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K. P. YOUNG,

AND

M. C. FLEMINGS

S i m p l e t h e o r i e s d e v e l o p e d in a s e r i e s of r e c e n t p a p e r s , d e s c r i b i n g s o l i d / l i q u i d i n t e r f a c e s t a b i l i t y and solute r e d i s t r i b u t i o n in t h r e e p h a s e t e r n a r y a l l o y s a r e s u m m a r i z e d and e x tended to a p p l y to t e r n a r y a l l o y s that a r e two p h a s e on e q u i l i b r i u m s o l i d i f i c a t i o n . C a l c u l a t e d r e s u l t s c o m p a r e w e l l with e x p e r i m e n t s on a l l o y s f r o m the A l - r i c h c o r n e r of the A 1 - C u - N i t e r n a r y p h a s e d i a g r a m . T h e s e a l l o y s r e p r e s e n t the t y p e c u r r e n t l y u n d e r d e v e l o p m e n t for gas t u r b i n e a p p l i c a t i o n s . A t s u f f i c i e n t l y high v a l u e s of G/R ( t h e r m a l g r a d i ent d i v i d e d by growth r a t e ) the a l l o y s g r o w with a fully p l a n e f r o n t . At m o d e r a t e v a l u e s of G/R t h e y exhibit a unique s t r u c t u r e of s i n g l e p h a s e c e l l s and two p h a s e i n t e r c e l l u l a r r o o t m a t e r i a l which g r o w s with a " p l a n e f r o n t " . F u r t h e r d e c r e a s e s in G/R p r o d u c e m o r e t y p i c a l t e r n a r y c e l l u l a r s t r u c t u r e s with r e g i o n s of one, two and t h r e e p h a s e s .

U N I D I R E C T I O N A L L Y s o l i d i f i e d t u r b i n e b l a d e s , with c o l u m n a r d e n d r i t i c s t r u c t u r e have now " c o m e of a g e " . T h e y a r e m a d e in p r o d u c t i o n f o u n d r i e s in l a r g e q u a n t i t i e s and a r e u s e d both on m i l i t a r y and c o m m e r c i a l a i r c r a f t . C a r e f u l c o n t r o l in p r o c e s s i n g is r e q u i r e d to o b t a i n 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 , but t h e r e i s the p r a c t i c a l a d v a n t a g e that only r e l a t i v e l y low r a t i o s of t h e r m a l g r a d i e n t to growth r a t e (G/R) a r e r e q u i r e d . 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 - l i k e "in situ c o m p o s i t e s " have p o t e n t i a l for a c h i e v i n g h i g h e r t e m p e r a t u r e s and a r e e n v i s i o n e d a s a n e x t - g e n e r a t i o n high temperature gas-turbine blade material. ~ However, t h e s e m a t e r i a l s have the d i s a d v a n t a g e (from a p r o c e s s ing standpoint) of r e q u i r i n g high v a l u e s of G/R d u r i n g solidification. A third, as yet relatively unexploited, intermediate s t r u c t u r e grown at G/R r a t i o s b e t w e e n the above two e x t r e m e s , is e m e r g i n g a s a p o s s i b l e c o m p r o m i s e high temperature material. These intermediate, "cellular in situ c o m p o s i t e s ''2'3 a r e a t t r a c t i v e s i n c e they m a y offer much of the m e c h a n i c a l p r o p e r t y a d v a n t a g e of the t r u e in situ c o m p o s i t e and y e t r e q u i r e l o w e r G/R r a t i o s in c a s t i