The effect of heat treatment on the fatigue and corrosion fatigue behavior of a CuNiCr alloy

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D. J. DUQUETTE

F a t i g u e e x p e r i m e n t s were conducted on a CuNiCr alloy (IN838) in a i r and in 0.5 N NaC1 s o l u t i o n s u n d e r conditions of free c o r r o s i o n and of applied anodic c u r r e n t s . The alloy was heat t r e a t e d to produce a s o l u t i o n i z e d s t r u c t u r e and a l s o to produce a p r e c i p i t a t i o n h a r d e n e d s t r u c t u r e . The fatigue b e h a v i o r of the s o l u t i o n i z e d alloy was unaffected by free c o r r o s i o n although i n c r e a s e d c o r r o s i o n r a t e s r e s u l t e d in a d e c r e a s e in fatigue r e s i s t a n c e for s m a l l applied anodic c u r r e n t s . The age h a r d e n e d alloy showed a d e c r e a s e in fatigue r e s i s t a n c e u n d e r free c o r r o s i o n conditions and a f u r t h e r d e c r e a s e in r e s i s t a n c e with s m a l l applied anodic c u r r e n t s . In both heat t r e a t m e n t s fatigue in a i r r e s u l t e d in mixed t r a n s g r a n u l a r - i n t e r g r a n u l a r c r a c k i n i t i a t i o n and p r o p a g a t i o n while c o r r o s i o n i n c r e a s e d the r e l a t i v e a m o u n t of i n t e r g r a n u l a r c r a c k i n g . T h e s e r e s u l t s can be explained by a cons i d e r a t i o n of p r e v i o u s l y developed fatigue and c o r r o s i o n fatigue models of pure copper and copper a l u m i n u m a l l o y s .

A r e c e n t study of the f u n d a m e n t a l a s p e c t s of c o r r o s i o n fatigue u t i l i z i n g p o l y c r y s t a l l i n e and s i n g l e c r y s t a l copper and a copper a l u m i n u m alloy s u g g e s t e d that a g r e s s i v e e n v i r o n m e n t s , in c o m b i n a t i o n with cyclic s t r e s s e s or s t r a i n s , affect s u r f a c e slip c h a r a c t e r as well as fatigue c r a c k i n i t i a t i o n and p r o p a g a t i o n . 1-~ A d d i t i o n a l l y , c o r r o s i o n a l t e r s the c r a c k path f r o m t r a n s c r y s t a l l i n e in l a b o r a t o r y a i r to e i t h e r p a r t i a l l y or totally i n t e r g r a n u l a r . T h e s e r e s u l t s were explained on the b a s i s of e n h a n c e d localized d i s s o l u t i o n of s t r a i n e d r e g i o n s a s s o c i a t e d with p e r s i s t e n t slip b a n d s or with g r a i n b o u n d a r y r e g i o n s , and it was suggested that the d e f o r m a t i o n c h a r a c t e r a n d / o r s t r u c t u r e of the m e t a l or alloy, as well as the alloy c o m p o s i t i o n a r e i m p o r t a n t f a c t o r s in the c o r r o s i o n fatigue p h e n o m e n o n . T h i s work r e p o r t s a c o n t i n u a t i o n of that study to a copper alloy which can be heat t r e a t e d to produce a homogeneous s t r u c t u r e (solutionized, wavy slip) or a p r e c i p i t a t i o n h a r d e n e d s t r u c t u r e (aged, p l a n a r slip). A d d i t i o n a l l y the models developed for pure m e t a l s and alloys w e r e extended to a r e l e v a n t e n g i n e e r i n g alloy. EXPERIMENTAL

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