Interlamellar spacing measurements in the Sn-Pb and Al-CuAl 2 Eutectic Systems
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a u t h o r s have r e c e n t l y p r e s e n t e d m e a s u r e m e n t s of i n t e r l a m e l l a r spacing, X, as a function of growth velocity, v, for s e v e r a l c o m p o s i t i o n s in the l a m e l l a r S n - C d and P b - C d e u t e c t i c s . 1,2 The X2v c o m p o s i t i o n relationships derived from these m e a s u r e m e n t s were found to be in a g r e e m e n t with those p r e d i c t e d by the e x t r e m u m condition of the Hunt and J a c k s o n t h e o r y a of e u t e c t i c growth. P r i o r to this work, J o r d a n and Hunt 4 had p r e s e n t e d s i m i l a r m e a s u r e m e n t s for the S n - P b and A1-CuAlz s y s t e m s in this j o u r n a l . R e a s o n a b l e a g r e e m e n t was found b e t w e e n t h e o r e t i c a l and e x p e r i m e n t a l v a l u e s of X2v for a r a n g e of c o m p o s i t i o n s in the S n - P b s y s t e m and a l l t h r e e solutions of the Hunt and J a c k s o n a n a l y s i s w e r e found to d e s c r i b e the m e a s u r e d v a r i a t i o n of X2v with c o m p o s i t i o n . However, the v a r i a t i o n m e a s u r e d for the A1-CuA12 s y s t e m and r e p r e s e n t e d by the full c i r c l e s in Fig. 1 was too l a r g e to be c o m p a t i b l e with t h e o r y . T h i s d i s c r e p a n c y was a t t r i b u t e d to the n e g l e c t of the k i n e t i c u n d e r c o o l i n g in the t h e o r y . The n e g l e c t e d u n d e r c o o l i n g t e r m i s a s s o c i a t e d with f a c e t i n g of the CuA12 phase. T h i s s e e m s q u e s t i o n a b l e as this p h a s e has an entropy of s o l u t i o n at the e u t e c t i c t e m p e r a t u r e of 15.1 J / m o l / K and faceting is c o n s i d e r e d to o c c u r for v a l u e s e x c e e d i n g 23 J / m o l / K . In addition, Steen and Hellawell 5 have a r g u e d that the k i n e t i c u n d e r c o o l i n g c o n t r i b u t i o n to the total u n d e r c o o l i n g is s m a l l in a n o n f a c e t i n g / f a c e t i n g s y s t e m . F i n a l l y , if the k i n e t i c u n d e r c o o l i n g is not a function of /% the i n c l u s i o n of such a t e r m in the equation for the t o t a l i n t e r f a c e u n d e r c o o l i n g will not affect the i n t e r l a m e l l a r spacing. The points in Fig. 1 p r o d u c i n g the c u r v a t u r e in the r e l a t i o n s h i p for the A1-CuA12 s y s t e m may be i n a c c u r a t e for other r e a s o n s . The a c c u r a c y of the point for the 28 wt pct Cu alloy is q u e s t i o n a b l e as the l i n e a r log X log v r e l a t i o n s h i p u s e d to calculate X2v is b a s e d on one e x p e r i m e n t a l point only (see Fig. 6(a), Ref. 4). The point for the 33 wt pct Cu alloy is not an e x p e r i m e n t a l point but was t a k e n from data due to Cooksey e t al that did not obey a XZv = const, r e l a t i o n s h i p - t h e r e l a t i o n s h i p p r e d i c t e d by a l l t h r e e solutions of the Hunt and J a c k s o n a n a l y s i s . The point for the 33.8 wt pct
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