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M. ALLEN

AND

JOHN

B. VANDER

SANDE

The weak b e a m 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 technique was employed to study the e a r l y s t a g e s of p r e c i p i t a t i o n on d i s l o c a t i o n l i n e s in A1-3.87 wt pct Z n - 1 . 7 9 wt pct Mg. The h e t e r o g e n e o u s p r e c i p i t a t i o n s e q u e n c e was found to follow the homogeneous sequence in this alloy. The i n t e r a c t i o n b e t w e e n the i n i t i a l c o h e r e n t p r e c i p i t a t e p a r t i c l e s and the s t r a i n fields of the c a t a l y z i n g d i s l o c a t i o n s produced " g a p s " of b a c k g r o u n d i n t e n s i t y at p r e c i p i t a t e locations along the o t h e r w i s e continuous weak b e a m i m a g e s of the d i s l o c a t i o n l i n e s . A s i m p l e model was developed to r e l a t e a d i s t r i b u t i o n of m e a s u r e d weak b e a m gap lengths to a p a r t i c l e size d i s t r i b u t i o n at a given aging t r e a t m e n t . In this m a n n e r the growth k i n e t i c s of the i n i t i a l p r e c i p i t a t e phase was o b s e r v e d ; it was found that the p r e c i p i tation followed the C o t t r e l l - B i l b y t 2Is law, s u g g e s t i n g that m a t r i x d i s l o c a t i o n s may a s s i s t the growth of h e t e r o g e n e o u s p r e c i p i t a t e s in a m a n n e r analogous to g r a i n b o u n d a r y " c o l l e c t o r p l a t e s . " Weak b e a m m i c r o s c o p y was found to be s u p e r i o r to s t a n d a r d b r i g h t field m i c r o s c o p y for the c u r r e n t study. P a r t i c l e s too s m a l l to be v i s i b l e in b r i g h t field were r e v e a l e d in weak b e a m . Weak b e a m o b s e r v a t i o n s also indicated that the c o h e r e n t p r e cipitate p a r t i c l e s w e r e positioned a s y m m e t r i c a l l y about the d i s l o c a t i o n c o r e s .

2. EXPERIMENTAL

i . INTRODUCTION

WEAK beam microscopy is a dark field transmission electron microscopy technique whose strain-sensitive nature lends itself to the resolution of complex microstructures. The technique has been successfully applied to the observation of dislocation dissociations in fcc metals, 1,2 dislocation reactions during work hardening, 3 interphase boundaries, 4 grain boundaries, %6 and fine precipitate structure. %s One advantage of the weak beam technique in such studies is that theoretical considerations appear to justify a fairly straightforward geometrical interpretation of weak beam images .9 The current paper reports the results of experiments in which weak b e a m m i c r o s c o p y was used to e x a m i n e the growth k i n e t i c s of fine s e c o n d - p h a s e p a r t i c l e s which had f o r m e d h e t e r o g e n e o u s l y along d i s l o cation lines in an A l - Z n - M g alloy. D e t a i l e d studies of the growth k i n e t i c s of homogeneously nucleated, e a r l y stage p r e c i p i t a t e s in this alloy s y s t e m have been p e r f o r m e d p r e v i o u s l y . 1~ The aging