The cohesion of previously fractured Fcc metals in ultrahigh vacuum
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AN u n d e r s t a n d i n g of the
cohesion* of m e t a l s is i m -
*The term cohesion is used loosely here to define both the bonding of a metal or alloy to itself and the bonding of dissimilar metals or alloys (adhesion).
p o r t a n t to such a r e a s as f r i c t i o n and w e a r , and bonding, j o i n i n g , and cladding. In addition, the s u b j e c t of c o h e sion in an u l t r a h i g h v a c u u m (defined h e r e as a p r e s s u r e < 10 -8 t o r r 1) has b e c o m e e s p e c i a l l y i m p o r t a n t in r e c e n t y e a r s in r e g a r d to the b e h a v i o r of c o m p o n e n t s and s y s t e m s in a space e n v i r o n m e n t and to the p o s s i b i l i t y of j o i n i n g m a t e r i a l s in space by cold weldingfl Studies of the c o h e s i o n of m e t a l s can a l s o p r o v i d e i n f o r m a t i o n c o n c e r n i n g the n a t u r e and b e h a v i o r of s u r f a c e s , which is of v a l u e to u n d e r s t a n d i n g such p h e n o m e n a as c a t a l y s i s , oxidation, and c o r r o s i o n . The objective of the p r e s e n t r e s e a r c h p r o g r a m was to develop a b e t t e r u n d e r s t a n d i n g of the cohesion of m e t a l s through a study of cold welding under u l t r a h i g h v a c u u m . I n i t i a l s t u d i e s have c o n c e n t r a t e d on m e t a l s with the fcc c r y s t a l s t r u c t u r e , n a m e l y Ag, A1, Cu, and Ni. T h e s e m a t e r i a l s r e p r e s e n t a r a n g e in such p r o p e r t i e s as e l e c t r o n i c s t r u c t u r e , e l a s t i c m o d u l u s , s t a c k ing fault e n e r g y , and s u r f a c e e n e r g y , all of which could be i m p o r t a n t in cohesion. M o r e o v e r , they were t e s t e d in c o n d i t i o n s and with m i c r o s t r u c t u r e s r e p r e s e n t i n g a r a n g e of m e c h a n i c a l p r o p e r t i e s , s i n c e s t r e n g t h and h a r d n e s s have b e e n c l a i m e d to be of i m p o r t a n c e in coh e s i o n . F o r all of these m a t e r i a l s , r o o m t e m p e r a t u r e is below the t e m p e r a t u r e w h e r e s e l f - d i f f u s i o n is e x p e c t e d to play a s i g n i f i c a n t r o l e . Since in cohesion s t u d i e s it is d e s i r a b l e to study s u r f a c e s which a r e f r e e of c o n t a m i n a t i n g g a s e s or f i l m s , it was decided to i n v e s t i g a t e the cohesion of s p e c i m e n s which has b e e n p r e v i o u s l y f r a c t u r e d in a v a c u u m , s i m i l a r to the e x p e r i m e n t s of Ham. 3 In this way, c l e a n s u r f a c e s can be p r o d u c e d with r e l a t i v e e a s e . P r e l i m i n a r y r e s u l t s on the cold welding of copper w e r e p r e s e n t e d in an e a r l i e r p a p e r 4 The p r e s e n t p a p e r c o v e r s a d d i t i o n a l m e a s u r e m e n t s on copper and r e s u l t s HANS CONRAD and LARRY RICE are Chairman and Research Assistant, respectively, Department of MetallurgicalEngineeringand Materials Science, University of Kentucky
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