Creep behavior of electron-beam-melted rhenium
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R. W I T Z K E AND P E T E R L. R A F F O
Creep deformation in electron-beam-melted polycrystaltine rhenium sheet was evaluated at 2200° to 4200°F (1477° to 2588°K) and 4 to 40 ksi (28 to 276 MN per sq m). Comparisons were made with powder metallurgy rhenium under similar conditions. Changes in creep-rupture behavior resutting from electron beam melting of rhenium were greater ductility, higher primary creep rate, and longer rupture life, especially at lower temperatures. The activation energy for creep was 72 kcal per mole for electron-beam-melted rhenium and 64 kcal per mole for powder metallurgy rhenium. T H E p o t e n t i a l usefulness o f rhenium as a n a e r o s p a c e m a t e r i a l c a n b e e x p r e s s e d in t e r m s o f i t s r e f r a c t o r y character, stiffness, and low temperature ductility. Rhenium has a hcp c r y s t a l s t r u c t u r e and a melting p o i n t o f 5 7 5 0 ° F (3450°K), t h e s e c o n d - h i g h e s t o f t h e m e t a l s . I t s m o d u l u s of e l a s t i c i t y a t r o o m t e m p e r a t u r e a p p r o x i m a t e s 67 x l 0 s p s i (4.6 x 105 M N p e r sq m ) a n d it h a s e n o u g h d u c t i l i t y t o be f a b r i c a b l e a t r o o m t e m p e r a t u r e . A l t h o u g h it i s c o s t l y a n d s c a r c e , r h e n i u m a n d i t s a l l o y s c a n be e x p e c t e d t o be u s e d i n s p e c i a l i z e d d e s i g n applications. T h e mechanical properties of unalloyed rhenium h a v e not b e e n i n v e s t i g a t e d i n a s m u c h d e t a i l a s t h o s e of t h e o t h e r refractory m e t a l s , W , M o , Cb(Nb), a n d T a . L i t e r a t u r e r e v i e w s o f r h e n i u m up t o 19601'2 i n d i cate a s m a l l amount of data on the tensile and s t r e s s r u p t u r e p r o p e r t i e s of p o w d e r m e t a l l u r g y w i r e a s t h e only i n f o r m a t i o n available o n t h e high t e m p e r a t u r e p r o p e r t i e s of r h e n i u m . S i n c e t h a t t i m e , a d d i t i o n a l mechanical property data including c r e e p and s t r e s s r u p t u r e d a t a on p o w d e r m e t a l l u r g y s h e e t a t h i g h t e m p e r a t u r e s h a v e b e e n r e p o r t e d , s-5 B e c a u s e electron beam m e l t i n g has shown i m p r o v e d p r o p e r t i e s i n o t h e r m e t a l s due t o p u r i f i c a t i o n , t h i s vacuum m e l t i n g p r o c e s s c o u l d possibly r e n d e r i m p r o v e m e n t s i n r h e n i u m a l s o . In g e n e r a l , e l e c t r o n beam m e l t e d m a t e r i a l s h a v e been d e s c r i b e d a s h a v i n g s u p e r i o r d u c t i l i t y t o a r c - c a s t o r s i n t e r e d s t o c k .6 T h e o b j e c t i v e o f t h e p r e s e n t s t u d y w a s to c h a r a c t e r ize t h e high t e m p e r a t u r e c r e e p d e f o r m a t i o n o f p o l y crystalline rhenium sheet r o l l e d from an electronb e a m - m e l t e d (EB) ingot, C r e e p r u p t u r e , step-load, a n d s t e p - t e m p e r a t u r e c r e e p t e s t s were p e r f o r m e
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