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s t r u c t u r e s induced by shock d e f o r m a t i o n have been e x a m i n e d in fcc m e t a l s to a g r e a t e r d e g r e e than in any o t h er c l a s s of m a t e r i a l s . It is well known that the p a s s a g e of a shock wave through th e s e m a t e r i a l s r e s u l t s in a highly d e f e c t e d t e r m i n a l s t r u c t u r e . At low p r e s s u r e s , the s t r u c t u r e s a r e c h a r a c t e r i z e d by high d i s l o c a t i o n d e n s i t i e s , but beyond a c e r t a i n l e v e l twinning a p p e a r s as an additional d e f o r m a t i o n m o d e . At h i g h e r shock p r e s s u r e s , the t r a n s i e n t t e m p e r a t u r e a s s o c i a t e d with the shock wave r e s u l t s in a d e c r e a s e in the i nc i d en ce of twinning, and in s o m e i n s t a n c e s to c o m p l e t e or p a r t i a l spontaneous annealing of the s h o c k - d e f o r m e d s t r u c t u r e . This s e q u e n c e has been m o s t c l e a r l y identified in nickel. ~ In the c a s e of co p p e r , the s t u d i e s have been l e s s c o m p r e h e n s i v e , though a n u m b e r of s t u d ie s have been published, s i n c e the initial work of Smith 2 upon the c h a r a c t e r of s h o c k - l o a d e d s i n g l e c r y s t a l s of OFHC c o p p e r . Twinning has been o b s e r v e d in OFHC and Tough P i t c h c o p p e r a f t e r s h o c k loading s i n g l e and poly c r y s t a l l i n e s a m p l e s . 2-8 T h e i n c i d e n c e of twinning in p o l y c r y s t a l l i n e m a t e r i a l is p a r t i c u l a r l y m a r k e d above 250 kbar. 3 In s i n g l e c r y s t a l s , twinning has been r e p o r t e d in s a m p l e s shocked along the