Brittle fracture in iridium
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F A C E - c e n t e r e d - c u b i c m e t a l s a r e t y p i c a l l y cons i d e r e d to be v e r y d u c t il e o r m a l l e a b l e . The e a s e of h a m m e r i n g gold into thin f o i l s is one e x a m p l e of m a l l e a b i l i t y . It is not g e n e r a l l y known that i r i d i u m and r h o d i u m , two of the noble r e f r a c t o r y fcc m e t a l s , e x hibit b r i t t l e f r a c t u r e o v e r a wide r a n g e of t e m p e r a ture. D o u g l a s s , K r i e r and J a f f e e 1 found that I r s i n g l e c r y s t a l s f a i l e d by t r a n s g r a n u l a r c l e a v a g e at r o o m t e m p e r a ture after substantial plastic deformation. Polycryst a l t i n e I r and Rh t e n s i l e s p e c i m e n s f a i l p r e d o m i n a n t l y by b r i t t l e i n t e r g r a n u l a r f r a c t u r e at t e m p e r a t u r e s b et w e e n - 196 and 1000~ Although the b r i t t l e f r a c t u r e b e h a v i o r of I r has r e c e i v e d c o n s i d e r a b l e a tt e n ti o n in the l i t e r a t u r e , 1-~3 it is c o n s p i c u o u s l y a b s e n t in t r e a t i s e s and textbooks on b r i t t l e f r a c t u r e . U n d e r s t a n d i n g the f r a c t u r e b e h a v i o r of I r has b e c o m e i n c r e a s i n g l y i m p o r t a n t in r e c e n t y e a r s b e c a u s e it is u s e d 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 f o r 238PUO2 in the heat s o u r c e s of r a d i o i s o tope t h e r m o e l e c t r i c g e n e r a t o r s . ~4 In this p a p e r we e x a m i n e the q u e s t i o n of t r a n s g r a n u l a r c l e a v a g e in I r on the b a s i s of s e v e r a l a n a l y t i c a l m o d e l s of b r i t t l e f r a c t u r e . We a l s o p r e s e n t e x p e r i m e n t a l e v i d e n c e to show that i n t e r g r a n u l a r f r a c t u r e in I r is i n t r i n s i c r a t h e r than a r e s u l t of i m p u r i t y s e g r e gation.
TRANSGRANULAR CLEAVAGE Experimental Observations D o u g l a s s e t a l ~ r e p o r t e d c l e a v a g e f r a c t u r e of I r s i n gle c r y s t a l s on (100) p l a n e s with many c l e a v a g e s t e p s . T h e y did not r u l e out the e x i s t e n c e of s e c o n d a r y c l e a v age p l a n e s . We f r a c t u r e d l a r g e s i n g l e g r a i n s of p u r e I r (taken f r o m an a r c m e l t e d button) by bending t h e m with p l i e r s . We a l s o c l e a v e d l a r g e s i n g l e g r a i n s of h i g h - p u r i t y I r - 0 . 3 pct W with a c h i s e l . T h e c h e m i c a l a n a l y s e s of t h e s e two m a t e r i a l s a r e p r e s e n t e d in T a b l e I. Both m a t e r i a l s c l e a v e d r e a d i l y a f t e r s o m e S. S. HECKER and D. L. ROHR are Staff Members, Los Alamos Scientific Laboratory, Los Alamos, NM 87544, and D. F. STEIN is Vice President of Academic Affairs, Michigan Technological University, Houghton, MI 49931. Mansucript submitted May 18, 1977. METALLURGICALTRANSACTIONSA
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