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R. T. HOLT AND E. TEGHTSOONIAN

C o b a l t s i n g l e c r y s t a l s have b e e n d e f o r m e d and s u b s e q u e n t l y h e a t e d (or c o o l e d ) t h r o u g h the t r a n s f o r m a t i o n b e f o r e f u r t h e r d e f o r m a t i o n . It h a s b e e n found t h e flow s t r e s s d o e s not c h a n g e on p a s s i n g t h r o u g h the t r a n s f o r m a t i o n and i s dependent only on the d e f o r m a t i o n h i s t o r y . On the o t h e r hand, the w o r k h a r d e n i n g r a t e O/G i s a l w a y s found to be low (-~0.3 • 10 -4) in t h e cph p h a s e and m u c h h i g h e r in the fcc p h a s e , f r o m 0.5 • 10 -3 to 2.8 • 10 -3 depending on the d e f o r m a t i o n and t r a n s f o r m a t i o n h i s t o r y . The r e s u l t s a r e d i s c u s s e d in t e r m s of f o r e s t m o d e l s of w o r k h a r d e n i n g and l a t e n t h a r d e n i n g t h e o r i e s .

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a r e c e n t p a p e r 1 the t e n s i l e p r o p e r t i e s of s i n g l e c r y s t a l s of c o b a l t w e r e r e p o r t e d a s a function of t e m p e r a t u r e . An i m p o r t a n t r e s u l t c o n c e r n s the condition g o v e r n i n g the o p e r a t i o n of s l i p in the fcc f o r m at T > 420~ The s l i p s y s t e m that o p e r a t e s in t h e fcc f o r m i s not n e c e s s a r i l y the one with the h i g h e s t Schmid f a c t o r . The s i t u a t i o n can b e s t be d e s c r i b e d in t e r m s of a p a r a m e t e r we have c a l l e d the " t r a n s f o r m a t i o n f a c t o r " , which i s defined a s the r a t i o of the Schmid f a c t o r on the m o s t f a v o r a b l y o r i e n t a t e d {111} fcc s l i p s y s t e m to the S c h m i d f a c t o r of the {111} fcc s l i p s y s t e m c o r r e s p o n d i n g to t h e t r a n s f o r m e d {0001} b a s a l p l a n e in the cph f o r m . Thus our e a r l i e r o b s e r v a t i o n s showed that the t r a n s f o r m e d b a s a l p l a n e i s f a v o r e d a s the o p e r a t i v e {111} fcc s l i p p l a n e , even when the t r a n s f o r m a t i o n f a c t o r i s a s high a s 1.3. Only when the t r a n s f o r m a t i o n f a c t o r e x c e e d s 1.3 d o e s the fcc c r y s t a l d e f o r m in a p r i m a r y s l i p s y s t e m in a way to be e x p e c t e d on the b a s i s of the u s u a l Schmid law c o n s i d e r a t i o n s . The p u r p o s e of t h i s p a p e r i s to r e p o r t on the m o r e d e t a i l e d i n v e s t i g a t i o n of t h i s f e a t u r e , and in p a r t i c u l a r , to d e s c r i b e the w a y s in which the d e f o r m a t i o n b e h a v i o r in cobalt in e i t h e r p h a s e i s influenced by p r e v i o u s d e f o r m a t i o n in the o t h e r p h a s e . 15 E X P E R I M E N T A L PROCEDURE AND RESULTS Both the p r e p a r a t i o n of s i n g l e c r y s t a l s of c o b a l t in an e l e c t r o n b e a m floating zone f u r n a c e and the g e n e r a l c o n d i t i o n s of high t e m p e r a t u r e t e n s i l e t e s t i n g have b e e n previ