An investigation of strengthening by spherical coherent G. P. Zones
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I n c r e a s e s in the c r i t i c a l r e s o l v e d s h e a r s t r e s s of A1-Zn single c r y s t a l s w e r e m e a s u r e d at 77~ as a function of the v o l u m e f r a c t i o n and a v e r a g e s i z e of the G u i n i e r - P r e s t o n (G.P.) zone population. Zone v o l u m e f r a c t i o n s r a n g i n g f r o m 1.45 to 13 pct w e r e st u d i ed which included a v e r a g e zone s i z e s of f r o m 8 to 90A. The G.P. zone s i z e s w e r e d e t e r m i n e d by an X - r a y s m a l l angle s c a t t e r i n g technique d e v e l o p e d by H a r k n e s s et al. The i n c r e a s e in the c r i t i c a l r e s o l v e d s h e a r s t r e s s was shown to be d e t e r m i n e d by the i n c r e a s e d Zn-A1 i n t e r f a c e c r e a t e d by d i s l o c a t i o n s h e a r through the G . P . zone. The s u r f a c e e n e r g y of the c o h e r e n t b o u n d a r y b e t w e e n zone and m a t r i x was found to be 320 e r g s p e r sq cm by this a p p r o a c h . F u r t h e r m o r e , a m o d e l b a s e d on s u r f a c e c r e a t i o n was shown to be c o n s i s t e n t with o t h e r g e n e r a l f e a t u r e s of the d e f o r m a t i o n process.
T H E d r a m a t i c s t r e n g t h e n i n g a c c o m p a n y i n g the p r e s ence of G u i n i e r - P r e s t o n (G.P.) z o n e s has a r o u s e d much r e c e n t i n t e r e s t . Many w o r k e r s have b e e n q u i ck to r e a l i z e that t h e s e s y s t e m s d i f f e r s i g n i f i c a n t l y f r o m o t h e r age h a r d e n i n g a l l o y s . Of p r i m a r y i m p o r t a n c e is the fact that slip d i s l o c a t i o n s cut through the z o n e s due to t h e i r c o m b i n a t i o n of c o h e r e n c y , s m a l l s i z e , 10 to 100/~, and high n u m e r i c a l d e n s i t y , 10 t8 p e r cu c m . It is the p u r p o s e of this p a p e r to d e t e r m i n e the s t r e n g t h e n i n g i n t e r a c t i o n s r e s p o n s i b l e f o r the l a r g e i n c r e a s e s in c r i t i c a l r e s o l v e d s h e a r t s t r e s s (CRSS) which a c c o m p a n y the zonal state in the A1-Zn s y s t e m . In e v a l u a t i n g s t r e n g t h e n i n g m e c h a n i s m s it i s h e l p ful to c o n s i d e r the p o s s i b l e i n t e r a c t i o n b e t w e e n a slip d i s l o c a t i o n and a c o h e r e n t G . P . zone in a fcc s y s t e m . The t h r e e m a j o r ones a r e : 1) As a s l i p d i s l o c a t i o n a p p r o a c h e s a c o h e r e n t z o n e , it m u s t i n t e r a c t with c o h e r e n c y s t r e s s e s c a u s e d by the zone. H a b e r k o r n and G e r a l d 2 show that this i n t e r action will r e a c h a m a x i m u m at s o m e s m a l l d i s t a n c e f r o m the s u r f a c e of the zone. 2) As the d i s l o c a t i o n b e g i n s to cut the z o n e , Z n - Z n bonds will be b r o k e n and A I - Z n bonds c r e a t e d . T h i s i n t e r a c t i o n will r e a c h a m a x i m u m at a d i s t a n c e , b, into the zo n e, s i n c e a f t e r this point an i n
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