The effect of solute convection upon macrosegregation in off-eutectic composite growth
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K. KURTZ
KINGERY,
AND
R. HOFER
Vertical solidification experiments have been carried out on Pb-Sn and Pb-Cd alloys at hypereutectic compositions where solute convectionwas present. A significant amount of macrosegregation was observed due to the convection,and the macrosegregation was found to be independentof tube diameter for those diameters studied, 3 to 6 mm. The resulting macrosegregation correlated well with existing theories. The experiments employed a temperature gradient of 360~ and it was found that thermotransport made a significant contribution to the observed macrosegregation at growth rates below around 5/am/s.
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was f i r s t d e m o n s t r a t e d by Mollard and F l e m i n g s 1 that aligned c o m p o s i t e alloys could be p r e p a r e d by d i r e c t i o n a l s o l i d i f i c a t i o n of e u t e c t i c alloys at c o m p o s i tions o t h er than t h e i r e u t e c t i c composition. T h e i r a n a l y s i s 2 concluded that s t e e p t e m p e r a t u r e gradient, slow growth r a t e and the a b s e n c e of c o n v e c t i o n w e r e r e q u i r e d for s u c c e s s f u l a li g n e d c o m p o s i t e growth at off-eutectic c o m p o s i t i o n s . Subsequent t h e o r e t i c a l 3 and e x p e r i m e n t a l work 4 d e m o n s t r a t e d that o f f - e u t e c t i c c o m posite growth could o c c u r in the p r e s e n c e of convection. The t h e o r e t i c a l a n a l y s i s 3 i n d i c a t e s that a m a c r o s e g r e gation will be p r o d u c e d by the s t i r r i n g in the c a s e of n o r m a l f r e e z i n g , i.e. s o l i d i f i c a t i o n of a b a r f r o m one end. Davies and F r y z u k 4 have shown that the c o n v e c tion induced m a c r o s e g r e g a t i o n m a y be r e m o v e d by ut i l i z in g a zone m e l t i n g technique. However, it is not always p o s s i b l e to u t i l i z e such a technique in growing c o m p o s i t e s for p r a c t i c a l a p p li c a ti o n s , e . g . , turbine b l ad es . In addition, c o n v e c t i o n induced by solute d e n s i t y v a r i a t i o n s , will be i n h e r e n t l y p r e s e n t in many o f f - e u t e c t i c s y s t e m s , depending on the c o m p o s i t i o n - d e n s i t y dependence, and which s i d e of the eut e c t i c one is on. Consequently, t h e r e is s o m e p r a c t i cal as well as t h e o r e t i c a l i n t e r e s t in d e t e r m i n i n g the d e g r e e of m a c r o s e g r e g a t i o n which can be expected in n o r m a l f r e e z i n g as a r e s u l t of solute induced c o n v e c tion in o f f - e u t e c t i c c o m p o s i t e growth. EXPERIMENTAL E x p e r i m e n t s w e r e c a r r i e d out on Pb-Sn and P b - C d alloys. In both c a s e s the c o m p o s i t i o n s w e r e chosen on the P b - r i c h side of the e u t e c t i c w h e r e solute conve c ti on was expected to be p r e s e n t . The s t a r t i n g m e t a l s had quoted p u r i t i e s of 99.999 pct. 1) Pb-Sn E x p e r i m e n t s The e x p e r i m e n t s w e r e c a r r i e d out
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