The effects of sodium on the growth velocity and growth morphology of silicon in Al-Si alloys
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M. F. X. GIGLIOTTI, JR. is Metallurgist,General Electric Research and Development Center,Schenectady, N. Y. G. A. COLLIGANis Associate Dean and Professor of Engineering,Thayer School of Engineering, Dartmouth College, Hanover, N. H. Manuscript submitted June 10, 1971. METALLURGICALTRANSACTIONS
m a r y s i l i c o n c r y s t a l s f r o m {111} faceting to {100}. is The p r i m a r y s i l i c o n a p p e a r s a s equant b l o c k s .
PREVIOUS WORK ON A1-Si ALLOYS T h e r e a r e two chief schools of thought c o n c e r n i n g the effects of s o d i u m . Sodium is thought to e i t h e r c a u s e the growth of s i l i c o n to be r e s t r i c t e d o r to have some effect on the n u c l e a t i o n of s i l i c o n f r o m the m e l t . T h a l l and C h a l m e r s , 4 Ghosh and Kondic, 16'17 and P l u m b and L e w i s 5 identify the e u t e c t i c s i l i c o n a s g l o b u l a r , and they r e g a r d the m a j o r a c t i o n of sodium a s a r e s t r i c t i o n on the growth of s i l i c o n . T h a l l and C h a l m e r s 4 obtained cooling c u r v e s for A1Si a l l o y s and showed that the e u t e c t i c cooling a r r e s t t e m p e r a t u r e was l o w e r e d with s o d i u m a d d i t i o n s . T h i s was e x p l a i n e d a s sodium l o w e r i n g the i n t e r r a c i a l e n ergy, and a s the eutectic g r o w s , the a l u m i n u m c o n t i n ually p i n c h e s off the s i l i c o n , f o r c i n g it to r e n u c l e a t e . The a r r e s t t e m p e r a t u r e is d e t e r m i n e d by that t e m p e r a t u r e at which the n u c l e a t i o n of s i l i c o n is high enough to p r o d u c e continual growth. Ghosh and Kondic 16 o b s e r v e d that at v e r y slow cooling r a t e s , even s o d i u m - t r e a t e d a l l o y s of A1-Si would solidify at the e q u i l i b r i u m e u t e c t i c t e m p e r a t u r e . The A1-Si a l l o y s with s o d i u m , in g e n e r a l , did not tend to u n d e r c o o l . The f r e e z i n g a r r e s t t e m p e r a t u r e was felt to be a function of cooling r a t e , with s o d i u m - t r e a t e d a l l o y s having the lowest a r r e s t t e m p e r a t u r e for a given cooling r a t e . The effect of sodium was held to be that of slowing the growth r a t e of the e u t e c t i c s i l i c o n by slowing the diffusion of s i l i c o n in the m e l t . U n d e r c o o l e d below the eutectic t e m p e r a t u r e , the n u c l e a t i o n f r e q u e n c y of s i l i c o n is high, and t h i s , coupled to the low diffusion r a t e of s i l i c o n , p r o d u c e s a fine d i s p e r s i o n of g l o b u l e s .16,17 P l u m b and Lewis, 5 as m e n t i o n e d above, r e g a r d that a n o t h e r m a j o r effect of sodium i s to a l t e r the n u c l e a t i o n b e h a v i o r of the s i l i c o n . K i m and H e i n e , la C r o s l e y and Mondolfo, 19 Davies and W e s t , 2~ and Chadwick 21 a l s o r e g a r d a m a j o r effect of sodium a s a l t e r i n g the n u c l e ation b e h a v i o r of s i l i c o n . C r o s l
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