High voltage metallography of stress corrosion failure in a Ni-16.5 At. Pct Au alloy
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I N r e c e n t y e a r s s e v e r a l i n v e s t i g a t o r s have used the e l e c t r o n m i c r o s c o p e to study the m o r p h o l o g i c a l changes which occur when alloys a r e exposed to c o r r o s i v e e n v i r o n m e n t s . The published m i c r o s t r u c t u r e s g e n e r a l l y show m o r e detailed i n f o r m a t i o n than can be obtained by o t h e r m e t a l l o g r a p h i c t e c h n i q u e s but the t h i n n e s s of electron microscope specimens makes their relevance to bulk c o r r o s i o n b e h a v i o r somewhat u n c e r t a i n . I n t e r p r e t a t i o n is e s p e c i a l l y difficult for thin s p e c i m e n s which a r e s t r e s s e d d u r i n g exposure since it has been o b s e r v e d that the modes of d e f o r m a t i o n and f r a c t u r e of thin f i l m s and bulk m a t e r i a l s a r e different. In p a r t i c u l a r , slip is l e s s homogeneous and the f r a c t u r e path i s c r y s t a l l o g r a p h i c and r e s u l t s from l a r g e a m o u n t s of s h e a r on single glide p l a n e s . 1 T h u s , c o r r o s i o n m a y n u c l e a t e along slip s t e p s in s t r e s s e d e l e c t r o n m i c r o scope s p e c i m e n s but not in the same m a t e r i a l when s t r e s s e d in bulk. Recently, F u j i t a 2 has shown that s p e c i m e n s which a r e just thick enough to exhibit bulk d e f o r m a t i o n b e h a v i o r can be p e n e t r a t e d by a 500 kv e l e c t r o n b e a m and produce an a c c e p t a b l e e l e c t r o n i m a g e . It t h e r e fore a p p e a r s possible that one of the above d e f i c i e n cies of the e l e c t r o n m i c r o s c o p e technique can be o v e r come in s t r e s s c o r r o s i o n s t u d i e s . In the p r e s e n t i n v e s t i g a t i o n the U.S. Steel high voltage m i c r o s c o p e was used to e x a m i n e the s t r e s s c o r r o s i o n b e h a v i o r of a Ni-16.5 at. pct Au alloy. An a t tempt was made to c o r r e l a t e e l e c t r o n m i c r o s t r u c t u r e s with light m i c r o s t r u c t u r e s which were found to be s i m i l a r to those obtained by Bakish and R o b e r t s o n 3,4 in t h e i r s t u d i e s of the s t r e s s c o r r o s i o n f a i l u r e of Cu3Au single c r y s t a l s . T h e s e w o r k e r s concluded that e m b r i t t l e m e n t o c c u r s as a r e s u l t of the s e l e c t i v e r e moval of copper f r o m the zone of complex d e f o r m a tion at the leading edge of the c r a c k . They found that although c r a c k s n u c l e a t e d at slip steps they tended to propagate n o r m a l to the t e n s i l e s t r e s s . P. R. SWANNformerly with Fundamental Research Laboratory, U. S. Steel Corporation, Monroeville,Pas., is Lecturer in Metallurgy, Imperial College, London. W. R. DUFF is Head Technician, Fundamental Research Laboratory, U. S. Steel Corporation. Manuscript submitted July 23, 1969. 4ETALLURGICALTRANSACTIONS
E X P E R I M E N T A L PROCEDURE T h i n film s p e c i m e n s of a Ni-16.5 at. pct Au alloy
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