Deformation twins in the Ni 3 Nb phase

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Nb Phase

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C. GROSSIORD AND M. T U R P I N T H E i m p o r t a n c e of the {211} twinning mode of the i n t e r m e t a l l i c p h a s e in the t e n s i l e d e f o r m a t i o n of the N i NiaNb e u t e c t i c h a s b e e n r e p o r t e d in p r e v i o u s p a p e r s . 1'2 The s a m p l e s w e r e t e s t e d along t h e i r growth d i r e c t i o n which is v e r y n e a r l y p a r a l l e l to the [100] a x i s of NiaNb. A r e c e n t w o r k b y Gangloff a is in a g r e e m e n t with the r e p o r t e d twinning mode and shows c o n c l u s i v e l y that the {112} t y p e i n i t i a l l y p r o p o s e d b y Hoover and H e r t z b e r g 4 was to be a t t r i b u t e d to i n a d e q u a t e e x p e r i m e n t a l m e t h o d s . Along i d e a s i n i t i a l l y put f o r w a r d b y J a s w o n and Dove, 5 G r o s s i o r d et alfl a l s o p r o p o s e d a s h e a r i n g m e c h a n i s m in the i n t e r m e t a l l i c p h a s e which is c o n s i s t e n t with the o b s e r v e d twinning e l e m e n t s and the s y m m e t r y of the c r y s t a l l a t t i c e . F o r the (211) twin of the f i r s t kind, 7/1 is an i r r a t i o n a l twinning d i r e c t i o n n e a r to [i-0, 7, 13] and 7/2 is the r a t i o n a l [111] d i r e c t i o n . The s m a l l e s t p o s s i b l e s h e a r is about 0.4. R e a r r a n g e m e n t s of a t o m s follow t h i s s h e a r motion; t h e y a r e p e r p e n d i c u l a r to the K1 p l a n e f o r any a t o m in a 2n p l a n e and p a r a l l e l to K1 for any a t o m in a 2n + 1 p l a n e (Fig. 8 of Ref. 2). They have b e e n c o m p u t e d a s 0.37 and 0.50,~. The i n c r e a s e with t e m p e r a t u r e of the t h i c k n e s s of {211} twins h a s b e e n a s s u m e d to c o m e f r o m a d e c r e a s e in the v a l u e of the s h e a r f l However high t e m p e r a t u r e m e a s u r e m e n t s of the l a t t i c e p a r a m e t e r s 6 show, that for any type of twins, the s h e a r r e m a i n s n e a r l y independent of t e m p e r a t u r e , thus d i s p r o v i n g t h i s h y p o t h e s i s . F u r t h e r o b s e r v a t i o n s on the s a m e s y s t e m showed e v i d e n c e of a s e c o n d s y s t e m of d e f o r m a t i o n twinning, Fig. 2(a); t h e s e twins w e r e p r e s e n t o n l y in p r i m a r y c r y s t a l s of the i n t e r m e t a l l i c p h a s e which a p p e a r with i n s t a b i l i t i e s in the growth c o n d i t i o n s of the d i r e c t i o n a l l y s o l i d i f i e d e u t e c t i c . The twins w e r e a l m o s t p a r a l l e l to the growth and t e n s i l e d i r e c t i o n , which a r e s l i g h t l y d i s o r i e n t e d with r e s p e c t to the [100] d i r e c t i o n of Ni3Nb. Using an 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 with a zone a x i s , the twinning p l a n e K1 was i d e n t i f i e d a s (011), F i g s . l ( a ) and (b). Since the s y m m e t r y of the c r y s t a l m u s t r e m a i n unchanged b y twinning, the p l a n e of s y

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