Creep-rupture behavior of iridium at elevated temperatures
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not g e n e r a l l y c o n s i d e r e d as a s t r u c t u r a l m a t e r i a l b e c a u s e of it s e x p e n s e . M o r e o v e r , even though it is an fcc t r a n s i t i o n m e t a l , i r i d i u m is b r i t t l e at r o o m t e m p e r a t u r e , 1 which m a k e s it d if f i c u lt to f a b r i c a t e . Its p r i m a r y use has b e e n as an a l l o y i n g e l e ment, as a c r u c i b l e m a t e r i a l , and as a coating, z Howe v e r , b e c a u s e of its e x t r a o r d i n a r y i n e r t n e s s and c o r r o s i o n r e s i s t a n c e (it is the m o s t c o r r o s i o n r e s i s t a n t m e t a l known), 2 its use as a s t r u c t u r a l c o n t a i n m e n t m a t e r i a l has b e e n c o n s i d e r e d f o r a p p l i c a t i o n s w e r e e x c e l l e n t c o r r o s i o n r e s i s t a n c e is r e q u i r e d at e l e v a t e d t e m p e r a t u r e s f o r long p e r i o d s of t i m e . I r i d i u m has been s t u d ie d in s o m e d e t a i l r e c e n t l y by B r i t i s h i n v e s t i g a t o r s , t, ~,4 with its unusual b r i t t l e n e s s at t e m p e r a t u r e s as high as the r e c r y s t a l l i z a t i o n t e m p e r a t u r e (of the o r d e r of 1200~ r e c e i v i n g the m o s t attention. The b r i t t l e n e s s , which is m a n i f e s t e d by int e r g r a n u l a r f r a c t u r e a f t e r s t r a i n s of the o r d e r of s e v e r a l pet, is a t t r i b u t e d to g r a i n b o u n d a r y i m p u r i t y c o n c e n t r a t i o n s . 1 The c o n s t i t u e n t r e s p o n s i b l e f o r this b e h a v i o r has not b e e n identified. It m a y be noted that b e c a u s e its b e h a v i o r is s e n s i t i v e to i m p u r i t i e s , the p r o p e r t i e s of i r i d i u m can v a r y w i d e l y depending on the m e t h o d of f a b r i c a t i o n , s i n c e t h i s a f f e c t s i ts i m p u r i t y content. This h a s b e e n d i s c u s s e d by B r o o k e s , et al. ~
T h e r e is v e r y l i t t l e i n f o r m a t i o n a v a i l a b l e on the c r e e p and r u p t u r e b e h a v i o r of i r i d i u m , a s u b j e c t of v i t a l i m p o r t a n c e when c o n s i d e r i n g its use at e l e v a t e d t e m p e r a t u r e as a s t r u c t u r a l m a t e r i a l . To the a u t h o r s ' knowledge, the only i n f o r m a t i o n a v a i l a b l e is co n t ai n ed in a c o n t r a c t r e p o r t which is d if f i c u lt to obtain; s s o m e of its contents have been s u m m a r i z e d in Ref. 2. A c c o r d i n g l y , c r e e p - r u p t u r e t e s t s of i r i d i u m w e r e c o n ducted at e l e v a t e d t e m p e r a t u r e s f o r r e l a t i v e l y s h o r t t i m e s ( r u p t u r e t i m e < 1000 h). The t e s t t e m p e r a t u r e s w e r e 1288~ 1093~ and 982~ B e c a u s e of the s h o r t t e s t d u r a t i o n , low c r e e p s t r a i n s (~ ~ 1 pct) w e r e not obtained. H o w e v e r , s u f f i c i e n t data w e r e obtained to d e v e l o p a p a r a m e t r i c m a s t e r r u p
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