Microstructural observations of metal powders using analytical electron microscopy
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w h e r e a is the r a d i u s of the c r a c k , and this is d i s p l a y e d g r a p h i c a l l y in F i g . 2. C o m p a r i s o n with F i g . 1 i n d i c a t e s that a g r e e m e n t i s good, although it is p r o b a b l e that the c r a c k f r o n t s a r e not p r e c i s e l y s e m i c i r c u l a r in p r a c t i c e . The a p p r o x i m a t e l y s e m i c i r c u l a r c r a c k p r o f i l e p r e s u m a b l y r e s u l t s f r o m the pinning effect of the thin l a y e r s of d u c t i l e / 3 p h a s e which s e p a r a t e the o~ p l a t e s . The c l e a v a g e s t e p s a r e c o n t i n u a l l y c r e a t e d at the c r a c k front and t e r m i n a t e at the a-/3 i n t e r f a c e . T h e o r i g i n of the s t e p s is not u n d e r s t o o d , a l though it i s p o s s i b l e that they a r e g e n e r a t e d when the crack intersects dislocations. B a s e d on this model, the f r a c t o g r a p h y within an a g g r e g a t e of t~ p l a t e s can be i n t e r p r e t e d . F o r e x a m p l e , the d i r e c t i o n of the c l e a v a g e s t e p s in F i g . 3 i n d i c a t e s that a c r a c k t r a v e l l i n g f r o m r i g h t to left in c~-plate A e n t e r e d a d j a c e n t p l a t e 13 at C, w h e r e it s p r e a d in opp o s i t e d i r e c t i o n s within that p l a t e . It follows that r e gions of plate A f r a c t u r e d b e f o r e n e i g h b o r i n g r e g i o n s on the o t h e r s i d e of the/3 f i l m in p l a t e B, r e q u i r i n g that the c r a c k - t i p o p e n i n g d i s p l a c e m e n t f o r c l e a v a g e of the a be l e s s than the s t r a i n n e c e s s a r y to r u p t u r e the i3. S p r e a d i n g of c r a c k s into n e i g h b o r i n g p l a t e s was c o m m o n l y o b s e r v e d , and o c c u r r e d at r e g i o n s w h e r e the ~3 f i l m was a b s e n t . Thus it a p p e a r s that, in our e x p e r i m e n t s , c r a c k s w e r e confined to s i n g l e ~ p l a t e s by the e n v e l o p e of d u c t i l e /3, a c c o u n t i n g for the g e n e r a l f a n - l i k e m o r p h o l o g y of the c l e a v a g e s t e p s , and s p r e a d into n e i g h b o r i n g p l a t e s at b r e a k s in the d u c t i l e env e l o p e . It i s c l e a r f r o m F i g . 3 and f r o m the study of many s p e c i m e n s that the c r a c k f r o n t is f r e q u e n t l y i r r e g u l a r on a m i c r o s c o p i c s c a l e . It is concluded that the s i m i l a r i t i e s b e t w e e n the f r a c t u r e s u r f a c e s p r o d u c e d b y SCC and SSRHE and the r e a d y i n t e r p r e t a t i o n of t h e s e s u r f a c e s by the c l e a v a g e m o d e l l e a v e s l i t t l e doubt that the p r o p a g a t i o n of SCC in this a l l o y is m e c h a n i c a l . F u r t h e r , the c o m p a r i s o n p r o v i d e s s t r o n g i n d i r e c t e v i d e n c e that SCC a l s o inv o l v e s h y d r o g e n , g e n e r a t e d in this c a s e by the aqueous solution. The a u t h o r s wish to acknowledge U. / 3 e r t o
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