Stress corrosion cracking of aluminum alloys
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O. SPEIDEL
S t r e s s c o r r o s i o n c r a c k i n g of a l u m i n u m alloys is reviewed. An e x t e n s i v e f a i l u r e a n a l y s i s shows how m a n y s e r v i c e f a i l u r e s o c c u r r e d in the a e r o s p a c e i n d u s t r y over a t e n y e a r p e r i o d and what kind of alloys and s t r e s s e s led to i n i t i a t i o n and p r o p a g a t i o n of s t r e s s c o r r o s i o n c r a c k s which c a u s e d t h e s e s e r v i c e f a i l u r e s . The paper c o n t a i n s m o s t of the r e s u l t s of s t r e s s c o r r o s i o n t e s t s with a l u m i n u m alloys that have b e e n obtained to date with f r a c t u r e m e c h a n i c s t e c h n i q u e s . S t r e s s c o r r o s i o n c r a c k growth r a t e m e a s u r e m e n t s a r e c o m p a r e d with the r e s u l t s f r o m smooth s p e c i m e n t e s t i n g and it is shown that the c o r r e l a t i o n b e t w e e n the different t e s t r e s u l t s is v e r y s a t i s f a c t o r y . The p r e s e n t and l i m i t e d s t a t u s of t h e o r e t i c a l u n d e r s t a n d i n g of s t r e s s c o r r o s i o n c r a c k i n g is outlined. A m a j o r p a r t of the paper is devoted to the r e s u l t s of the l a t e s t alloy development. High s t r e n g t h a l u m i n u m alloys of d r a m a t i c a l I y i n c r e a s e d s t r e s s c o r r o s i o n r e s i s t a n c e a r e a v a i l able now. In the n e a r f u t u r e , s t r e s s c o r r o s i o n r e s i s t a n t alloys of even higher s t r e n g t h might b e c o m e a v a i l a b l e . What is s t i l l lacking is a d e t a i l e d u n d e r s t a n d i n g of the m e c h a n i s m s b y which s t r e s s c o r r o s i o n c r a c k s initiate and propagate.
ALUMINUM
and all its alloys can fail by c r a c k i n g along g r a i n b o u n d a r i e s when s i m u l t a n e o u s l y exposed to specific e n v i r o n m e n t s and s t r e s s e s of sufficient magnitude. 1'2 Well known specific e n v i r o n m e n t s i n clude w a t e r v a p o r , aqueous s o l u t i o n s , o r g a n i c liquids, and liquid m e t a l s . S t r e s s e s sufficient for c r a c k i n i t i a tion and c r a c k growth can be f a r below the s t r e s s e s r e q u i r e d for g r o s s yielding, e s p e c i a l l y in those a l l o y / e n v i r o n m e n t c o m b i n a t i o n s which a r e of p r a c t i c a l i m p o r tance, e . g . , high s t r e n g t h a l u m i n u m alloys in a i r . This p h e n o m e n o n of e n v i r o n m e n t - i n d u c e d i n t e r g r a n u l a r c r a c k i n g is often c a l l e d s t r e s s c o r r o s i o n c r a c k i n g (SCC). It was f i r s t r e p o r t e d in 1919. 3 M e t a l l u r g i c a l p r o g r e s s in r e c e n t y e a r s has led to a m a r k e d i m p r o v e m e n t in the r e s i s t a n c e to s t r e s s c o r r o s i o n c r a c k i n g of high s t r e n g t h a l u m i n u m alloys. This has r e s u l t e d in a s i g n i f i c a n t r e d u c t i o n of s t r e s s c o r r o s i o n
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