Effect of purity and processing procedure on the mechanical properties of beryllium sheet
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M. HERMAN,
AND
H. CONRAD
The purpose of this research was to assess what improvements might be obtained in the bend ductility of beryllium sheet by starting with high purity commercial powder and consolidating it into sheet by techniques that minimized deformation and texturing. Sound sheet was achieved with reduction ratios from 2:1 to 12:1 at temperatures from 700~ I050~ by forging, upsetting and compression roiling. Compression rolling is accomplished by successively changing the roiling direction by 30 deg after each pass so that no preferred rolling direction was developed. It was found that as the plastic bend ductility increased (up to 4 pct) the yield strength decreased. The higher bend ductilities corresponded to lower reduction ratios, lower degrees of preferred orientation and a larger subgrain size as determined by transmission electron microscopy. Sheet prepared by commercial rolling practices from the less pure commercial powder exhibited similar behavior. The ductile-to-brittletransitioh temperature for bending decreased with improved purity and reduced texture. T H E g r e a t potential of b e r y l l i u m for a e r o s p a c e a p p l i c a t i o n s , and its s u p e r i o r i t y r e l a t i v e to o t h e r m e t a l s has been e x t e n s i v e l y r e v i e w e d . ~-7 T h e s u p e r i o r i t y of b e r y l l i u m a r i s e s f r o m i ts high e l a s t i c m o d u l u s (43 x 10~ psi) and its v e r y low d e n s i ty (0.067 lb p e r cu in.) coupled with good s t r e n g t h , e . g . , y i e l d s t r e n g t h s b e tween 50 • 103and 90 x 103psi. Sheet is one of the s t r u c t u r a l f o r m s of p r i m e i n t e r e s t tn a e r o s p a c e a p p l i c a t i o n s . The l a c k of ductility in the t h i c k n e s s d i r e c t i o n of c o m m e r c i a l b e r y l l i u m sheet, h o w e v e r , s e v e r e l y l i m i t s its u s e f u l n e s s . B e c a u s e of this l i m i t e d ductility the s t r u c t u r a l r e l i a b i l i t y of the s h e e t is poor. This b e h a v i o r is due p r i n c i p a l l y to the fact that thinning is r e q u i r e d in the t h i c k n e s s d i r e c t i o n of s h e e t w h e n e v e r it is bent u n d e r conditions a p p r o a c h ing plane s t r a i n . The t e x t u r e p r o d u c e d in b e r y l l i u m s h e e t h o w e v e r is such that the c - a x e s a r e c o n c e n t r a t e d about the s h e e t n o r m a l and s i n c e the b e r y l l i u m c r y s t a l is i nc a p ab l e of p l a s t i c c o m p r e s s i o n in this d i r e c t i o n v e r y l i t t l e thinning can o c c u r . C o n s e q u e n t ly , b e r y l l i u m s h e e t f r a c t u r e s at v e r y low v a l u e s of p l a s t i c bend s t r a i n . The o b s e r v e d bend d u c ti li ty of b e r y l l i u m s h e e t is thus r e l a t e d to 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 , s The m o r e highly t e x t u r e d is the s h e e t , the l o w e r is its ductility. 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 in b e r y
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