The effect of trace element additions on the grain boundary composition of Ir + 0.3 pct W alloys
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IRIDIUM
base alloys are currently used as cladding and post impact containment m a t e r i a l for radioactive fuel in r a d i o i s o t o p e t h e r m o e l e c t r i c g e n e r a t o r ( R T G ) h e a t s o u r c e s .~ A n i m p o r t a n t r e q u i r e m e n t f o r t h e R T G f u e l c l a d d i n g i s that i t h a v e a d e q u a t e h i g h t e m p e r a t u r e i m p a c t (HTI) d u c t i l i t y . O t h e r r e q u i r e m e n t s i n c l u d e a high m e l t i n g p o i n t , high t e m p e r a t u r e s t r e n g t h , oxidation resistance, and comparability with radioisotope fuels. L i u a n d I n o u y e1'2 h a v e r e p o r t e d d r a m a t i c i m p r o v e m e n t s i n t h e H T I d u c t i l i t y of I r + 0.3 p c t W a l l o y s a s a r e s u l t of a d d i t i o n s of 40 p p m A 1 , 8 0 p p m F e , 30 p p m T h , 16 p p m N i , a n d 75 p p m R h ( d e s i g n a t e d D O P - 4 type I r + 0.3 p c t W ) . F i g u r e 1 c o m p a r e s s c a n n i n g e l e c t r o n micrographs of HTI fracture s u r f a c e s of undoped and D O P - 4 type I r + 0 . 3 p c t W a s r e p o r t e d by L i u a n d I n o u y e . 1'2 T h e u n d o p e d I r + 0 . 3 p c t W h a s c l e a r l y f a i l e d intergranularly w h i l e the D O P - 4 alloy has failed t r a n s granularly, with extensive necking p r i o r t o f a i l u r e . I m p r o v e d HTI ductility has also been r e p o r t e d f o r I r + 0.3 p c t W a l l o y s d o p e d w i t h v a r i o u s c o m b i n a t i o n s o f the d o p a n t s u s e d i n t h e D O P - 4 a l l o y .3-5 T h e c o m m o n f a c t o r in all o f t h e a l l o y s s h o w i n g i m p r o v e d H T I duct i l i t y i s t h e p r e s e n c e of a t l e a s t a f e w p a r t s p e r m i l l i o n of thorium. L i u a n d I n o u y e~'2 r e p o r t s p a r k - s o u r c e m a s s s p e c t r o m e t r i c (SSMS) a n a l y s i s of t h e f r a c t u r e s u r f a c e s of a n a l l o y s i m i l a r t o D O P - 4 that i n d i c a t e s e n r i c h m e n t of t h e f r a c t u r e s u r f a c e s in T h , A1, T a , a n d Fe.* T h e y *This alloy had approximately the same bulk concentration of aluminum and iron as DOP.4, but only about 6 wt ppm Th. also c i t e s o m e o f t h e A E S a n a l y s i s r e p o r t e d in t h i s p a p e r , t h a t h a s c o n f i r m e d t h e s e g r e g a t i o n of T h t o t h e g r a i n b o u n d a r y r e g i o n s i n the D O P - 4 a l l o y . T h e p u r p o s e of t h i s p a p e r i s t o r e p o r t t h e r e s u l t s of a s t u d y of the g r a i n b o u n d a r y c o m p o s i t i o n o f I r + 0.3 pct W alloys that were doped with v a r i o u s concentrat i o n s a n d c o m b i n a t i o n s o f A1, F e , T h , N i , a n d R h . T h e s e alloys all fail intergranularly at r o o m temperature, all o w i n g A E S t o be u s e d t o s t u d y t h e i r g r a i n b o u n d a r y c o m p o s if ion. CALVIN L. WHITE and ROBERT E. CLAUSING are Research Staff Members, and LEE HEATHERLY is Senior Laboratory Technician, Metals and Ceramic
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