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D e f o r m a t i o n of tungsten single c r y s t a l s a s a function of s t r a i n , t e m p e r a t u r e , and a l l o y i n g was s t u d i e d by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . Single c r y s t a l s o r i e n t e d for (]01)[111] s l i p w e r e grown by e l e c t r o n b e a m zone r e f i n i n g . C o m p r e s s i o n s p e c i m e n s of t u n g s t e n , W-1 and 3 pct Re and W - 1 and 3 pct T a w e r e d e f o r m e d to 2 pct s t r a i n at 150 ~ 300 ~ and 590~ (0.04, 0.08, and 0.16 Tin). S p e c i m e n s w e r e a l s o s t r a i n e d to 0.5 and 5.0 pet s t r a i n at 300~ Transmission mic r o s c o p y r e v e a l e d that the d i s l o c a t i o n s u b s t r u c t u r e s in s i n g l e - c r y s t a l tungsten a r e s i m i l a r to s u b s t r u c t u r e s in o t h e r r e f r a c t o r y m e t a l s when c o m p a r e d on a h o m o l o g o u s t e m p e r a t u r e b a s i s . At t e m p e r a t u r e s g r e a t e r than 0.1 Tin, the s u b s t r u c t u r e i s c h a r a c t e r i z e d p r i m a r i l y by edge d i p o l e s . At t e m p e r a t u r e s l e s s than 0.1 Tin, long s c r e w d i s l o c a t i o n s lying p a r a l l e l to the p r i m a r y [111] s l i p d i r e c t i o n c h a r a c t e r i z e the s u b s t r u c t u r e . R h e n i u m a d d i t i o n s to tungsten p r o m o t e f o r m a t i o n of edge d i p o l e s at t e m p e r a t u r e s of 300 ~ and 150~ and i n c r e a s e d i s l o c a t i o n d e n s i t y at a l l t h r e e t e m p e r a t u r e s . In a d d i t i o n , d i s l o c a t i o n s c o n s i s t e n t with (112)[111] s l i p w e r e o b s e r v e d in the W - R e single c r y s t a l s a f t e r d e f o r m a t i o n at 150~ T a n t a l u m a d d i t i o n s had a l e s s e r effect on the d i s l o c a t i o n s u b s t r u c t u r e c o m p a r e d to r h e n i u m a d d i t i o n s . The W - 1 and 3 pct T a a l l o y s e x h i b i t e d h i g h e r d i s l o c a t i o n d e n s i t i e s than u n a l l o y e d tungsten a f t e r s i m i l a r s t r a i n s and, at 150~ W - 3 pct T a c o n t a i n e d a few d i s l o c a t i o n s c o n s i s t e n t with (112)[111] s l i p . It is c o n c l u d e d that the r e d u c t i o n in d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e of p o l y c r y s t a l l i n e tungsten c o n t a i n i n g dilute r h e n i u m a d d i t i o n s , 1 to 5 pet, can be a t t r i b u t e d to an i n c r e a s e in d i s l o c a t i o n m o b i l i t y at t e m p e r a t u r e s l e s s than 0.1 Tin. D E F O R M A T I O N b e h a v i o r of the r e f r a c t o r y m e t a l s h a s b e e n the s u b j e c t of a n u m b e r of i n v e s t i g a t i o n s o v e r the p a s t d e c a d e . D i s l o c a t i o n a r r a n g e m e n t s have b e e n s t u d i e d by the technique of t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , m o s t f r e q u e n t l y in p o l y c r y s t a l l i n e m e t a l s d e f o r m e d by r o l l i n g and s u b s e q u e n