Hydrogen effects and cadmium segregation to grain boundaries in 7075-T6 aluminum
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Communications
Hydrogen Effects and Cadmium Segregation to Grain Boundaries in 7075-T6 Aluminum
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EDWIN C. POW, JAMES C. SCHWANEBECK, AND WILLIAM W. GERBERICH
Crack initiation s i t e
T h i s p r e l i m i n a r y study r e p r e s e n t s the f i r s t c l e a r e v i d e n c e of enhanced g r a i n b o u n d a r y s e g r e g a t i o n of c a d m i u m in a c o m m e r c i a l a l u m i n u m alloy. Studies r e l a t e d to g r a i n b o u n d a r y s e g r e g a t i o n in a l u m i n u m alloys a r e few.~ G r e e n and Montague 2 p r e sented m a g n e s i u m and zinc depth p r o f i l e s in t h e i r study of s t r e s s c o r r o s i o n c r a c k i n g in A 1 - Z n - M g alum i n u m a l l o y s . They also indicated that with a q u a t e r n a r y addition of 0.28 pct Cd, that an e x t r e m e l y s m a l l a m o u n t of Cd, p r o b a b l y l e s s than about 0.2 pct, was detected on the s u r f a c e but this does not indicate g r a i n b o u n d a r y e n r i c h m e n t . Joshi, e t al 3 r e l a t e d solute c o n c e n t r a t i o n s at g r a i n b o u n d a r i e s to heat t r e a t m e n t in a 7075 a l u m i n u m alloy but found no g r a i n b o u n d a r y s e g r e g a t i o n of c a d m i u m . It is e x t r e m e l y i m p o r t a n t to note that the 7075 alloy studied by J o s h i , e t al 3 was in the f o r m of 10 m m (3/8 in.) rod swaged to 3.66 m m (0.144 in.) and thus only the l o n g i t u d i n a l ( L - T ) o r i e n tation was e x a m i n e d . In G r e e n and M o n t a g u e ' s t e s t s , the s a m p l e s were cut f r o m s p e c i a l l y p r e p a r e d l a b o r a tory heats in the f o r m of sheet s p e c i m e n s , 0.8 m m thick, and thus were p r o b a b l y of the L - T o r i e n t a t i o n . As only one o r i e n t a t i o n was e x a m i n e d in both of the p r e v i o u s studies, it was decided to a n a l y z e the L - T , T - L and S - T o r i e n t a t i o n s f r o m a c o m m e r c i a l 7075 a l u m i n u m plate. The alloy was a c o m m e r c i a l l y a v a i l a b l e 7075-T6 a l u m i n u m alloy in plate f o r m with a bulk a n a l y s i s as shown in T a b l e I. Since the detection t h r e s h o l d of the bulk a n a l y s i s was 0.05 pct, the p r e s e n c e of c a d m i u m as a t r a m p e l e m e n t might have b e e n overlooked. Two s u b s e q u e n t t r a c e e l e m e n t a n a l y s e s showed no c a d m i u m up to 0.004 wt pct but a t h i r d with a s p e c i a l low cadm i u m s t a n d a r d u s i n g a t o m i c a b s o r p t i o n and a graphite c r u c i b l e p r o v i d e d a t r a c e of 0.0003 wt pct Cd. In the p r e s e n t study, S c a n n i n g A u g e r E l e c t r o n S p e c t r o s c o p y was used to e x a m i n e the f r a c t u r e s u r f a c e s . The s a m -
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Fig. 1--Raster locations on fracture surface.
Table I. 7075 Aluminum Alloying Element Composition, Wt Pct
Nominal Analy
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