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Fig. 2--Maerostructure of a Ag + 34.8 wt pet C u ingot (hypere u t e c t i e u n d e r e o o l e d t o 716°C (63°C b e l o w the e u t e c t i e t e m p e r a t u r e ; a p p r o x 95°C b e l o w the l i q u i d u s ) . T h e s p h e r i c a l form of the growth f r o n t i s evident. T h e m i e r o s t r u e t u r e of the m a t e r i a l b e h i n d the front s t r o n g l y r e s e m b l e s that s h o w n i n Fig. 3(a) of ttef. 1 . Magnification 1.5 t i m e s . T h e o r i g i n s o f t h e b r a n c h i n g g r o w t h m e c h a n i s m obs e r v e d i n c e r t a i n a l l o y s a r e not c l e a r but c r y s t a l l o g r a p h i c c o n s t r a i n t s , o p e r a t i v e in s i n g l e - p h a s e d e n d r i t e g r o w t h , do not p l a y a p a r t . T h e m o d e o f g r o w t h i n w h i c h the m i n o r p h a s e g r o w s f a s t e r t h a n , a n d b r a n c h e s out a h e a d of, t h e m a t r i x p h a s e m a y p e r h a p s a p p l y to t h e s o l i d i f i c a t i o n p r o c e s s in m o d i f i e d A1-Si a l l o y s . A1-Si is a highly a s y m m e t r i c s y s t e m a n d h a s b e e n s h o w n 1° t o e x h i b i t a n a s y m m e t r i c c o u p l e d z o n e . U n c o u p l e d g r o w t h of u n d e r c o o l e d e u t e c t i c c o m p o s i t i o n a l l o y s i s t h e r e f o r e t o be e x p e c t e d a n d it i s by n o w w e l l - e s t a b l i s h e d 1~ that t h e m i n o r p h a s e (Si) in such a l l o y s , a l t h o u g h i r r e g u l a r in m o r p h o l o g y , is c o m p l e t e l y c o n t i n u o u s within o n e e u t e c t i c g r a i n . 1. T. Z. Kattamis and M. C. Flemings: Met. Trans., 1970, vol. I, p . 1449. 2. G. L. Powell andL. M. Hogan: Z Inst. Metals, 1965, vol. 93, p . 505. 3. B. L. Jones, G. M. Weston, andR. T. Southin: Z CrystalGrowth, 1971, vol. 10, in press. 4 . A. Kofler: ,7,. Elektrochem., 1953, vol. 57, p. 721. 5 . E. Scheil: Z. Metallk., 1954, vol. 4 5 , p . 298. 6. L. M. Hogan: J. Australian Inst. Metals, 1964, vol. 9 , p . 228. 7 . M. C. Flemings: Abex Professor of Metallurgy and Materials Science, MI.T., Cambridge, Mass., private communication, 1971. 8 . B. L. Jones: 3[ Inst. Metals, 1971, vol. 99, p . 27. 9 . G. A. Chadwick: Progr. Mater. ScL, 1963, vol. 12, pt. 2 . 10. A. Moore: Ph.D. Thesis, Univ. of Manchester, 1967. 11. M. G. Day andA. Hellawell: Proc. Royal Soc., 1968, Series A, vo|. 305, p. 473.

An Electrochemical Etchant for Observation of Dislocations in 70-30 Brass N . V I D A L , G. M . A R N S T E I N , A N D G. C A S T R O T o t h e k n o w l e d g e o f the a u t h o r s o n l y t w o s a t i s f a c t o r y t e c h n i q u e s f o r r e s o l u t i o n o f d i s l o c a t i o n s in C u - Z n a l loys have been reported.

Fig. 1--Ag + 25.2 wt pet C u undereooled t o 711°C b e f o r e n u cleation (58°C b e l o w the eutectic t e m p e r a t u r e ; a p p r o x 80°C b e l o w the l i q u i d u s ) . T h e m i c r o g r a p h s h o w s b r a n c h e d g r o w t h of uncoupled e u t e e t i c together with the e q u i l i b r i