Homogenization of primary microsegregation
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feature of m i c r o s e g r e g a t i o n s u b s t r u c t u r e s called nodes a p p e a r s at the onset of constitutional s u p e r c o o l ing and p e r s i s t s throughout f r e e z i n g u n d e r m o r e s e v e r e conditions. ~ T h e s e nodes a r e g e n e r a l l y the r e g i o n s of highest solute c o n c e n t r a t i o n , each localized within a few m i c r o n s in c r o s s - s e c t i o n and all d i s t r i b u t e d about a mean spacing of ~ 50 ~z for cubic m e t a l s (but nodes a r e not n e c e s s a r i l y s e e n for the u n c o m m o n s o l u t e s with k0 ~ 1). This p e r v a s i v e f e a t u r e of nodes as a n e l e m e n t of s u b s t r u c t u r e s in g e n e r a l , provides a unique c h a r a c t e r i s t i c for all kinds of solidification s u b s t r u c t u r e s in the a s - c a s t state. F u r t h e r m o r e , if it can be e s t a b l i s h e d that nodes a r e r e p r e s e n t a t i v e d u r i n g d i s solution, then any study of the homogenization of a solidified s e g r e g a t e d alloy can r e l i a b l y be b a s e d upon nodes. A n a l y s e s of nodes and of a total s u b s t r u c t u r e a r e both p e r f o r m e d h e r e on A1-0.34 pct Ca, a b i n a r y alloy that p r e s e n t s all the p o s s i b l e solidification s u b s t r u c t u r e f e a t u r e s through a c e l l u l a r d e n d r i t i c type. The r e s u l t s s e e m to be g e n e r a l for the evolution of the m i c r o s e g r e gation p a t t e r n of any b i n a r y alloy following t h e r m a l homogenization. Fine c h a r a c t e r i s t i c s of nodes had to be taken into a c count in designing the e x p e r i m e n t s p e r f o r m e d . As is well known, ~ nodes a r e g e n e r a l l y prolate s p h e r o i d s , exhibiting an a p p r o x i m a t e l y c i r c u l a r c r o s s - s e c t i o n in a cut n o r m a l to the d i r e c t i o n of growth. Any l a r g e enough s a m p l e of nodes will show the following f e a t u r e s : a) v a r i a b l e s i z e s , both n o r m a l and p a r a l l e l to the growth d i r e c t i o n ; b) v a r i a b l e size of c r o s s - s e c t i o n s along the s a m e node, both as a consequence of s e c t i o n and of morphology; c) soIute c o n c e n t r a t i o n s that v a r y f r o m node to node, independently of their s i z e , and that v a r y in magnitude f r o m within the range of solid solution, up to much higher v a l u e s ; a d) different c o n c e n t r a t i o n p r o f i l e s even for such nodes whose m e a s ured m a x i m u m c o n c e n t r a t i o n s a r e the s a m e . As a r e s u l t of the c h a r a c t e r i s t i c s e n u m e r a t e d , d i s solution m u s t p r o g r e s s inhomogeneously throughout s m a l l v o l u m e s of an alloy s a m p l e and henc% the s p a D. FAiNSTEIN, currently on leave from CNEA, and G. F, BOLLING are with the Scientific Research Staff, Ford Motor Company, Dearborn, Mich. H, BILONI is with the Departmento de Metalurgia, ComisfonNacional de Energia At6mica (CNEA), Buenos A
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