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w h e r e ~ is the s h e a r s t r a i n r a t e , A is a c o n s t a n t w h o se v a l u e is 680 • 20 (MN/m2) -4"s. The s t r e s s e x p o n e n t n --- 4.3 ~- 0.3 and the a c t i v a t i o n e n e r g y Q = 2.05 • 0.15 eV. It is p r o p o s e d t h a t the high t e m p e r a t u r e p r i s m a t i c s l i p in Zr is c o n t r o l l e d by a g l i d e - c l i m b p r o c e s s w h e r e t h e r a t e of p l a s t i c flow is d e t e r m i n e d by the r a t e of c l i m b of d i s l o c a t i o n s .

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is w e l l e s t a b l i s h e d now that f i r s t o r d e r p r i s m a t i c s l i p is t h e p r i m a r y d e f o r m a t i o n m o d e in ~ - Z r at a l l t e m p e r a t u r e s . 1'2 The low t e m p e r a t u r e r a t e c o n t r o l l i n g m e c h a n i s m , 3 the n a t u r e of w o r k h a r d e n i n g , 4 and the e f f e c t of i n t e r s t i t i a l s o l u t e s s,~ on th e e a s e of p r i s m a t i c slip have been examined earlier using single crystals. The r a t e c o n t r o l l i n g m e c h a n i s m at e l e v a t e d t e m p e r a t u r e s h a s , h o w e v e r , not b e e n 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 in s i n g l e c r y s t a l s , although M i l l s and C r a i g s h a v e m e a s u r e d t h e 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 and the a c t i v a t i o n v o l u m e at t h e o n s e t of p r i s m a t i c s l i p at t e m p e r a t u r e s up to 950 K. T h e s e a u t h o r s h a v e a l s o s u g g e s t e d that ab o v e 800 K the r a t e c o n t r o l l i n g m e c h a n i s m m a y be the a r r i v a l of v a c a n c i e s at j o g s . P o l y c r y s t a l l i n e Zr, on the o t h e r hand, h a s b e e n e x a m i n e d in g r e a t e r d e t a i l at e l e v a t e d t e m p e r a t u r e s and the r e s u l t s of c r e e p t e s t s 7-9 as w e l l as t h o s e of t e n s i o n I~ ~ and c o m p r e s s i o n "2 s t u d i e s a r e a v a i l a b l e . The a i m of t h e p r e s e n t i n v e s t i g a t i o n was to d e f o r m in t e n s i o n o r i e n t e d s i n g l e c r y s t a l s of Zr in p r i s m a t i c s l i p at t e m p e r a t u r e s b e t w e e n 473 and l l l 3 ~ and o v e r a wide r a n g e of s t r a i n r a t e s , and to i d e n t i f y the d i s l o c a t i o n m e c h a n i s m r e s p o n s i b l e f o r p l a s t i c flow at e l e vated temperatures. E X P E R I M E N T A L METHOD C r y s t a l b a r Zr in t h e f o r m of 4.5 m m r o d s and 22 c m in length was u s e d as the s t a r t i n g m a t e r i a l . Single c r y s t a l s up to 20 c m in length w e r e g r o w n f r o m the m e l t u s i n g an e l e c t r o n b e a m f l o a t i n g zone t e c h n i q u e . Suitable o r i e n t a t i o n s w e r e c h o s e n out of r a n d o m l y g r o w n c r y s t a l s . T h e d e t a i l s of c r y s t a l g r o w t h and specimen preparation have been described elsewherefl'4 The fi n al s p e c i m e n s p r i o r to t e s t i n g had a r e d u c e d g a g e length of 2.54 c m and a gage d i a m of 2.