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GRAIN BOUNDARY STRUCTURE AND MECHANICAL PROPERTIES OF THE TiAl
INTERMETALLIC COMPOUND
R. Imayev, G. Salishchev
To cite this version:
R. Imayev, G. Salishchev. GRAIN BOUNDARY STRUCTURE AND MECHANICAL PROPERTIES
OF THE TiAl INTERMETALLIC COMPOUND. Journal de Physique Colloques, 1990, 51 (C1),
pp.C1-519-C1-524. �10.1051/jphyscol:1990181�. �jpa-00230349�
COLLOQUE DE PHYSIQUE
C o l l o q u e C l , s u p p l e m e n t a u n o l , Tome 51, janvier 1 9 9 0
GRAIN BOUNDARY STRUCTURE AND MECHANICAL PROPERTIES OF THE TiAl INTERMETALLIC COMPOUND
R.M. IMAYEV and G.A. SALISHCHEV
I n s t i t u t e of M e t a l s S u p e r p l a s t i c i t y P r o b l e m s , URSS A c a d e m y
of
Sciences, U f a 450001, U.R.S.S.Resume
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L'Btude d e s p r o p r i d t 6 s mdcaniques du compose i n t e r m e t a l l i q u e T i A l a e t 6 f a i t e d e l a t e m p e r a t u r e ambiante B 1050°C. Ces p r o p r i d t d s dependent f o r t e m e n t d e l a s t r u c t u r e d e s j o i n t s d e g r a i n . En p a r t i c u l i e r , l e comportement m6canique du composi? i n t e r m e t a l l i q u e est r e l i d 8. l a c a p a c i t d d e s j o i n t s B a b s o r b e r l e s d i s l o c a t i o n s .A b s t r a c t
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An i n v e s t i g a t i o n o f t h e TiAl i n t e r m e t a l l i c compound mechanical p r o p e r t i e s h a s been undertaken i n t h e t e m p e r a t u r e i n t e r v a l o f 20-1050°C. Its p r o p e r t i e s have been found .to be c o n s i d e r a b l y dependent on t h e s t r u c t u r e o f g r a i n b o u n d a r i e s . Absorption p r o p e r t i e s o f g r a i n b o u n d a r i e s and mechanical behaviour o f t h e i n t e r m e t a l l i c m a t e r i a l a r e shown t o be i n t e r r e l a t e d .1
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INTRODUCTIONP r e s e n t l y , g r a i n boundaries a r e undoubtedly ,recognized t o be t h e most i m p o r t a n t component o f a p o l y c r y s t a l s t r u c t u r e e x e r t i n g c o n s i d e r a b l e i n f l u e n c e on t h e mechanical p r o p e r t i e s o f poly- c r y s t a l s 11-51. A t t h e same t i m e , it is e v i d e n t t h a t t h e s e p r o p e r t i e s depend n o t only on t h e l e n g t h o f GB, b u t a l s o on t h e i r S t r u c t u r e 15-71. T h i s paper p r e s e n t s e x p e r i m e n t a l d a t a which prove t h a t g r a i n boundary s t r u c t u r e and p r o p e r t i e s s h o u l d be t a k e n i n t o account when p r e d i c t - i n g mechanical behaviour o f p o l y c r y s t a l s .
The TiAl i n t e r m e t a l l i c compound ( s u p e r l a t t i c e L ) was s p e c i a l l y chosen a s an o b j e c t f o r i n - v e s t i g a t i o n . T h i s a l l o y i s b r i t t l e and s t r a i n - r & i s t a n t even a t high t e m p e r a t u r e s 1 8 , 91.
However, when c o n v e r t e d t o a f i n e - g r a i n e d s t a t e , it can d i s p l a y s u p e r p l a s t i c i t y (SP) i n c e r - t a i n t e m p e r a t u r e - r a t e c o n d i t i o n s o f deformation / 1 0 / . T h i s m a t e r i a l was chosen t o e n a b l e a s p e c i a l kind o f experiment with t h e end o f comparing t h e a l l o y mechanical p r o p e r t i e s i n states w i t h c l o s e g r a i n s i z e s , but c o n s i d e r a b l y d i f f e r e n t i n t h e i r GB s t r u c t u r e s 1 1 1 , 121.
2 - MATERIAL AND EXPERIMENTAL PROCEDURE
The Ti-36 wt% A 1 a l l o y ( f u r t h e r denoted a s TiA1) i n t h e form of i n g o t s and b l a n k s o b t a i n e d by t h e compaction o f g r a n u l e s was used. The r e q u i r e d s t r u c t u r a l s t a t e s were c r e a t e d by means of s p e c i a l t r e a t m e n t whose modes a r e g i v e n i n t h e Table.
Table. Modes o f t h e TiAl Alloy Treatment.
S t a t e Modes o f Treatment
I 80% h o t comgreasion s t r a i n i n g o f t h e compacted a l l o y a t 1000°C and a t t h e s t r a i n r a t e o f 1 0 S
I1 Sane a s i n I + a n n e a l i n g a t 1050°C f o r 2 h o u r s
II! Same a s i n I , b u t f o r t h e c a s t a l l o y
+
70% r e s t r a i n i n g under t h e same c o n d i t i o n s and with a change o f t h e d i r e c t i o n o f deformation t o 90'I V Same a s i n 1 1 1 , b u t t h e r e s t r a i n i n g t a k e s p l a c e a t llOO°C V Same a s i n 111
+
a n n e a l i n g a t 1050°C f o r 2 h o u r s- -
Mechanical t e n s i l e t e s t s were conducted on t h e I n q t r q n machine i n t h e t e m p e r a t u r e i n t e r v a l o f 20-1050°C a t t h e i n i t i a l s t r a i n r a t e of48,2 X 10- S-
.
A t 1025OC t e s t s were conducted a t t h e i n i t i a l s t r a i n r a t e s o f (1.6-8.3) X 10 S.
T e n s i l e diagrams were used t o d e f i n e : maximal r e l a t i v e e l o n g a t i o n t o r u p t u r e ,
g ;
t r u e f l o w s t r e s s a t t h e r e l a t i v e e l o n g a t i o n o f 50%, d t i m l t q m a l W,dt.
The s t r a i n r a t e s e n s i t i v i t y c o e f f i c i e n t , m, was e v a l u a ed by t h e s l o p e o f l g d O - l g g c u r v e s . For mecha- nical- t%ts f l a t specimens with t h e workins p o r t i o n dimensions o f18
X 5 X 2 mm were used.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990181
Cl-520 COLLOQUE DE PHYSIQUE
These specimens were prepared by grinding with subsequent e l e c t r o p o l i s h i n g .
To know t h e absorption p r o p e r t i e s of GB, t h e temperature of r e l a x a t i o n , t
,
i . e . " b l u r r i n g "of e l e c t r o n microscopic c o n t r a s t , of trapped l a t t i c e d i s l o c a t i o n s
(TLD)
ig GB was measured.To do t h i s , t h e a l l o y specimens i n s t a t e s I V and V a f t e r 2.5% deformation a t room temperature were annealed f o r 15 minutes a t various temperatures.
3 -RESULTS
3 . l-lastisity o f TiA1.
S t a t e s I and I1 were s t u d i e d .
Mechanical p r o p e r t i e s . The temperature dependence of t h e a l l o y mechanical p r o p e r t i e s i n is a s follows. With t h e r i s e of t e s t i n g temperature t h e r e l a t i v e elongation t o r u p t u r e , f i r s t gradually grows, but a f t e r t h e maximum (250%) has been reached a t 1025OC, it abruptly decreases. Flow s t r e s s decreases monotonously with t h e r i s e of temperature.
Flow s t r e s ~ , d ~ ~ , l a r g e l y depends on t h e s t r a i n r a t e a t 1025OC (Fig. l a ) , t h e s t r a i n r a t e s e n s i t i v i t y of d50 i n s t a t e I , contrary t o s t a t e 1 1 , i n a c e r t a i n s t r a i n r a t e i n t e r v a l being increased.
m
6 % ~ ;
200.
I50 .
100
m - ' IW
Fig. 1
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The e f f e c t o f s t r a i n r a t e on t h e TiAl mechanical p r o p e r t i e s : a-
s t a t e I ; o - s t a t e I1 (t=1025OC)Thus, f o r s t a t e I t h e m values i n t h e s t r a i n r a t e i n t e r v a l of (0.83-1.6) X 10-~s-' vary from 0.33 t o 0.43 (Fig. l b ) . Higher values of m a r e matched with t h e l a r g e s t r e l a t i v e e l o n g a t i o n s t o r u p t u r e (200-250%). For s t a t e 1 1 , m i s constant throughout t h e considered s t r a i n r a t e i n - t e r v a l and i t s value i s 0.26 and r e l a t i v e e l o n g a t i o n only s l i g h t l y depends on s t r a i n r a t e
( 6=130-160%)
.
Thus, i n s t a t e I t h e a l l o y d i s p l a y s SP f e a t u r e s while i n s t a t e I1 SP behaviour i s l e s s pro- nounced.
Fig. 2
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The flow-stress-deformation curyqs-pf t h e TiAl a l l o y i n s t a t e s I (1) and I1 (2) (t=1025OC, s t r a i n r a t e 8 . 3 X 10 S )Fig. 2 shcrzs
Gs-
d curves c h r g the m i l e strajnirg of the alloy specimers in states I and 11 a t an optimdl s t r a i n r a t e . The s t a g e o f s t e a d y s t a t e f l o w i s p r e c e d e d by a p r o l o n g e d s t a g e o f s t r e n g t h e n - i n g . I n s t a t e I1 a t t h e i n i t i a l s t a g e , t h eGs
peak i s o b s e r v e d . With f u r t h e r d e f o r m a t i o n t h e f l o w s t r e s s monotonously d e c r e a s e s and a p p r o a c h e s t h eCifs
l e v e l i n s t a t e I.M i c r o s t r u c t u r e . When s p e c i m e n s a r e h e a t e d up t o t h e o p t i m a l t e m p e r a t u r e o f 1025OC t h e i r mean g r a i n s i z e d i n s t a t e I i n c r e a s e s from 5 t o 8 pm. S u b s e q u e n t s o a k i n g a t t h i s t e m p e r a t u r e f o r f i v e hans c a u s e s no f u r t h e r growth o f
3.
I n s t a t e 1 1 , m i c r o s t r u c t u r e a t 1025OC i s s t a b l e w i t h - d=9.5 pm.F i g . 3
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M i c r o s t r u c t u r e and h i s t o g r a m s o f g r a i n s i z e d i s t r i b u t i o n TiAl a l l o y i n t h e head o f s p e c i m e n s 160% deformed a t 1025OC and g = 8 . 3 X 1 0 S : a-
s t a t e I ; b - s t a t e 1 1 ; c : m - s t a t e I , o - s t a t e 11.F i g . 3 shows t h e m i c r o s t r u c t u r e and h i s t o g r a m s o f t h e g r a i n s i z e d i s t r i b u t i o n i n b o t h s t a t e s i n specimen h e a d s . B e s i d e s t h e main r - p h a s e (TiA1) t h e a l l o y a l s o c o n t a i n s a b o u t 3 v o l . % o f d 2 - p h a s e ( T i A1
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s u p e r l a t t i c e DO ) i n t h e form o f p a r t i c l e s s i z e d 0.1-3 pm l o c a t e d mainly i n GB. No d i f s e r e n c e i n t h e a m o ~ n t l ; ~ d i s t r i b u t i o n , s i z e , a n d s h a p e o f t h e - p h a s e p a r t i c l e s i s o b s e r v e d . Comparison o f t h e g r a i n s i z e d i s t r i b u t i o n h i s t o g r a m s o f t h e a l l o y s t a t e s shows t h a t t h e y a r e p r a c t i c a l l y i d e n t i c a l . Twin b o u n d a r i e s a r e o b s e r v e d i n t h e m i c r o s t r u c t u r e o f t h e a l l o y s t a t e s . T h e i r f r a c t i o n i n s t a t e s I and I1 i s 1 0 and 35% r e s p e c t i v e l y o f t h e t o t a l number of g r a i n s . No d 2 - p a r t i c l e s a r e o b s e r v e d i n t w i n b o u n d a r i e s . S i n c e s t a t e s I and I1 a r e c h a r a c t e r i z e d by c l o s e g r a i n s i z e s , s i m i l a r amount, s h a p e , s i z e , and d i s t r i b u t i o n o fd
-phase particles, the difference of the alloy in the said states are obviously due to the sbxcture of GB? i.e.d i f f e r e n t number o f t w i n b o u n d a r i e s i n e a c h s t a t e . 3 . 2 T o u g h - b r i t t l e t r a n s i t i o n i n TiA1.
S t a t e s 1 1 1 - V were s t u d i e d .
M i c r o s t r u c t u r e . F i g . 4 shows t h e m i c r o s t r u c t u r e and h i s t o g r a m s o f g r a i n s i z e d i s t r i b u t i o n o f t h e s t a t e s . T h e r e a r e no d i f f e r e n c e s i n t h e amount, d i s t r i b u t i o n , s i z e s , and s h a p e o f t h e d 2 - p h a s e p a r t i c l e s e i t h e r . The mean g r a i n s i z e i s 5 pm i n s t a t e 1 1 1 , 9 pm i n s t a t e IV, 11 pm i n s t a t e V. The comparison of g r a i n s i z e d i s t r i b u t i o n h i s t o g r a m s i n s t a t e s I11 and I V h a s shown t h a t t h e y a r e p r a c t i c a l l y i d e n t i c a l . Annealing t w i n s a r e o b s e r v e d i n t h e m i c r o s t r u c t u r e o f the s t a t e s . Most of them a r e i n s t a t e f i v e making a b o u t 50% of b o u n d a r i e s w h i l e i n s t a t e s I11 a n d IV t h e f r a c t i o n o f t w i n b o u n d a r i e s d o e s n o t e x c e e d 5%. No d 2 - p h a s e p a r t i c l e s p r e c i p i t a t e i n t w i n b o u n d a r i e s .
M e c h a n i c a l p r o p e r t i e s . Mechanical p r o p e r t i e s o f t h e a l l o y i n s t a t e s 1 1 1 - V a r e shown i n F i g . 5 . The dependence o f c o n d i t i o n a l t e m p o r a l r e s i s t a n c e , g t , on t e m p e r a t u r e f o r a l l t h e s t a t e s i s o f an anomalous c h a r a c t e r . With t h e r i s e o f t e m p e r a t u r e
g
t i n c r e a s e s r e a c h i n g t h e maximum a tCl-522 COLLOQUE DE PHYSIQUE
F i g . 4 - M i c r o s t r u c t u r e s and h i s t o g r a m s o f t h e g r a i n s i z e d i s t r i b u t i o n o f t h e T i A l a l l o y : a - s t a t e 1 1 1 ; b
-
s t a t e IV; c : A - s t a t e 1 1 1 , m-
s t a t e I V , o-
s t a t e V .F i g . 5 - T i A l m e c h a n i c a l p r o p e r t i e s : A - s t a t e 111,
-
s t a t e IV, 0 - s t a e V .750°C, b u t w i t h t h e f u r t h e r r i s e o f t e m p e r a t u r e it d e c r e a s e s . I n t h e same t e m p e r a t u r e i n t e r - v a l where b t , i n c r e a s e s , t h e growth o f
d
t a k e s p l a c e i n d i c a t i n g t h a t t h e t o u g h b r i t t l e t r a n - s i t i o n (TBT) is i n p r o g r e s s .The o r i g i n a l m i c r o s t r u c t u r e i n f l u e n c e s t h e a l l o y p r o p e r t i e s which depend t o a g r e a t e r e x t e n t
on t h e GB s t r u c t u r e t h a n on t h e g r a i n s i z e . T h u s , f o r e x a m p l e , s t a t e s I11 and I V b e i n g c o n s i - d e r a b l y d i f f e r e n t i n g r a i n s i z e s , have p r a c t i c a l l y i d e n t i c a l p r o p e r t i e s . A t t h e same t i m e i f we t a k e s t a t e s I V and V, t h e d i f f e r e n c e i n t h e i r p r o p e r t i e s is c o n n e c t e d w i t h t h e d i f f e r e n c e i n t h e GB s t r u c t u r e s i n c e g r a i n s i z e s i n t h e s e s t a t e s a r e q u i t e c l o s e . S t a t e s I11 a n d I V a r e a r e c h a r a c t e r i z e d by h i g h e r v a l u e s o f
d
andGt
a s c o ~ ~ i p a r e d t o s L a t e V . Only a t t>750°C,Gt
f o r s t a t e V i s h i g h e r t h a n i n s t a t e s I11 and I V ,
A b s o r p t i o n p r o p e r t i e s o f GB. The s t u d y o f t h e r e l a x a t i o n o f TLD i n GB d u r i n g a n n e a l i n g shows t h a t i n t w i n b o u n d a r i e s c o n t r a s t d u e t o TLD d o e s n o t d i s a p p e a r e v e n a f t e r t h e a n n e a l i n g a t llOO°C ( F i g . 6 a ) . A t 1000°C and more t h e TLD s y s t e m i n t w i n b o u n d a r i e s becomes o r d e r e d form- i n g q u a s i p e r i o d i c a l rows and n e t s . O t h e r b o u n d a r i e s which a r e o b v i o u s l y random a r e d e p r i v e d of TLD a l r e a d y a t 600°C ( F i g . 6 b ) .
F i g . 6
-
The r e l a x a t i o n o f TLD i n GB: a - t a n n = l l O O O C , t w i n boundary; b-
600°C, 15 m i n u t e s . R e s u l t s o b t a i n e d d u r i n g t h e s t u d y o f TLD r e l a x a t i o n i n GB a l l o w t o d e f i n e t h e f r a c t i o n o f t w i n and random b o u n d a r i e s i n s t a t e s I V and V o f t h e a l l o y . It was e s t a b l i s h e d t h a t t h e f r a c t i o n o f t w i n b o u n d a r i e s i n t h e s e s t a t e s c o n s t i t u t e s 3 and 50% r e s p e c t i v e l y , t h e r e s t b e i n g random b o u n d a r i e s . Hence, t h e s e r e s u l t s a g r e e w i t h t h o s e o b t a i n e d r n e t a l l o g r a p h i c a l l y .4
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DISCUSSIONI n t h e TBT i n t e r v a l and t h e SP c o n d i t i o n s t h e T i A l m e c h a n i c a l p r o p e r t i e s depend t o a g r e a t e x t e n t on t h e s t r u c t u r e o f GB. The growth o f t h e f r a c t i o n o f t w i n b o u n d a r i e s l e a d s t o a con- s i d e r a b l e i n c r e a s e o f t h e a l l o y p l a s t i c i t y above 600°C.
With t h e r i s e o f t e m p e r a t u r e t h e a b i l i t y o f GB t o a b s o r b d i s l o c a t i o n s becomes v e r y i m p o r t a n t f o r t h e growth o f p o l y c r y s t a l p l a s t i c i t y 1 4 , 5 1 . The s t u d y o f t h e a b s o r p t i o n p r o p e r t i e s o f GB h a s shown t h a t t w i n b o u n d a r i e s a r e l e s s e f f i c i e n t a s s i n k s f o r d i s l o c a t i o n s t h a n random boun- d a r i e s . T h a t i s why t h e g r o w t h o f t h e i r f r a c t i o n i n t h e a l l o y l e a d s t o t h e d e c r e a s e o f its p l a s t i c i t y i n t h e TBT i n t e r v a l above 600°C. Under t h e SP c o n d i t i o n s , a c c o r d i n g t o t h e model of g r a i n boundary d i s l o c a t i o n ' ( G B D ) motion k i n e t i c s , GB a c t n o t o n l y a s s i n k s f o r d i s l o c a t i o n s , b u t a l s o a s a " c h a n n e l " a l o n g which p l a s t i c d e f o r m a t i o n i s e f f e c t u a t e d t h r o u g h the motion o f GBD 1131.
S i n c e during t h e SP f l o w g r a i n boundary s l i d i n g i n t w i n b o u n d a r i e s is impeded d u e t o a low m o b i l i t y o f GBD i n t h e s e b o u n d a r i e s 1 4 , 5 1 , t h e increase o f t h e i r f r a c t i o n i n t h e a l l o y l e a d s t o t h e d e c r e a s e of t h e a l l o y p l a s t i c i t y u n d e r t h e g i v e n c o n d i t i o n .
According t o t h e e x p e r i m e n t , t h e TLD r e l a x a t i o n i n random b o u n d a r i e s o c c u r s a t t e m p e r a t u r e s which a r e l o w e r t h a n t h o s e o f TBT. A t t h e same t i m e t h e p r o c e s s of TLD r e l a x a t i o n i n GB i s known 1 4 , 5 1 t o b e i n d i c a t i v e o f t h e enhanced m o b i l i t y o f GB0 i n t h e b o u n d a r i e s . Hence, t h e g r o w t h o f t h e T i A l p l a s t i c i t y i n t h e p o l y c r y s t a l l i n e s t a t e w i t h t h e r i s e o f t e m p e r a t u r e i n t h e TBT i n t e r v a l i s r e l a t e d t o t h e enhancement o f t h e GBD m o b i l i t y i n GB.
5
-
CONCLUSIONS1. M e c h a n i c a l p r o p e r t i e s o f t h e T i A l i n t e r m e t a l l i c compound a r e l a r g e l y d e p e n d e n t on tt-estruc- t u r e o f g r a i n b o u n d a r i e s . The g r o w t h o f t h e f r a c t i o n o f t w i n b o u n d a r i e s i n t h e a l l o y c o n s i - d e r a b l y d e c r e a s e s i t s p l a s t i c i t y a t t e m p e r a t u r e s above 600°C.
2. The r e l a x a t i o n o f TLD i n T i A l o c c u r s a t t e m p e r a t u r e s which a r e l o w e r t h a n t h o s e o f TBT.
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