The effects of temperature and strain rate on the strength of beryllium sheet
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AND
F.W.
COOKE
The s t r e s s - s t r a i n b e h a v i o r and the d e f o r m a t i o n d y n a m i c s of c o m p r e s s i o n r o l l e d and hot upset b e r y l l i u m s h e e t p r e p a r e d from SR powder w e r e i n v e s t i g a t e d in tension o v e r the r a n g e 25 ~ to 355~ The t r u e s t r e s s - t r u e s t r a i n c u r v e s w e r e a p p r o x i m a t e d by the r e l a t i o n = ~(0) + hE ~/~. Both ~(0) and h d e c r e a s e d with t e m p e r a t u r e , a(0) was higher at a l l t e m p e r a t u r e s for the c o m p r e s s i o n r o l l e d s h e e t , w h e r e a s h w as the s a m e for the two m a t e r i a l s . The d i f f e r e n c e in a(0) was in the a t h e r m a l c o m p o n e n t of the s t r e s s . The e f f e c t s of t e m p e r a t u r e and s t r a i n r a t e on the f l o w - s t r e s s of the p o l y c r y s t a l l i n e sh eet a g r e e d with t h o se f o r p r i s m slip in s i n g l e c r y s t a l s . T h e r m a l a c t i v a t i o n a n a l y s i s of the d e f o r m a t i o n d y n a m i c s yielded v a l u e s of 123 to 158b 3 for the a c t i v a t i o n v o l u m e , 1.5 x 107 see "t for the p r e e x p o n e n t i a l f a c t o r and 1.8 ev (0.18 pb ~) for the a c t i v a t i o n e n e r g y , Ho.
I T has not yet been possible to fully utilize the very desirable properties of beryllium (low density, high modulus, and relatively high strength) because of its brittleness. Since this brittleness is related to the deformation characteristics,~'2 an understanding of the plastic flow behavior is desired. Especially important is an understanding of the deformation dynamics, i.e. the effects of strain rate and temperature on the flow stress, which reflects the mobiIRy of dislocations. Therefore, a study of the deformation dynamics of beryllium sheet was undertaken in the present investigation. The thermally activated approach 3 was used, since it offers a useful way of analyzing the data. This investigation is part of a larger program dealing with the effects of fabricating procedures on the structure and properties of beryllium sheet. 4 M ATER I AL S AND P R O C E D U R E The two s h eet m a t e r i a l s (0.050 in. thick) e m p l o y e d in this i n v e s t i g a t i o n w e r e p r e p a r e d f r o m P e c h i n e y SR (Secondarily Refined) b e r y l l i u m powder by c o m p r e s s i o n r o l l i n g and by hot upsetting and had a BeO content of about 1 pct by wei g h t . D e t a i l s of the s h e e t f a b r i c a t i o n a r e d e s c r i b e d in a co m p a n io n p a p e r . * The g r a i n s in both t yp es of s h eet w e r e p a n c a k e - s h a p e d , being 25 p in diam in the plane of the s h e e t arid about 5;~ in the t h i c k ness direction. The t e x t u r e s w e r e the usual s y m m e t r i c a l b a s a l t e x t u r e exhibited by b e r y l l i u m s h e e t . The d e g r e e of p r e f e r r e d o r i e n t a t i o n was h i g h e r for the c o m p r e s s i o n r o l l e d sheet than the hot upset s h e e t , s i n c e the f
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