Selective sublimation as a means of revealing eutectic structures: The Cd-CuCd 3 eutectic
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Fig. 1--Colony microstructure of CuCd 8 fibers after removal of cadmium-matrix by sublimation. Growth-rate 5 cm/h. Magnification 220 times.
PROCEDURE
Cd-CuCd 3 eutectlc alloys were directionally-solidif l e d in s e a l e d P y r e x t u b e s , u s i n g the B r i d g m a n t e c h nique. F o r t h e s e p r e l i m i n a r y r u n s , 99.95 p c t Cd and 99.99 p c t Cu w e r e u s e d . Growth r a t e s r a n g e d f r o m 1 to 6 c m / h . The t e m p e r a t u r e - g r a d l e n t was 20~ Longitudinal and t r a n s v e r s e s e c t i o n s of the s o l i d i f i e d i n g o t s w e r e cut and m e c h a n i c a l l y p o l i s h e d on s i l i c o n c a r b i d e p o l i s h i n g w h e e l s and a l u m i n a . To r e m o v e the c a d m i u m by s u b l i m a t i o n , the s a m p l e s w e r e h e a t e d at 220~ f o r 30 m i n under a d y n a m i c v a c uum of 10-4 t o r t . At t h i s t e m p e r a t u r e , the v a p o r p r e s s u r e of p u r e c a d m i u m i s 8 x 10 -3 t o r r while the v a p o r p r e s s u r e of CuCd 3 is s e v e r a l o r d e r s of magnitude l o w e r . The s u b l i m a t i o n t r e a t m e n t r e m o v e d the c a d m i u m and thus f r e e d the CuCd 3 p h a s e to a m a x i m u m depth of 0.3 m m . O b s e r v a t i o n s w e r e m a d e with a s c a n n i n g e l e c t r o n m i c r o s c o p e . Good p e r s p e c t i v e v i e w s of the CuCd~ p h a s e w e r e obtained, b e c a u s e it r e p r e s e n t s a r e l a t i v e l y low v o l u m e - f r a c t i o n (7.4 pct) of the t o t a l s o l i d . G. HAOUR and J. W. RUTTER are Graduate Student and Professor, respectively, Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada. Manuscript submitted July 16, 1973. METALLURGICAL TRANSACTIONS
Fig. 2--Col0ny-boundary of Fig. 1. Magnification 5700 times.
RESULTS The morphology of the CuCd 3 phase appears fibrous, a s one m i g h t e x p e c t f r o m a p h a s e with a low v o l u m e f r a c t i o n , s Fig. 1 shows the " s e a w e e d - l i k e " a p p e a r ance of the CuCd 3 p h a s e at low m a g n i f i c a t i o n ; the r e m o v a l of the C a d m i u m - m a t r i x h a s f r e e d the f i b e r s and opened up the s t r u c t u r e along the c o l o n y - b o u n d a r i e s . The i m p u r i t i e s a c c o u n t f o r the colony m i c r o s t r u c t u r e . Higher m a g n i f i c a t i o n of the s a m e a r e a (Fig. 2) indic a t e s that the f i b e r s have quite s m a l l d i a m e t e r s , f r o m VOLUME 5, FEBRUARY 1974-515
0.3 to 1 m i c r o n . No s p h e r o i d i z a t t o n of the f i b e r s , due to the h e a t t r e a t m e n t d u r i n g s u b l i m a t i o n , h a s b e e n o b served. F u r t h e r w o r k will involve study of the g r o w t h - m o r p h o l o g i e s in t h i s s y s t e m , a s c o m p o s i t i o n and g r o w t h c o n d i t i o n s a r e v a r i e d . It is p l a n n e d to i n v e s t i g a t e the c r y s t a l l o g r a p h i c r e l a t i o n s b e t w e e n p h a s e s and t
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