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T H E c r y s t a l l o g r a p h y of s l i p in b c c m e t a l s h a s b e e n the s u b j e c t of m a n y i n v e s t i g a t i o n s which have p r o d u c e d an a s s o r t m e n t of o b s e r v a t i o n s c o n c e r n i n g the a c t i v e s l i p p l a n e s . T h i s s i t u a t i o n h a s l e d to c o n s i d e r a b l e c o n fusion in s p e c i f y i n g the s l i p p l a n e s in b c c m e t a l s . (See Ref. 1 f o r a t a b u l a t i o n of the r e p o r t e d s l i p p l a n e s in bcc m e t a l s ) . R e c e n t findings that m a y c l a r i f y the s l i p p r o c e s s in b c c m e t a l s a r e t h o s e c o n c e r n e d with the a s y m m e t r y of s l i p ; t h e e a s e with which s l i p o c c u r s d e p e n d s on the s e n s e and the d i r e c t i o n of the a p p l i e d s t r e s s . T h i s e f fect h a s b e e n o b s e r v e d in s i n g l e c r y s t a l s o f / 3 - b r a s s , 2 F e - S i , S-7 Nb, l'a-m Mo, 1a-15 W, 16-1a T a , 13'19 and F e 20-~. T h e s e s t u d i e s have a l s o shown that the c r i t i c a l r e s o l v e d s h e a r s t r e s s law (i.e., s l i p b e g i n s at a c r i t i c a l v a l u e of the s h e a r s t r e s s on a s l i p plane) d o e s not at)pear to be o b e y e d in b c c m e t a l s , e s p e c i a l l y at low t e m p e r a t u r e s (see Ref. 25 for a r e c e n t r e v i e w of the e x p e r i m e n t a l findings). T h i s a s y m m e t r y of s l i p in b c c m e t a l s h a s b e e n o b s e r v e d in v a r i o u s w a y s ; d i f f e r e n t s l i p 0 l a n e s o p e r a t e in c r y s t a l s of s i m i l a r o r i e n t a t i o n depending on w h e t h e r they a r e d e f o r m e d in t e n s i o n o r c o m p r e s sion; the c r i t i c a l r e s o l v e d s h e a r s t r e s s f o r s l i p on a {112} (111) s y s t e m is d i f f e r e n t in t e n s i o n and c o m p r e s sion, the c r i t i c a l r e s o l v e d s h e a r s t r e s s in t e n s i o n f o r s l i p on a {112}(111) s y s t e m i s s m a l l e r if the (111) d i r e c t i o n c o r r e s p o n d s to a twinning d i r e c t i o n . In t h i s study we u s e d t h i s l a s t m e t h o d to o b s e r v e a s y m m e t r y of s l i p d u r i n g t e n s i l e t e s t i n g of i r o n s i n g l e c r y s t a l s . T h e s e had m a x i m u m r e s o l v e d s h e a r s t r e s s p l a n e s (MRSSP) that v a r i e d f r o m ([[2) to (211) along the g r e a t c i r c l e of the [111] zone. The s l i p p r o c e s s e s w e r e i n v e s t i g a t e d s y s t e m a t i c a l l y a s a function of o r i e n t a t i o n W. A. SPITZIG is with the Edgar C. Bain Laboratory for Fundamental Research, U. S. Steel Corp., Research Center, Monroeville, Pa. A.S. KEH, formerly with the Edgar C. Bain Laboratory for Fundamental Research, is deceased. Manuscript submitted April 2, 1970. METALLURGICAL TRANSACTIONS

and t e m p e r a t u r e . To r e d u c e the influence of d i f f e r e n c e s in p u r i t y on the o p e r a t i v e s l i p s y s t e m a l l s p e c i m e n s w e