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f a c t o r s c o n t r o l l i n g the c r e e p r a t e of b e r y l l i u m s u c h as p u r i t y g r a i n size and d i s l o c a t i o n d e n s i t y have b e e n p r e v i o u s l y d e s c r i b e d . 1 In this i n v e s t i g a t i o n the o b s e r v a b l e m e c h a n i s m s of c r e e p a r e d e s c r i b e d . One of t h e s e m e c h a n i s m s , i.e. l o c a l i z e d def o r m a t i o n a r o u n d a liquid g r a i n b o u n d a r y s t r e s s conc e n t r a t i o n m a y be unique to b e r y l l i u m . 1.0 MATERIALS The m a t e r i a l s used i n this i n v e s t i g a t i o n a r e l i s t e d i n TabIe I. RR243, BSP9 and BTP5 a r e hot i s o s t a t i c a l l y p r e s s e d (HIP) p o w d e r s . T30 and EF1 a r e c a s t m a t e r i a l s , and w e r e t e s t e d in the a s - c a s t condition with the s t r e s s axis p e r p e n d i c u l a r to the long d i m e n s i o n s of the g r a i n s . EF1 was also tested as a fully r e c r y s t a l l i z e d upset f o r g i n g having a n e g l i g i b l e t e x t u r e and a n equiaxed g r a i n s i z e of a p p r o x i m a t e l y 200 g m . The r e m a i n i n g s a m p l e s in T a b l e I w e r e p r o duced by v a c u u m hot p r e s s i n g of p o w d e r s . 2.0 E X P E R I M E N T A L TECHNIQUE C o m p r e s s i o n c r e e p t e s t s w e r e p e r f o r m e d in a n a r gon a t m o s p h e r e on 1.27 cm diam s p e c i m e n s at t e m p e r a t u r e s between 922 K and 1366 K. The load was m a i n t a i n e d constant at l e v e l s chosen to give c r e e p r a t e s of 10~ to 10"~ pct p e r s. S t r e s s e s b e t w e e n 0.5 and 200 mN//m 2 w e r e u s e d . Length changes w e r e m e a s u r e d by a q u a r t z e x t e n s o m e t e r of 2.54 cm gage length. The d e t a i l e d c r e e p r e s u l t s a r e d e s c r i b e d e l s e w h e r e . 1 In this work the r e s u l t s a r e used only where they r e late to m i c r o s t r u c t u r a l and m e c h a n i s m i c c h a n g e s . M i c r o s t r u c t u r a l c o m p a r i s o n s w e r e made on s p e c i m e n s which had been t e s t e d u n d e r s i m i l a r c o n d i t i o n s of s t r e s s and t e m p e r a t u r e and which p o s s e s s e d s i m i l a r g r a i n s i z e s . However c h a n g e s in g r a i n size, oxide content, s t r e s s and t e m p e r a t u r e between 1000 and 1366 K did not a p p e a r to o v e r r i d e the e s s e n t i a l m e c h a n i s m i c changes due to low m e l t i n g point i m p u r i ties. S p e c i m e n s on which the s u r f a c e effects of d e f o r m a t i o n w e r e to be studied w e r e e n c a p s u l a t e d i n v a c u u m in a g l a s s chosen to be p l a s t i c at the t e m p e r a t u r e of t e s t i n g . In this way the b e r y l l i u m could be p l a s t i c a l l y DONALD WEBSTERand DONALD D. CROOKS are Research Scientists in the Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304. Manuscript submitted October 2, 1975. METALLURGICALTRANSACTIONSA

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