The growth rate of dendrites in undercooled tin
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k i n e t i c s of d e n d r i t i c growth is an i m p o r t a n t but p o o r l y u n d e r s t o o d a s p e c t of the s o l i d i f i c a t i o n of m e t a l lic m e l t s . Velocity data for pure tin, for example, r e p o r t e d by R o s e n b e r g and Winegard, 1 O r r o k 2 and Nikonova and T e m k i n 3 v a r y by a s m u c h a s a n o r d e r of magnitude at a given d e g r e e of u n d e r c o o l i n g . None of t h e s e r e s e a r c h e r s e x a m i n e d the s a m p l e a f t e r each v e l o c i t y m e a s u r e m e n t to c o n f i r m that the s a m p l e was a single c r y s t a l and that the p r e f e r r e d growth d i r e c t i o n w a s p a r a l l e l to the s p e c i m e n axis. F o r t h e s e r e a s o n s , t h e r e a r e at l e a s t two p o s s i b l e e x p l a n a t i o n s for the wide s c a t t e r in the r e p o r t e d data. If m u l t i p l e c r y s t a l n u c l e a t i o n o c c u r s such that d i f f e r e n t c r y s t a l s t r i g g e r the velocity m e a s u r e m e n t probes, a r e s u l t higher than the t r u e f i g u r e could be obtained. Should a single c r y s t a l nucleate both p r o b e s but its f a s t e s t growth d i r e c tion, that is, the p r e f e r r e d growth d i r e c t i o n , lie at a n angle r a t h e r than p a r a l l e l to the s p e c i m e n axis, it is to be expected that the r e c o r d e d value will be l e s s than the t r u e figure. Accordingly, it might be anticipated that the t r u e value for the r a t e of growth of d e n d r i t e s of tin l i e s b e t w e e n the r e p o r t e d m a x i m u m and m i n i m u m v a l u e s . 1-3 T h i s p a p e r r e p o r t s the r e s u l t s of d e n d r i t i c growth r a t e e x p e r i m e n t s in which the s a m p l e w a s e x a m i n e d a f t e r a m e a s u r e m e n t to e n s u r e that the two m e a s u r i n g p r o b e s w e r e a c t i v a t e d by a single c r y s t a l . The p r e f e r r e d d i r e c t i o n of d e u d r i t e growth w a s d e t e r m i n e d and where the d e n d r i t e growth d i r e c t i o n was not p a r a l l e l to the s p e c i m e n axis, the t r u e growth velocity was d e t e r m i n e d by c o r r e c t i n g for the a n g u l a r d i f f e r e n c e between the growth d i r e c t i o n and the s p e c i m e n axis. E X P E R I M E N T A L PROCEDURE The m e l t i n g p r o c e d u r e , by which l a r g e (>40~ and c o n s i s t e n t u n d e r c o o l i n g was achieved, was a s d e s c r i b e d p r e v i o u s l y by the a u t h o r s . 4 The e x p e r i m e n t a l a p p a r a t u s for v e l o c i t y m e a s u r e m e n t is shown s c h e m a t i c a l l y in Fig. 1. Most s p e c i m e n s w e r e c o n t a i n e d in h o r i z o n t a l v i t r e o u s s i l i c a b o a t s of a p p r o x i m a t e d i m e n s i o n s 12 • 5 • 70 ram. T i n of n o m i n a l p u r i t y 99.999 pct was GRAHAM L. F. POWELL is Senior Research Associate, GEORGE A. COLLIGANis Professor of Engineering,VICTORA. SURPRENANT is Research Associate
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