Protective oxide formation on Cu-7.5Al-2Si alloy
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The s t r u c t u r e and composition of oxide f i l m s f o r m e d on C u - 7 . 5 A1-2 Si in a i r over the t e m p e r a t u r e r a n g e 100 ~ to 800~ have b e e n s t u d i e d by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y of s t r i p p e d f i l m s , r e p l i c a s , and thin foils. At the h i g h e s t t e m p e r a t u r e , the f o r m a t i o n of a thin (250/~) film c o n s i s t i n g m a i n l y of y a l u m i n a is followed within the f i r s t m i n u t e by the n u c l e a t i o n of single c r y s t a l s of c u p r o u s oxide e p i t a x i a l l y o r i e n t e d to the ot phase of the alloy. T h e s e grow to a depth of about 6000~. The location of these crystals~ and the m e c h a n i s m by which they a r e f o r m e d , have b e e n studied by e x p e r i m e n t s involving s e l e c t i v e oxidation and s h o r t e x p o s u r e t i m e s . An e x t e n s i v e d i s c u s s i o n of p o s s i b l e m e c h a n i s m s i s included. While s i l i c o n has a b e n e ficial effect upon the oxidation r e s i s t a n c e of C u - A l , no compound of s i l i c o n was detected i n the oxide f i l m s .
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i s well known that additions of a l u m i n u m i m p r o v e the oxidation r e s i s t a n c e of copper c o n s i d e r a b l y . 1-4 T e r n a r y additions of s i l i c o n to dilute Cu-A1 a l l o y s i m prove the oxidation r e s i s t a n c e s t i l l f u r t h e r , s-7 Additions of s i l i c o n a l o n e , h o w e v e r , do not i m p r o v e the oxidation of copper to such a l a r g e extent. 8 D u r i n g a c o m p r e h e n s i v e study of the oxidation r e s i s t a n c e of a wide r a n g e of Cu-A1, Cu-Si, and C u - A l - S i a l l o y s , one p a r t i c u l a r a l l o y , C u - 7 . 5 Al-2 Si, was studied in detail s i n c e it exhibited v e r y good oxidation r e s i s t a n c e at t e m p e r a t u r e s in the r a n g e 600~ to 800~ The objectives of the i n v e s t i g a t i o n were to e s t a b lish the oxide p h a s e s that f o r m on the alloy and to deduce the m e c h a n i s m of t h e i r growth. EXPERIMENTAL S m a l l q u a n t i t i e s of the C u - 7 . 5 A1-2 Si a l l o y were made up under l a b o r a t o r y conditions in a g a s - f i r e d c r u c i b l e f u r n a c e . E l e c t r o l y t i c a l l y pure copper p e l l e t s and high purity a l l o y i n g additions were used. Small ingots of the alloy were hot forged in the t e m p e r a t u r e r a n g e 860 ~ to 600~ and hot r o l l e d and cold r o l l e d down to a t h i c k n e s s of 0.75 ram. S p e c i m e n s of the alloy m e a s u r i n g 3 by 3 c m were p r e p a r e d for oxidation b y m e c h a n i c a l l y p o l i s h i n g down to 2/0 grade e m e r y p a p e r , followed by a v a c u u m a n n e a l at 800~ and a c h e m i c a l polish in a m i x t u r e cont a i n i n g 2 p a r t s c o n c e n t r a t e d n i t r i c acid, 1 p a r t g l a c i a l a c e t i c acid, and 1 p a r t water. F i n a l l y the s u r f a c e s were thoroughly washed in d i s t i l l e
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