Defect structures in fatigued Al-6.5 at. pet Zn alloy

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T H E R E e x i s t s a c o n s i d e r a b l e body of e v i d e n c e that the type and c o n c e n t r a t i o n of l a t t i c e d e f e c t s g e n e r a t e d by c y c l i c s t r a i n i n g d i f f e r f r o m those induced by u n i d i r e c t i o n a l s t r a i n i n g and that the c y c l i c a l l y induced d e f e c t s a c c e l e r a t e v a r i o u s p r o c e s s e s 1 such as p r e c i p i t a t i o n 2 and o r d e r i n g . 3 It has b e e n shown, for e x a m p l e , that the t o - a n d - f r o motion of jogged d i s l o c a t i o n s at s m a l l c y c l i c s t r a i n s a l t e r s s u b s t a n t i a l l y the d i s l o c a t i o n s u b s t r u c t u r e by enhanced d i s l o c a t i o n c l i m b , and that such motion has a m a r k e d e f f e c t on the s t r e s s c o n centration near internal boundaries. 4 T h i s p a p e r a i m s to c l a r i f y the e f f e c t s which p r e e x i s t ing l a t t i c e d e f e c t s e x e r t on the d e f e c t s t r u c t u r e p r o duced by c y c l i c s t r a i n i n g . A1-6.5 at. p c t Zn w a s s e l e c t e d as the a l l o y for this i n v e s t i g a t i o n , s i n c e a r e c e n t d e t a i l e d study has shown that the d e f e c t s t r u c t u r e i n duced in t h i s a l l o y c a n be w e l l d e f i n e d and that i t c o n s i s t s of q u e n c h e d - i n d i s l o c a t i o n l o o p s , h e l i c a l d i s l o c a t i o n s , and r e a c t i o n p r o d u c t s of t h e s e d e f e c t s , 5 a s w e l l a s of G. P . z o n e s . 6'7 P a r t i c u l a r a t t e n t i o n wiU be f o c u s e d in this study on the e f f e c t s of c y c l i c s t r a i n i n g upon both the p r e e x i s t i n g d e f e c t s t r u c t u r e s and the newly g e n e r a t e d d e f e c t s l o c a t e d in the v i c i n i t y of g r a i n boundaries. E X P E R I M E N T A L PROCEDURE An A1-6.5 at. p c t Zn alloy s u p p l i e d by R e y n o l d s M e t a l s C o m p a n y w a s c o l d - r o l l e d to s h e e t s of 0.25 m m t h i c k n e s s . S p e c i m e n s of s i z e 0.25 by 8 by 32 c u m m p r e p a r e d f r o m t h e s e s h e e t s w e r e h o m o g e n i z e d at 550~C in flowing a r g o n g a s f o r two h r and w e r e s u b s e q u e n t l y quenched into i c e - w a t e r . T h e quenched s p e c i m e n s w e r e aged at r o o m t e m p e r a t u r e for m o r e than two d a y s b e f o r e being s u b j e c t e d to the fatigue t e s t . The a v e r a g e g r a i n s i z e of the quenched and r o o m - t e m p e r a t u r e aged s p e c i m e n s w a s about 0.8 m m in d i a m e t e r . S p e c i m e n s w e r e c y c l i c a l l y s t r a i n e d by t e n s i o n and c o m p r e s s i o n along t h e i r l o n g e s t a x i s with z e r o m e a n s t r e s s . In o r d e r to p r e v e n t the s p e c i m e n s f r o m b u c k l i n g , a s p e c i a l a t t a c h m e n t was u s e d for mounting on S. WEISSMANNis Professor and Direc!or of Materials Research Laboratory, College of Engineering, Rutgers University, New Brunswick