Structure and high temperature mechanical behavior of Ni-Ni 3 Nb unidirectional eutectic

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The s t r u c t u r e and m e c h a n i c a l b e h a v i o r of d i r e c t i o n a l l y s o l i d i f i e d Ni-NisNb e u t e c t i c h a v e b e e n i n v e s t i g a t e d . T h e l a m e l l a r e u t e c t i c g r o w s with the Ni-NisNb i n t e r f a c e s e m i c o h e r e n t and, within e x p e r i m e n t a l e r r o r , p a r a l l e l to c l o s e - p a c k e d p l a n e s in both p h a s e s . When t h e t e m p e r a t u r e i s l o w e r e d , s u p e r s a t u r a t i o n in the n i c k e l - r i c h p h a s e i s r e l i e v e d m o s t l y b y p r e c i p i t a t i o n . M e c h a n i c a l twinning i s t h e t y p i c a l d e f o r m a t i o n m o d e of the NisNb p h a s e and i s a l s o o b s e r v e d in the n i c k e l p h a s e b e t w e e n 700 ~ and 1000~ The u n u s u a l l y high d u c t i l i t y of the r e g u l a r s t r u c t u r e b e l o w 600~ i s r e l a t e d to t h e high s t r a i n h a r d e n i n g r a t e due to m e c h a n i c a l twinning of the i n t e r m e t a l l i c and the d i f f i c u l t y of c r a c k p r o p a g a t i o n .

B E C A U S E of t h e i r p r o m i s e a s high t e m p e r a t u r e m a terials,directionally solidified eutectics have received c o n s i d e r a b l e a t t e n t i o n in r e c e n t y e a r s . R e g u l a r , o r i e n t e d t w o - p h a s e s t r u c t u r e s a r e obtained which show e n h a n c e d t h e r m a l s t a b i l i t y up to t e m p e r a t u r e s c l o s e to t h e i r m e l t i n g point. 1'2 R e i n f o r c e m e n t of the m a t r i x by the f i b e r s o r p l a t e s of the s e c o n d p h a s e i s o b t a i n e d , a s r e p o r t e d by many i n v e s t i g a t o r s , s'4'but t h e s t r e n g t h ening m e c h a n i s m s h a v e r a r e l y been s t u d i e d in d e t a i l . Though m u l t i c o m p o n e n t s y s t e m s a r e of g r e a t e r p r a c t i c a l i m p o r t a n c e , s i m p l e b i n a r y e u t e c t i c s a r e of i n t e r e s t in o r d e r to i n v e s t i g a t e the growth m e c h a n i s m s and the m e c h a n i c a l b e h a v i o r of t h i s s p e c i a l k i n d of c o m p o s i t e m a t e r i a l . W h i l e t h i s i n v e s t i g a t i o n w a s in p r o g r e s s , Quinn e t a l . s r e p o r t e d s o m e r e s u l t s on the s t r u c t u r e and the m e c h a n i c a l p r o p e r t i e s of the Ni-NiaNb e u t e c t i c . T h e c r y s t a l s t r u c t u r e w a s d e s c r i b e d by the following statements:

by a s t a i n l e s s s t e e l rod, w a s f i t t e d t h r o u g h a w a t e r c o o l e d s t e e l r i n g which e n t e r e d into the hot zone, F i g . 1. A f t e r m e l t i n g the a s s e m b l y w a s d r a w n d o w n w a r d s at a c o n t r o l l e d r a t e . T h e t e m p e r a t u r e of the hot zone w a s c o n t r o l l e d by a t h e r m o c o u p l e . A s e c o n d one, p r o t e c t e d by a 3 m m OD a l u m i n a s h e a t h , e n t e r e d through the u p p e r h e a d of the f u r n a c e and w a s s e t c l o s

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