DISLOCATION CONTRIBUTION TO THE DAMPING PHENOMENA ACCOMPANYING RECRYSTALLIZATION IN Al

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DISLOCATION CONTRIBUTION TO THE DAMPING PHENOMENA ACCOMPANYING

RECRYSTALLIZATION IN Al

E. Bonetti, P. Gondi, R. Montanari

To cite this version:

E. Bonetti, P. Gondi, R. Montanari. DISLOCATION CONTRIBUTION TO THE DAMPING PHE-

NOMENA ACCOMPANYING RECRYSTALLIZATION IN Al. Journal de Physique Colloques, 1985,

46 (C10), pp.C10-363-C10-366. �10.1051/jphyscol:19851081�. �jpa-00225466�

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JOURNAL DE PHYSIQUE

Colloque C10, supplkment au n012, Tome 46, dkcembre 1985 page C10-363

DISLOCATION CONTRIBUTION TO T H E DAMPING PHENOMENA ACCOMPANYING RECRYSTALLIZATION I N A1

E

.

^BONETTI*

^,

^P

.

^GONDI(⁾ and R. MONTANARI*

' D i p a r t i m e n t o d i F i s i c a , U n i v e r s i t d d i B o l o g n a , V i a I r n e r i o 4 6 ,

? o l o g n a , I t a l y

D i p a r t i m e n t o d i I n g e g n e r i a Meccanica, 11 U n i v e r s i t a d i Roma, T o r V e r g a t a , Roma, I t a l y

Resum6 - On a suivi les premieres Gtages de la recristallisation de lames

-- minces dl aluminium par observations de frottement interieur,module elastique, metallographie et X.R.D.. Les resultats sont interprete en termes d'emission des dislocations par les joints de grains,associe a la nucleation.

Abstract - The initial recrystallization stages of cold rolled A1 sheets have been followed by internal friction,elastic modulus,metallographic and X-ray diffraction observations.The results are interpreted in terms of dislocation emission from the grain boundaries,connected with nucleation.

I -INTRODUCTION

'.,any authors/l/ have observed Q-' maxima accompanying recrystallization_ Different behaviours of the modulus have been observed in correspondence of the Q maxima, either a drop or a minimum. In general the various results have been related to dis- 1ocations;in particular Isore et a1./2/ refer the increase of internal friction to a high mobility degree of the dislocations in the freshly recrystallized domains, behind the migrating grain boundary.

Questions remain open,in particular on the contribution of nucleation to these re- laxation phenomena. Hence internal friction and dynamic modulus measurements have been made by us during the recrystallization of cold rolled A1 sheets,togheter with metallographic observations and analyses of the X-ray diffraction line profiles, in particular in the initial recrystallization stages. The results are discussed also with reference to the possibility of grain boundary sliding contributions,given that in experiments with increasing temperature the recrystallization peaks present cor- respondence with the so called grain boundary peaks/3/.

I1 - EXPERIWENTAL

Observations and measurements were carried out on sheets of A1 99.9%,obtained

by

cold rolling with

90%

reduction.

Internal friction and elastic modulus were measured directly during different iso- therms,in vacuum6.The vibrations were flexura1,vibration frequencies ca. 500 Hz, amplitudes

<

10- . The elastic modulus was derived from the resonance frequencies.

Errors reported in the results are inclusive of the scatter between subsequent measurements. The metallographic observations and the X-ray diffraction analyses were made not directly,during isothermal heating,but on different samples of the same material ,each heated for given times and then cooled for examinations.

Fourier analyses of the diffraction line profiles were made with the Warren-Averbach

( l ) ~ r u p p o Nazionale di Struttura della Materia

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19851081

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C10-364 JOURNAL DE PHYSIQUE

method on some specimens before t h e thermal treatments.

I 1 1

-

RESULTS

Isothermal behaviour o f Q-' and dynamic modulus Gd (expressed by t h e r a t i o over Go, modulus before h e a t i n g ) are considered f i r s t . As shown i n f i g . 1 twopronouncedpeaks, P1 and P2,maxima o f Q-1 vs time appear. The r a t i o between t h e h e i g t h s o f P1 and P2 decreases by i n c r e a s i n g temperature. A t a l l temperatures a r e l a t i v e l y small peak,PO, i s appreciable,preceding peak PI. The dynamical modulus presents i n general r e l e v a n t

progressive decreases a t t h e lower times before the peak P1,with some changes i n t r e n d r e f e r a b l e t o PO;

a f t e r t h i s i n i t i a l t r e n d a satura- t i o n l e v e l i s reached w i t h shallow minima i n correspondence o f peaks PI and P2;measurements e r r o r s o f Gd were p a r t i c u l a r y h i g h i n t h e range corresponding t o t h e maxima o f such peaks,as due t o i n s t a b i l i t i e s dur- i n g t h e measurements. The continous l i n e diagram i n f i g . 1 represents t h e corresponding r e c r y s t a l l i z a t i o n

0.95

behaviour;two stages a r e d i s t i n - guished : i n t h e f i r s t o n e , l e f t o f

O m

t h e arrow,the increase i n average g r a i n dimensions i s mainly due t o t h e increases o f t h e zones where r e c r y s t a l l i z a t i o n i s e v i d e n t ; i n t h e r e c r y s t a l 1 iz e d zones t h e g r a i n sizes remained approximately con- s t a n t d u r i n g t h i s stage. I n t h e second stage t h e gkains present u n i form increases. Peak PI covers t h e

0 10 20 30 ₄₀ 50 60 min

whole f i r s t r e c r y s t a l l i z a t i o n stage whereas peak P2 appears a t t h e be- g i n n i n g o f t h e s e c o n d stage o f r e - Fig. l a - b - c )

-

I n t e r n a l f r i c t i o n c o e f f i c i e n t c r y s t a l l i z a t i o n . I n f i g . 2 t h e l o g - (-o-), dynamic modulus Gd ( i n t h e f r a c t i o n arithms o f time vs i n v e r s e tempera- over i n i t i a l modulus Go) ( r r r ) , and average t u r e are r e p o r t e d f o r a l l t h e peaks g r a i n dimension, D (-) d u r i n g isothermal examined; apparent a c t i v a t i o n en- heating, a t t h e temperaturesindicated, o f ergies,derived from t h e average c o l d r o l l e d A1 sheets. slopes are i n d i c a t e d i n t h e f i g u r e ;

t h e apparent energy i s higher f o r PO (=0.5 eV);for the o t h e r peaks t h e energies a r e lower =0.3,0.2 eV.

!Jith p a r t i c u l a r a t t e n t i o n t o d i s l o c a t i o n c o n t r i b u t i o n s X-ray d i f f r a c t i o n l i n e spec- t r a have been examined on the same specimens used f o r the metallographic observa- t i o n s . Widths o f the (220) l i n e s are shown i n f i g . 3 , w i t h t h e Q-1 behaviour repro- duced f o r comparison.

The i n t e r n a l f r i c t i o n maxima correspond t o w e l l gronouncedmaxima o f t h e l i n e widths.

F o u r i e r analyses o f the d i f f r a c t i o n l i n e p r o f i l e s have shown that,in. p a r t i c u l a r a f t e r t h e i n i t i a l r e c r y s t a l l i z a t i o n stage,the c o n t r i b u t i o n o f the average q u a d r a t i c strains,i.e. t h e c o n t r i b u t i o n o f t h e d i s l o c a t i o n s p r e v a i l s over the one connected w i t h g r a i n o r subgrain dimensions.

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F i g . 2

-

A r r h e n i u s p l o t o f t h e times o f peak maxima.

f i g . 3

-

The v e r t i c a l l i n e s r e p r e s e n t t h e h a l f h e i g t h w i d t h o f t h e X-ray d i f f r a c t i o n 1 in e (220) a f t e r i s o t h e r m a l h e a t i n g a t 538 K f o r thetimes i n a b z i s s a . F o r comparison t h e c o r r i s p o n d e n t b e h a v i o u r o f Q-1 i s reported(-).

Other a u t h o r s have c o n s i d e r e d a l s o i s o c h r o n a l r e c r y s t a l l i z a t i o n behaviour.

Analogous measurements have been made a l s o b y us; i n t h i s r e s p e c t r e f e r e n c e i s made t o r e s u l t s r e p o r t e d i n a p r e v i o u s paper/4/,showing t h e development o f an i n t e r n a l f r i c t i o n peak d u r i n g r e c r y s t a l l i z a t i o n b y i n c r e a s i n g t e m p e r a t u r e ; f o r t h e f r e q u e n c i e s used t h i s peak l i e s i n t h e temperature range o f t h e peak a t t r i b u i t e d b y many a u t h o r s t o g r a i n boundary s l i d i n g .

I V

-

DISCUSSION

The Q-I i n c r e a s e s , o c c u r r i n g d u r i n g i s o t h e r m a l r e c r y s t a l l i z a t i o n , a s w e l l as t h o s e o f modulus decrement,Go Gd,and o f d i f f r a c t i o n l i n e width,L,are a l l c o n s i s t e n t w i t h d i s - l o c a t i o n damping,due e i t h e r t o d i s l o c a t i o n s a l r e a d y p r e s e n t and p a s s i n g t o r e l a x e d c o n d i t i o n s because o f i n c r e a s e s o f t h e f r e e l e n g t h s o f v i b r a t i o n , o r t o new d i s l o c a - tions,as m i g h t be d i s l o c a t i o n s e m i t t e d f r o m t h e g r a i n boundaries.

R e c r y s t a l l i z e d g r a i n s a r e i n general c o n s i d e r e d f r e e f r o m d i s l o c a t i o n s as w e l l as f r o m p o l y g o n i z a t i o n wa1ls;so t h e a t t r i b u t i o n o f t h e r e l a x a t i o n phenomena observed t o d i s l o c a t i o n s e m i t t e d f r o m t h e g r a i n boundaries seems more r e l i a b l e f o r peak P2, which appears a f t e r f u l l f i l l m e n t o f t h e f i r s t r e c r y s t a l l i z a t i o n stage,when t h e whole volume i s occupied by r e c r y s t a l l i z e d g r a i n s o f t h e f i r s t stage.

F o r peak P1 analogous phenomena o f d i s l o c a t i o n e m i s s i o n a r e p r o b a b l y i n v o l v e d n o t o n l y f r o m g r a i n boundaries b u t a l s o f r o m subboundaries,polygonization w a l l s , p r e l i m i - n a r t o t h e coalescence phenomena observed by many authors/5/.

O f course t h e Q-l,Go/Gd, Ldecreases a f t e r t h e maxima,can be due n o t o n l y t o t h e disappearance o f t h e d i s l o c a t i o n s e m i t t e d b u t a l s o t o t h e i r g r o u p i n g i n d i s l o c a t i o n w a l l s . T h i s i n t r o d u c e s us t o t h e q u e s t i o n o f n u c l e a t i o n , s i n c e Bollman observed nu- c l e i i n p a r t w i t h l a r g e a n g l e - , i n p a r t w i t h l o w angle-boundariesL5/.

I n t h i s r e s p e c t i t i s w o r t h r e c a l l i n g a l s o t h e o b s e r v a t i o n s o f many authors, i n d i - c a t i n g t h a t new g r a i n s do n o t n u c l e a t e u n i f o r m e l y i n t h e volume b u t o n l y i n p r e f - e r e n t i a l s i t e s , m a i n l y on g r a i n boundaries and t r i p l e p o i n t s /5/.

C o n s i s t e n t l y w i t h a l l t h e s e o b s e r v a t i o n s as w e l l as w i t h t h e r e s u l t s discussed h e r e i t seems reasonable t o assume t h a t n u c l e a t i o n i s connected w i t h t h e e m i s s i o n of d i s l o c a t i o n s f r o m g r a i n and s u b - g r a i n boundaries.

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Other i n d i c a t i o n s may be drawn from the X-ray d i f f r a c t i o n r e s u l t s , on t h e p o s s i b i l - i t i e s o f o t h e r mechanisms. As mentioned, t h e r e c r y s t a l l i z a t i o n peak coming about by i n c r e a s i n g temperature can be observed i n correspondence o f the s o - c a l l e d g r a i n boundary peak. I t might be thought thus t h a t t h e r e c r y s t a l l i z a t i o n peak depends on damping phenomena connected w i t h g r a i n boundary s l i d i n g , w i t h p o s s i b l e c o n t r i b u t i o n s a l s o t o modulus r e l a x a t i o n . However, the observed maxima o f t h e d i f f r a c t i o n l i n e widths seem h a r d l y r e f e r a b l e t o g.b. s l i d i n g ; hence t h i s p o s s i b i l i t y i s neglected.

Further, e q u i v a l e n t c o n t r i b u t i o n s t o i n t e r n a l f r i c t i o n and modulus r e l a x a t i o n can be obtained w i t h s h o r t d i s l o c a t i o n segments i n h i g h d e n s i t i e s o r w i t h long d i s l o c a t i o n segments i n low d e n s i t i e s . The increases o f t h e X-ray d i f f r a c t i o n l i n e widths present evidence i n f a v o u r o f the f i r s t a l t e r n a t i v e , i.e. o f d i s l o c a t i o n s i n l a r g e d e n s i t i e s .

The a c t i v a t i o n energies d e r i v e d from t h e isotherms ( f i g . 2) a r e r a t h e r low. O f course t h e y are o n l y apparent; h i g h e r energies would be obtained by a s u i t a b l e choice o f t h e i n i t i a l c o n d i t i o n s . I n t h i s r e s p e c t more v a l i d appears t h e value r e l a t i n g t o the shallow maximum PO, f o r which more r e l i a b l e i s t h e reference t o zero time f o r t h e beginning o f t h e processes involved, and t h i s energy can be r e l a t e d t o d i f f u s i o n i n g r a i n boundaries.

V

-

ACKNOWLEDGEMENTS

Our thanks a r e due t o Mr. F. B a r b i e r i f o r the assistance i n t h e whole research.

V I

-

BIBLIOGRAPHY

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/4/ B o n e t t i E., E v a n g e l i s t a E., Gondi P. and Tognato R., Phys. S t a t . Sol. ( a )

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G o r e l i k G.S., R e c r y s t a l l i z a t i o n i n Metals and A1 loys; Ploscow (1981) (review).