The effect of anodic coating on the creep behavior of strong aluminum alloys
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AND MENAHEM NATHAN
The e a r l y o b s e r v e d i n c r e a s e in c r i t i c a l r e s o l v e d s h e a r s t r e s s of c a d m i u m m o n o c r y s t a l s when coated with a thin oxide s u r f a c e l a y e r l'z was followed by n u m e r o u s s t u d i e s on the effect of e n v i r o n m e n t and s u r face c o n d i t i o n s on the m e c h a n i c a l p r o p e r t i e s of m a t e r i a l s . V a r i o u s i n v e s t i g a t i o n s w e r e c a r r i e d out with r e g a r d to c r e e p of coated v e r s u s uncoated m e t a l s . C r e e p r a t e s of coated c a d m i u m and zinc s i n g l e c r y s t a l s w e r e s l o w e r than those of uncoated c r y s t a l s , s-9 yet, c o a t i n g had no effect on c r e e p of p o l y c r y s t a l s of t h e s e two m e t a l s , s'6 An high s t r e n g t h s i n g l e c r y s t a l of a n i c k e l alloy exhibited f a s t e r i n i t i a l p r i m a r y :creep r a t e s in a i r but a s l o w e r s e c o n d a r y c r e e p r a i e in v a c uum. ~~ This i n c o n s i s t e n t b e h a v i o r was r e l a t e d to the f o r m a t i o n of an oxide l a y e r d u r i n g c r e e p in a i r . C r e e p r a t e s of cold worked and p a r t i a l l y a n n e a l e d p o l y c r y s t a l l i n e Z i r c a l o y - 4 w e r e always s l o w e r in a i r than in high v a c u u m , n The s l o w e r c r e e p r a t e s in a i r w e r e a t t r i b u t e d to the c o n t i n u o u s f o r m a t i o n of a t e n a c i o u s oxide l a y e r which s e r v e d as an o b s t a c l e to e g r e s s of d i s l o c a t i o n s to the s u r f a c e of the m e t a l . Annealed p o l y c r y s t a l l i n e n i c k e l had a l o w e r c r e e p r a t e when t e s t e d at low s t r e s s e s in a i r than in v a c u u m , t2 The r e v e r s e was o b s e r v e d at high s t r e s s l e v e l s . ~z C e r t a i n c o m m e r c i a l i r o n - b a s e , n i c k e l - b a s e and c o b a l t - b a s e alloys w e r e c r e e p - t e s t e d in a i r , n i t r o g e n , h e l i u m and in v a c u u m a f t e r r e c e i v i n g t h e i r u s u a l heat t r e a t m e n t which p r o d u c e d the p r o p e r t i e s at which they a r e n o r m a l l y used. ~s With a few exceptions, the m i n i m u m (secondary) c r e e p r a t e s w e r e s l o w e s t in a i r but no c o n s i s t e n t effect was p r o d u c e d in o t h e r a t m o s p h e r e s . It was s u g g e s t e d that t h e r e is a n o p t i m a l a m o u n t of oxygen in a t e s t i n g a t m o s p h e r e to p r o d u c e the h i g h e s t c r e e p and r u p t u r e strength.~S Slower c r e e p r a t e s in v a c u u m of a i r - a n n e a l e d high p u r i t y p o l y c r y s t a l l i n e copper w i r e s as c o m p a r e d with those of v a c u u m - a n n e a l e d w i r e s w e r e a t t r i b u t e d to the f o r m a t i o n of an oxide s u r f a c e l a y e r d u r i n g a i r - a n n e a l i n g . 14 The lack of d i f f e r e n c e s in c r e e p r a t e s of high p u r i t y s i l v e r wire a n n e a
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