Production and mechanical behavior of very fine-grained copper
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Behavior of Very Fine-Grained Copper ANTHONY W. BACKOFEN
THOMPSON
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WALTER
A.
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s i z e of a b o u t 1 3 p m , w h i c h i s t y p i c a l of t h e l o w e r l i m i t s o b s e r v e d in the l i t e r a t u r e f o r p u r e c o p p e r . 2'~ S m a l l e r g r a i n s i z e s w e r e t h e n o b t a i n e d b y t w i s t i n g a r o d of 1 3 p r o g r a i n s i z e to a l a r g e s t r a i n , a n d r e c r y s t a l l i z i n g again. This torsion technique has the advantage that l a r g e s t r a i n s c a n b e a p p l i e d w i t h o u t c h a n g e s in s p e c i men shape. Standard tensile specimens were machined from the s w a g e d r o d , p o l i s h e d , a n n e a l e d to 13 p m g r a i n s i z e , a n d twisted in a lathe. The surface shear V is given by 7 = t O ~ l , w h e r e r = s p e c i m e n r a d i u s , 0 = r o t a t i o n in r a d i a n s , a n d l = l e n g t h of t h e t w i s t e d r e g i o n . S p e c i m e n s were twisted through 0/2 and untwisted the same a m o u n t . T h e r e s u l t a n t s t r a i n i s e q u i v a l e n t to t h a t p r o d u c e d in a u n i d i r e c t i o n a l t w i s t , b u t a v o i d s a l i g n i n g s u b microscopic flaws and particles along helical paths a n d t h e r e b y a f f e c t i n g f r a c t u r e b e h a v i o r . 4'5 S h e a r s t r a i n s r a n g i n g f r o m y = 2 to V = 10 w e r e i n vestigated. Although the shear strain depends linearly on radius, the grain size after recrystallization does n o t , a s i l l u s t r a t e d in F i g . 1. T h e g r a i n s i z e w a s m e a s u r e d a s t h e m e a n l i n e a r i n t e r c e p t , l , a n d c o n v e r t e d to true grain diameter, d, by d = 1.68T. 6 The uniform s m a l l g r a i n s i z e , i n t h i s c a s e 7.2 t~m, c o m p r i s e s a b o u t 75 p c t of t h e c r o s s - s e c t i o n a l a r e a ; t h e c e n t e r i s c o a r s e r , up to a b o u t 1 3 p m . T h i s u n i f o r m o u t e r v o l u m e w a s n o t o b s e r v e d b e l o w y --- 4, a n d d i d not i n c r e a s e s i g n i f i c a n t l y i n e x t e n t a b o v e v = 8. F o r t h i s r e a s o n , a t o t a l s h e a r s t r a i n of V = 8.0 w a s c h o s e n f o r t h e t o r s i o n a l cold work. The recrystallization temperature was established for e a c h t o r s i o n a l s t r a i n . F o r y = 8.0, t h e t e m p e r a t u r e w a s ANTHONY W. THOMPSON, formerly with Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass., is now with Metallurgy Division, Sandia Laboratories, Livermore, Calif. WALTER A. BACKOFEN is Professor, Department of Metallurgy and Materials Science, M.I.T. Manuscript submitted December 7, 1970. 2, J U L Y 1 9 7 1
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THE preparation of very fine-grained materials has received attention because of the considerable increases in yield strength due to grain refinement predicted by the Petch equation, ay = ao + kd -I12, where cr = stress and d = grain size. It is difficult to refine the grain size of pure metals, h
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