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THE DISLOCATION-GRAIN BOUNDARY INTERACTION AND ITS EFFECT ON THE
BOUNDARY DYNAMIC PROPERTIES
R. Valiev, V. Gertsman
To cite this version:
R. Valiev, V. Gertsman. THE DISLOCATION-GRAIN BOUNDARY INTERACTION AND ITS
EFFECT ON THE BOUNDARY DYNAMIC PROPERTIES. Journal de Physique Colloques, 1990,
51 (C1), pp.C1-679-C1-683. �10.1051/jphyscol:19901108�. �jpa-00230015�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au n0l, Tome 51, janvier 1990
THE DISLOCATION-GRAIN BOUNDARY INTERACTION AND ITS EFFECT ON THE BOUNDARY DYNAMIC PROPERTIES
R.Z. VALIEV and V.Yu. GERTSMAN
Institute of Metals Superplasticity Problems, URSS Academy of Sciences, Ufa 450001, U. R. S. S.
A b s t r a c t
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The problem o f t h e i n t e r a c t i o n o f d i s l o c a t i o n s w i t h g r a i n boundaries i s d i s - cussed on t h e b a s i s o f t h e concept o f g r a i n boundary n o n - e q u i l i b r i u m s t a t e g e n e r a t i o n and r e c o v e r y . According t o t h i s concept, t r a p p e d l a t t i c e d i s l o c a t i o n s , b e i n g non-equi- l i b r i u m d e f e c t s , a r e absorbed by t h e boundaries as t h e y d i s s o c i a t e i n t o g r a i n boundary d i s l o c a t i o n s which a r e t h e n rearranged i n t o e q u i l i b r i u m c o n f i g u r a t i o n s . During t h e ab- s o r p t i o n o f l a t t i c e d i s l o c a t i o n s , g r a i n boundaries a r e shown t o be i n an n o n - s t a t i o n a r y , e x c i t e d s t a t e , which causes an abrupt a c c e l e r a t i o n o f some o f t h e g r a i n boundary dynamic process such as d i f f u s i o n , m i g r a t i o n , and s l i d i n g .The i n t e r a c t i o n o f g r a i n boundaries (GB) w i t h l a t t i c e d i s l o c a t i o n s (LD) u s u a l l y i m p l y such processes as n u c l e a t i o n o f LD, p e n e t r a t i o n o f LD t h r o u g h GB and t h e i r a b s o r p t i o n . The l a t t e r process i s o f s p e c i a l importance when s t r u c t u r a l changes and behaviour o f s o l i d s a t h i g h e r temperatures a r e s t u d i e d , i n p a r t i c u l a r d u r i n g t h e s o l i d phase r e a c t i o n , r e c r y s t a l l i z a t i o n , and high-temperature deformation /l, 21. The process o f LD a b s o r p t i o n i n t h i s case i s respon- s i b l e n o t o n l y f o r t h e r e d u c t i o n o f t h e d e n s i t y o f d i s l o c a t i o n s i n t h e s t r u c t u r e , b u t a l s o f o r s i g n i f i c a n t changes i n such dynamic p r o p e r t i e s o f GB as d i f f u s i o n , m i g r a t i o n , and s l i d - i n g . These problems have been t h e a u t h o r s ' primary concern l a t e l y and t h e r e s u l t s o f t h e i r s t u d i e s a r e summarized i n t h i s paper.
The study o f t h e d i s l o c a t i o n - b o u n d a r y i n t e r a c t i o n and i t s r e l a t i o n t o t h e GB dynamic proper- t i e s i n c l u d e d b o t h t h e a n a l y s i s o f t h e LD a b s o r p t i o n mechanism / 3 , ' 4 / and experiments i n o r - der t o c o n f i r m t h e e x i s t e n c e o f t h i s r e l a t i o n and t o d e f i n e i t s n a t u r e /l, 21. L e t us con- s i d e r t h e s e two p o i n t s i n t u r n .
1. Our a n a l y s i s o f t h e LD a b s o r p t i o n mechanigms i s based on t h e concept o f t h e GB non-equi- l i b r i u m s t a t e g e n e r a t i o n and recovery 14, 5/
.
According t o t h i s concept, t r a p p e d l a t t i c e d i s l o c a t i o n s (TLD), being n o n - e q u i l i b r i u m d e f e c t s , a r e absorbed by t h e boundaries, t h e boun- dary energy being reduced t o t h e e q u i l i b r i u m s t a t e values, through r a t h e r complicated r e a r - r a n g e m e n t ~ i n GB s t r u c t u r e . The t y p e o f t h e s e rearrangements depends on t h e c r y s t a l l o g e o m e t r y o f GBand c o n d i t i o n s o f t h e LD a b s o r p t i o n . It should be noted t h a t t h e t e r m "recovery" used w i t h r e f e r e n c e t o t h e process o f LD a b s o r p t i o n i s r a t h e r c o n v e n t i o n a l s i n c e i n transforma- t i o n s o f t h e f o l l o w i n g t y p e : an e q u i l i b r i u m GB-
LD e n t e r i n g t h e GB - absorption n f TLD by t h e GB s t r u c t u r e , t h e r f s u l t a n t e q u i l i b r i u m GB s t r u c t u r e must d i f f e r from t h e o r i g i n a l m /4/.Numerous experiments / 2 / prove t h a t d u r i n g h e a t i n g TLD a r e u n s t a b l e i n a l l t h e GB, b u t t h e observed changes however depend on t h e GB c r y s t a l l o g e o m e t r y . I n c o i n c i d e n c e and n e a r - c o i n c i - dence GB t h e d i s s o c i a t i o n o f TLD i n t o p e r f e c t e x t r i n s i c g r a i n boundary d i s l o c a t i o n s (EGBD) w i t h t h e DSc-Burgers v e c t o r s o r p a r t i a l ones a r e o f t e n observed; i n GB w i t h v i s i b l e i n t r i n s i c g r a i n boundary d i s l o c a t i o n (IGBD) networks t h e i n t e r a c t i o n o f EGBD w i t h IGBD and i n c o r p o r a - t i o n o f t h e former i n t o e q u i l i b r i u m IGBD networks were observed. I n random GB i t i s h a r d l y p o s s i b l e t o see t h e d i s s o c i a t i o n o f TLD i n t o EGBD. The experiments on t h e i n s i t u h e a t i n g o f f o i l s r e v e a l e d t h e spreading o f t h e e l e c t r o n microscope TLD c o n t r a s t : .dith h e a t i n g , t h e TLD (l-fhe n o n - e q u i l i b r i u m s t a t e o f GB can be d e s c r i b e d i n more d e t a i l i n t h e framework o f d i s -
c l i n a t i o n - s t r u c t u r a l u n i t model proposed by t h e authors i n /6/ and developed i n / 7 / .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901108
Cl-680 COLLOQUE DE PHYSIQUE
images i n random GB w i l l g r a d u a l l y widen and fade u n t i l t h e d i f f r a c t i o n c o n t r a s t disappears completely.
I t should be emphasized t h a t spreading and disappearance o f t h e e l e c t r o n microscope images o f TLD do n o t a t t e s t t o t h e complete TLD a b s o r p t i o n , i . e . t o t h e recovery o f t h e GB s t r u c t u r e . T h i s c o n c l u s i o n was drawn d i r e c t l y from t h e experiments which demonstrate t h a t some o f t h e GB p r o p e r t i e s remain n o n - e q u i l i b r i u m a f t e r t h e spreading o f TLD images (see below).
The e l u c i d a t i o n o f t h e mechanism o f LD a b s o r p t i o n i n GB i m p l i e s t h e c o n s i d e r a t i o n o f b o t h t h e c r y s t a l l o g e o m e t r i c a l rearrangements o f GB s t r u c t u r e and energy aspects o f t h i s i n t e r a c t i o n 1 4 , 5, 61.
I n 15, B/ v a r i a t i o n s o f t h e GB energy d u r i n g t h e TLD a b s o r p t i o n was analyzed on t h e b a s i s o f . t h e model o f t h e d i s s o c i a t i o n o f TLD i n t o EGBD. The energy o f a n o n - e q u i l i b r i u m GB was r e p r e - sented as a t o t a l o f t h e f o l l o w i n g components: t h e energy o f t h e o r i g i n a l e q u i l i b r i u m GB; t h e energy o f e l a s t i c d i s t o r t i o n s i n t r o d u c e d e i t h e r by t h e TLD o r by t h e complex o f EGBD, i . e . products o f TLD d i s s o c i a t i o n ; t h e v a r i a t i o n o f t h e energy o f o r i g i n a l GB due t o t h e change i n m i s o r i e n t a t i o n s provoked by t h e d e f e c t s ; t h e energy o f t h e i n t e r a c t i o n o f EGBD w i t h each o t h e r ; t h e energy o f t h e i n t e r a c t i o n o f GB r e g i o n s occupied by t h e products o f TLD d i s s o c i a - t i o n w i t h t h e GB r e g i o n s o f t h e o r i g i n a l GB s t r u c t u r e , i . e . t h e energy o f t h e t r a n s i t i o n r e - g i o n . The energy o f GB decreases d u r i n g t h e r e d i s t r i b u t i o n o f EGBD i n GB, i . e . d u r i n g t h e spreading o f TLD d i s s o c i a t i o n products. Assuming t h a t t h e e l a s t i c i t y t h e o r y - a p p l i e s t o h i g h - angle GB, t h e energy v a r i a t i o n v a l u e f o r TLD w i t h t h e Burgers v e c t o r normal t o t h e GB w i l l be as shown i n F i g . 1. During t h e a b s o r p t i o n o f TLD, t h e GB energy does n o t change monotonously:
t h e r e i s a c e r t a i n p o t e n t i a l b a r r i e r a t t h e beginning o f t h e s e p a r a t i o n o f TLD d i s s o c i a t i o n products. It means t h a t TLD i n GB a r e r e l a t i v e l y s t a b l e and t o be absorbed by t h e GB s t r u c - t u r e , t h e y have t o o v e r c o m e ' t h e . p o t e n t i a 1 b a r r i e r due t o t h e r m a l a c t i v a t i o n .
p. --. .
'
-pF i g . 1
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Dependence o f t h e GB energy on t h e w i d t h o f t h e EGBD complex (H) formed d u r i n g t h e TLD d i s s o c i a t i o n . The energy a t H/L=1 (L i s t h e GB l e n g t h ) corresponds t o t h e new e q u i l i b r i - um GB s t r u c t u r e w i t h t h e corresponding number, n, o f TLD d i s s o c i a t i o n p r o d u c t s .The most i m p o r t a n t c o n l u s i o n drawn from t h e r e s u l t s obtained i n
/a/
i s t h a t t h e n o n - e q u i l i b - r i u m s t a t e o f t h e GB s t r u c t u r e may be e i t h e r r e l a t i v e l y s t a b l e ( q u a s i - s t a b l e ) i f TLD a r e l o c a l i z e d , o r u n s t a b l e i f t h e s e p a r a t i o n o f t h e TLD d i s s o c i a t i o n p r o d u c t s has s t a r t e d and t h e necessary p o t e n t i a l b a r r i e r has been overcome. A s i m i l a r c o n c l u s i o n was made e a r l i e r on t h e b a s i s o f t h e q u a l i t a t i v e a n a l y s i s : /g/ and l a t e r / I D / and o t h e r ; t h e corresponding s t a t e s o f GB s t r u c t u r e a r e c a l l e d n o n - e q u i l i b r i u m s t a t e s o f t h e f i r s t and second o r d e r r e s p e c t i v e l y . Besides, i n 12, 5/ t h e s t a t e o f GB s t r u c t u r e where t h e process o f TLD a b s o r p t i o n i s i n p r o - g r e s s i s c a l l e d n o n - e q u i l i b r i u m o r n o n - s t a t i o n a r y s t a t e .2. L e t us d w e l l now on t h e r e l a t i o n s between t h e TLD a b s o r p t i o n and GB dynamic p r o p e r t i e s . The e x i s t e n c e o f such r e l a t i o n s c o u l d be expected i f we s u p p o r t t h e view t h a t t h e non-equi- l i b r i u m o r n o n - s t a t i o n a r y s t a t e o f GB i s formed i n t h e course o f LD a b s o r p t i o n . To c o n f i r m t h i s statement we have performed a s e r i e s o f experiments.
G r a i n Boundary D i f f u s i o n . The e f f e c t o f t h e TLD a b s o r p t i o n on t h e d i f f u s i o n was determined on t h e b a s i s o f t h e data o b t a i n e d t h r o u g h t h e study o f GB p r e c i p i t a t e n u c l e a t i o n and growth k i - n e t i c s d u r i n g t h e ageing o f an Al-base a l l o y /11/. Two s t a t e s o f t h e a l l o y were s t u d i e d : as- annealed w i t h t h e g r a i n s i z e o f 15 pm and as-annealed w i t h subsequent 2% s t r a i n i n g a t room
temperature. Such sma 1 c o l d deformation p r o v i d e s f o r t h e r u n n i n g o f TLD w i t h t h e average d e n s i t y o f 3 X 105cm-a i n GB. These d i s l o c a t i o n s were u n s t a b l e d u r i n g t h e h e a t i n g and a l r e a d y a t 90°C t h e spreading o f t h e s e TLD s t a r t e d . The k i n e t i c s o f p h a s e - p r e c i p i t a t i o n s were s t u d i e d a t 130°C (see t h e T a b l e ) .
Table
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The F r a c t i o n s o f G r a i n Boundaries Containing Disperse P a r t i c l e s ( X ) and t h e Volume F r a c t i o n o f MgZn2 P r e c i p i t a t e s ( f ) Depending on t h e Time o f Annealing a t 130°C.S t a t e Time o f Annealing (min)
2. As-annealed
+
2% c o l dp r e s t r a i n i n g 18 6 65 11 100 14
A c o n s i d e r a b l e a c c e l e r a t i o n o f t h e f o r m a t i o n and growth o f p r e c i p i t a t e s i n GB i n t h e pre- s t r a i n e d specimen i s observed, which may be d i r e c t l y connected w i t h t h e a b s o r p t i o n o f TLD i n t h e course o f subsequent annealing. The a l l o y phase p r e c i p i t a t i o n k i n e t i c s i s c o n t r o l l e d by t h e d i f f u s i o n o f p r e c i p i t a t e forming elements Mg and Zr i n GB. It i s w e l l known t h a t when p r e c i p i t a t e s a r e s m a l l ( u n i t i e s o r t e n s o f nanometers) t h e r o l e o f g r a i n boundary d i f f u s i o n i n t h e i r f o r m a t i o n and growth i s dominating as compared t o volume d i f f u s i o n . Basing on t h i s statement i t was shown i n Ill/ t h a t due t o t h e a b s o r p t i o n o f TLD i n t h e a l l o y , d i f f u s i o n ac- c e l e r a t e s more t h a n 10 t i m e s . It i s i m p o r t a n t t o emphasize t h a t t h e a c c e l e r a t i o n o f g r a i n boundary d i f f u s i o n d u r i n g t h e a b s o r p t i o n o f TLD may cause a b r u p t changes i n t h e k i n e t i c s and c h a r a c t e r o f s o l i d - p h a s e r e a c t i o n s which proceed w i t h t h e involvement o f g r a i n boundaries as i t has r e c e n t l y been shown on t h e Ni-base a l l o y 1121.
G r a i n Boundary S l i d i n g . Zinc b i c r y s t a l s were used f o r i n v e s t i g a t i o n 113, 141. T h i s m e t a l w i t h an h.c.p. l a t t i c e has o n l y one p r e f e r e n c i a l s l i p p l a n e ( b a s a l p l a n e ) , t h u s t h e study o f t h e d e f o r m a t i o n o f t h e s e b i c r y s t a l s a l l o w s t o d i s t i n g u i s h between t h e d e f o r r n a t i o n a l processes i n t h e boundary and w i t h i n t h e g r a i n s .
F i g . 2
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Scheme o f c u t t i n g ( a ) and t y p e s (b) o f t h e i n v e s t i g a t e d Zn b i c r y s t a l s w i t h 90°<1010>tilt boundary.
It can be seen t h a t i n t y p e I b i c r y s t a l t h e b a s a l s l i p i s banned. Thus, d u r i n g t h e t e n s i l e s t r a i n i n g , t h e "pure" s l i d i n g develops h e r e w i t h o u t i n t r a g r a n u l a r s l i p . At t h e same t i m e , i n t y p e I1 b i c r y s t a l s , simultaneous development o f s l i d i n g and b a s a l s l i p t a k e s p l a c e d u r i n g t h e d e f o r m a t i o n .
It t u r n e d o u t t h a t a t t h e same shear s t r e s s o f 1 . 4 MPa f o r t y p e I1 specimens t h e GBS r a t e was 50-60 t i m e s as h i g h as t h e r a t e o f GBS f o r t y p e I specimens. I n t h e former case c o n t r a r y t o t h e l a t t e r one an i n t e n s i v e i n t e r a c t i o n o f g r a i n boundaries w i t h t h e i n t r a g r a n u l a r s l i p t o o k p l a c e and, consequently, an a c t i v e a b s o r p t i o n o f LD i n GB was observed 1141. The data obtained
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g i v e d i r e c t evidence f o r t h e s t i m u l a t i n g e f f e c t o f t h e a b s o r p t i o n o f TLD on t h e development o f g r a i n boundary s l i d i n g .
T h i s e f f e c t may be due t o t h e a c c e l e r a t i o n o f d i f f u s i o n and t h e increase o f t h e d e n s i t y o f mobile g r a i n boundary d i s l o c a t i o n s as a r e s u l t o f t h e n o n - e q u i l i b r i u m s t a t e o f g r a i n bound- a r i e s induced d u r i n g t h e a b s o r p t i o n o f TLD.
M i g r a t i o n o f Grain Boundaries. The g r a i n boundary m i g r a t i o n was assesed by t h e data obtained from t h e i n v e s t i g a t i o n o f g r a i n growth i n t h e magnesium a l l o y w i t h t h e g r a i n s i z e o f 10 pm i n t h e f o l l o w i n g two i n i t i a l s t a t e s : as-annealed and 3% p r e s t r a i n e d a t room temperature 1151.
It t u r n e d o u t t h a t t h e annealed a l l o y displayed no n o t i c e a b l e v a r i a t i o n s o f g r a i n s i z e w i t h f u r t h e r h e a t i n g and annealing a t 400°C.
F i g . 3
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Dependence o f t h e average g r a i n s i z e i n t h e magnesium a l l o y specimen 3.3% s t r a i n e d on t h e t i m e o f annealing a t 670 K.The case was q u i t e d i f f e r e n t w i t h t h e p r e s t r a i n e d m a t e r i a l ( F i g . 3 ) . At t h e s t a r t o f anneal- i n g t h e r e was an i n c u b a t i o n p e r i o d o f about 10 minutes, t h e n an i n t e n s i v e g r a i n growth s t a r t - ed, a f t e r which t h e g r a i n s i z e became s t a b l e . This g r a i n growth cannot be explained e i t h e r from t h e p o i n t o f view o f t h e d i s s o l u t i o n o f p r e c i p i t a t e s i n t h e a l l o y o r by t h e i n f l u e n c e of t h e volume d e n s i t y o f d i s l o c a t i o n s 1151. However, a d i s t i n c t c o r r e l a t i o n was observed between t h e g r a i n growth and t h e TLD behaviour i n g r a i n boundaries.It t u r n e d o u t t h a t a t t h e s t a r t o f annealing t h e TLD i n t h e boundaries were s t a b l e , b u t a f t e r t h e 10 minutes o f annealing t h e i r spreading s t a r t e d , f o l l o w e d by t h e i r absorption. The i n v e s t i g a t i o n s showed t h a t t h e beginning o f t h e TLD a b s o r p t i o n was t h e main reason f o r an a c t i v e m i g r a t i o n o f boundaries and g r a i n growth.
Conclusions.
The absorption o f TLD seems t o be r e l a t e d t o complicated rearrangements i n G8 d u r i n g which t h e G8 s t r u c t u r e i s i n a n o n - e q u i l i b r i u m and non-stationary s t a t e .
2. The experimental data obtained i n d i c a t e t h a t considerable a c t i v a t i o n o f such dynamic pro- p e r t i e s o f G8 as d i f f u s i o n , s l i d i n g , and m i g r a t i o n seems t o t a k e p l a c e d u r i n g t h e a b s o r p t i o n o f TLD i n GB.
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