Nucleation, undercooling and homogeneous structures in rapidly solidified powders
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c o n n e c t i o n with r e c e n t t e c h n o l o g i c a l d e v e l o p m e n t s in p r o d u c t i o n of s u p e r a l l o y s f r o m m e t a l p o w d e r s , t h e r e h a s b e e n a f u n d a m e n t a l i n t e r e s t in c o n t r o l of the d e g r e e of h o m o g e n e i t y of a l l o y p o w d e r s by c o n t r o l l i n g the c o o l i n g r a t e d u r i n g s o l i d i f i c a t i o n . ~'6 M o s t a t t e n t i o n h a s b e e n d i r e c t e d to u n d e r s t a n d i n g growth r e l a t e d e f f e c t s . One such effect is the t r a n s i t i o n f r o m d e n d r i t i c to c e l l u l a r to p l a n a r growth, in a g r e e m e n t with the s t a n d a r d t h e o r y of s o l i d i f i c a t i o n , 7 a s the r a t i o G / R inc r e a s e s . H e r e , G i s the t e m p e r a t u r e g r a d i e n t in the liquid a d j a c e n t to the s o l i d i f y i n g i n t e r f a c e and R is the m e a n growth r a t e n o r m a l to the i n t e r f a c e . A s e c o n d effect is the d e c r e a s e of d e n d r i t i c a r m s p a c i n g with r o u g h l y the o n e - t h i r d p o w e r of the. c o o l i n g r a t e , ~'3'8 T, for a n u m b e r of m a t e r i a l s f o r T up to at l e a s t 10 4 to l 0 s K / s . T h e s e t r e n d s have s p u r r e d the d e v e l o p m e n t of t e c h n i q u e s f o r m a k i n g r a p i d l y cooled, fine p o w d e r to a c h i e v e g r e a t e r h o m o g e n e i t y of the s o l i d i f i e d powder.2 I n d i c a t i o n s a r e that i n c r e a s e d h o m o g e n e i t y y i e l d e d b e t t e r c r e e p p r o p e r t i e s in s e v e r a l s u p e r a l l o y s p r o duced f r o m p o w d e r . 2 H o w e v e r , b e c a u s e of p r o b l e m s in p o w d e r handling and c o n t a m i n a t i o n , e f f o r t to e x c e e d the T ~ 10 5 K / s v a l u e , p a r t i c u l a r l y if it r e q u i r e s m a k ing p o w d e r s with d i a m e t e r s ~10 /an, has been q u e s tioned. N e v e r t h e l e s s , a n o t h e r f a c t o r s u g g e s t s that s t i l l l a r g e r Iv v a l u e s can be i m p o r t a n t in a c h i e v i n g h o m o g e n e o u s m i c r o s t r u c t u r e s . T h i s f a c t o r is the o c c a s i o n a l a p p e a r a n c e of a u n i f o r m - c o m p o s i t i o n , inv e r s e - s e g r e g a t i o n r e g i o n at the c e n t e r of d e n d r i t e s , in a l l o y s which w e r e c h i l l c a s t , 9'~~ s p l a t cooled, n o r gas a t o m i z e d to p o w d e r . '2 T h e o b s e r v a t i o n s i n d i c a t e that f o r s o m e p e r i o d following n u c l e a t i o n , g r o w t h p r o c e e d e d with conditions of u n d e r c o o l i n g at the g r o w i n g s o l i d - l i q u i d i n t e r f a c e . 3'4'9-~2 E v e n t u a l l y , r e l e a s e of the heat of fusion r a i s e d the t e m p e r a t u r e of the i n t e r f a c e to the n e a r - e q u i l i b r i u m v a l u e and n o r m a l , s e g r e g a t e d , d e n d r i t i c growth e n s u e d . 9-~2 T h e finding of the c e n t r a l
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