On the microstructure of rapidly solidified IN-100 powders
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r o u t i n e e x a m i n a t i o n of t h e m i c r o s t r u c t u r e o f p o w d e r p a r t i c l e s p r o d u c e d by a n e w l y d e v e l o p e d r a p i d solidification r a t e (RSR) centrifugal a t o m i z a t i o n p r o c e s s ,z i t w a s n o t e d that t h e p a r t i c l e s f r e q u e n t l y e x hibited a seemingly dendrite free microcrystalline s t r u c t u r e , in a d d i t i o n to t h e m o r e c h a r a c t e r i s t i c a n d f a m i l i a r c o a r s e g r a i n e d dendritic structure. This intriguing r e s u l t led quite naturally t o much speculation as to t h e o r i g i n o f t h e m i c r o c r y s t a l l i n e m a t e r i a l . A n i m p o r t a n t clue t o t h e a c t u a l m e c h a n i s m i n v o l v e d w a s p r o v i d e d by t h e o b s e r v a t i o n that the f r a c t i o n of p a r t i c l e s h a v i n g the m i c r o c r y s t a l l i n e s t r u c t u r e i n c r e a s e d with d e c r e a s i n g melt superheat d u r i n g atomization. A c c o r d i n g l y , it was d e c i d e d to p r e p a r e two b a t c h e s o f p o w d e r r e p r e s e n t i n g t w o e x t r e m e s of m e l t s u p e r h e a t d u r i n g atomization and compare t h e i r microstructures. T h i s p a p e r d o c u m e n t s the p r i n c i p a l f i n d i n g s of t h e study. EXPERIMENTAL
r e s i d e n c e t i m e o f t h e m e l t o n t h e d i s c of t h e r o t a r y atomizer. C a l c u l a t i o n s b a s e d on t h e i n f o r m a t i o n p r e s e n t e d i n T a b l e I i n d i c a t e d that the m e l t t e m p e r a t u r e must h a v e f a l l e n b e l o w t h e liquidus t e m p e r a t u r e in L S - 2 6 , c o r r e s p o n d i n g w i t h a b o u t 17 p c t s o l i d i n t h e a t o m i z e d s p r a y . O n the o t h e r h a n d , s i m i l a r c a l c u l a t i o n s f o r the c a s e o f H S - 4 3 s h o w e d that t h e t e m p e r a t u r e m u s t h a v e r e m a i n e d a b o v e the l i q u i d u s t e m p e r a t u r e d u r i n g a t o m i z a t i o n ; t h e e s t i m a t e d r e d u c t i o n in s u p e r h e a t i n t h i s c a s e w a s only ~ 1 0 0 ° C . T h e o b s e r v e d b u i l d - u p of s o l i d s k u l l o n t h e d i s c of t h e r o t a r y a t o m i z e r in t h e two c a s e s was c o n s i s t e n t w i t h t h e calculations. T h e two b a t c h e s o f p o w d e r s were carefully screened, u s i n g f i n e w i r e m e s h s i e v e s , a n d s e p a r a t e d into d i f f e r e n t l y s i z e d f r a c t i o n s . T h e y i e l d o f p a r t i c l e s 850 -850 + 177 177+ 105 -105 + 62 -62 + 45
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