Mechanism of UAI 3 to UAI 4 transformation
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The compound, UAI4, which f o r m s d u r i n g the p e r i t e c t i c r e a c t i o n (Liquid + UAI3 ~ UA14), has a long n e e d l e l i k e shape and r e v e a l s the c h a r a c t e r i s t i c morphology of a f i s h t a i l when cut along the long axis. When cut p e r p e n d i c u l a r to the long axis, it shows an i n t e r n a l c o r e of a n o t h e r phase, which has b e e n identified as a p r i m a r y a l u m i n u m solid solution. B e c a u s e of the i n t e r e s t i n g r e l a t i o n s h i p b e t w e e n the c r y s t a l l o g r a p h i c s t r u c t u r e s of UA13 and UA14, the p e c u l i a r UA14 morphology can be explained on the b a s i s of a p r o p o s e d m e c h a n i s m . The m e c h a n i s m of UA13 to UA14 t r a n s f o r m a t i o n has b e e n identified as s h e a r and diffusion c o n trolled transformation.
L E I G H L Y and B r a m f i t t I have studied the s o l i d i f i c a t i o n of a s e r i e s of c a s t , a l u m i n u m - b a s e d , u r a n i u m alloys by u s i n g m e t a l l o g r a p h i c methods. They published a s e r i e s of p h o t o m i c r o g r a p h s for the v a r i o u s a l l o y s . When the u r a n i u m content is g r e a t e r than 18 pct, a p e r i t e c t i c r e action o c c u r s , as can be s e e n in Fig. 1, and UA14 is f o r m e d ~2 Most of the UA14 c r y s t a l s have a f i s h t a i l r e e n t r a n t angle at one end or a p a r a l l e l o g r a m with an i n t e r n a l c o r e as s e e n in Fig. 2. Leighly and B r a m f i t t have shown by e l e c t r o n m i c r o p r o b e a n a l y s i s that this i n t e r n a l c o r e is a p r i m a r y a l u m i n u m solid s o l u t i o n . R u n n a l l s and Boucher 3 have p r e s e n t e d i n f o r m a t i o n that i n d i r e c t l y d e m o n s t r a t e s the p o s s i b l e e x i s t e n c e of a low d e n s i t y i n t e r n a l c o r e in t h e i r s a m p l e s . The v a l u e s of t h e i r m e a s u r e m e n t s of the d e n s i t y of UA14 a r e s u b s t a n t i a l l y l e s s than the c a l c u l a t e d v a l u e s . B o r i e 4 d e t e r m i n e d that the c r y s t a l l o g r a p h i c s t r u c t u r e of UA14 is b o d y - c e n t e r e d o r t h o r h o m b i c and has 20 a t o m s p e r unit cell. The l a t t i c e p a r a m e t e r s a r e : a = 4.41A, b = 6.27A, c = 13.17,~. Kaufman and Gordon 5 found UA13 to be s i m p l e cubic isoty~e with Cu3Au and to have a l a t t i c e p a r a m e t e r of 4.26A. P r o j e c t i o n s of t h e s e unit c e l l s a r e shown in Fig. 3. The UA14 c r y s t a l m o r p h o l o g y has been e x a m i n e d in t e r m s of the c r y s t a l l o g r a p h i c s t r u c t u r e of both UA1a and UAI4. On the b a s i s of the p e r i t e c t i c r e a c t i o n , the c r y s t a l l o g r a p h i c s t r u c t u r e of the two compounds, and the m o r p h o l o g y of UA14, a m e c h a n i s m for the r e a c t i o n has b e e n developed.
E X P E R I M E N T A L MATERIALS AND METHODS The alloy used in this i n v e s t i
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