Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb 2 C eutectic composites
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II (112)i~b
II [010]Nb2C or [120]Nb2C
and for the r o d - l i k e c a r b i d e morphology rod i n t e r f a c e ( m a j o r axis) I1{110}Nb IL (001)Nb2C growth d i r e c t i o n
II( l l 2 ) N b II [01011~b2C or [210]Nb2 C .
T h e t r a n s i t i o n i n morphology of the c a r b i d e phase is d i s c u s s e d i n t e r m s of the r e l a t i v e volume f r a c t i o n of the p h a s e s , growth r a t e , and o r i e n t a t i o n r e l a t i o n s h i p s . The c a r b i d e morphology is i n f l u e n c e d by the growth r a t e and c a r b o n content. F o r c o n s t a n t growth r a t e i n c r e a s i n g the v o l u m e f r a c t i o n of the c a r b i d e phase f a v o r s the l a m e l l a r morphology. At low growth r a t e s the l a m e l l a r morphology is favored, and at high growth r a t e s the r o d - l i k e m o r p h o l o g y is favored. Growth c r y s t a l l o g r a p h y has no d i r e c t influence on the t r a n s i t i o n i n c a r b i d e morphology,
IT
has b e e n d e m o n s t r a t e d for v a r i o u s eutectic s y s t e m s that in situ c o m p o s i t e growth under steady state conditions o c c u r s with a p r e f e r r e d r e l a t i v e c r y s t a l o r i e n t a t i o n between the s o l i d phases.X I n v e s t i g a t i o n s of in situ c o m p o s i t e s have focused on the growth c r y s t a l l o g r a p h y of eutectic p h a s e s s i n c e these s t u d i e s could lead to a b e t t e r u n d e r s t a n d i n g of the growth m o r p h o l o g i e s and s t a b i l i t y of the r e s u l t i n g m u l t i p h a s e s t r u c t u r e when s u b j e c t e d to v a r i o u s t h e r m a l and m e c h a n i c a l t r e a t m e n t s . D u r i n g the u n i d i r e c t i o n a l f r e e z i n g of e u t e c t i c s s o m e a l l o y s have b e e n o b s e r v e d to f r e e z e with a l a m e l l a r morphology, e'~ some to f r e e z e with a r o d - l i k e m o r phology, 4'5 and s t i l l o t h e r s to change morphology as the growth conditions o r c o m p o s i t i o n s a r e changed. 2'6'7 The o r i g i n s of the l a m e l l a r and rod m o r p h o l o g i e s have b e e n a s u b j e c t of c o n s i d e r a b l e s p e c u l a t i o n and e x p e r i m e n t a l study. R e s u l t s for v a r i o u s alloy s y s t e m s i n d i c a t e that morphology depends on a) growth r a t e , 4'6'7 b) i m p u r i t y e l e m e n t s , 8 c) r e l a t i v e volume f r a c t i o n s and a n i s t r o p y of s u r f a c e e n e r g i e s of the p h a s e s , 2'7'9 and d) growth out of p r e f e r r e d o r i e n t a t i o n as the r e sult of local or m a c r o s c o p i c i n t e r f a c e c u r v a t u r e . 8-n Influence of a l l t h e s e f a c t o r s on the t r a n s i t i o n m a y be r a t i o r m l i z e d i n t e r m s of m i n i m i z a t i o n of the u n d e r cooling at the solidifying i n t e r f a c e . The n i o b i u m - n i o b i u m c a r b i d e s y s t e m , which has a eutectic t e m p e r a t u r e of 2350~ exhibits a t r a n s i t i o n i n c a
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