An observation by transmission electron microscopy of diffusional creep in a magnesium alloy
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i g . 1 - - 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 g r a p h of a r e g i o n at a g r a i n b o u n d a r y that w a s u n d e r t e n s i l e s t r e s s . T h e s t r e s s d i r e c t i o n i s i n d i c a t e d by the a r r o w . N o t e that the zone i s d e p l e t e d of p r e c i p i t a t e s but not completely d e n u d e d .
F i g . 2 - - 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 g r a p h y of a longitudinal g r a i n boundary. T h e s t r e s s d i r e c t i o n i s i n d i c a t e d by the a r r o w . N o t e the i n c r e a s e i n precipitate concentration in the vicinity of the g r a i n boundary.
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P. E. RAINEY, formerly Materials Engineer, Army Materials and Mechanics Research Center, Watertown,Mass., is now Metallurgist, Army Construction Engineering Research Laboratory, Champaign, Ill. D. L. H O L T , formerly Assistant Professor, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass., is now Associate Professor, School of Engineering, University of Auckland, Auckland, New Zealand. Manuscript submitted July 1, 1971. 3238-VOLUME 2 , NOVEMBER 1971
t e s t e d s p e c i m e n s ) w h i c h w e r e not f o u n d i n s p e c i m e n s q u e n c h e d f r o m that t e m p e r a t u r e . H o w e v e r , i t w a s not p o s s i b l e to i d e n t i f y t h e p h a s e g i v i n g r i s e t o t h e s e p e a k s . O n t h e b a s i s of t h e d i f f e r e n c e b e t w e e n t h e X - r a y d i f f r a c t i o n p a t t e r n s f r o m slowly c o o l e d a n d q u e n c h e d s p e c i m e n s , it is s u g g e s t e d t h a t t h e p r e c i p i t a t e s in t h e z o n e s f o r m d u r i n g c o o l i n g . T h e y a r e probably of m a g n e s i u m and zinc w h i c h w o u l d mean t h a t z o n e s f o r m by d i f f u s i o n o f z i n c a s w e l l a s m a g n e s i u m . If t h e s e e x t r a p r e c i p i t a t e s f o r m d u r i n g c o o l ing, t h e n additional t r a n s m i s s i o n electron microscopy METALLURGICAL TRANSACTIONS
on t e s t e ds p e c i m e n s s h o u l d r e v e a l completely d e n u d e d z o n e s . I t w a s not p o s s i b l e t o r a p i d l y c o o l o u r s p e c i mens a f t e r testing. D e n u d e d z o n e s were n o t s e e n in all g r a i n s , n o r w a s t h e w i d t h of t h e z o n e s t h e s a m e f o r a l l z o n e s . T h e a v e r a g e w i d t h o f t h e z o n e s w a s f o u n d t o be 3.6 p . T h i s i s h i g h e r t h a n t h e a v e r a g e z o n e w i d t h p e r g r a i n of 2.6 ~t m e a s u r e d by l i g h t m i c r o s c o p y . 8 T h e d i f f e r e n c e c o u l d be a c c o u n t e d f o r by t h e f a c t t h a t i n t h e p r e s e n t w o r k g r a i n s i n w h i c h z o n e s w e r e not o b s e r v e d w e r e nbt counted. T h e z o n e s f o r m e d m a i n l y at g r a i n b o u n d a r i e s n o r m a l to t h e s t r e s s a x i s ; h o w e v e r , a f e w were o b s e r v e d t o f o r m at b o u n d a r i e s p a r a l l e l to t h e s t r e s s a x i s . This probably c
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