Crystal multiplication in undercooled Cu + 2 Pct Sn alloy
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1. INTRODUCTION W H E N a liquid m e t a l o r a l l o y is u n d e r c o o l e d s u b s t a n t i a l l y below its f r e e z i n g t e m p e r a t u r e o r liquidus the g r a i n s t r u c t u r e s o b s e r v e d a f t e r s o l i d i f i c a t i o n m a y be v e r y d i f f e r e n t f r o m t h o s e e x p e c t e d when the s a m e a l l o y f r e e z e s with s m a l l u n d e r c o o l i n g , a s u n d e r n o r r e a l c a s t i n g c o n d i t i o n s . One effect that has b e e n obs e r v e d in a n u m b e r of a l l o y s y s t e m s is a r e f i n e m e n t of g r a i n s i z e when the u n d e r c o o l i n g (AT) i s l e s s than 100~ The m e c h a n i s m inducing g r a i n r e f i n e m e n t is not known in a l l c a s e s but its effect, and indeed the AT r a n g e in which it o c c u r s , a r e o b s e r v e d to v a r y in diff e r e n t a l l o y s . H y p o t h e s e s in the l i t e r a t u r e g e n e r a l l y a s s u m e a f r a g m e n t a t i o n of d e n d r i t e s which i s v a r i o u s l y a t t r i b u t e d to fluid motion, r e m e l t i n g o r c o a r s e n ing. T h e r e h a s a l s o b e e n m e n t i o n of r e c r y s t a l l i z a t i o n of m a t e r i a l containing d e f e c t s i n t r o d u c e d by high s p e e d g r o w t h o r fluid m o t i o n induced d e f o r m a t i o n , but it i s u n l i k e l y t h a t c o m p l e t e r e c r y s t a l l i z a t i o n (as d i s t i n c t f r o m g r a i n b o u n d a r y m o v e m e n t ) is o p e r a t i v e in t h i s r a n g e of r e l a t i v e l y m o d e r a t e a T . G r a i n r e f i n e m e n t of e l e c t r o m a g n e t i c a l l y s t i r r e d and u n s t i r r e d S n - P b a l l o y ~,2 was d i s c u s s e d by T i l l e r and O ' H a r a s in t e r m s of d e n d r i t e b e n d i n g and s e g m e n t a t i o n ( f r a g m e n t a t i o n ) . T h e y e n l a r g e d on the conc e p t of d e n d r i t e r e m e l t i n g i n t r o d u c e d by J a c k s o n e t a l . ~ C o m p l e t e d i s i n t e g r a t i o n of f r e e d e n d r i t e s in a S n - B i a l l o y was o b s e r v e d d i r e c t l y by S c h a e f f e r and G l i c k s m a n . s T a r s h i s e t ale a t t r i b u t e d g r a i n r e f i n e m e n t in u n d e r c o o l e d C u - N i a l l o y s to an u n s p e c i f i e d type of d e n d r i t e f r a g m e n t a t i o n . K a t t a m i s and F l e m i n g s 7 c o n c l u d e d that g r a i n s i z e c h a n g e s in v a r i o u s i r o n and n i c k e l a l l o y s could be a t t r i b u t e d to d e n d r i t e a r m c o a r s e n i n g . J o n e s and W e s t o n , s'~~ Southin and W e s t o n ~1 A. J. McLEOD, formerly with the University of Queensland, Australia, and the National Research Council of Canada, Ottawa, is now Staff Metallurgist, Alcan Australia Limited, Granville, N.S.W., Australia and L. M. HOGAN is Reader in Physical Metallurgy at the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Queensland, Australia. Manuscript submitt
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