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PHASE TRANSFORMATIONS IN Ti50Ni50-xFex ALLOYS
M. Nishida, T. Honma
To cite this version:
M. Nishida, T. Honma. PHASE TRANSFORMATIONS IN Ti50Ni50-xFex ALLOYS. Journal de
Physique Colloques, 1982, 43 (C4), pp.C4-225-C4-230. �10.1051/jphyscol:1982429�. �jpa-00222143�
JOURNAL DE PHYSIQUE
CoZtoque C4, suppte'ment au n o
12,Tome
43,de'cembre
1 9 8 2page
C4-225PHASE TRANSFORMATIONS I N Ti50Ni50-,Fe, ALLOYS
M. Nishida and T.
onm ma*
Graduate SchooZ, Tohoku University, Japan
* ~ e s e a r c h I n s t i t u t e of Mineral Dressing and MetaZZurgy, Tohoku University, Sendai, Japan
(Accepted 9 August 1982)
A b s t r a c t .
-
The phase t r a n s f o r m a t i o n s i n T i 5 o N i 5 o-xFex a l l o y s have been s t u d i e d by means of e l e c t r i c a l r e s i s t i v i t y measurement, d i f f e r e n t i a l scanning calorim- e t r y (DSC), d i l a t o m e t r y , o b s e r v a t i o n of shape changes, o p t i c a l microscopy, t r a n s m i s s i o n e l e c t r o n microscopy (TEM) and e l e c t r o n d i f f r a c t i o n . During cool- i n g , t h e e l e c t r i c a l r e s i s t i v i t y r a p i d l y i n c r e a s e d a t M & ( s t a r t i n g temperature f o r t h e p a r e n t t o i n t e r m e d i a t e phase t r a n s f o r m a t i o n ) and t h e n reached maximum a t Ms ( s t a r t i n g temperature f o r t h e i n t e r m e d i a t e t o m a r t e n s i t e phase t r a n s f o r - m a t i o n ) . M's and Ms corresponded with temperatures of a b r u p t r i s i n g of two exothermic peaks and two breaks i n DSC and thermal expansion curve on c o o l i n g , r e s p e c t i v e l y . The shape changes d u r i n g h e a t i n g and c o o l i n g f o r T i 5 0 N i 4 7 . 5 F e 2 . 5 were devided i n t o two s t e p s . Correspondingly, two k i n d s of s u r f a c e r e l i e f were d i s t i n c t l y observed. In t h e p a r e n t t o i n t e r m e d i a t e phase t r a n s f o r m a t i o n , i t s m e t a l l o g r a p h i c f e a t u r e e x h i b i t e d t h e t h e r m o e l a s t i c n a t u r e . In e l e c t r o n d i f f r a c t i o n , many e x t r a r e f l e c t i o n s occured c l o s e t o o n e - t h i r d and one-haef p o s i t i o n s of t h e B2 r e c i p r o c a l l a t t i c e on c o o l i n g . A s an i n t e r m e d i a t e s t a t e , t h e n e e d l e shaped domains with l/3B2 e x t r a s p o t s were observed i n e l e c t r o n microscopy. Judging from morphology and temperature range of t h e s t r u c t u r a l change, t h e s e domains coincided w i t h t h e s u r f a c e r e l i e f of t h e p a r e n t t o i n - t e r m e d i a t e phase t r a n s f o r m a t i o n i n o p t i c a l microscopy. The c r y s t a l s t r u c t u r e , i n t e r n a l d e f e c t s and morphology of m a r t e n s i t e s were a l s o d e s c r i b e d . A s an ap- p l i e d i n v e s t i g a t i o n f o r development of p r a c t i c a l use of TiNi a l l o y s , t h e d r a s - t i c r e v e r s i b l e shape memory (RSM) e f f e c t a s s o c i a t e d with t h e two-step t r a n s - formation was found t o be o b t a i n e d by a c o n s t r a i n e d a g i n g i n N i r i c h TiNi a l l o y s .I n t r o d u c t i o n . - Recently, t h e p r a c t i c a l a p p l i c a t i o n s of shape memory e f f e c t s have been developed n o t only i n t e c h n o l o g i c a l b u t a l s o medical and d e n t a l f i e l d s . TiNi a l l o y i s one of t h e most e x c e l l e n t shape memory m a t e r i a l s . This a l l o y e x h i b i t s unique p h y s i c a l p r o p e r t i e s , depending on t h e composition and t h e h e a t t r e a t m e n t . By complete and incomplete thermal c y c l e s , Ms f a l l s and t h e i n t e r m e d i a t e phase w i t h rhombohedra1 d i s t o r t i o n appears between p a r e n t [B2] and m a r t e n s i t e phase [monoclinic
I .
In t h i s temperature r a n g e , t h e e l e c t r i c a l r e s i s t i v i t y remarkably i n c r e a s e s oncooling. M b i s d e f i n e d a s t h e temperature a t which e l e c t r i c a l r e s i s t i v i t y i n c r e a s - e s on cooling. These phenomena i n c l u d e d i f f u s e s c a t t e r i n g s t r e a k s , e x t r a d i f f r a c t i o n s p o t s a t l / 3 and 1/2 p o s i t i o n s of t h e B2 r e c i p r o c a l l a t t i c e , t h e i n t e n s i t i e s of which i n c r e a s e on cooling. When N i atoms a r e s u b s t i t u t i o n by Fe atoms, ,yL and Ms a r e c l e a r l y s e p a r a t e d [ l l . Therefore i t i s s u i t a b l e t o use TisoNisa-xFex f o r t h e s y n t h e s i z e d i n v e s t i g a t i o n of phase t r a n s f o r m a t i o n i n t h e TiNi a l l o y , e s p e c i a l l y t h e p a r e n t t o i n t e r m e d i a t e phase t r a n s f o r m a t i o n . The p r i n c i p a l purpose of t h e p r e s e n t work i s t o i n v e s t i g a t e t h e r o l e o f t h i s t r a n s f o r m a t i o n f o r t h e shape memory e f f e c t and i t s m e t a l l o g r a p h i c f e a t u r e s , and t o c l a r i f y t h e d r a s t i c RSM a s s o c i a t e d with two- s t e p t r a n s f o r m a t ions.
Experimental procedure.- Sample p r e p a r a t i o n and specimens f o r o p t i c a l and e l e c t r o n microscopy have been r e p o r t e d i n our p r e v i o u s papers [ 1 , 2 ] . The d e t e r m i n a t i o n s of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982429
C4-226 JOURNAL DE PHYSIQUE
t r a n s f o r m a t i o n t e m p e r a t u r e s were made'hy t h e e l e c t r i c a l r e s i s t i v i t y measurement and DSC. Thermal expansion was measured by a d i f f e r e n t i a l d i l a t o m e t e r u s i n g r e f e r e n c e sample o f q u a r t z rod. Specimens f o r shape change experiments were i n i t i a l l y formed i n t o a U-shape i n a s t a i n l e s s p i p e and then memorized i t s shape f o r 1 . 8 k s a t 773K, f i n a l l y quenched i n t o i c e water. To observe t h e shape change behaviour, t h e follow- i n g method was accepted [ 3 1 , t h e U-shaped specimen was n e a r l y s t r a i g h t e n e d by hand i n t h e mixture of e t h a n o l and dry i c e a t about 170K, and then warmed up i n t h e above mixture, while t h e shape change was monitored o p t i c a l l y and e l e c t r i c a l r e s i s t i v i t y was measured. The e l e c t r o n microscopic o b s e r v a t i o n was c a r r i e d o u t with JSEMZOOB o p e r a t i n g 200KV and f i t t e d a s i n g l e t i l t i n g c o o l i n g device. Generating procedure of RSM w i l l be d e c r i b e d l a t e r .
Dilatometry and shape change behaviour.- Figure 2 shows thermal expansion of TisoNi47.sFe2.5. A 1 i s t h e d i f f e r e n c e of expansion o r c o n t r a c t i o n between specimen and r e f e r e n c e sample, and 1 i s t h e l e n g t h of a specimen. In TisoNi47. sFe2 . s , grad- u a l c o n t r a c t i o n s t a r t s a t Ms' and then s e v e r e expansion o c c u r s a t Ms on c o o l i n g . I t seams- t h a t t h e s e r e s u l t s correspond t o t h e p a r e n t t o i n t e r m e d i a t e phase transforma- t i o n with rhombohedra1 d i s t o r t i o n and 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 , r e s p e c t i v e l y . The r e v e r s i b l e changes occur during h e a t i n g . Transformation temperatures c o i n c i d e with e l e c t r i c a l r e s i s t i v i t y changes and DSC r e s u l t s . Figure 3 shows t h e shape change be- haviour and corresponding e l e c t r i c a l r e s i s t i v i t y v s . temperature curve f o r
T i 5 0 N i 4 7 . 5 F e 2 . 5 . The e l e c t r i c a l r e s i s t i v i t y d i d n o t change d u r i n g s t r a i g h t e n i n g specimen. I t means t h a t t h e specimen have been transformed t o m a r t e n s i t e . The r e - Experimental r e s u l t s and d i s c u s s i o n .
I n Figure 1 broken and s o l i d l i n e s re- p r e s e n t e l e c t r i c a l r e s i s t i v i t y v e r s u s tem- p e r a t u r e and DSC c u r v e s , r e s p e c t i v e l y , f o r T i 5 o N i 5 -xFex. We d e f i n e d t h e i n d i c a t i o n s of t r a n s f o r m a t i o n temperature [ l l , a s shown i n F i g . l ( b ) . The e l e c t r i c a l r e s i s - t i v i t y d e c r e a s e s l i n e a r l y , and t h e n begins t o i n c r e a s e a t M ' s on c o o l i n g . A t MS t h e
A e l e c t r i c a l r e s i s t i v i t y i s maximum. Two a b r u p t r i s i n g s of peaks a r e observed i n
- = 5
DSC c u r v e s f o r T i S O N i 4 8 . 5 F e 1 . 5 and Tis0Ni4 ?. ~ F e 2 . ~ on c o o l i n g , corresponding E with MIs and Ms i n e l e c t r i c a l r e s i s t i v i t yg
curves. In T i 5 0 N i 4 6 ~ e 4 , M I S i s 216K, b u t Ms i s n o t observed u n t i l l i q u i d n i t r o g e n-
temperature. E l e c t r i c a l r e s i s t i v i t y be- g i n s t o a b r u p t l y i n c r e a s e a t As ( s t a r t i n g temperature f o r r e v e r s e t r a n s f o r m a t i o n of5
m a r t e n s i t e t o i n t e r m e d i a t e p h a s e ) , and de- c r e a s e a t A's ( s t a r t i n g temperature f o r r e - v e r s e t r a n s f o r m a t i o n of i n t e r m e d i a t e t o p a r e n t p h a s e ) , i n r e v e r s e t r a n s f o r m a t i o n on h e a t i n g . The temperature d i f f e r e n c e of As and A', i s narrow i n t h e specimen con- t a i n i n g low c o n c e n t r a t i o n s of Fe atoms.T h e r e f o r e , a s i n g l e peak appears on h e a t - ing i n DSC curve f o r T i S 0 N i b 8 . ~ F e 1 . ~ . AS Measurement of t r a n s f o r m a t i o n t e m ~ e r a t i n - e . -
Temperature I K
Fe atoms i n c r e a s e , s e p a r a t i o n of As and A's F i g . 1 DSC and e l e c t r i c a l r e s i s t i v i t y vs-becomes c l e a r e r a s shown i n F i g . l ( b ) . The temperature curve f o r TisoNiso-xFex- thermal h y s t e r e s i s i n t h e p a r e n t t o i n t e r - ( a ) X = 1 . 5 , ( b ) X = 2 . 5 , ( c ) X = 4 . mediate phase t r a n s f o r m a t i o n (A', : 296K -
MTs : 287K) and t h a t of i n t e r m e d i a t e t o m a r t e n s i t e (As : 264K - Ms : 246K) a r e 9K and 18K f o r T i 5 0 N i 4 7 . ~ F e 2 . 5 , r e s p e c t i v e l y . These r e s u l t s c o i n c i d e w i t h e l e c t r i c a l . r e s i s t i v i t y d a t a . The former t r a n s f o r m a t i o n a l s o h a s small thermal h y s t e r e s i s , which i s c l e a r i n DSC and e l e c t r i c a l r e s i s t i v i t y curve f o r TisoNiqsFes.
1.2-
1.0-
. 0.8
O.S.C.
( a )
-
/-*I . I
2 'L K
- -
,
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,',
5
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2 ,
I1.0-
__--
_ r --+
I I I
100 200 300 400
1 I I I
200 300 4w
Temperature I K
Fig.2 Thermal expansion d a t a f o r Ti50Ni97. 5Fe2.5.
I I I
I I
200 300 400
Tempemture IK
F i g . 3 The shape change and corresponding e l e c t r i c a l r e s i s t i v i t y v s . temperature curve f o r TiSONit,7. 5Fe2 . s .
covery a n g l e r a p i d l y d e c r e a s e s n e a r As and g r a d u a l l y b e g i n s t o reach c o n s t a n t value above A's d u r i n g h e a t i n g . During c o o l i n g , c l e a r two-step shape change o c c u r s . From t h e s e r e s u l t s , t h e p a r e n t t o i n t e r m e d i a t e phase t r a n s f o r m a t i o n c o n t r i b u t e s t o shape change, e s p e c i a l l y i n RSME, which a r e c o n s i s t e n t w i t h t h e a n o t h e r i n v e s t i g a t i o n [31.
O p t i c a l microscopy r e s u l t s . - The t y p i c a l examples of o p t i c a l micrographs observed f o r Ti50Nik7. 5Fe2 . 5 a r e shown i n F i g u r e 4. The s u r f a c e i s f l a t i n p a r e n t phase a t about 304K, e x c e p t f o r e t c h p i t s and i n c l u s i o n s , a s shown i n F i g . 4 ( a ) . A s t h e tem- p e r a t u r e d e c r e a s e s , a s t r a i g h t - b a n d e d r e l i e f g r a d u a l l y appears. S t a r t i n g temperature f o r t h i s s t r u c t u r a l change c o i n c i d e s with M I s . On f u r t h e r c o o l i n g , t h e banded r e - g i o n s expand i n h o r i z o n t a l and l o n g i t u d i n a l d i r e c t i o n s , and t h e i r c o n t r a s t s become c l e a r e r . A t n e a r 2 8 5 K corresponding t o A ' f , t h e banded r e l i e f suspends growth, a s shown i n F i g . 4 ( b ) . These bands c o n s i s t of a p a i r of l i g h t and dark c o n t r a s t s , and t h e l i g h t and dark bands a r e t w i n - r e l a t e d two v a r i a n t s of i n t e r m e d i a t e phase. Upon
Fig.4 O p t i c a l micrographs f o r T i 5 0 N i k 7 . 5 F e 2 . 5 .
f u r t h e r c o o l i n g , t h e s u r f a c e r e l i e f f o r m a r t e n s i t i e ap- p e a r s a t M s . Its morphology
i s s i m i l a r t o t h a t of mar- t e n s i t e i n TiNi. Reverse t r a n s f o r m a t i o n o c c u r s on subsequent h e a t i n g . F i r s t , m a r t e n s i t e r e l i e f complete- l y d i s a p p e a r s a t 296K, and then straight-banded r e l i e f d i s a p p e a r e s a t 306K. On t h e second thermal c y c l e , s t r a i g h t banded r e l i e f h a s a complete m i c r o r e v e r s i b i l i - t y b u t m a r t e n s i t e r e l i e f does n o t , a s shown i n Fig.4
( d ) . These r e s u l t s s u g g e s t t h a t t h e p a r e n t t o interme- d i a t e phase t r a n s f o r m a t i o n should be considered n o t a s a premonitory b u t a s an in- dependent phenomenon.
E l e c t r o n microscopy r e s u l t s . - The s e r i e s of d i f f r a c t i o n p a t t e r n s taken from t h e t h r e e most d e n s e l y r e c i p r o -
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Fig.5 Changes i n d i f f r a c t i o n p a t t e r n s Fig.6 Changes i n e l e c t r o n micrographs on c o o l i n g from 320K t o l O O K f o r f o r Ti5 o N i 1 , 7 . s F e 2 . 5 , corresponding t o Ti50Ni47. 5Fez. 5 . d i f f r a c t i o n p a t t e r n s i n Fig.5 ( a ) t o ( e )
.
c a l l a t t i c e p l a n e s f o r t h e B2 s t r u c t u r e i n Ti50Nirt7.5Fe2.5 on c o o l i n g , a r e reproduc- ed i n F i g u r e 5. F i g . 5 ( a ) t o ( e l , (f) t o (j), and ( k ) t o ( 0 ) a r e taken from [llO]B2,
F l l l l B 2 , and [0011B2 zone a x i s , r e s p e c t i v e l y . The d i f f r a c t i o n p a t t e r n s taken from p a r e n t phase a r e shown i n F i g . l ( a ) , (f), and ( k ) . The 1/3 110 e x t r a s p o t s come o u t a t v i c i n i t y of room temperature, a s shown i n Fig. 5 ( b )
,
ig),
and ( 1 ).
On c o o l i n g un- d e r M I s , t h e e x i s t i n g e x t r a s p o t s i n t e n s i f y , except f o r [llO]B2 zone a x i s , and new 1 / 3 e x t r a s p o t s make appearance, f o r example l / 3 l l l , l / 3 1 1 2 , e t c , a s shown i n Fig.5 ( c ) , ( h ) , and ( 9 ) . Appearance and disappearance of t h e 1 / 3 e x t r a s p o t s a r e very complicated. I t i s due t o domain e f f e c t s , which w i l l be d e s c r i b e d l a t e r . On f u r t h e r c o o l i n g j u s t above Ms temperature, t h e 1/2 e x t r a s p o t s emerge i n [001]B2 and [ l l l ] B 2 zone a x i s , a s shown i n F i g . 5 ( i ) and ( n ) , b u t t h e s e s p o t s can n o t b e observed i n [1101 B2 zone a x i s . The a l l p a r t s of specimen d o n ' t always transform i n above sequence.
Appearance of 1/2 e x t r a s p o t s is n o t f r e q u e n t . A t l a s t , t h e m a r t e n s i t e s p o t s come o u t under t h e Ms temperature, a s shown i n Fig. 5 ( e )
,
( j ),
and ( 0 ).
These and some o t h e r d i f f r a c t i o n p a t t e r n s taken from m a r t e n s i t e phase can b e indexed using t h e l a t -t i c e parameters given by Otsuka e t a l . [41. The c r y s t a l s t r u c t u r e i s d e s c r i b e d a s of d i s t o r t e d B19 o r modified 2H type with a monoclinic u n i t c e l l . Therefore t h e p r e s - ence of i n t e r m e d i a t e s t a t e h a s l i t t l e i n f l u e n c e on t h e c r y s t a l s t r u c t u r e of marten- s i t e . The i n t e r n a l d e f e c t s a l s o c o i n c i d e w i t h t h o s e of TiNi m a r t e n s i t e [ 2 , 4 ] . The b r i g h t f i e l d images i n Figure 6 ( a ) t o ( e l , correspond t o t h e d i f f r a c t i o n p a t t e r n s taken from [llOIB2 zone a x i s i n F i g . 5 i a ) t o ( e l , r e s p e c t i v e l y . E l e c t r o n micrograph a t about 320K proves t h a t t h e specimen a r e s i n g l e phase of B2 s t r u c t u r e a s shown i n
~ i g . 5 ( a ) and 6 ( a ) . On c o o l i n g t o room temperature, an obvious change i s n o t observ- ed i n b r i g h t f i e l d image a s shown-in F i g . G ( b ) , b u t f i n e i r r e g u l a r domains a r e observ- ed i n dark f i e l d image u s i n g 1/3110 e x t r a ' s p o t . On f u r t h e r c o o l i n g , n e e d l e shaped domains a r e observed with l / 3 1 i l ~ 2 r e f l e c t i o n , a s shown i n ~ i g . G ( c ) . I t seems t h a t t h e s e two phenomena correspond t o t h e incommensurate and commensurate rhombohedra1 phase, r e s p e c t i v e l y , which a r e r e p o r t e d by Wayman e t a l . [ 5 ] . An i n t e r m e d i a t e p r o d u c t appearing w i t h 1/2B2 e x t r a s p o t s i n [1113B2 and 10011B2 zone a x i s e x i s t s t o g e t h e r needle shaped domain i n very few i n s t a n c e s . I t s morphology i s i n d e f i n i t e shape and
Fig.7 Spontaneous and r e v e r s i b l e shape changes i n Ti4qNi
-- Fig.8 ( a ) E l e c t r o n micrograph taken 14.4ks. M : m a r t e n s i t e
5 1 . s c a l e : 50mm
~ o o k f o r Ti,,9Nis1 aging a t 673K f o r ( b ) 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 taken from ( a ) .
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DE
PHYSIQUEm i c r o s t r u c t u r e i s n o t c l e a r i n dark f i e l d Image, u s i n g 1/2B2 e x t r a s p o t s . According t o morphology and temperature range of appearance, n e e d l e shaped domains c o i n c i d e with t h e s t r a i g h t banded r e l i e f i n o p t i c a l micrographs. A s t h e temperature more de- c r e a s e s , t h e image of t h e needle shaped domains and i n t e n s i t y of l / 3 l i l ~ 2 r e f l e c - t i o n s become s t r o n g e r , and t h e n m a r t e n s i t e phase appears a c r o s s t h e n e e d l e shaped domains, a s shown i n F i g . G ( e ) .
RSM i n N i r i c h TiNi o b t a i n e d by c o n s t r a i n e d aging.- I n N i r i c h TiNi, it i s known t o p r e c i p i t a t e second phase p a r t i c l e s and e x h i b i t two-step t r a n s f o r m a t i o n s behaviour by means of a g i n g [61. S e v e r a l g e n e r a t i n g methods of RSM have common p r i n c i p l e s i n r e s p e c t of i n t r o d u c i n g i n t e r n a l s t r e s s f i e l d i n t o p a r e n t phase. T h e r e f o r e , it i s very l i k e l y t h a t above p r e c i p i t a t i o n s g e t i n t e r n a l s t r e s s f i e l d . Ribbon specimens
(3 x 0.2- 0.5 X 90mm3 ) of T i 4 9 N i 5 1 (MS = 175K) were prepared by c o l d work, which were homogenized a t 1073K f o r 7.2ks i n an evacuated q u a r t z and quenched i n t o i c e water.
They a r e b e n t i n a c i r c u l a r form and f i x e d by copper p i p e a s shown i n F i g u r e 7 ( b ) , and t h e n which a r e h e a t t r e a t e d a t 573
-
873K. F i g . 7 ( c ) t o ( x ) show spontaneous and r e v e r s i b l e shape changes i n Tik9Ni51 generated by c o n s t r a i n e d aging a t 573- 873K f o r 3.6ks. I n t h e case of a g i n g a t 573K, RSM o c c u r s , b u t it i s r e v e r s e phenomenon i n r e s p e c t of one-way memory. On a g i n g a t 673K and 773K, t h e specimens r e v e r s e themselves between Af' and M f , i . e . t h e c u r v a t u r e t u r n s upside down, a s shown Fig.7( i ) t o f n ) and (0) t o ( t ) . I t i s noteworthy t h a t a l a r g e amount o f shape change t a k e s p l a c e even i n t h e p a r e n t t o i n t e r m e d i a t e phase t r a n s f o r m a t i o n . On aging a t 873 K , t h e RSM does n o t occur because of a s i n g l e phase. The c h a r a c t e r and c a p a c i t y of RSM a r e markedly v a r i e d by a g i n g c o n d i t i o n , i n i t i a l c o n s t r a i n e d s t r a i n and composi- t i o n . Figure 8 shows e l e c t r o n micrograph taken a t l O O K f o r T i + 9 N i s l a g i n g a t 673K f o r 14.4ks. Banded c o n t r a s t s show m a r t e n s i t e s and small i r r e g u l h r p a r t i c l e s a r e second phase i n F i g . 8 ( a ) . Morphology of m a r t e n s i t e i s d i f f e r e n t from t h a t of s i n g l e phase, a s shown i n F i g . G ( e ) . F i g . 8 ( b ) shows 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 taken from F i g . B ( a ) . It i n d i c a t e s t h a t t h e p a r e n t , i n t e r m e d i a t e and m a r t e n s i t e phase c o e x i s t . The l/3B2 e x t r a s p o t s d e r i v e d from i n t e r m e d i a t e phase a r e observed along o n l y one (110) d i r e c t i o n . I t means t h a t t h e v a r i a n t of i n t e r m e d i a t e phase a r e arranged along a p r e f e r e n t i a l o r i e n t a t i o n by t h e i n t e r f a c i a l s t r a i n s between t h e m a t r i x and t h e p r e - c i p i t a t i o n s . These r e s u l t s s u g g e s t t h a t t h e t r a n s f o r m a t i o n behaviour and t h e mor- phology of i n t e r m e d i a t e and m a r t e n s i t e phase i n N i r i c h TiNi a r e a f f e c t e d by second phase p a r t i c l e s , which haveenough i n t e r n a l s t r a i n f i e l d s t o g e n e r a t e RSM.
Acknowledgment : The a u t h o r s would l i k e t o thanks D r . M. Matsumoto and M r . Y. Shugo a t Research I n s t i t u t e of Mineral Dressing and Metallurgy, Tohoku U n i v e r s i t y , f o r u s e f u l s u g g e s t i o n s . T h i s work i s p a r t l y supported by t h e Grant-in-Aid f o r Fundamen- t a l S c i e n t i f i c Research from t h e M i n i s t r y of Education of Japan.
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