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MARTENSITIC TRANSFORMATIONS IN A TiNi THIN FOIL
P. Moine, E. Goo, R. Sinclair
To cite this version:
P. Moine, E. Goo, R. Sinclair. MARTENSITIC TRANSFORMATIONS IN A TiNi THIN FOIL.
Journal de Physique Colloques, 1982, 43 (C4), pp.C4-243-C4-248. �10.1051/jphyscol:1982432�. �jpa-
00222146�
MARTENSITIC TRANSFORMATIONS I N A TiNi THIN FOIL
P . Moine, E. GOO* and R. s i n e l a i r *
l h i v e r s i t e ' de P o i t i e r s , Laboratoire de Me'taZZurgie Physique, 86022 P o i t i e r s , France
*&oxford University, Department o f Materials Science, Stanford University, Stanford, CaZifornia 94305, U.S.A.
(Accepted 9 August 1982)
A b s t r a c t . - The i n t e r m e t a l l i c compound TiNi h a s l o n g been known t o e x h i b i t "ano- m a l i e s " above t h e M t e m p e r a t u r e
-
t h e s o - c a l l e dre-martensitic
phenomena.I n p a r t i c u l a r e l e c t g o n d i f f r a c t i o n p a t t e r n s show e x t r a r e f l e c t i o n s and t h e Trans- m i s s i o n E l e c t r o n Microscopy (TEM) images some m o t t l i n g , "tweed-like" s t r u c t u r e . . . However, i t i s s u s p e c t e d t h a t some of t h e s e e f f e c t s a r e n o t p r e m o n i t o r y f o r t h e m a r t e n s i t i c t r a n s f o r m a t i o n i n t h e b u l k m a t e r i a l . R e c e n t l y i t was p o i n t e d o u t
t h a t e x t r a r e f l e c t i o n s o b s e r v e d i n Cu-%n-A1 e l e c t r o n d i f f r a c t i o n p a t t e r n s were due t o a s u r f a c e e f f e c t . To c l a r i f y a l l t h e s e p r e t r a n s i t i o n phenomena i n TEM s p e c i m e n s , we have c a r r i e d o u t an i n v e s t i g a t i o n on p h a s e t r a n s f o r m a t i o n s i n a Ti-50.5 a t % N i t h i n f o i l . I n o r d e r t o know t h e i n f l u e n c e o f t h e i n i t i a l s t a t e a t Room Temperature (RT) on t h e r r a r t e n s i t i c f o r m a t i o n i n a t h i n f o i l d u r i n g a n
" i n s i t u " c o o l i n g p r o c e s s , d i f f e r e n t t h e r m a l t r e a t m e n t s were performed i n t h e microscope. TEM samples h e a t e d up t o 400°C and c o o l e d down t o RT e x h i b i t i n t h e i r e l e c t r o n d i f f r a c t i o n p a t t e r n s s t r o n g 112 100 and 114 210 t y p e e x t r a r e f l e c t i o n s , and do n o t t r a n s f o r m i n t o m a r t e n s i t e when t h e y a r e c o o l e d down t o L i q u i d N i t r o g e n
(LN) t e m p e r a t u r e . The same t h e r m a l t r e a t m e n t performed on t h e b u l k d o e s n o t g i v e r i s e t o t h e s e e x t r a r e f l e c t i o n s and does n o t s u p ? r e s s t h e m a r t e n s i t i c t r a n s f o r - mation when t h e specimen i s c o o l e d d o , . t o LM t e m p e r a t u r e . I t i s t h o u g h t t h a t t h i s t h i n f o i l b e h a v i o r i s due t o c o n t a m i n a t i o n d u r i n g t h e " i n s i t u " t h e r m a l t r e a t m e n t . I n t e r s t i t i a l o r d e r i n g of atoms ( ~ o s s i b l y oxygen), which would adequa- t e l y a c c o u n t f o r t h e e x p e r i m e n t a l o b s e r v a t i o n s , i s s u g g e s t e d .
I n t r o d u c t i o n . - E l e c t r o n d i f f r a c t i o n p a t t e r n s of t h e B2 TiNi phase c o n t a i n , i n a d d i t i o n t o t h e B2 s p o t s , many e x t r a r e f l e c t i o n s and some s t r e a k i n g e f f e c t s (1-6). These phenomena have been a t t r i b u t e d t o new o r d e r e d p h a s e s (7-9) o r t o a l a t t i c e dynamical i n s t a b i l i t y o r s o - c a l l e d s o f t phonon mode (1-6) and have o f t e n been t h o u g h t t o a r i s e from p r e m a r t e n s i t i c o r p r e m o n i t o r y e f f e c t s . I I o w e v e r , t h i s b e h a v i o r i s n o t y e t w e l l u n d e r s t o o d and i s q u i t e complex. Moreover, t h i n f o i l e f f e c c s must be t a k e n i n t o a c c o u n t , when T r a n s m i s s i o n E l e c t r o n Microscopy (TEM) e x p e r i m e n t s a r e performed.
I n a ~ r e v i o u s p a p e r (4) we n o t i c e d t h a t some e x t r a r e f l e c t i o n s which we a n a l y z e d a s L a t t i c e Displacement Waves (LDIJ) a r e u n r e l a t e d t o t h e major p h a s e t r a n s f o r m a t i o n and s o c o u l d n o t b e rem monitory phenomena ( v i z . 112 100, 114 210 and p o s s i b l y 113 110 t y p e r e f l e c t i o n s ) . I n t h i s p a p e r , we a t t e m p t t o d e t e r m i n e t h e o r i g i n o f t h e s e l a t t e r supplementary s p o t s d u r i n g a s t u d y of phase t r a n s f o r - mations i n a TiNi t h i n f o i l . H e a t i n g and c o o l i n g e x p e r i m e n t s a r e performed
1 1 .
I n s i t u " i n t h e microscope. Some comparison w i t h t h e b u l k m a t e r i a l i s a l s o pre- s e n t e d .
E x p e r i m e n t a l p r o c e d u r e .
Ti-50.5 a t % N i s t r i ~ s were ? r o v i d e d by Qaychem Co (:?e1110 P a r k , U.S.A.).
They were a n n e a l e d a t 9 0 0 t f o r 10 m i n u t e s and water-quenched. The l a y e r o f o x i d e was c h e m i c a l l y removed. D i s c s 3 m i n d i a m e t e r f o r TEM o b s e r v a t i o n s were s p a r k
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982432
C4-244 JOURNAL DE PHYSIQUE
c u t , then mechanically thinned and e l e c t r o p o l i s h e d a t room temperature using a p e r c h l o r i c - a c e t i c a c i d s o l u t i o n . The TEM experiments were performed with a P h i l i p s EM 400, 120 kv microscope equipped with c o o l i n g and h e a t i n g s t a , ~ e s .
R e s u l t s and d i s c u s s i o n .
"In s i t u " h e a t i n g experiments : A t Room Temperature (RT), t h e a l l o y i s mainly m a r t e n s i t i c with some a u s t e n i t i c g r a i n s . Figure 1 i s t y p i c a l of t h e micro-struc- t u r e a t RT. When a TEM sample i s h e a t e d t h e m a r t e n s i t e d i s a p p e a r s around 100°C and l e a v e s "ghost m a r t e n s i t e " (10) (Fig. 2 ) .
Pig. 1 : Typical f e a t u r e s of T i M i micro- Fig. 2 : Bright F i e l d (BF) and SAD s t r u c t u r e a t RT showing i n the t o p l e f t p a t t e r n showing "ghost m a r t e n s i t e "
hand c o m e r a g r a i n of o x i d e and i n t h e ( i n a specimen heated up t o 219OC) low l e f t hand corner m a r t e n s i t e ~ l a t e s and e x t r a r e f l e c t i o n s among t h e B2 and i n s e t d i f f r a c t i o n p a t t e r n r e l a t e d r e f l e c t i o n s
.
t o t h e l a t t e r .
When t h e temperature reaches 3 0 0 " ~ , theTTp,host m a r t e n s i t e " has d i s a p p e a r e d , t h e sample becomes f u l l y a u s t e n i t i c and t h e S e l e c t i o n Area D i f f r a c t i o n p a t t e r n s show e x t r a r e f l e c t i o n s t o g e t h e r with B2 r e f l e c t i o n s (Fig. 3 ) . These e x t r a re- f l e c t i o n s a r e d i f f u s e and elongated along < l o o > o r <110> d i r e c t i o n s . We can no- t i c e e x t r a r e f l e c t i o n s l o c a t e d a t h a l f of t h e < l o o > fundamental spacings ( c a l - l e d 112 100 r e f l e c t i o n s ) a t 114 of t h e <210> fundamental spacings ( c a l l e d
114 210 r e f l e c t i o n s ) and a t 113 and 1/2 of t h e <110> fundamental s p a c i n g s ( c a l l e d
Fig. 3 : B.F. and SAD p a t t e r n i n a specimen heated up t o 3 2 3 ' ~ showing small i s o l a t e d r e g i o n s and 112 100, 114 210 e x t r a r e f l e c t i o n s r e s p e c t i - v e l y .
( o r 112 010) r e f l e c t i o n i n support of t h i s c o n t e n t i o n . The 112 100 r e f l e c t i o n s a r e l o c a t e d s l i g h t l y away from r a t i o n n a l p o s i t i o n s . The 113 110 r e f l e c t i o n s a r e observed when t h e specimen i s s l i g h t l y t i l t e d from a p e r f e c t Bragg o r i e n t a t i o n
d
Fig. 4 : a) P e r f e c t 111 zone a x i s o r i e n t a t i o n SAD p a t t e r n - b) t i l t e d
-
6' - c) t i l t e d + 6"-
d) r e l a t e d B.F. from a specimen heated up t o 323OC.I n o r d e r t o answer t h e q u e s t i o n : " a r e t h e regions which give r i s e t o t h e s e e x t r a r e f l e c t i o n s , s u r f a c e e f f e c t s ?"Centered Dark f i e l d s t e r e o - p a i r images have been taken (1 l o a p a r t ) u s i n g t h e s t r o n g e x t r a r e f l e c t i o n 312 100 and t h e super- l a t t i c e r e f l e c t i o n 100. The s m a l l p a r t i c l e s r e l a t e d t o t h e 312 100 r e f l e c t i o n ( a s e s t a b l i s h e d by a 2 112 D experiment ( 5 ) ) a r e seen a t d i f f e r e n t h e i g h t s when examined i n a s t e r e o viewer, and i t i s concluded t h e r e f o r e t h a t they a r e n o t s u r f a c e phenomena.
Cken t h e temperature reaches 400°C ( i n t h e microscope) t h e i n t e n s i t y of t h e 112 100 type r e f l e c t i o n s and 114 210 type r e f l e c t i o n s i n c r e a s e s ( F i g . 5 ) ; they become s h a r p e r b u t t h e i r p o s i t i o n s remain t h e same. A t t h e same time, t h e number of small p a r t i c l e s a s s o c i a t e d with t h e s e r e f l e c t i o n s i n c r e a s e s considera- b l y . When t h e sample i s cooled down t o R.T. t h i s new s t r u c t u r e remains p r e s e n t and 113 (110) r e f l e c t i o n s appear. I f t h e i n i t i a l s t a t e of t h e specimen a t R.T. i s f u l l y a u s t e n i t i c (depending on the thermal h i s t o r y ) , t h e behavior i s t h e same e x c e p t , of course, a t t h e beginning of t h e h e a t i n g p r o c e s s .
Fi5.5 : B.F. and SAD p a t t e r n i n a W s p e c i m e n heated up t o 398OC
showing a l o t of small p a r t i c l e s and s t r o n g e x t r a r e f l e c t i o n s . Several v a r i a n t s of l a t t i c e displacement wa- ves a r e p r e s e n t : 112 <100><100>,
112 <010><010> and f o u r v a r i a n t s of 115 <210> waves.
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" ~ n s i t u " c o o l i n g e x p e r i m e n t s : The s p e c i m e n , h e a t e d up t o 400°C and c o o l e d down t o R.T.,does n o t t r a n s f o r m t o m a r t e n s i t e when it i s cooled down t o L i q u i d N i t r o g e n
(LN2) t e m p e r a t u r e . But i n a sample n o t h e a t e d b e f o r e t h e c o o l i n g p r o c e s s a u s t e n i - t i c g r a i n s surrounded by m a r t e n s i t i c g r a i n s w i l l t r a n s f o r m t o "wavy m a r t e n s i t e "
( F i n . 6 a . b ) .
k'lg. b : a) B.F. showing an a u s t e n i t e F i g . 7 : T.E.M. TiNi specimen h e a t e d g r a i n s u r r o u n d e d by m a r t e n s i t e a t R.T. and =2oo0c and c o o l e d down t o LN r e l a t e d SAD p a t t e r n . b) B.F. showing wavy t e m p e r a t u r e : a ) B.F. showing d e n d r i - m a r t e n s i t e a t LN t e m p e r a t u r e and r e l a t e d t i c s t r u c t u r e s and r e l a t e d SAD p a t - SAD p a t t e r n . c )
it
R.T. t h e wavy m a r t e n s i t e t e r n . b ) B.F. showing " ~ a t c h w o r k h a s d i s a p p e a r e d i n t h e i n i t i a l a u s t e n i t e q u i l t " s t r u c t u r e and r e l a t e d SAD p a t -g r a i n . t e r n . c ) B.F. showing d e n d r i t i c s t r u c -
t u r e s ; t h e r e l a t e d SAD p a t t e r n s show s t r o n g 113 110 r e f l e c t i o n s .
These s t r u c t u r e s d i s a p p e a r a t R.T. This behavior i s d i f f e r e n t from t h a t of t h e bulk m a t e r i a l because r e g a r d l e s s of t h e thermal h i s t o r y , Ti-50.5 % a t N i a l l o y transforms t o m a r t e n s i t e upon c o o l i n g t h e b u l k m a t e r i a l down t o LN temperature.
I n o r d e r t o know i f t h e small r e g i o n s r e l a t e d t o 112 100 and 112 2?0 r e f l e c t i o n s could be r e s p o n s i b l e f o r t h e s u p p r e s s i o n of t h e m a r t e n s i t i c t r a n s f o r m a t i o n , bulk m a t e r i a l was heated up t o 4 0 0 ' ~ f o r 112 hour and cooled down t o R.T. A f t e r e l e c - t r o p o l i s h i n n f o r T.E.M., we d i d n o t n o t i c e 112 100 and 114 210 type r e f l e c t i o n s b u t 113 110 t y p e r e f l e c t i o n s a r e p r e s e n t i n a[001]zone a x i s o r i e n t a t i o n S e l e c t i o n Area D i f f r a c t i o n (S.A.D.) p a t t e r n . So it appears t h a t t h e 112 100 and 114 210 r e f l e c t i o n s a r e due t o a t h i n f o i l e f f e c t b u t n o t t h e 113 110 r e f l e c t i o n s . I t can be thought t h a t t h e r e i s a contamination of t h e whole t h i n f o i l which a f f e c t s t h e subsequent m a r t e n s i t i c t r a n s f o r m a t i o n . I n t h e bulk m a t e r i a l t h i s contamina- t i o n w i l l occur only on t h e s u r f a c e and s o , w i l l n o t a f f e c t t h e m a r t e n s i t e forma- t i o n . I n f a c t , T.E.M. samples l e f t a t R.T. f o r s e v e r a l months show i n t e n s e 112 100 and 1/4 2 1 3 type r e f l e c t i o n s . This contamination i s a p p a r e n t l y due t o t h e ambient atmosphere of t h e specimen and s o could be due t o oxygen, n i t r o g e n
...
atoms. Wasilewski (11) h a s demonstrated t h a t i n c r e a s i n g t h e oxynen contamina- t i o n l e v e l can lower t h e M temperature of TiNi. I n t e r s t i t i a l atom o r d e r i n g ( p o s s i b l y oxygen) could prgduce s t a t i c l o n g i t u d i n a l displacement waves which would adequately account f o r t h e experimental o b s e r v a t i o n s . For i n s t a n c e , f i g u r e 8 a , we show i n a diagram how atoms i n o c t a h e d r a l i n t e r s t i c e s of BCC TiNi can g i v e r i s e t o a 112 [loo] [loo] LDW and so e x p l a i n why t h e 112 100 s p o t i n t e n s i t y i s lower t h a n t h e 312 100 s p o t i n t e n s i t y . P e r i o d i c p l a n e f a u l t s i n t h i s s t r u ~ t u - r e , l y i n g i n every o t h e r (010) p l a n e (Fig. 8 b) g i v e r i s e t o LDrJ's, with a q v e c t o r 114 <210> which could accnimt f o r 114 210 e x t r a r e f l e c t i o n s . W e t h e r o r n o t t h i s model f u l l y accounts f o r a l l t h e observed r e f l e c t i o n s i n 3-D r e c i p r o c a l s?a- c e remains t o be seen. 113 110 type r e f l e c t i o n s which appear i n[ o o ~ ]
,[I 111and[ll0] zone a x i s o r i e n t a t i o n S.A.D. p a t t e r n a t room temperature occur i n T.E.M.
specimens thermally t r e a t e d i n t h e microscope a s w e l l a s i n t h e bulk m a t e r i a l . Chandra e t Purdy ( 6 ) have a l r e a d y observed t h e s e e x t r a r e f l e c t i o n s , assuming t h a t they appear a s a r e s u l t of i n t e r s e c t i o n s of s p i k e s o r s t x e a k s with t h e Ewald s?he- r e and i n t e r p r e t i n g them i n terms of " s o f t phonon"<321><111>. These e x t r a r e f l e c - t i o n s , a l s o , o f t e n p r e s e n t a t R.T. a r e n o t premonitory f o r t h e m a r t e n s i t i c t r a n s - formation ( 4 , 5 ) , b u t r a t h e r c h a r a c t e r i s t i c of a n o t h e r n e t a s t a b l e phase which may compete with t h e m a r t e n s i t i c t r a n s f o r m a t i o n .
When t h e T.E.M. specimen i s only heated up t o 200°c-250°C, we can assume t h a t t h e contamination i s l e s s r a p i d : t h e 112 100 and 114 210 supplementary re- f l e c t i o n s a r e n o t very s t r o n g (FiG.7 c ) .
4-1 pool
7 2
,
+
\I2
001~ i g . 8 a : Diagram showing how i n t e r s - Fig. 8 b : Diagram showing how p e r i o d i c t i t i a l o r d e r i n g of atoms can c r e a t e a p l a n e d e f e c t (010) (period = 2a) can L.D.W. 112 100 100
.
c r e a t e i n t h e s t r u c t u r e of t h e f i g u r e114 210
.
JOURNAL DE PHYSIQUE
However, d u r i n g t h e c o o l i n g p r o c e s s t h e u s u a l wavy m a r t e n s i t e does n o t form even a t LN2temperature, b u t a d e n d r i t i c and a "patchwork q u i l t " s t r u c t u r e a p p e a r . These s t r u c t u r e s d i s a p p e a r when t h e specimen i s h e a t e d up a g a i n t o R.T. T h i s b e h a v i o r s u g g e s t s t h e f o r m a t i o n of a new m a r t e n s i t e whose s t r u c t u r e p r e s e n t s some s i m i l a r i t i e s w i t h t h e 3R m a r t e n s i t e d e s c r i b e d by Gupta and a l . ( 1 2 ) . We do n o t know i f t h i s new s t r u c t u r e competes w i t h t h e u s u a l m a r t e n s i t e , o r i s premoni- t o r y f o r t h e m a r t e n s i t e f o r m a t i o n below LN7 t e m T e r a t u r e , o r s u p p r e s s e s t h e u s u a l m a r t e n s i t i c t r a n s f o r m a t i o n . I t i s w o r t h n o f i c i n g t h a t t h e s t r o n g 113 110 t y p e e x t r a r e f l e c t i o n s a s s o c i a t e d w i t h t h e d e n d r i t i c s t r u c t u r e s observed t o g e t h e r w i t h t h e fundamental B2 s p o t s , a p p e a r i n p e r f e c t Bragg o r i e n t a t i o n SAD p a t t e r n s which i s t h e o p p o s i t e of t h e ! / 3 110 e x t r a r e f l e c t i o n s when t h e specimen i s a t R.T. o r above. I t may a l s o be n o t i c e d t h a t 113 112 r e f l e c t i o n s appear i n [ l 1 1 1 zone a x i s o r i e n t a t i o n SAD p a t t e r n s .
Conclusion.
T.E.M. phenomena observed above M t e m p e r a t u r e i n a TiNi t h i n f o i l have t o b e a n a l y z e d v e r y c a r e f u l l y ; varkoious phe:omena o c c u r c o n c u r r e n t l y s p e c i a l l y i n T.E.M. specimens. We ~ o i n t o u t , f o r i n s t a n c e , t h a t 112 100 and 114 210 t y p e ex- t r a r e f l e c t i o n s o b s e r v e d i n a <001> zone a x i s o r i e n t a t i o n SAD p a t t e r n a f t e r hea- t i n g t h e specimen " i n s i t u " up t o 400°C a r e n o t observed i n t h e b u l k m a t e r i a l a f t e r t h e same t h e r m a l t r e a t m e n t . I t i s t h o u g h t t h a t t h e s e e f f e c t s may a r i s e from c o n t a m i n a t i o n o f t h e whole t h i n f o i l ( p o s s i b l y i n t e r s t i t i a l o r d e r i n g o f oxygen atoms) which w i l l s u p p r e s s t h e m a r t e n s i t e f o r m a t i o n d u r i n g a s u b s e q u e n t c o o l i n g p r o c e s s down t o LN t e m p e r a t u r e ( o r d e p r e s s t h e Ms t e m p e r a t u r e below LN2 t e m p e r a t u r e ) . Conversely, $he 113 110 t y p e e x t r a r e f l e c t i o n s a r e o b s e r v e d i n T.E.M. specimens and b u l k m a t e r i a l a t room t e m p e r a t u r e . We do n o t know y e t what r o l e t h i s i n t e r m e d i a t e s t a t e p l a y s i n t h e m a r t e n s i t i c t r a n s f o r m a t i o n .
I n a t h i n f o i l a s opposed t o t h e b u l k m a t e r i a l t h e m a r t e n s i t e f o r m a t i o n i s v e r y s e n s i t i v e t o i t s t h e r m a l h i s t o r y . It i s t h o u g h t t h a t t h i s i s due t o a t h i n f o i l c o n t a m i n a t i o n d u r i n g t h e " i n s i t u " t h e r m a l t r e a t m e n t s . Depending on t h e c o n t a m i n a t i o n l e v e l new m a r t e n s i t i c s t r u c t u r e s can a p p e a r d u r i n g t h e c o o l i n g p r o c e s s o r t h e r e may b e no t r a n s f o r m a t i o n a t a l l .
Acknowledgements: F i n a n c i a l s u p p o r t f o r t h i s work was p r o v i d e d by t h e NSF-MRL program through t h e C e n t e r o f M a t e r i a l s Research, S t a n f o r d U n i v e r s i t y . A s s i s t a n - c e from Raychem Co was g r e a t l y a p p r e c i a t e d . One o f us (PM1) i s v e r y g r a t e f u l t o P r o f e s s o r R. S i n c l a i r f o r h i s s u p p o r t w h i l e d o i n g r e s e a r c h i n h i s l a b o r a t o r y a t S t a n f o r d U n i v e r s i t y .
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