Colloque Cl, suppl6ment au n0l, Tome 51, janvier 1990. MECHANISMS OF THE FORMATION OF HIGH ANGLE GRAIN BOUNDARIES DURING THE HOT.
COLLOQUE DE PHYSIQUE Colloque Cl, suppl6ment au n0l, Tome 51, janvier 1990
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.
Abstract - The paper d e a l s with t h e formation o f g r a i n boundaries during hot deformat i o n of 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 formation 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 flow s t r u c t u r a l changes a r e defined by m a t e r i a l and s t r a i n temperature. A t low temperatures 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 of a two-phase m a t e r i a l t h e formation o f fragmented s t r u c t u r e occurs. 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 , while i n a u s t e n i t e only a t considerably high t e m p e r a t u r e s . Mechanisms of s t r u c t u r e formation 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 - INTRODUCTION
It is known now t h a t deformation of 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 21. The authors/3,8/, having observed t h e formation o f f i n e g r a i n e d s t r u c t u r e r a l changes /l, 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 has 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 of 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 formation 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 migration of t h e i n i t i a l g r a i n boundaries. 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 has 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 , cause t h e formation 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 presence of g r a i n boundaries g i v i n g a d i s l o c a t i o n canton t h e electron-microscopic 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 degrees. 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 dipoles. 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 flow it r e s u l t s i n t h e formation o f "knife" boundaries with t h e space 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 space fragmentation 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. This i s due t o complex c o o p e r a t i v e motion processes 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 with s t r u c t u r a l changes during hot 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 has 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 val. 2 - EXPERIMENTAL PROCEDURE AND MATERIALS 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 have been used i n t h i s study ( 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
Chemical composition, wt. %
Mark
Grain s i z e
MO CJ ...................................................................................... C
Austenitic
12X18HlOT
0,12
Ferritic
15X25T
0,15
Cr
18 25
Ni
Ti
10
0,9
-
-
0,9
-
-
110 230
Ferritic+K72 austenitic 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
-
RESULTS
Deformation 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 temp 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 spac 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)
tee1 after deforl~~a~lur~ a ) T=1173K, E =20%: b) T=1173K, & =50%: c ) T=1273K, &=50%: d) T=1173K,& =70%. There is 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 observed. 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-
t e r i c t i c o f b o t h t h e new g r a i n s and n o n - r e c r s t a l l i z e d p a r t ( F i g . I d ) . The s u b g r a i n s a r e 2 t o 3 times s m a l l e r i n s i z e t h a n t h e r e c r y s t a l l i z e d g r a m s . I n f e r r i t i c steel 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 temper 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 ) .
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 a ) T=12731
