Cleavage in beryllium monocrystals
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E R Y L L I U M and its a l l o y s a r e p o t e n t i a l l y useful m a t e r i a l s for a e r o s p a c e a p p l i c a t i o n s b e c a u s e of t h e i r high s t r e n g t h to d e n s i t y r a t i o , e.g., ~15 • 105 in. at 25~ 1 At the p r e s e n t t i m e , h o w e v e r , t h e i r use is l i m i t e d by t h e i r e x t r e m e b r i t t l e n e s s , f a i l u r e usually o c c u r r i n g by c l e a v a g e on the b a s a l plane.~ E v e n so, few s y s t e m a t i c s t u d i e s of the f r a c t u r e b e h a v i o r of b e r y l l i u m have b e e n r e p o r t e d , and t h e r e f o r e the p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to gain insight into the f r a c t u r e p r o c e s s . The p r i n c i p a l a i m was to study the p ro p ag a t io n of b a s a l c l e a v a g e c r a c k s in m o n o c r y s t a l s using the double c a n t i l e v e r t e c h n i q u e , 2 and so d e t e r m i n e the c l e a v a g e s u r f a c e e n e r g y . In addition, a study was made of c r a c k initiation in d a m a g e - f r e e and in " i n d e n t e d " m o n o c r y s t a l s .
to ~800 psi for the P e c h i n e y SR g r a d e m o n o c r y s t a l s . An a l l o y e d m o n o c r y s t a l containing c o p p e r a l s o was p r e p a r e d f r o m P e c h i n e y SR g r a d e rod. F o l l o w i n g z o n e r e f i n i n g (3 p u r i f i c a t i o n p a s s e s w e r e made at a r a t e of -~ iph), e v e n l y s p a c e d h o l e s w e r e d r i l l e d on o p p o s i t e s i d e s of the c r y s t a l , and an amount of high p u r i t y c o p p e r e q u i v a l e n t to 1.5 wt pct was i n s e r t e d into t h e s e . S e v e r a l h o m o g e n i z i n g p a s s e s w e r e then m a d e at a r a t e of 1 iph in a l t e r n a t e d i r e c t i o n s . Subsequent m a c r o e t c h i n g of the c r y s t a l in 500 m l of H3PO4 containing 59 g of CrO3 and 25 m l H2SO4 at 44~ 3 r e v e a l e d a m a r k e d s u b s t r u c t u r e . T h i s was r e m o v e d by m e a n s of a s e e d i n g p a s s i n i t i a t e d in a high p u r i t y SR g r a d e unalloyed m o n o c r y s t a l s e e d . The r e s u l t i n g m o n o c r y s t a l co n t ai n ed ~1.49 wt pct Cu, and had a usable length of ~2.5 in., with • -~ 5 deg.
I) M A T E R I A L S The m o n o c r y s t a l s w e r e grown by N u c l e a r M e t a l s Inc., C o n c o r d , M a s s . M a t e r i a l of two l e v e l s of purity was used, n a m e l y P e c h i n e y SR g r a d e b a r s (1~ in diam and 4 in. long) p u r c h a s e d f r o m G e n e r a l A s t r o m e t a l s , Y o n k e r s , N.Y., and d i s t i l l e d b e r y l l i u m p r o d u c e d by N u c l e a r M e t a l s Inc. C h e m i c a l a n a l y s e s f o r t h e s e m a t e r i a l s a r e given in T a b l e I. Four monocrystals were prepared from Pechiney SR g rad e e x t r u d e d b a r s using a z o n e - r e f i n i n g t e c h nique. 3 The angle, X, b e tw e e n the b a s a l plane and the long a x i s of the c r y s t a l (also the t e n s i l e a
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