Diffusion creep by dislocation climb in beryllium and Be-Cu single crystals
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W H E N a s i n g l e c r y s t a l of an hcp m e t a l i s c o m p r e s s e d along the ~ a x i s , glide can only o c c u r on the p y r a m i d a l p l a n e s of the s e c o n d kind {119.2}. In the c a s e of b e r y l l i u m w h e r e t h i s type of s l i p is v e r y difficult, c r e e p t e s t s conducted on s i n g l e c r y s t a l s c o m p r e s s e d along the ~ a x i s n e v e r t h e l e s s y i e l d high c r e e p r a t e s and c r e e p was shown to p r o c e e d b y p u r e c l i m b of ~ d i s l o c a t i o n s ( B u r g e r s v e c t o r [0001]) l y i n g in the b a s a l p l a n e s of the c r e p t s a m p l e s . Such e x p e r i m e n t s a r e t h e r e f o r e e x p e c t e d to i n c r e a s e our knowledge on the c l i m b p r o c e s s i t s e l f , but t h e i r q u a n t i t a t i v e a n a l y s i s is not s t r a i g h t f o r w a r d b e c a u s e of the a b s e n c e of s t e a d y - s t a t e c r e e p . * In the p r e s e n t work, we g a t h e r e d d a t a o v e r a w i d e r r a n g e of t e m p e r a t u r e and s t r e s s than was p r e v i o u s l y done and s t u d i e d the effect of c o p p e r a d d i t i o n on the a c t i v a t i o n e n e r g y for c r e e p . A c r e e p m o d e l is p r o p o s e d which a c c o u n t s for the o b s e r v e d b e h a v i o r .
PART I) CREEP OF BERYLLIUM SINGLE CRYSTALS
- - a t the b e g i n n i n g of the t e s t , the c r e e p r a t e d e c r e a s e s a s s t r a i n i n c r e a s e s . This p a r t of the c u r v e n e v e r e x t e n d s o v e r m o r e than 3 p c t s t r a i n , Fig. 1, and m a y even be v e r y s m a l l . - - a f t e r a few p e r c e n t s t r a i n , the c r e e p r a t e i n c r e a s e s with t i m e . Steady s t a t e is n e v e r obtained. 2) VARIATIONOF CREEP RATE WITH TIME It can be seen on the curve 4 vs t, Fig. 2, that the creep rate linearly increases with time over a wide strain range (~ is a constant). However ~ often suddenly increases at the end of the test (7 pct < e < 25 pct). In this case we always observe a macroscopic kinking of the specimen.
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I) E x p e r i m e n t a l Methods B e r y l l i u m S. R. s i n g l e c r y s t a l s w e r e grown b y the C z o c h r a l s k y method. A low m i s o r i e n t a t i o n (< 15') of the s u b s t r u c t u r e about the ~ a x i s was o b t a i n e d b y o r i enting the ~ a x i s of the s e e d p e r p e n d i c u l a r to the growth a x i s . 2 P a r a l l e l e p i p e d i c s p e c i m e n s w e r e s p a r k cut. To a v o i d b a s a l s l i p , the c o m p r e s s i o n f a c e s w e r e p a r a l l e l to the b a s a l p l a n e within 15'. The o r i e n t a t i o n technique has been described elsewherefl The s a m p l e s w e r e t e s t e d at c o n s t a n t s t r e s s (within 1 pct) in the t e m p e r a t u r e r a n g e 350 ~ to 1000~ at s t r e s s e s r a n g i n g f r o m 2.5 to 80 k g f / m m a. The t e m p e r a t u r e was m a i n t a i n e d c o n s t a n t to • and the s e n s i t i v i t y was about 0.01 p e t s t
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