The role of oxide microstructure and growth stresses in the high-temperature scaling of nickel
- PDF / 1,802,562 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 14 Downloads / 180 Views
T H E f o r m a t i o n of oxide s c a l e s on unalloyed m e t a l s at e l e v a t e d t e m p e r a t u r e s is g e n e r a l l y b e l i e v e d to p r o c e e d a c c o r d i n g to the W a g n e r m e c h a n i s m w h e r e i n m a t e r i a l i s t r a n s p o r t e d by diffusion a c r o s s a single h o m o g e n e o u s l a y e r of r e a c t i o n p r o d u c t . 1 Two c o m m o n l y o b s e r v e d p h e n o m e n a a r e not a c c o u n t e d for by this m o d e l : 1) the g e n e r a t i o n of s t r e s s in s c a l e l a y e r s , and 2) the n o n p a r a b o l i c r a t e of oxide growth d u r i n g l o n g - t e r m oxidation. The a u t h o r s b e l i e v e that t h e s e a n o m a l i e s m a y be r a t i o n a l i z e d if the g r a i n s t r u c t u r e of the s c a l e l a y e r and its effect upon m a t e r i a l t r a n s p o r t a r e taken into account. The p r o b l e m of s t r e s s g e n e r a t i o n d u r i n g s c a l e f o r m a t i o n has long b e e n of c o n c e r n to those i n v o l v e d with the oxidation of m e t a l s ; h o w e v e r , no single concept of the o r i g i n of these s t r e s s e s has yet a t t a i n e d g e n e r a l a c c e p t a n c e . P i l l i n g and Bedworth were a m o n g the f i r s t i n v e s t i g a t o r s to r e c o g n i z e the i m p o r t a n c e of oxidation growth s t r e s s e s in the s c a l i n g p r o c e s s . 2 T h e y p o s t u lated that the s t r e s s e s a r o s e as a r e s u l t of v o l u m e d i l a t a t i o n upon the c o n v e r s i o n of m e t a l to its oxide. Dankov and C h u r a e v l a t e r showed that the oxidation of thin m e t a l l i c l a y e r s p r o d u c e d s t r e s s e s which c a u s e d b e n d i n g of the s p e c i m e n s 3 and E v a n s showed that a thin oxide f i l m , when r e m o v e d f r o m its n i c k e l s u b s t r a t e , tended to c u r l 4 i n d i c a t i n g the p r e s e n c e of both a s t r e s s and a s t r e s s g r a d i e n t . Moore found that a loop of n i c h r o m e w i r e , c i r c u m f e r e n t i a l l y p l a c e d a r o u n d a copper s p h e r e , i s r e d u c e d in its r i n g d i a m e t e r a s copper oxide f o r m s and e n v e l o p e s the wire loop 5 and E n g e l l and Wever o b s e r v e d that h e l i c a l s p e c i m e n s of i r o n tend to " w i n d u p " d u r i n g h i g h - t e m p e r a t u r e oxidation. 6 Both of t h e s e effects may be a t t r i b u t e d to those f o r c e s which m a i n t a i n contact b e t w e e n m e t a l F. N. RHINES is Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Florida, GainesviUe,Fla. J. S. WOLF is Associate Professor of Materials Engineering, Divisionof Interdisciplinary Studies, Clemson University, Clemson, S. C. This paper is based in part on a doctoral dissertation submitted to the Graduate School of the University of Florida by J. S. WOLFin partial fulf'dlment of the requirements for the degree of Doctor of Philosopy. Manuscript submitted November 21, 1
Data Loading...
