Some observations on the growth of directionally solidified eutectic alloy

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Some Observations on the Growth of Directionally Solidified Eutectic Alloy O. P. ARORA Attention has r e c e n t l y b e e n f o c u s e d on both n a t u r a l and m a n - m a d e c o m p o s i t e s to p r o v i d e i m p r o v e d m a t e r i a l s f o r f u t u r e a d v a n c e d gas t u r b i n e e n g i n e s . It has b e e n w i d e l y d e m o n s t r a t e d that a l l o y s g r o w n by d i r e c tional s o l i d i f i c a t i o n p o s s e s s v e r y a t t r a c t i v e p r o p e r t i e s when c o m p a r e d to the c o n v e n t i o n a l m o d e s of f a b r i c a t i o n . T h e r m a l s t a b i l i t y at v e r y high t e m p e r a t u r e s m a k e s this c l a s s of n a t u r a l c o m p o s i t e s p r i m e c a n d i d a t e s f o r f u t u r e high t e m p e r a t u r e s t r u c t u r a l m a t e r i a l s . The p r e s e n t note d e s c r i b e s s t r u c t u r a l v a r i a t i o n s o b s e r v e d in d i r e c t i o n a l l y s o l i d i f i e d C o - C r e u t e c t i c at v a r i o u s s o l i d i f i c a tion s p e e d s and its m i c r o s t r u c t u r a l s t a b i l i t y at e l e v a t e d temperatures. A n u m b e r of s t u d i e s h a v e r e c e n t l y b e e n m a d e to d e v e l o p c o b a l t b a s e c o m p o s i t e m a t e r i a l u s in g 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 t e c h n i q u e s to obtain d e s i r e d p r o p e r t i e s . 1-4 The a p p r o a c h in t h e s e s t u d i e s h a s b e e n e i t h e r to u se m o n o c a r b i d e s as r e i n f o r c i n g m a t e r i a l s f o r s t r e n g t h e n ing o r using m o n o v a r i a n t e u t e c t i c s o l i d i f i c a t i o n to o b tain d e s i r e d c o m p o s i t e s . The a p p r o a c h u s e d in this i n v e s t i g a t i o n was to s e l e c t a e u t e c t i c a l r e a d y p o s s e s s i n g good hot c o r r o s i o n r e s i s t a n c e and c o n t a i n i n g a p h ase which would p r o v i d e the d e s i r e d m e c h a n i c a l p r o p e r t i e s when p r o p e r l y aligned. The p r e s e n t s t u d i e s looked at 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 C o - C r e u t e c t i c (Te = m e l t i n g point = 1400~ e f f e c t of s p e e d of g r o w t h on m i c r o s t r u c t u r e s and t h e r m a l s t a b i l i t y at e l e v a t e d t e m peratures. M a s t e r a l l o y s of c o b a l t - c h r o m i u m (Co-42.5 wt p ct Cr) w e r e p r e p a r e d by a r c m e l t i n g buttons f r o m high p u r i t y c o b a l t (99.98 pct) and c h r o m i u m (99.98 pet). The a r c m e l t e d buttons w e r e h o m o g e n i z e d f o r one w e e k in a high v a c u u m f u r n a c e . The buttons w e r e then cut into s m a l l p i e c e s and d i r e c t i o n a l l y s o l i d i f i e d in 1/4 in. d i a m a l u m i n a tubes in a f u r n a c e , d e s c r i b e d e l s e w h e r e in the l i t e r a t u r e . 5 The 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 was a c c o m p l i s h e d by l o w e r i n g a 12 in. long c r u c i b l e t h r o u g h

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