Dislocation cell formation and recrystallization in tungsten
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ly N. A. D. P A R L E E W e w e r e p r e p a r e d to expect s o m e N i O precipitation unless w e used sufficiently low P O2 pressures. T h u s , *M. M . A. EL-NAGGAR and N. A. D. PARLEE, Met. Trans., 1 9 7 1 , vol. 2 , p p . 909-11. A. D. KULKARNI is Staff Scientist, Casting Laboratory, Chase Brass & Copper Co., Cleveland, Ohio. Discussion submitted April 2 9 , 1 9 7 1 . METALLURGICAL
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we c a l c u l a t e d t h e e x p e c t e d e q u i l i b r i u m v a l u e s a n d t h e n c h o s e w h a t we c o n s i d e r e d t o be s a f e P O w o r k i n g p r e s s u r e s p a r t i c u l a r l y f o r t h e 3,3 a t , p c t N ~ a n d 5.9 a t . p e t N i r u n s , w h i c h w o u l d be s u f f i c i e n t t o e s t a b l i s h t h e i n t e r action p a r a m e t e r . W e u s e d different thermodynamic d a t a than K u l k a r n i w h i c h g a v e us a g o o d d e a l g r e a t e r m a r g i n of s a f e t y . E v e n i f we u s e K u l k a r n i ' s e s t i m a t e d PO2 e q u i l i b r i u m v a l u e of 2.6 × 10 -7 a t 1 2 0 0 ° C f o r 10 a t . p c t N i a s a b a s i s , we f i n d that no N i O p r e c i p i t a t i o n i s t o be e x p e c t e d a t 3.3 a n d 5.9 a t . p c t N i . O n h i s b a s i s a s i m p l e c a l c u l a t i o n s h o w s t h e e q u i l i b r i u m PO2 f o r 3.3 a t . p c t N i (in e q u i l i b r i u m w i t h NiO) s h o u l d be 2.1 × 10 -~ a t m ; w e a c t u a l l y u s e d 5.3 × 10 -7 a t m w h i c h i s w e l t b e l o w that v a l u e . A g a i n , o n K u l k a r n i ' s b a s i s f o r t h e 5.9 a t . p c t N i t h e e q u i l i b r i u m PO2 i s 7 . 5 × 10 -7, w h e r e a s we u s e d 7 × 10 -7 w h i c h a d m i t t e d l y i s c l o s e . W e w e r e s u r p r i s e d o u r s e l v e s that e v e n h i g h e s t N i a l l o y (19 p e t ) s h o w e d no i n d i c a t i o n o f N i O p r e c i p i t a t i o n , n e i t h e r f r o m v i s u a l e x a m i n a t i o n of t h e m e l t s a t temperature or a f t e r cooling, nor from examination of t h e a b s o r p t i o n c u r v e s w h i c h followed t h e u s u a l diffusion behavior, nor from any u n u s u a l uptake of oxygen such a s e x p e c t e d if NiO f o r m e d . O n e p o s s i b l e e x p l a n a t i o n , o t h e r t h a n e r r o r s due t o t h e r m o d y n a m i c e s t i m a t i o n s , i s that N i O did not n u c l e a t e a n d g r o w due t o i n sufficient driving f o r c e . It s h o u l d be n o t e d a g a i n that t h e e x t r a p o l a t e d d a t a o f T a n k i n s , E r t h a l a n d T h o m a s ( R e f . 3 in our c o m m u n i c a t i o n ) s u g g e s t a n e Ni v a l u e o f z e r o o r s l i g h t l y p o s i t i v e a t 1 2 0 0 ° C . It a p p e a r s that we m a y h a v e t o look d e e p e r for t h e real c a u s e of t h e s e discrepancies. 5 . C. W. Schultz, G. R. Zellers, S. L. Payne, andE. F. Foerster; U.S. Bur. Mines, Rept. Invest. No. 6410, 1964. 6 . R. Hultgren, R. L. Orr, andK:K. Kelley: Supplement to Selected Values o f Thermodynamic Properties o fMetals and All
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