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M. I. de VRIES is a Scientist and A. MASTENBROEKis a Technician with the Netherlands Energy Research Foundation, ECN, Westerduinweg 3, Petten (N-H), The Netherlands. Manuscript submitted February 22, 1977. METALLURGICALTRANSACTIONSA

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M. I. DE VRIES AND A. MASTENBROEK P l a s t i c d e f o r m a t i o n a r o u n d fatigue c r a c k s can be o b s e r v e d with different t e c h n i q u e s . P i n e a u and P e l loux have m e a s u r e d p l a s t i c zones r a n g i n g f r o m 2 m m to 0.2 m m in fatigue t e s t e d F e - N i alloys and type 301 s t a i n l e s s s t e e l with h a r d n e s s i n d e n t a t i o n s . ~ D i s l o c a tion s u b s t r u c t u r e s can be o b s e r v e d in v e r y l i m i t e d t h i n n e d a r e a s by m e a n s of t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM). Awatani and S h i r a i s h i o b s e r v e d d i s l o c a t i o n s u b s t r u c t u r e s a r o u n d fatigue c r a c k s in type 304 s t a i n l e s s s t e e l with a 2000 kV e l e c t r o n m i croscope.2 In this paper S E M - o b s e r v a t i o n s of p l a s t i c d e f o r m a t i o n in the v i c i n i t y of fatigue c r a c k s grown in C e n t e r Notched (CN) s p e c i m e n s a r e c o m p a r e d with r e s u l t s f r o m m i c r o h a r d n e s s m e a s u r e m e n t s and with T E M o b s e r v a t i o n s of m a t e r i a l j u s t below the f r a c t u r e s u r f a c e of a b r o k e n CN s p e c i m e n and of bulk m a t e r i a l f r o m low cycle fatigue s p e c i m e n s . The p o s s i b i l i t y to o b s e r v e l a t t i c e defects by the use of back s c a t t e r e d e l e c t r o n s has b e e n d e m o n s t r a t e d for the f i r s t t i m e by C l a r k e and S t e r n in 1971 and 1972. 3,4 We have o b s e r v e d that this technique is e s p e c i a l l y v a l u a b l e for o b s e r v a t i o n of s u b s t r u c t u r e s like d i s location n e t w o r k s and c e l l s , often s e e n in fatigue t e s t e d m a t e r i a l s by m e a n s of TEM. Although T E M r e v e a l s m o r e d e t a i l s , SEM o b s e r v a t i o n s of such defects a g r e e v e r y well with TEM o b s e r v a t i o n s . This is dem o n s t r a t e d in Fig. 1, which shows the d i s l o c a t i o n subs t r u c t u r e of a fatigue t e s t e d s a m p l e as o b s e r v e d by T E M and SEM. SEM has the advantage that l a r g e a r e a s , l i m i t e d only by the d i m e n s i o n s of the s a m p l e , can be studied. A type 304 fully a u s t e n i t i c s t a i n l e s s s t e e l with 18 pct Cr and 11 pct Ni was used for the e x p e r i m e n t s . The m a t e r i a l was in the a s - r e c r y s t a l l i z e d condition with ASTM g r a i n size n u m b e r r a n g i n g f r o m 3 to 5. F a t i g u e t e s t s were p e r f o r m e d at r o o m t e m p e r a t u r e on CN s p e c i m e n s with d i m e n s i o n s W = 45 m m , B = 6 mm, L = 340 m m and c r a c k s t a r t e r notch = 10 m m . P l a s t i c d e f o r m a t i o n a r o u n d c r a c k tips was e