Fatigue Crack initiation and microcrack growth in 2024-T4 and 2124-T4 aluminum alloys
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ation a n d advancing c r a c k s o f t e n p a s s e d through such voids. G r o s s k r e u t z a n d S h a w ,2 on t h e o t h e r h a n d , o b s e r v e d that a l l f a t i g u e c r a c k s i n 2 0 2 4 - T 4 i n i t i a t e d in t h e r e g i o n o f c o n s t i t u e n t p a r t i c l e s . T h e i r s t u d i e s w e r e on notched s p e c i m e n s (elastic s t r e s s concentration fact o r , Kt, of 2) s t r e s s e d i n p u s h - p u l l a t a n e t s e c t i o n s t r e s s of 94 M N / m z. O p t i c a l m i c r o s c o p e e x a m i n a t i o n of t h e n o t c h s u r f a c e a n d o f r e p l i c a s s t r i p p e d f r o m t h e s u r f a c e were periodically m a d e . Electron microscope s t u d i e s o f t h e r e p l i c a s a n d of t h i n n e d s p e c i m e n s n e a r a d i m l ~ l e o n t h e b a s e of t h e n o t c h w e r e a l s o m a d e . E v e n t h o u g h t h e n o m i n a l a p p l i e d s t r e s s was o n e - h a l f t h e y i e l d s t r e s s o r l e s s , f i n e slip l i n e s in m a n y g r a i n s were o b s e r v e d . T h e r e was a m u c h h e a v i e r c o n c e n t r a t i o n o f s l i p l i n e s a r o u n d m a n y of the c o n s t i t u e n t p a r t i c l e s . T h e h e a v i l y c o n c e n t r a t e d s l i p l i n e r e g i o n s did not a p p e a r t o c o n t r i b u t e t o f a t i g u e c r a c k i n i t i a t i o n . A s o b s e r v e d with t h e optical microscope, t h e fatigue c r a c k s a l l i n i t i a t e d n e a r p a r t i c l e s w h i c h did not h a v e such c o n c e n t r a t e d slip l i n e r e g i o n s . C r a c k e d inclus i o n s w h i c h f o r m e d d u r i n g p r o c e s s i n g a l s o did not p l a y a r o l e in t h e i n i t i a t i o n . G r o s s k r e u t z a n d S h a w s u g g e s t e d t h a t t h e fatigue c r a c k s nucleated at t h e int e r f a c e b e t w e e n the m a t r i x a n d t h e p a r t i c l e s by i n t e r f a c e d e b o n d i n g a l t h o u g h t h e y did not p r e s e n t c o n v i n c i n g evidence f o r t h i s m o d e l . T h e y also o b s e r v e d t h a t l a r g e r c o n s t i t u e n t particles, t h a t is, p a r t i c l e s g r e a t e r t h a n 1 /~m i n s i z e , w e r e m o r e e f f e c t i v e i n i n i t i a t i n g fatigue c r a c k s t h a n s m a l l e r c o n s t i t u e n t p a r t i c l e s . T h e r e s u l t s of t h e p r e s e n t w o r k w h i c h w a s a n in situ optical quantitative metallographic s t u d y at magnificat i o n s up t o 800 t i m e s s h o w that w h e t h e r t h e f a t i g u e c r a c k s in 2 0 2 4 - T 4 i n i t i a t e at c o n s t i t u e n t p a r t i c l e s o r at c o a r s e slip b a n d s u n a s s o c i a t e d w i t h c o n s t i t u e n t particles depends upon t h e s t r e s s at t h e n o t c h t i p , t h e g r a i n s i z e , a n d t h e s i z e a n d d i s t r i b u t i o n of t h e c o n s t i tuent p a r t i c l e s . EXPERIMENTAL
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T h e 2024 a n d 2 1 2 4 " a l u m i n u m b a s e a l l o y s w e r e C. Y. KU
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