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T H E f o r m a t i o n of an equiaxed g r a i n s t r u c t u r e in a c a s t i n g r e q u i r e s that n u c l e i be p r e s e n t i n the m e l t and that, u n d e r the p r e v a i l i n g t h e r m a l c o n d i t i o n s , the growth r a t e of equiaxed g r a i n s be higher than that of c o l u m n a r g r a i n s . ~ The n u m b e r and s i z e s of floating equiaxed c r y s t a l s as well as the shape of the a d v a n c ing c o l u m n a r s o l i d i f i c a t i o n front a r e thought to be d e t e r m i n i n g f a c t o r s in the c o l u m n a r - t o - e q u i a x e d t r a n s i t i o n (CET). 2 P r i o r s t u d i e s in n o r m a l and r e duced g r a v i t y e n v i r o n m e n t s have shown that g r a v i t a t i o n a l f i e l d s can play a n i m p o r t a n t role in this t r a n s i tion.2-s C u r r e n t l y , NASA Space P r o c e s s i n g P r o g r a m s have made a v a i l a b l e a r e d u c e d g r a v i t y e n v i r o n m e n t for e x p e r i m e n t a t i o n p u r p o s e s ; this e n v i r o n m e n t allows a c r i t i c a l e x a m i n a t i o n of the v a r i o u s f a c t o r s which i n fluence the CET. In addition, the p r o j e c t e d p o s s i b i l i t y of e x t r a t e r r e s t r i a l c o n s t r u c t i o n or p r o c e s s i n g o p e r a t i o n s e n c o u r a g e s d e v e l o p m e n t of a c a p a b i l i t y for p r e dicting the s o l i d i f i c a t i o n b e h a v i o r of m a t e r i a l in r e duced g r a v i t y . T h u s , the e x p e r i m e n t d e s c r i b e d in this p a p e r was u n d e r t a k e n in o r d e r to e x a m i n e the effect of r e d u c e d g r a v i t y on the CET in s m a l l c a s t i n g s and to a t t e m p t to d e t e r m i n e which v a r i a b l e s will be of p r i mary importance. Nuclei can be g e n e r a t e d in the m e l t in v a r i o u s ways. If the m e l t i s inoculated, h e t e r o g e n e o u s n u c l e a t i o n n o r m a l l y takes place. 9 A l t e r n a t i v e l y , at low m e l t s u p e r heat, u n d e r c o o l i n g m a y o c c u r d u r i n g p o u r i n g , leading to a l a r g e n u m b e r of h o m o g e n e o u s l y f o r m e d n u c l e i that a r e d i s t r i b u t e d t h r o u g h the liquid a c c o r d i n g to local flow p a t t e r n s ( " b i g b a n g " n u c l e a t i o n ) . 4 C o n v e c tion, caused by t h e r m a l g r a d i e n t s , can induce r e m e l t ing and b r e a k - o f f of d e n d r i t e a r m s f r o m the growing c o l u m n a r d e n d r i t i c i n t e r f a c e . T h e s e a r m s a r e then JOHN M. PAPAZIANis Staff Scientist, Research Department, Grumman Aerospace Corp., Bethpage, NY 11714. THEODOULOS Z. KATTAMISis Professor, Department of Metallurgy, Institute of Materials Science, Universatyof Connecticut, Storrs, CT 06268. Manuscript submitted June 15, 1979. METALLURGICAL TRANSACTIONSA

t r a n s p o r t e d to the c e n t e r of the c a s t i n g and act as n u c l e i for the f o r m a t i o n of equiaxed g r a i n s . 4 C o n v e c t i o n m a y