On the nature of freckles in nickel base superalloys
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F R E C K L E S a r e m a c r o s c o p i c defects which o c c u r in s e v e r a l alloy s y s t e m s u n d e r c e r t a i n s o l i d i f i c a t i o n cond i t i o n s . P r e v i o u s l i t e r a t u r e on the s u b j e c t of f r e c k l i n g p r o v i d e s n e i t h e r a c o m p l e t e d e s c r i p t i o n of the n a t u r e of f r e c k l e s in nickel b a s e alloys nor a s a t i s f a c t o r y theory explaining t h e i r o r i g i n . In 1959, S m e l t z e r 1 r e f e r r e d to f r e c k l i n g defects in c o n s u m a b l y a r c m e l t e d A-286 (iron base, see Table I) and identified them as a r e a s of localized s e g r e g a t i o n . In 1965, Gould 2 d i s c u s s e d s i m i l a r ingots and p r o p o s e d m o d e l s to explain the f o r m a t i o n of f r e c k l e t r a i l s dependent upon the evolution of gas bubbles or the f o r m a t i o n of i m m i s c i b l e fluids d u r i n g solidification. In 1967, P r e s t o n 3 d i s c u s s e d the location of f r e c k l e s in iron b a s e a r c m e l t e d ingots. It was shown that the f r e c k l e d r e g i o n s w e r e c l o s e to eutectic c o m p o s i t i o n and had d e n s i t y v a l u e s which differed from those of the bulk m a t e r i a l . C r e m i s i o 4 (1967) followed by D e V r i e s and Mumau 5 (1968) d i s c u s s e d the o c c u r r e n c e of s e g r e g a t i o n and f r e c k l i n g in i r o n and nickel b a s e a r c m e l t e d ingots. In this investigation, a detailed a n a l y s i s has been m a d e which defines the s t r u c t u r e and c o m p o s i t i o n of f r e c k l e s in u n i d i r e c t i o n a l l y solidified nickel b a s e a l loys. The o c c u r r e n c e of the f r e c k l i n g p h e n o m e n o n has been c o r r e l a t e d with p a r t g e o m e t r y , s o l i d i f i c a t i o n cond i t i o n s and alloy c o m p o s i t i o n . E X P E R I M E N T A L RESULTS A) F r e c k l e Location F r e c k l e s have been o b s e r v e d in u n i d i r e c t i o n a l l y solidified p o l y c r y s t a l l i n e (columnar) and m o n o c r y s t a l line r o d s , b a r s , ingots and shaped p a r t s of s e v e r a l n i c k e l b a s e alloys, m a n y of the s u p e r a l l o y type. T h e s e c a s t i n g s were made u s i n g s h e l l molds and a chill plate to e s t a b l i s h the p r e f e r r e d [001] growth d i r e c t i o n . F r e c k l e s a r e m o s t e a s i l y detected in single c r y s t a l
c a s t i n g s and a p p e a r on the e x t e r n a l s u r f a c e as long v e r t i c a l c h a i n s of equiaxed g r a i n s . T h e s e d e f e c t s a r e n o r m a l l y not p r e s e n t in s i n g l e c r y s t a l wire o r s m a l l d i a m e t e r rods. In b a r s the n u m b e r of f r e c k l e l i n e s i n c r e a s e s with i n c r e a s i n g b a r d i a m e t e r . In s i m p l e shapes such as b a r s , once a given f r e c k l e line i n i t i a t e s , it g e n e r a l l y p e r p e t u a t e s i t s e l f throughout the length of the b a r in a f a i r l
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