t s movement i n d u c e d by t h e a p p l i e d e x t e r n a l s t r e s s ( t l ) . When t h e y r e a c h some s t a g e o f development, t h e y l o o s e t h e a b i l i t y o f f o l l o w i n g d i s l o c a t i o n movement a n d become f i x e d p i n s ( t 2 ) ( n e i t h e r movement n o r b r e a k away t a k e p l a c e ) . I n this h y p o t h e s i s , t h e r e l a x a t i o n i n t e n s i t y and l i m i t r e l a x a t i o n time a r e g i v e n by: ( 1 1 )

A b ~ 2 and A = 1 2 y

where

A = d i s l o c a t i o n d e n s i t y Do

-

^limitd i f f u s i o n c o n s t a n t b

-

B u r g e r ' s v e c t o r k = Boltzmann c o n s t a n t

Y = l i n e t e n s i o n L = d i s l o c a t i o n l i n e l e n g h t b e t v e e n f i x e d p i n s TP = a b s o l u t e peak t e m p e r a t u r e II = d i s l o c a t i o n l i n e l e n g h t between mobile p i n s

The e x p e r i m e n t a l r e s u l t s may b e i n t e r p r e t e d a s f o l l o w s :

peak h e i g h t i n c r e a s e s w i t h t h e amount of p r i o r d e f o r m a t i o n b e c a u s e o f a n i n c r e a s e i n A .

a s some young z o n e s ( m o b i l e p i n s ) t r a n s f o r m t o f i x e d p i n s by growing d u r i n g t h e f i r s t h e a t i n g i n t h e i n t e r n a l f r i c t i o n measurement, i n s u b s e q u e n t c o o l i n g and r e h e a t i n g L i s s m a l l e r and s o i s peak h e i g h t . T h i s f a c t a l s o makes t h e peak s h i f t t o l o w e r t e m p e r a t u r e .

The f a c t t h a t t h e peak i s s l i g h t l y s m a l l e r and s h i f t e d t o lower t e m p e r a t u r e s i n w a t e r quenched samples i n comparison w i t h t h a t observed i n a i r quenched specimen, c a n be e x p l a i n e d by a f a s t e r zone growth a s a r e s u l t o f d i f f u s i o n enhanced by vacancy s u p e r s a t u r a t i o n . When t h e sample i s s t o r e d a t low t e m p e r a t u r e a f t e r quench, growth i s a l s o slowed down l e a d i n g t o h i g h e r v a l u e s of L. I n t h i s c a s e , a h i g h e r peak, l o c a t e d a t a l s o h i g h e r t e m p e r a t u r e i s e x p e c t e d , a s was found ( c u r v e 3 , F i g . 4). The s l i g h t d i f f e r e n c e between t h e peak i n w a t e r quenched and a i r quenched samples s u g g e s t s t h a t vacancy s u p e r s a t u r a t i o n h a s a s m a l l e f f e c t on zone n u c l e a t i o n on d i s l o c a t i o n s .

Lower S i e x c e s s h a s a s i m i l a r e f f e c t on zone growth r a t e ( 4 ) a s t h a t o f low t e m p e r a t u r e s t o r a g e l e a d i n g a l s o t o a l a r g e r peak s h i f t e d t o h i g h e r t e m p e r a t u r e s ( c u r v e 4 , Fig. 4).

I m m e d i a t e l y a f t e r w a t e r quench, an i n i t i a l d e n s i t y o f c l u s t e r s o f a b o u t 1018 h a s been r e p o r t e d (12) and t h e same v a l u e i s assumed i n o u r samples. I h a s e e n o b s e r v e d a l s o t h a t a f t e r a g e i n g a t t e m p e r a t u r e below 473 K a s much a s 1"'' cm-' 8 "

z o n e s a r e p r e s e n t ( 5 ) . We assume t h i s v a l u e a s t h e v o l u m e t r i c d e n s i t y o f z o n e s t h a t a c t a s f i x e d p i n s d u r i n g t h e second h e a t i n g , b e c a u s e i t i s e x p e c t e d t h a t z o n e s t h a t have r e a c h e d c r i t i c a l s i z e t o be f i x e d p i n s d u r i n g second h e a t i n g a r e t h o s e t h a t s u r v i v e ( n o t d i s s o l v e ) and grow a t h i g h e r a g e i n g t e m p e r a t u r e s .

I n w a t e r que ched specimens, deformed 18% i n e l o n g a t i o n a d i s l o c a t i o n d e n s i t y of a b o u t

lo8

cm-I i s commonly e s t i m a t e d 3 ) o t h i s v a l u e i s assumed h e r

T h e n , i f d e n s i t y o f f i x e d p i n s i s 10f'cm-', L i s c a l c u l a t e d t o be q-S.cm and t h e p e a k h e i g h t d u r i n g t h e s e c o n d h e a t i n g

it

c a l c u l a t e d t o be 8 x 10- i n e x c e l l e n t agreement w i t h t h e measured one: 7.8 x 10-

.

P e a k h e i g h t , d u r i n g f i r s t h e a t i n g i s 22 x 10-4 w h i c h i s c o n s i s t e n t w i t h a v o l u m e t r i c d e n s i t y o f grown z o n e s of a b o u t 3 x 1014 cm-3. T h i s means t h a t one i n t e n thousand c l u s t e r s n u c l e a t e d a f t e r quench, have reached t h e c r i t i c a l s t a g e of f i x e d p i n d u r i n g room t e m p e r a t u r e s t o r a g e and a b o u t 7 x 1014 cm-3 became f i x e d p i n s d u r i n g t h e f i r s t h e a t i n g and s u b s e q u e n t c o o l i n g .

(6)

r e m a i n s a l m o s t c o n s t a n t . W i t h t h i s h y p o t h e s i s , t h e change i n L between t h e f i r s t a n d second h e a t i n g , l e a d s t o p r e d i c t a s h i f t i n g o f -12K i n p e a k t e m p e r a t u r e which i s t h a t e x p e r i m e n t a l l y o b s e r v e d .

C o n c e r n i n g t h e r e s u l t s r e p o r t e d i n A1-Si ( I ) , i f t h i s mechanism i s a l s o a c t i v e , t h e peak i s more u n s t a b l e a g a i n s t a n n e a l i n g b e c a u s e i n t h i s s y s t e m n o i n t e r m e d i a t e m e t a s t a b l e p h a s e i s p r e s e n t ; G.P. z o n e s t r a n s f o r m e a r l y i n p u r e S i . T h i s , a d d e d t o t h e f a c t t h a t t h e e f f e c t o f Mg i n s l o w i n g S i d i f f u s i o n i s a b s e n t l e a d s t o a n a r r o w t i m e r a n g e i n which z o n e s b e h a v e a s m o b i l e p i n s . F o r t h e less c o n c e n t r a t e d a l l o y ( 0 , 1 4 % S i ) t h e p e a k i s n o t o b s e r v a b l e a f t e r q u e n c h i n g f r o 2 673K, w h i c h i s 180K below t h e t e m p e r a t u r e f o r w h i c h t h e p e a k i s a b o u t 2 0 x 10-

.

T h i s c h a n g e i n s o l u t i o n t e m p e r a t u r e b f o r e qu n c h i n g l e a d s t h e v o l u m e t r i c c o n c e n t r a t i o n o f p r e c i i t a t e s ( 1 0 ) f r o m 1015cm-' t o 1 0 ' ~ m - ~ . T h i s l a s t v a l u e l e a d s t o L v a l u e s a b o u t 10-'cm w h i c h i s l a r g e r t h a n d i s l o c a t i o n l e n g t h b e t w e e n o t h e r h a r d e r f i x e d p i n s and t h e p r o p o s e d mechanism c a n n o t a c t i n t h i s c a s e .

F i g . 1: I n t e r n a l f r i c t i o n p e a k ( P O ) a t 1 Hz, F i g . 2: E f f e c t o f s u c c e s s i v e h e a t i n g s w i t h s t r a i n a m p l i t u d e

-

d u r i n g t h e o n Pg p e a 5 a t 1 Hz, s t r a i n a m p l i t u d e

f i r s t h e a t i n g s .

-

⁵^x ^10-

^,

w i t h p r i o r t e n s i l e

Curve 0: non deformed s t a t e , a f t e r quench. d e f o r m a t i o n o f 18%.

Curve 1: p r i o r t e n s i l e deformed s t a t e ( 4 % ) . Curve 1: f i r s t h e a t i n g Curve 2 : p r i o r t e n s i l e deformed s t a t e ( 1 5 % ) . Curve 2: s u b s e q u e n t c o o l i n g Curve 3: p r i o r t e n s i l e d e f o r m e d s t a t e ( 1 8 % ) . Curve 3 ; immediate s e c o n d h e a t i n g

REFERENCES

1- K.M. E n t w i s t l e , P. F u l l e r , P. Brough, I.: A c t a Met. Vol 2 6 , 1055 ( 1 9 7 8 ) . 2- A. G h i l a r d u c c i , S. U r r e t a , H. B e r t o r e l l o : J. d e P h y s i q u e

66

^C10-371, ( 1 9 8 5 ) . 3- A. L u t t s . A c t a M e t . 9 N06, 3 7 7 , ( 1 9 6 1 ) .

4- T.V. S h c h e g o l e v a . ~ i i . M e t a l l . M e t a l l o v e d

5

no 2 , 246 ( 1 9 6 8 ) . 5- G. Thomas. J. I n s t . M e t a l s ,

2,

5 7 ( 1 9 6 1 ) .

6- S. C e r e s a r a , E. Di Russo, P. F i o r i n i , A. G i a r d a . M a t e r . S c i . Eng.

2 ,

220 (1970).

7- I. Kovacs, J. L e n d v a i , E. Nagy. A c t a M e t .

g ,

^{N o}⁷( 1 9 7 2 ) .

8- J . B e l s o n , D. L e m e r c i e r , P. Moser, P. V i g i e r . Phys. S t a t . S o l .

0,

647 (1970).

9- P.A. Beaven, A.P. D a v i d s o n , E.P. B u t l e r . P r o c . o f a n I n t . Conf. S o l i d S o l i d P h a s e T r a n s f o r m a t i o n s , e d i t e d by H. Aaronson, D. L a u g h i n , R. S e k e r k a , C. Wayman, 6 6 1 , ( 1 9 8 1 ) .

10- E. Ozawa, H. Kimura. A c t a Met.

18,

995, No 9 , ( 1 9 7 0 ) . 11- C. D i a l l o , T h e s i s EPFL, Lausanne, S u i z a (1983).

12- R - J . AcuEa, F.L. D a r o q u i , M e t a l u r g i a Moderna

2,

^{N O} 1, 2 1 ( 1 9 8 6 ) . 13- F r i e d e l . D i s l o c a t i o n s Vol. 3, Pergamon P r e s s 1964.

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CS-462 JOURNAL

DE

PHYSIQUE

3

0

' CURVE 2

2

1 CURVE 1

F i g . 3 : A m p l i t u d e e f f e c t o n Po p e a k a t F i g . 4: Po p e a k d u r i n g t h e f i r s t h e a t i n g 1 H z , w i t h o u t p r i o r d e f o r m a t i o n Curve 1: a f t e r w a t e r q u e n c h

Curve 1: s t r a i n a m p l i t u d e = 7 8 x Curve 2 : a f t e r a i r q u e n c h .

Curve 2: s t r a i n a m p l i t u d e = 8 x Curve 3: a f t e r l o w t e m p e r a t u r e s t o r a g e . Curve 3: s t r a i n a m p l i t u d e = 5 x Curve 4: a l l o y w i t h l o w e r S i e x c e s s

F i g . 5: A 100 keV TEM m i c r o g r a p h y o f t h e a s q u e n c h s t a t e s h o w i n g f r e s h l y d i s l o c a t i o n s .

'1 C I

. .

_t₁

m _i

_L, _!

.+

t 2

TQTF

t

L 2

t 2 st,

,

L 2 < L l I 12

-

¹¹

* d ~pms r fixed pins

2 4 2.5 2 6 '2.7 2 4

F i g . 6: A r r h e n i u s p l o t o f t h e Po p e a k

F i g . 7: S c h e m e o f t h e d i s l o c a t i o n d r a g g i n g mechanism a t t i m e t l , when young c l u s t e r s a c t a s m o b i l e p i n s . At t i m e t 2 , d e v e l o p e d c l u s t e r s become f i x p i n s .