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MECHANISMS OF THE FORMATION OF HIGH ANGLE GRAIN BOUNDARIES DURING THE HOT
DEFORMATION OF STAINLESS STEELS
A. Belyakov, R. Zaripova, R. Kaibyshev, G. Salishchev
To cite this version:
A. Belyakov, R. Zaripova, R. Kaibyshev, G. Salishchev. MECHANISMS OF THE FOR- MATION OF HIGH ANGLE GRAIN BOUNDARIES DURING THE HOT DEFORMATION OF STAINLESS STEELS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-435-C1-438.
�10.1051/jphyscol:1990167�. �jpa-00230334�
MECHANISMS OF THE FORMATION OF HIGH ANGLE GRAIN BOUNDARIES DURING THE HOT DEFORMATION OF STAINLESS STEELS
A. BELYAKOV, R. ZARIPOVA, R. KAIBYSHEV and G. SALISHCHEV
Institute of Metals superplasticity Problems, URSS Academy of Sciences, Ufa 450001, U.R.S.S.
A b s t r a c t - The paper d e a l s with t h e formation o f g r a i n boundaries d u r i n g h o t deforma- t i o n o f a u s t e n i t i c , f e r r i t i c and a u s t e n i t i c - f e r r i t i c s t e e l s . It i s shown t h a t i n a l l m a t e r i a l s under i n v e s t i g a t i o n t h e f o r m a t i o n o f high a n g l e boundaries o c c u r s a f t e r c o n s i d e r a b l e s t r a i n i n g . A t t h e e a r l i e r s t a g e s o f p l a s t i c f l o w s t r u c t u r a l changes a r e d e f i n e d by m a t e r i a l and s t r a i n t e m p e r a t u r e . A t low t e m p e r a t u r e s i n a u s t e n i t i c s t e e l and i n t h e a u s t e n i t i c phase o f a two-phase m a t e r i a l t h e f o r m a t i o n o f fragmented s t r u c t u r e o c c u r s . Developed s u b s t r u c t u r e is formed i n f e r r i t e i n a l l t h e t e m p e r a t u r e - s t r a i n r a t e i n t e r v a l under s t u d y , w h i l e i n a u s t e n i t e only a t c o n s i d e r a b l y high t e m p e r a t u r e s . Mechanisms o f s t r u c t u r e f o r m a t i o n i n p l a s t i c flow i n s t a i n l e s s s t e e l s a r e d i s c u s s e d .
1
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INTRODUCTIONIt is known now t h a t deformation o f d i f f e r e n t s t e e l s is accompanied by c o n s i d e r a b l e s t r u c t u - r a l changes /l, 21. The authors/3,8/, having observed t h e f o r m a t i o n o f f i n e g r a i n e d s t r u c t u r e i n a u s t e n i t i c and f e r r i t i c s t a i n l e s s s t e e l s , and i n maraging s t e e l c o n t a i n i n g 18% N i a s w e l l connect t h i s p r o c e s s with dynamic r e c r y s t a l l i z a t i o n . I n /4/ it h a s been shown t h a t depending on t h e t e m p e r a t u r e - s t r a i n r a t e c o n d i t i o n s t h e r e can e x i s t two mechanisms o f t h e r e c r y s t a l l i z ed g r a i n formation: t h e f o r m a t i o n o f t h e new g r a i n c e n t r e s and t h e i r growth, a n t t h e l o c a l m i g r a t i o n o f t h e i n i t i a l g r a i n b o u n d a r i e s . However no d e t a i l e d i n v e s t i g a t i o n s on mechanisms o f high a n g l e g r a i n boundary formation have been undertaken.
It h a s been e s t a b l i s h e d /5/ t h a t a t low t e m p e r a t u r e s high s t r a i n s o f f . c . c . and b . c . c . m e t a l s , i n c l u d i n g s t a i n l e s s s t e e l s , c a u s e t h e f o r m a t i o n o f fragmented s t r u c t u r e . Such s t r u c - t u r a l s t a t e i s c h a r a c t e r i z e d by t h e p r e s e n c e o f g r a i n boundaries g i v i n g a d i s l o c a t i o n cant- on t h e e l e c t r o n - m i c r o s c o p i c p i c t u r e s , b u t having a m i s o r i e n t a t i o n a n g l e i n a few t e n s o f d e g r e e s . The a u t h o r s connect t h e appearance o f such boundaries with t h e motion of d i s c l i n a - t i o n d i p o l e s .
A t t h e i n i t i a l s t a g e o f p l a s t i c f l o w it r e s u l t s i n t h e f o r m a t i o n o f " k n i f e " boundaries w i t h t h e s p a c e i n a few t e n s o f microns between them. With t h e s t r a i n i n c r e a s e t h e r e o c c u r s a s p a c e f r a g m e n t a t i o n between them. A t high s t r a i n s t h e appearance o f s e p a r a t e r e c r y s t a l l i z ed g r a i n s is observed. T h i s i s due t o complex c o o p e r a t i v e motion p r o c e s s e s o f c r y s t a l l a t - t i c e d e f e c t s and t h e i r i n t e r a c t i o n with each o t h e r .
The p r e s e n t paper d e a l s w i t h s t r u c t u r a l changes d u r i n g h o t deformation o f a u s t e n i t i c , f e r - r i t i c and a u s t e n i t i c - f e r r i t i c s t e e l s . The absence o f polymorphic t r a n s f o r m a t i o n s i n t h e s e m a t e r i a l s h a s allowed t o c a r r y o u t i n v e s t i g a t i o n s i n a broad t e m p e r a t u r e - s t r a i n r a t e i n t e r - v a l .
2
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EXPERIMENTAL PROCEDURE AND MATERIALSA u s t e n i t i c , f e r r i t i c and a u s t e n i t i c - f e r r i t i c s t e e l s have been used i n t h i s s t u d y ( Table 1 )
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990167
Cl-436 COLLOQUE DE PHYSIQUE
Table 1
Type of s t e e l Mark Chemical composition, wt. % Grain s i z e
C C r N i Ti
M O
CJ...
A u s t e n i t i c 12X18HlOT 0,12 18 10 0 , 9
- -
110F e r r i t i c 15X25T 0,15 25
-
0 , 9 --
230F e r r i t i c -
a u s t e n i t i c +K72
Numerator i s a u s t e n i t e g r a i n s i z e , denominator i s f e r r i t e g r a i n s i z e . Austenite t o f e r r i t e r a t i o i s 50150.
Specimens 10x15 mm were upset on t h e I_NSTRON dynamometer i n t h e temperature range o f 1073 - 1373K and a t s t r a i n r a t e s of 5.10
-
2 . 1 0 - ~.
To r e g i s t e r t h e s t r u c t u r e of t h e m a t e r i a l being formed due t o deformation instantaneous water quenching of specimens was used. Time from stopping of t h e t e s t machine t r a v e r s e t o t h e beginning of quenching d i d not exceed 0.2 s e c . M i c r o s t r u c t u r a l i n v e s t i g a t i o n s were conducted on t h e NEOPHOT-2 o p t i c a l microscope and EPIQUANT automatic s t r u c t u r e a n a l y s e r . The d i s l o c a t i o n s t r u c t u r e was s t u d i e d on t h i n f o i l s prepared by means of j e t e l e c t r o p o l i s h i n g on e l e c t r o n microscopes"JEM2000Xn and "TESLA BS-540".
3
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RESULTSDeformation of a u s t e n i t i c s t e e l causes considerable s t r u c t u r a l changes. The formation of r e c r y s t a l l i z e d g r a i n s i s observed i n a l l t h e temperature-strain r a t e i n t e r v a l , and t h e i r s i z e and volume f r a c t i o n i n c r e a s e with temperature r i s i n g and s t r a i n r a t e decreasing.
New g r a i n s appear i n t h e v i c i n i t y of boundaries, on twins and gradually i n t h e body of t h e i n i t i a l g r a i n s . O i s l o c a t i o n s t r u c t u r e formation depends on s t r a i n temperature. A t tem- p e r a t u r e s lower than T=1273K coexistence of s u b s t r u c t u r e , c e l l u l a r and fragmented s t r u c - t u r e s t u r e s i s observed. A t t h e i n i t i a l s t a g e of p l a s t i c flow fragmentation t a k e s p l a c e .
"Knife" boundaries appear, t h e i r length c o i n c i d e s with t h e s i z e of i n i t i a l g r a i n s . In spa- c i n g s of 4-20 microns between them fragments, c e l l s and subgrains with t h e s i z e of 1-4 mic rons occur [ F i g . l a , b)
.
'3)
t e e 1 a f t e r d e f o r l ~ ~ a ~ l u r ~
a ) T=1173K, E =20%: b) T=1173K, & =50%: c ) T=1273K, &=50%: d) T=1173K,& =70%.
There i s high d i s l o c a t i o n d e n s i t y . Deformation temperature r i s i n g changes t h e p i c t u r e of s t r u c t u r e formation. In a u s t e n i t i c s t e e l s subgrain boundaries a s d i s l o c a t i o n walls a r e ob- served. Density of l i n e a r d e f e c t s i s low, c e l l u l a r and fragmented s t r u c t u r e s a r e absent ( F i g . l c ) . A t higher s t r a i n s t h e m a t e r i a l i s c h a r a c t e r i z e d by t h e presence of equiaxed elongated s u b g r a i n s , broken boundaries. The presence of developed s u b s t r u c t u r e is charac-
I n f e r r i t i c s t e e l s u b s t r u c t u r e is formed a t t h e i n i t i a l s t a g e o f d e f o r m a t i o n i n t h e tempe- r a t u r e - s t r a i n r a t e u n d e r s t u d y . A f t e r s t r a i n ( &=SO%) t h e s u b s t r u c t u r e i s r e v e a l e d b o t h m e t a l l o g r a p h i c a l l y and by e l e c t r o n microscopy ( F i g . 2 a ) .
-3 -i F i g . 2 . The s t r u c t u r e o f f e r r i t i c s t e e l a f t e r t h e d e f o r m a t i o n w i t h s t r a i n r a t e
E
= 5.10 C a ) T=12731<, E =50%: b ) T=1273K, & =75%.A t d i f f e r e n t a n g l e s o f f o i l i n c l i n a t i o n s s u b g r a i n s a r e o b s e r v e d a s d i s l o c a t i o n w a l l s and a s g r a i n s g i y i n g t h i c k n e s s c o n t o u r s . S u b g r a i n s i z e i s d e t e r m i n e d by s t r a i n c o n d i t i o n s a t T=1173K and & =5.10-'ci it is e q u a l t o 5 m i c r o n s . With t e m p e r a t u r e r i s i n g up t o T=1373K s u b g r a i n s i z e i n c r e a s e s up t o 25-30 m i c r o n s . Very low d i s l o c a t i o n d e n s i t y i s c h a r a c t e r i s t i c o f t h e deformed f e r r i t i c s t e e l . S t r a i n i n c r e a s e up t o & =75% r e s u l t s i n t h e f o r m a t i o n o f h i g h a n g l e g r a i n b o u n d a r i e s w i t h o u t e f f e c t i n g t h e s u b g r a i n s i z e . New g r a i n s a r e mainly g e n e r a t e d a t t h c i n i t i a l b o u n d a r i e s and a t t r i p l e p o i n t s . It i s i n t e r e s t i n g t o n o t e t h a t volume f r a c t i o n and s i z e o f r e c r y s t a l l i z e d g r a i n s a r e c l o s e t o t h o s e o f s u b g r a i n s a t E =50%. The s t r u c t u r e o f m a t e r i a l i s inhomogeneous. T o g e t h e r w i t h r e c r y s t a l l i z e d g r a i n s t h e r e e x i s t n o n - r e c r y s t a l l i z e d g r a i n s w i t h d e v e l o p e d s u b - s t r u c t u r e ( F i g . 2b)
.
S t r u c t u r a l t r a n s f o r m a t i o n s i n b o t h p h a s e s o f a u s t e n i t i c - f k r r i t i c s t e e l a r e i n t h e main a n a l o g o u s w i t h t h e mentioned a b o v e . L o c a l i z a t i o n o f p l a s t i c f l o w i n f e r r i t i c p h a s e i s o b s e r v e d , which c o r r e s p o n d i n g l y r e s u l t s i n t h e a p p e a r a n c e o f r e c r y s t a l l i z e d g r a i n s a t l o w e r s t r a i n s t h a n i n f e r r i t i c steels.
4 - DPSCUSSIO!d-
The a n a l y s i s o f t h e o b t a i n e d e x p e r i m e n t a l d a t a i n d i c a t e s t h a t d e p e n d i n g on t h e t e m p e r a t u r e a t t h e i n i t i a l s t a g e o f d e f o r m a t i o n d i f f e r e n t d i s l o c a t i o n s t r u c t u r e s a r e f o r m e d i n a u s t e n i t i c s t e e l s . F r a g m e n t e d s t r u c t u r e i s f o r m e d a t l o w t e m p e r a t u r e s , Rybin c o n n e c t s i t s a p p e a r a n c e w i t h d i s c l i n a n i o n moving. Being s t o r e d i n t h e g r a i n s t h e d i s l o c a t i o n s a r e g a t h e r e d i n t o c e l l s . Only some o f t h e s e d i s l o c a t i o n s , which form w a l l s , become s u b g r a i n b o u n d a r i e s . With t h e r i s e o f d e f o r m a t i o n t e m p e r a t u r e t h i s r e c o v e r y mechanism becomes p r e v a i l i n g , and t h e r o l e o f f r a g m e n t a t i o n i n t h e f o r m a t i o n o f s t r u c t u r e s s h a r p l y d e c r e a s e s . S t r a i n i n c r e a s e c a u s e s t r a n s f o r m a t i o n o f d i s l o c a t i o n b o u n d a r i e s i n t o random h i g h a n g l e g r a i n o n e s . T h i s p r o c e s s i s accompanied by i n t e n s i v e m i g r a t i o n of random h i g h a n g l e g r a i n b o u n d a r i e s . E n t r y o f l a t t i c e d i s l o c a t i o n s i n t o s u b g r a i n b o u n d a r i e s r e s u l t s i n t h e i n c r e a s e o f m i s o r i e n t a t i o n and g r a d u a l t r a n s f o r m a t i o n o f s u b g r a i n b o u n d a r i e s i n t o h i g h a n g l e o n e s . But t h e e x a c t mechanism o f boundary f r a g m e n t t r a n s f o r m a t i o n i n t o random b o u n d a r i e s r e m a i n s vague. However it c a n b e s u g g e s t e d t h a t t h i s t r a n s f o r m a t i o n i s c o n n e c t e d w i t h t h e i n t e r a c t i o n o f b o u n d a r i e s h a v i n g d i s c l i n a t i o n o r i g i n w i t h c r y s c t a l l a t t i c e d e f e c t s and w i t h t h e d i f f u s i o n p r o c e s s e s .
L a r g e s t a c k i n g f a u l t e n e r g y i n f e r r i t e c a u s e s c o n s i d e r a b l e d i s l o c a t i o n motion r e s p o n s i b l e f o r t h e f o r m a t i o n o f r e l a t . i v e l y l a r g e s u b g r a i n s . To t r a n s f o r m low a n g l e b o u n d a r i e s i n h i g h a n g l e o n e s c o n c e n t r a t i o n o f a g r e a t number o f d i s l o c a t i o n s i s n e c e s s a r y , it r e s u l t s i n t h e a p p e a r a n c e o f r e c r y s t a l l i z e d g r a i n s a t h i g h s t r a i n s o n l y and c o n c u r r e n t f o r m a t i o n w i t h i n t h e whole volume o f i n i t i a l g r a i n s . S i m i l a r mechanism o f dynamic r e c r y s t a l l i z a t i o n
Cl-438 COLLOQUE DE PHYSIQUE
has been e a r l i e r observed i n
P
- t i t a n i u m and i n t h e a l l a y Mg-6Zn-n,6Zr / 6 , 7 / . 5-
CONCLUSIONS1. The q a i n mechanism o f h i g h angle g r a i n boundary f o r m a t i o n i n s t a i n l e s s s t e e l s i n c l u d e s t h e development o f s u b s t r u c t u r e a t t h e i n i t i a l s t a q e o f deformation and g r a d u a l t r a n s f o r - mation o f l o w angle boundaries i n t o h i g h angle ones.
2. Durinq t h e deformation o f a u s t e n i t e a t l o w temperatures f r a g m e n t a t i o n precedes t h e f o r m a t i o n o f new q r a i n s .
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