Interdendritic structures of a directionally solidified cobalt-base alloy
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Table I. Composition of Co-Cr-Ni-Ta Alloy
Chemical Composttion, Wt Pct
Nominal composition Actual analysis
Co
C
Cr
Nt
Ta
Remainder Remainder
0.10 0.007
35.0 36.3
15.0 14.7
6.0 Not analyzed
S. J. DAPKUNAS
UNIDIRECTIONAL solidification of alloys has received considerable attention in recent years as one of the methods of enhancing the properties of materials s u i t a b l e for high t e m p e r a t u r e a p p l i c a t i o n s . P r e c i s i o n c a s t i n g t e c h n i q u e s using d i r e c t i o n a l s o l i d i f i c a t i o n show that 1 the p r o c e d u r e i m p r o v e s d u c t i l i t y , c r e e p p r o p e r t i e s , and t h e r m a l shock r e s i s t a n c e without l o s s of high temperature strength. A cobalt base alloy was selected for the p r e s e n t study b e c a u s e the a l l o y showed good s u l f i d a t i o n and oxidation r e s i s t a n c e f H o w e v e r , the a l l o y , w h o s e c o m p o s i t i o n i s shown in T a b l e I, is e x p e c t e d to contain e l e c t r o n compounds such a s a p h a s e a t 2200~ 2 and thus i s not v e r y s u i t a b l e for high t e m p e r a t u r e a p p l i c a t i o n s , a s d e m o n s t r a t e d by S i m s 3 and Sullivan. 4 T h e s e a u t h o r s have shown that t o p o l o g i c a l l y c l o s e p a c k e d p h a s e s , l i k e a, p r e c i p i t a t e in the f o r m of p l a t e l e t s and a r e u s u a l l y d e t r i m e n t a l to the m e c h a n i c a l p r o p e r t i e s of the a l l o y . H o w e v e r , it h a s b e e n r e c e n t l y shown by v a r i o u s a u t h o r s , 5-7 that t h e s e i n t e r m e t a l l i c c o m p o u n d s , if p r o p e r l y a l i g n e d by u n i d i r e c t i o n a l s o l i d i f i c a t i o n , can c o n t r i b u t e t o w a r d the o v e r a l l s t r e n g t h of the m a t e r i a l . T h i s note d e s c r i b e s the m i c r o s t r u c t u r e s p r o d u c e d in d i r e c t i o n a l s o l i d i f i c a t i o n of t h i s a l l o y at v a r i o u s s p e e d s and the v a r i a t i o n of m i c r o h a r d n e s s with r e s p e c t to the s p e e d of growth. The C o - C r - N i - T a a l l o y whose c o m p o s i t i o n i s shown in T a b l e I w a s o b t a i n e d in the f o r m of p i n s , -~1 in. in d i a m e t e r by 3 in. long. D i r e c t i o n a l s o l i d i f i c a t i o n of a 5 - i n . long s a m p l e w a s a c c o m p l i s h e d by d r a w i n g an a l u m i n a tube,(-~ in. d i a m ) , containing r e m e l t e d p i n s of the a l l o y t h r o u g h a g r a d i e n t f u r n a c e whose c o n s t r u c t i o n i s b a s e d on a p r e v i o u s d e s i g n . 8 The s a m p l e s w e r e grown u n d e r an a r g o n a t m o s p h e r e to r e d u c e oxidation and l o s s of c h r o m i u m . A f t e r growth, both l o n g i t u d i n a l and t r a n s v e r s e s e c t i o n s w e r e taken f r o m the m i d d l e of the s a m p l e for m i c r o s t r u c t u r a l e x a m i n a t i o n and h a r d n e s s t e s t s . D i r e c t i o n a
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