A stability analysis of the plate-to-needle morphological transition
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THE plate-to-needle morphological transition appears in such d i v e r s e s y s t e m s a s w a t e r - i c e , F e - C , C u - Z n , and d u r i n g a v a r i e t y of g r o w t h c o n d i t i o n s such a s s o l i d i f i c a t i o n , e l e c t r o c r y s t a l l i z a t i o n , and s o l i d - s o l i d t r a n s f o r m a t i o n s . The n e e d l e m o r p h o l o g y can n u c l e a t e and grow d i r e c t l y f r o m the p a r e n t p h a s e , g r o w f r o m grain boundary allotriomorphs or evolve from platelet i n s t a b i l i t y . E x p e r i m e n t a l e v i d e n c e s e e m s to s u p p o r t the l a t t e r m e c h a n i s m a s being d o m i n a n t d u r i n g the p l a t e - n e e d l e t r a n s i t i o n . W i l l i a m s o n and C h a l m e r s 1 n o t i c e d that l a r g e thin i c e d i s c s b e c a m e u n s t a b l e a t b a t h t e m p e r a t u r e s below - 0 . 4 ~ Sinusoidal protubera n c e s g r e w into d e n d r i t i c n e e d l e s which s u b s e q u e n t l y e x p e r i e n c e d s i d e b r a n c h i n g . Diggle 2 o b s e r v e d that, d u r i n g e l e e t r o c r y s t a l l i z a t i o n of zinc at c o n s t a n t t e m p e r a t u r e , d e n d r i t i c n e e d l e s r e p l a c e d the d i s c m o r p h o l ogy a s the zinc ion c o n c e n t r a t i o n w a s i n c r e a s e d . Founded in the e a r l y d a y s of m e t a l l o g r a p h y , the study of the s o - c a l l e d "Widmanst~/tten s t r u c t u r e " i s one of the o l d e s t m e t a l l u r g i c a l m i c r o s t r u c t u r a l i n v e s t i g a t i o n s . T h i s d e s i g n a t i o n p e r t a i n s to p l a t e - and n e e d l e - s h a p e d p r e c i p i t a t e s in f e r r o u s a l l o y s . T h e s e p r e c i p i t a t e s a r e g e n e r a l l y t e r m e d Widmanst~/tten f e r r i t e and WidmanstS:tten c e m e n t i t e for h y p o e u t e c t o i d and h y p e r e u t e c t o i d s t e e l s , r e s p e c t i v e l y , and a r e o b s e r v e d to grow f r o m e i t h e r l o w - a n g l e g r a i n b o u n d a r i e s or grain boundary allotriomorphs. F l e w i t t and T o w n e r 3 m e a s u r e d T T T c u r v e s for a n e e d l e s and p l a t e s p r e c i p i t a t i n g f r o m /3 C u - Z n a l l o y s . A C - c u r v e f o r n e e d l e s was found at high t e m p e r a t u r e s and a s i m i l a r c u r v e for p l a t e s w a s o b t a i n e d at low t e m p e r a t u r e s . T h e s e c u r v e s i n t e r s e c t e d with a s h a r p c u s p at a t e m p e r a t u r e c h a r a c t e r i z e d a s B s . Above B s p l a t e s w e r e n e v e r found w h e r e a s p l a t e s and n e e d l e s w e r e f r e q u e n t l y found at t e m p e r a t u r e s l e s s than B s . Slightly l e s s than B s p l a t e s n u c l e a t e d but soon y i e l d e d to the d e v e l o p m e n t of n e e d l e s which g r e w f r o m the
edge of t h e s e a p p a r e n t l y u n s t a b l e p l a t e s . It w a s found that the t i m e to the s t a r t of n e e d l e f o r m a t i o n
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