Lattice and grain boundary diffusion of cerium and neodymium in nickel
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PAUL
A N D R. P. A G A R W A L A
D i f f u s i o n of c e r i u m a n d n e o d y m i u m in n i c k e l h a s b e e n s t u d i e d by the s e r i a l s e c t i o n i n g t e c h n i q u e u s i n g r a d i o a c t i v e t r a c e r s ~4aCe a n d ~4VNd, in the t e m p e r a t u r e r a n g e s 700 ° to l l 0 0 ° C f o r v o l u m e a n d 500° to 8 7 5 ° C f o r g r a i n b o u n d a r y d i f f u s i o n r e s p e c t i v e l y . V o l u m e d i f f u s i v i t i e s c a n b e e x p r e s s e d as : DCe/Ni
= ( 0 . 6 6 + 0 . 1 8 ) exp (
6 0 , 8~0 0 + 8 1 0 ) a m 2 / s e c
59,820RT + 830) c m 2 / s e c D N d / N i = ( 0 . 4 4 + 0 . 1 3 ) exp ( a n d g r a i n b o u n d a r y d i f f u s i v i t i e s by:
Dgce/Ni = 0.11 exp (-- 29,550~RT/ c m 2 / s e c DgNd/Ni = 0.07 exp (-- 2 8 , 5 8 0 /~ c m 2 / s e c R e s u l t s of v o l u m e d i f f u s i o n h a v e b e e n c o m p a r e d with t h o s e c a l c u l a t e d f r o m t h e t h e o r i e s of diff u s i o n b a s e d on s i z e a n d c h a r g e d i f f e r e n c e b e t w e e n the s o l u t e a n d the s o l v e n t a t o m s . W h i p p l e a n d S u z u o k a m e t h o d s h a v e b e e n u s e d to e v a l u a t e the g r a i n b o u n d a r y d i f f u s i o n c o e f f i c i e n t s . Both t h e m e t h o d s give s i m i l a r r e s u l t s . D I F F U S I O N of r a r e e a r t h i m p u r i t i e s in f c c m e t a l s h a s y i e l d e d a n o m a l o u s r e s u l t s . 1-3 This a n o m a l y h a s b e e n a t t r i b u t e d3 to the l i m i t e d s o l i d s o l u b i l i t y of the d i f f u s i n g s p e c i e s in the s o l v e n t , thus c o n t r o l l i n g the flux of i m p u r i t y a t o m s t h r o u g h d i s l o c a t i o n s . H o w e v e r , a s p o i n t e d out by B a l l u f f i ,4 in m o s t of the m e t a l s , the s o l i d s o l u b i l i t i e s of the i m p u r i t i e s s t u d i e d a r e s u f f i c i e n t to e n a b l e the d i f f u s i n g a t o m s to m o v e t h r o u g h v a c a n c i e s a~ the t e m p e r a t u r e s of i n v e s t i g a t i o n . R e c e n t l y ,5-s i t has b e e n p o i n t e d out that the low d i f f u s i v i t i e s in a l u m i n u m (fe e m e t a l ) f o r the d i f f u s i o n of low s o l i d solubility e l e m e n t s m a y b e due to the " h o l d u p " p h e n o m e n o n a r i s i n g f r o m the o x i d a t i o n of the s p e c i m e n s u r f a c e w h i c h p r e v e n t s t r a c e r a t o m s to d i f f u s e a p p r e c i a b l y in t h e host lattice. In view of t h e s e c o n t r o v e r s i e s a n d to u n d e r s t a n d the m e c h a n i s m of l a t t i c e a n d g r a i n b o u n d a r y d i f f u s i o n in s u c h c a s e s , i t w a s c o n s i d e r e d d e s i r a b l e to s t u d y the d i f f u s i o n of c e r i u m a n d n e o d y m i u m in n i c k e l . F o r latt i c e d i f f u s i o n , t h e o r i e s b a s e d o n s i z e9 and c h a r g e1° h a v e b e e n a p p l i e d a n d they a g r e e w e l l with t h e e x p e r i m e n t a l results. G r a
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